Chapter 10c - Diseases

Diseases of Canola

Fungal, bacterial and viral diseases may attack canola at any time from seeding to maturity. These diseases come from:

• the soil
• infected seed
• infected crop residue from within the field or
• airborne spores from neighbouring canola fields

Most fields in the major production areas have some disease present each year. The pathogens that cause these diseases are almost always present because they may maintain themselves in the soil, survive on infected residues or multiply on tissues of other susceptible host plants. Many of these diseases also attack other Brassica crops, such as mustard and common cruciferous weeds. Consequently, diseases are usually more severe in areas of intensive canola production. Disease severity varies with the year and the location, and is greatly influenced by farming practices and environmental conditions.

Worldwide, the diseases that can attack canola are:

• bacterial
  • bacterial leaf spot - Pseudomonas syringae
  • bacterial soft rot - Erwinia marginalis
  • bacterial soft rot Pseudomonas - Pseudomonas marginalis
  • black rot - Xanthomonas campestris
• fungal
  • Alternaria black spot - Alternaria spp.
  • anthracnose - Colletrotrichum higginsianum
  • blackleg - Leptosphaeria maculans
  • black mold rot - Rhizopus stolonifer
  • black root - Aphanomyces raphani
  • cercospora leaf spot - Cercospora brassicicola
  • clubroot - Plasmodiophora brassicae
  • downey mildew - Peronospora parasitica
  • fusarium wilt - Fusarium avenaceum and F. oxysporum.
  • graymold - Botrytis cinerea
  • light leaf spot - Pyrenopeziza brassicae
  • phymatotrichum root rot - Phymatotrichopsisomnivora
  • phytophthora root rot - Phytophthora megasperma
  • powdery mildew - Erysiphe polygoni
  • ring spot - Mycosphaerella brassicicola
  • root rot complex - Rhizoctonia solani, Fusarium and Pythium spp.
  • seedling disease complex - Rhizoctonia solani, Fusarium and Pythium spp.
  • sclerotinia white stem rot - Sclerotinia sclerotiorum
  • southern blight - Sclerotium rolfsii
  • verticillium wilt - Verticillium albo-atrum
  • white leaf spot and gray stem - Pseudocercosporella capsellae
  • white rust and staghead - Albugo candida
  • yellows - Fusarium oxysporum
• viral
  • cauliflower mosaic virus
  • radish mosaic virus
  • turnip mosaic virus
  • beet Western yellows virus
• phytoplasma-like
  • aster yellows

Fortunately many of these diseases are not present in Canadian canola crops. In Canada, fungal diseases such as sclerotinia stem rot, virulent blackleg, Alternaria black spot, seedling disease complex and the root rot complex are the major concerns for canola crops (Table 1). Seedling disease complex is very dependent on weather conditions and crop rotation. Root rot complex diseases are destructive, but sporadic, and sometimes linked to root maggot infestations or high rainfall and waterlogged soil, particularly during flowering. Staghead can be an important disease of B. rapa varieties.

Table 1. Major Diseases of Canola

Disease	Appearance	Damage	Control
Seedling Disease Complex	Seed and seedlings fail to germinate and emerge Emerged seedlings have constricted roots just below soil surface, topple and die Stands can be thin, slow to emerge or patchy Young above ground plant parts may have a purple or chlorotic appearance	Healthy seedlings usually compensate for dead seedlings; however, yields are reduced in very uneven and patchy stands Severe loss may necessitate reseeding	Use certified seed Sow shallowly into a firm, moist, warm seedbed with adequate fertilizer Use fungicide treatments
Root Rot Complex	Foot rot: Hard brown lesions at stem base Salmon coloured spore masses often present in lesion	Losses are minor as lesions develop late in season; and early lesions cause premature ripening and reduced yields	Crop rotation Control volunteer canola and cruciferous weeds in rotation Use clean seed
	Brown girdling root rot: Light brown lesions on taprootand at bases of larger roots Tap root finally girdled, leaving a stump	A major disease of B. rapa varieties in the Peace River region Yield losses from pod abortion, premature ripening, seed weight loss and shrivelled seed	Use B. napus varieties where possible, as they are relatively less susceptible Use management practices for seedling disease complex Balanced fertility may help to lessen the impact
Blackleg	Weakly virulent: Whitish spots on leaves and stems peppered with small dark fruiting bodies Stem lesions may be shallow and grey or black	Weakly virulent blackleg is widespread, but of minor importance Infects plants very late in season	Use a minimum of a four-year rotation Control volunteer canola and cruciferous weeds Stubble and crop residue management necessary Seed treatments may reduce spread into areas where disease is absent Use resistant or moderately resistant varieties
	Virulent: Whitish spots on leaves and stems with small dark fruiting bodies Stem cankers deep, brown with a dark margin, and may cut plant off causing severe lodging and ripening with shrivelled seed	Virulent blackleg can cause severe early infections that result in high yield losses
Sclerotinia Stem Rot	Premature ripening of plants Stems bleached and tend to shred Hard black sclerotia inside stems near base of stalk and other bleached areas	Yield losses about equal to 0.5 x percentage infection Can be severe in higher rainfall areas of Alberta and Manitoba with up to 50% or more yield losses when conditions are ideal	Use a minimum four-year rotation with non-susceptible crops Use pedigreed seed Control weeds and volunteers Use B. rapa varieties Manage stubble and crop residue (direct seeding might reduce apothecia production Foliar fungicide treatments are effective
Alternaria Black Spot	Black, brown or greyish spots on leaves, stems and pods Pod splitting may occur	Very widespread in canola growing areas Can be severe if moist conditions occur during podding Severe early infections may kill plants Severe infections at ripening may cause yield loss	Sow well cleaned seed of high germination Use minimum three-year rotation Control volunteer canola and cruciferous weeds Early swathing will reduce seed loss B. napus types tend to be less affected

Other fungal diseases that may be present in a canola crop in western Canada, but seldom cause significant yield losses are gray stem, avirulent blackleg and downey mildew. Clubroot has only rarely been found in canola in western Canada. Bacterial, viral and aster yellows type diseases are usually minor concerns.

Seedling Disease Complex and Root Rot Complex

The same fungi, primarily Rhizoctonia solani, cause both diseases in this complex although other fungi Fusarium species and Pythium species may be present. Seedling disease complex exhibits several symptoms-seed decay, pre- and post-emergence damping-off (wirestem), seedling blight and seedling root rot.

The symptoms of root rot complex are foot rot, late root rot, root rot and brown girdling root rot. Research by Agriculture and Agri-Food Canada (AAFC) Beaverlodge, AB Research Centre found that nearly half of the microorganisms isolated from brown girdling root rot infected root tissue were identified as Rhizoctonia solani (Figure 1).

Figure 1. Microorganisms Isolated from Diseased Root Tissue at Hythe and Grande Prairie, AB 1991

Rhizoctonia solani exists as several strains with anastomosis group AG2-1 commonly identified in the Peace River region and the less virulent AG4 in other regions. In most of western Canada, the disease manifests itself primarily at the seedling stage, while in the Peace River region both seedling loss and the much more serious brown girdling root rot occur.

Seedling Disease Complex

The most widespread problem of canola production is stand establishment. Seedling disease complex accounts for most of the failure of viable seeds to emerge as seedlings and may result in partial to complete loss of plant stand. The problem appears to be greatest under cold conditions or when the seedbed is not firmly packed under dry, cool conditions. The disease complex is most often a problem in the northwestern prairies with prolonged low soil temperatures. If seedling losses are uniform throughout the stand, surviving canola plants will compensate by growing larger. If the loss is patchy and large areas die out, then compensation cannot take place and reseeding is required.

Symptoms

The symptoms of this complex appear as patchy emergence during the four weeks following seeding, or up to the fourleaf stage. Seeds fail to germinate and become soft and pulpy (seed decay). Seeds germinate but the developing seedlings decay and fail to emerge (pre-emergence damping-off). Seedlings emerge and appear normal above ground, but either the roots decay with rot moving rapidly up into the hypocotyl, or the young stem or hypocotyl may be partially or completely girdled with decay and shrivel at any point. The decay spreads in both directions (Figure 2). The young above ground parts of the canola seedling may also exhibit a purple or chlorotic discolouration. When the decay reaches the soil surface, the emerged part of the seedling topples over, wilts and dies (seedling blight, wirestem or post-emergence damping-off). The hypocotyl appears constricted or shrivelled and may be discoloured reddish brown. In moist topsoil, the shrivelled stem may persist for a while, but in dry windy conditions, the whole seedling disappears in a few days. Seedlings may also emerge and then stagnate in the two- to four-leaf growth stage even when growing conditions appear favourable. The disease complex also attacks seedling rootlets resulting in root pruning (seedling root rot).

Figure 2. Canola Seedlings with Seedling Disease Complex

Photo by Phil Thomas

Flea beetle and/or cutworm damage may occur together with the disease complex, but do not mistake them for disease. Flea beetles do not cause the hypocotyl to rot off or constrict at or below the soil line. However, they do eat portions out of the hypocotyl at or below the soil line. Young cutworms chew holes and notches into leaves while older cutworms eat into stems and usually sever them at or just above the soil surface.

The effects of seedling root rot and partial girdling of the hypocotyl may be less apparent in some cases. Plants with damaged root systems may stagnate for a while, then grow and mature in the usual way. The fungi may persist throughout the life of the plant on its feeding rootlets, reducing the efficiency of the root system in utilizing moisture and nutrients, and reducing tolerance to moisture stress in midsummer heat. This results in a stand with poor vigour and reduced yield.

Disease Cycle

Dry seeds become vulnerable to attack by fungi as soon as they take up water prior to germination. The fungi grow in the soil when conditions are suitable or are stimulated by secretions from germinating seeds or roots of host plants. The fungi are usually weak pathogens able to infect only young succulent roots and hypocotyl tissue. Once in the plant, the fungi multiply causing decay that damages or kills the seedling. The fungi can form microscopic resting bodies within or on the surface of the infected tissue following the death of the cells. The resting or dormant bodies allow the fungi to survive until another susceptible host becomes available. At the two- to four-leaf stage, the below ground parts of the plant become sufficiently woody to withstand further infections and are able to regenerate more rootlets than they lose.

Vigorous, fast-growing seedlings reach this stage quickly, reducing the susceptible period and withstand infection better than slow-growing plants.

Soil moisture and temperature influence disease severity. Cold damp soils favour Fusarium species while loose; cold, dry, well-worked soils favour Rhizoctonia solani; and wet, heavy soils favour Pythium species. Seed-borne fungi rarely cause canola emergence problems.

Management

For quick germination, emergence and vigorous seedling growth, plant high germination certified seed 10 to 20 mm (0.4 to 0.8") deep into firm, moist, adequately fertilized soil when the temperature is above 10°C. Stresses from extreme weather conditions or physical or chemical injury predispose seedlings to infection. Typical causes of stress are:

• deep seeding
• poor seed quality
• toxic herbicide residues
• flea beetle injury
• placement of excessive fertilizer with the seed

Soil temperatures below 10°C delay germination and emergence, reduce growth rate and vigour of seedlings, and prolong the period of seedling susceptibility. Avoid deep seeding which places seed into colder soil and forces it to grow through more soil to emerge. This increases the time that the seedling is dependent on food reserves of the seed and the period of susceptibility. Avoid deep seeding, which increases the infested crop residue particles that come into contact with the seedling. Greatest losses come from seeding early into cold soils and from deep seeding.

Maintain soil fertility and avoid harmful quantities of fertilizer with seed. Fertilizer placed with the seed may delay or reduce germination and emergence, prolong the period of susceptibility and increase infection. Avoid inadequate or unbalanced nutrients, which also favour the disease complex fungi.

Reseeding a crop destroyed by the seedling disease complex is usually successful if the soil temperatures and moisture conditions are favourable for rapid germination and vigorous seedling growth.

Rotate with non-cruciferous crops. Severe damage can result when canola is sown after canola or a canolasummerfallow rotation.

Seed treatment fungicides offer some protection against disease. However, the results may be unpredictable because no single fungicide is effective against all three fungi. Fungicide-insecticide combination seed treatments also provide early protection against flea beetle injury.

Root Rot Complex

Foot rot symptoms occur sporadically throughout canola growing areas. The fungi causing these symptoms are potentially serious, but so far have been a minor problem. Incidence, as well as intensity, is increasing throughout the prairies with some fields having over 50% of the plants infected late in the season. Of the root rots, only brown girdling root rot is of major economic importance.

Brown girdling root rot occurs only occasionally in the northern parkland zone, but in the Peace River region of Alberta and British Columbia it is an extremely widespread and a serious problem in B. rapa varieties. In this region, brown girdling root rot causes greater yield losses than all other diseases. For the period 1987-1993, 100% of surveyed fields were infected with estimated losses ranging from 56 to 784 kg/ha (1 to 14 bu/ac), depending on the year, with an average loss of approximately 448 kg/ha (8 bu/ac) or 30%. Yield losses of up to 55% in individual fields have occurred. In any one year, one-third of B. rapa fields may suffer significant losses, 10% in a season of normal moisture and up to 50% in a wet season.

Foot Rot Symptoms

In the growing season, brown, hard, clearly defined lesions occur near the stem bases of canola plants. The lesions may be black-bordered, and during periods of humid weather, pink spore masses may develop on diseased root tissues. Discolouration of the upper part of the taproot above the lateral roots may occur. In severe cases, the stem is girdled, killing the plant. Yield loss may occur when the stem is onehalf girdled. Yield losses are light when lesions occur late in the season. Affected plants may ripen prematurely as scattered plants or in patches in the field. Where foot rot develops late in the season, the earlier maturing B. rapa varieties may escape infection or lesions may not have time to develop to the point that yield is seriously reduced.

Root Rot Symptoms

The symptoms are variable in colour and shape but can be grouped into four general types:

• a light grey oval lesion of the upper taproot
• a dark grey discolouration of the lower taproot and internal tissue, later becoming black (Figure 3)
• a light brown, soft, diffuse taproot lesion
• a dark brown, sunken, sharply defined taproot lesion

Figure 3. Root Rot

Photo by Phil Thomas

Brown Girdling Root Rot Symptoms

Light brown lesions with irregular margins first appear at or after the onset of flowering, usually 7.5 cm (3") or more below the soil surface on the taproot or main lateral roots. Later lesions may appear anywhere on the taproot. As the lesions develop they expand, grow together, become sunken and eventually girdle the taproot (Figure 4). The sunken lesions become dark brown. Roots below the girdling rot off. The lesion continues to develop upwards, sometimes to the soil surface but never moves into the stem. Root tissues above the infected part become swollen.

Figure 4. Brown Girdling Root Rot

Photo by Beth Hoar

In moist conditions, the whole taproot may be destroyed up to the soil surface. Girdled plants may survive and even set some seed if they are not uprooted or blown over by wind.

In dry soil, a sound taproot stub remains. The above ground parts of the plant remain turgid as long as there is any root connection with moist soil. Taproot stubs that retain a few roots are sometimes capable of regenerating main laterals as well as fibrous roots. Plants with girdled taproots wilt, dry up and shrivel, even though soil moisture may be adequate for plants with normal root systems.

Yield losses will depend on the amount of root system lost by girdling. If only root stubs are present or brown lesions girdle the taproots, the disease will result in considerable yield loss. If brown spots are present, but do not girdle the root, actual disease losses may be minimal. Losses are a result of increased pod sterility, loss of seed weight, and seed shrivelling and plant death from desiccation or wind toppling. Girdled plants that survive ripen prematurely and tend to be pulled out of the ground rather than cut during swathing, increasing shatter losses. Damage from disease is usually greatest when wet soil conditions during first flowering permit extensive root rot development, followed by high temperatures and dry windy conditions.

Brown girdling root rot occurs on all principal soil types and textures of the Peace River region, in all crops. Disease development appears to be favoured by fine-textured heavy clay soils with high levels of copper (4 to 20 ppm). Research at the Agriculture and Agri-Food Canada (AAFC) Research Centre in Fort Vermilion, AB found that disease severity was similar for all tillage systems. Brown girdling root rot can be more severe when canola follows canola or in canola following clover or fescue than on canola following summerfallow. Decomposing fescue sod, for reasons not clear, is conducive to the development of this disease. It is unpredictable because it can also be severe where canola has never been grown previously, and even on freshly broken land that has never been cropped. Several cruciferous weeds, including stinkweed, shepherd's purse and ball mustard, also suffer from root rot.

Management of Root Rot Complex

Consider these measures for control of root rot:

• Maintain recommended N, P, K and S fertility levels in the soil. Nitrogen decreases disease severity. Liming the soil may be beneficial in reducing disease severity.
• Allow at least three years between canola crops, and control volunteer canola and cruciferous weeds during rotation. Consider including field peas in the rotation (to improve the nitrogen level) to reduce disease severity.
• Use B. napus varieties since they are only moderately susceptible to root rot, compared to the more susceptible B. rapa varieties.
• Use the management practices outlined for the seedling disease complex to establish a vigorous, uniform growing crop.
• Use a recommended seed fungicide treatment to control the seedling blight stage of this disease. No economical chemical controls are available for brown girdling root rot.

Blackleg

Blackleg is caused by the fungus Leptosphaeria maculans. It is a serious disease of canola and can cause significant yield losses in susceptible varieties. Blackleg occurs in a mild or weakly virulent strain and in a highly virulent strain. The weakly virulent, or mild type is widespread in western Canada. This strain usually infects plants very late in the season, rarely causing yield losses of even 1 to 2%. Therefore, it is considered a minor problem. Virulent or severe blackleg infects canola seedlings and progressively damages the growing crop in June and July. It causes major crop loss by severely damaging or killing infected plants. The virulent strain was first detected in 1975 in east central Saskatchewan. Since then virulent blackleg disease has become widespread throughout Saskatchewan. By the late 1980's, blackleg was present in almost all fields surveyed in Saskatchewan (Figure 5).

Figure 5. Mean Percentage of Standing Canola Fields with Blackleg Infected Plants and Infected Plants/Field in Saskatchewan (1975-1991)

Before the widespread adoption of moderately resistant to resistant varieties that became available in the early to mid 1990's it was estimated that blackleg disease caused over $500,000,000 in losses to Saskatchewan canola growers.

Surveys in Saskatchewan in 1989 found blackleg in 90% of the fields with an average of 52% of plants infected per field. A few fields were reported with 100% infection with yield losses higher than 50%. However, blackleg surveys in that province in the mid 1990's found that 75% of B. napus fields and greater than 90% of B. rapa fields had blackleg at infections levels of only 5% and 7%, respectively. In Alberta, Manitoba and Ontario virulent blackleg was identified in the early 1980's although most growers in these provinces did not experience the levels of losses experienced in Saskatchewan. Both provinces also reported declines in blackleg incidence in the early 1990's. This decline in virulent blackleg incidence in western Canada occurred due to grower adoption of more resistant varieties and sound disease management practices.

Symptoms

Blackleg infections may occur on cotyledons, leaves, stems and pods. Leaf spots are dirty white, round to irregularly shaped, and usually dotted with numerous small, black pycnidia (pepper-like spore-bearing structures (Figure 6). Pycnidia appear as tiny round specks that may be seen more easily with the aid of a hand lens. Under moist weather conditions, pink spore (pycnidia spores) masses ooze from the pycnidia.

Figure 6. Blackleg on Canola Leaf with Pycnidia

Photo by Beth Hoar

On stems, blackleg lesions can be quite variable, but are usually found at the base of the stem or at points of leaf attachment. Stem infection before the six-leaf stage is usually associated with serious yield loss. Stem lesions may be up to several inches in length, and are usually white or grey with a dark border. Numerous pycnidia form in the centre of the lesion (Figure 7). Basal stem lesions may also appear as a general blackening at the base, again with numerous pycnidia.

Figure 7. Blackleg Stem Lesions

Photo by Beth Hoar

Severe infection usually results in a dry rot or canker at the base of the stem. The weakly virulent form of the fungus usually infects plants near maturity, resulting in shallow stem lesions, rarely forming extended cankers that girdle the stem. The highly virulent form of the fungus attacks the crop earlier. If basal infection begins early, stem cankers appear from flowering onwards. As the season progresses, cankers penetrate, deepen and may girdle stem bases, often completely severing the plant. By mid-July, plants ripen prematurely and may start lodging. Less severely affected plants remain standing but have restricted moisture and nutrient flow. Hard, black fruiting perithecia, slightly larger than pycnidia, may form on the basal stem cankers late in the year or in the following year (Figure 8).

Figure 8. Blackleg Perithecia on a Canola Stem

Photo by Beth Hoar

Pods and seeds may also be infected. Infected pods ripen and shatter easily at harvest, resulting in seed loss. The seed beneath pod lesions may be sunken or shrivelled and pale grey. The earlier pod infection occurs, the less likely viable seed will be produced as the fungus infects the seed coat and embryo.

Disease Cycle

The disease cycle is shown in Figure 9.

Figure 9. Blackleg Disease Cycle

The fungus overwinters on infected seed and canola stubble. Seed infection is seldom more than 1 to 2% and is not regarded as a major source of infection within crops. However, infected seed can spread the disease into new areas. A 1% infected seed lot could have up to 25,000 infected seeds/ha (10,000/ac). A canola crop planted with diseased seed could develop scattered areas of infection over a field. Seed is infected by dormant mycelium (the vegetative phase of the fungus) on or under the seed coat and not by spores. Mycelium growth resumes when the seed germinates.

The major source of widespread heavy field infection that causes high yield loss is the spores produced from diseased stubble, especially infected root pieces. Stem infections eventually progress internally down into the taproot of the canola plant. The taproot being very woody in nature will resist decay and can help the blackleg pathogen survive for three years or more. Seedlings, infected either from diseased seed or by spores from previous crop stubble, develop fungal lesions on the cotyledons or stems. Under the right conditions, spore-producing structures called pycnidia then appear as pepper-like spots on the lesions. These pycnidia ooze out masses of tiny, short-lived summer spores called pycnidiospores. These spores are spread short distances (up to 1 m or 40") by wind and rain to nearby healthy plants to start new infections. Once the leaves are infected the fungus grows down the petiole into the stem, eventually leading to the most damaging phase of the disease, stem cankering, usually at ground level. This cuts off moisture and nutrient movement in the plant. Flowering plants may start lodging as the fungus girdles stem bases, often completely severing or cutting off the plant. Infection before the six-leaf stage is usually associated with serious yield loss. The earlier the infection occurs, the greater the likelihood of basal stem canker development. Infections initiated after the six-leaf stage cause less damage than early infections. Less severely affected plants may survive but produce less seed and these are of poor quality. Wounded stems from hail or insects may be infected directly from spores germinating in the wound. Pods may also be infected which can lead to a small level of infected seed.

Stubble from plants with stem cankers produce pycnidiospores the following year. Infected stubble can continue to produce pycnidiospores for three to five years. Another type of spore may also develop in infected stubble after the disease has been present in an area for some time. This larger and longer-lived sexual spore (ascospore) is produced from the hard, black perithecia on basal stem cankers. Ascospores result from the mating of two distinct strains of the virulent blackleg fungus that infected the same plant. Ascospores are very common in heavily infected regions of Saskatchewan. Ascospores are discharged from the previous year's stubble usually in July or August but occasionally in May or June. Infected stubble will produce ascospores for three to five years, with peak production normally occurring in the second year after infection. Twoyear- old or older infected stubble may begin discharging thousands of these spores into the air in April or May and can continue to produce spores until October (Figure 10).

Figure 10. Ascospore Production by Blackleg on One-Year-Old and Older Canola Stubble Residue (#/1.5 hr sample/mo.)

About 1 mm (0.04") of rain and moderate temperature (16 to 20°C) are required to induce a large ascospore discharge, although small discharges result from light showers or even heavy dew. Most ascospores are wind-borne only a few hundred metres (yards), although some may travel 5 km (3 miles) or more. Ascospores that land on canola or volunteer canola plants in the field or in nearby fields can begin new infections. Once infection is established the disease can spread within a crop by spores produced in pycnidia and distributed by rain splash. Canola stubble, especially larger root pieces, can take two to three years to break down and up to five years during a series of dry seasons.

Warm, dry conditions slow the spread of the disease while prolonged moist weather favours rapid spread and development. Wild mustard is the only important weed host of the virulent strain.

Management

Follow these steps to help control blackleg:
Resistant varieties
Grow B. napus varieties that are moderately resistant or resistant to blackleg in areas with a high potential for virulent blackleg. A resistant variety is not immune to the disease. Stems can still be infected as shown by the scale used for determining the blackleg classification of varieties (Table 2).

Table 2. Classification of Canola Cultivars for Blackleg Resistance

Class	% of Westar
Resistant	<28
Moderately Resistant	29 to 45
Moderately Susceptible	46 to 63
Susceptible	64 to 100

Varieties rated "R" or "MR" for blackleg will suffer less yield loss than those with less resistance. All current B. rapa varieties are susceptible to blackleg disease. In areas where virulent blackleg is at low levels or has not been found, all registered varieties are suitable. Mustard varieties are highly resistant to blackleg.

Rotation

Use a crop rotation of non-host crops with canola no more frequent than once every four years to allow the canola stubble to decompose. While there is always a possibility that blackleg could be introduced to a field by windborne spores, longer rotations reduce the probability of high levels of disease infection. Avoid planting canola within 1 km (0.6 mi) of infected land for three years. Research studies on susceptible varieties in 1986 by AAFC Research Centre in Saskatoon, SK showed that longer rotations increase the probability that low levels of disease infection will disappear when the infected stubble has rotted (Figure 11).

Figure 11. Effect of Rotation on Blackleg Disease

Observations from this study:

1. In a field where canola had not been grown as a previous crop, 39% of the plants were infected as spores were blown into the field from an adjacent field. Even with moderate levels of infection yield loss was slight due to low levels of severe basal stem cankers.
2. Similarly in a field with a three-year break between canola crops, 89% of plants were infected but few developed severe basal cankers. After four years much of the inoculum in the field had died out and plants often had been infected at a later growth stage when they were less susceptible to damage.
3. A field with a two-year break between canola crops showed a higher incidence of infection, moderate basal cankers and a 24% yield loss.
4. With only a one-year break between canola crops nearly all of the plants showed infection with a majority having severe basal stem cankers and a severe yield loss of 48%. Even with a light level of infection in the previous canola crop there was sufficient inoculum to cause severe infection two years later.
5. In all the fields that had canola previously, most of the infection appeared to come from old infected residue within the field itself and not from adjacent fields. Similar results were shown in a 1999 Manitoba AAFC study in which the incidence of blackleg dropped with longer rotations (Figure 12).

Figure 12. Effect of Rotation on the Incidence of Blackleg Disease (Manitoba 1999)

Since blackleg reproduces sexually, an infected canola field will have a population of many individuals that are the result of sexual recombination. When challenged with a resistant variety, the individuals that tend to survive and reproduce on that new variety are the more virulent in the population. Shorter rotations can increase the selection pressure to favour more virulent individuals in the blackleg population and may overcome resistance in the variety being grown.

Seed

Purchase pedigreed seed that has been tested for blackleg. Seed testing is an important preventative measure in areas where the disease is not established.

Fungicide Seed Treatments

Treat all canola seed with recommended fungicides. Even if seed does test blackleg-negative, there could be a few infected seeds in the total seed lot. A fungicide treatment will decrease the probability of blackleg surviving on the seed. Fungicides effectively reduce seed-borne blackleg but do not guarantee 100% control. Where virulent blackleg is already established, infected seed is of relatively little importance to disease development in comparison to ascospore infection from infected stubble. Seed treatment will protect plants from infected seed but will not protect seedlings from infection by airborne spores. Seed treatment is essential to keep the disease out of areas where it is not established. Seed provides an effective means of long distance spread and introduction of the fungus into areas where it does not occur.

Weed Control

Practice proper weed control. Control volunteer canola as the disease can live from one year to the next on volunteer canola and wild mustard. Without proper weed control a long crop rotation will be ineffective.

Tillage

Bury canola stubble in the top 12 cm (5") of soil in the field. This speeds stubble decomposition and reduces disease infection potential in nearby fields. Where soil erosion is a problem incorporate the stubble just before planting. In the following three years, use shallow tillage or direct seeding to avoid bringing infected residue, especially root pieces, to the surface.

Foliar Fungicide

A registered foliar fungicide can be applied at the rosette stage for blackleg control, however, there is unlikely to be much evidence of the disease at this stage. While it may be possible to identify blackleg lesions on the leaves, stem cankers are generally not visible at this growth stage. The fungicide will provide about three weeks of protection that could delay the development of stem cankers and result in higher yields for susceptible varieties. The plants can still be infected at later growth stages so some stem cankers may be visible later in the season, and pod and seed infections could still occur.

Sclerotinia Stem Rot

The fungus Sclerotinia sclerotiorum that occurs in all the canola growing areas of Canada causes stem rot. Stem rot of canola is a sporadic but often severe disease in Canada. The disease is usually most severe in the higher rainfall areas of Alberta and Manitoba. Infections are usually light in the dryland areas and low to moderate elsewhere (Figure 13).

Figure 13. Percentage of Fields Infected and Average Field % Infection by Region in Manitoba 1997

However, the severity of stem rot varies from year to year, and even from field to field within a region (Figure 14).

Figure 14. Sclerotinia Infection Variations within Areas of Alberta 1996

In a random survey of about 300 Alberta growers in the early 1980's, 35% reported problems with sclerotinia stem rot; one-half with moderate to heavy damage. The yields of those with moderate to heavy damage were 224 to 336 kg/ha (4 to 6 bu/ac) lower than those reporting little or no damage.

Plants are infected when the canola crop is in bloom. Variation in the percentage of infected plants is due to:

• differences in the quantity of infectious spores
• plant population
• crop height and vigour
• severity of lodging
• rainfall
• soil moisture
• temperature

With the right combination of crop density and weather conditions or irrigation, heavy infections can develop almost anywhere. Even after plants are infected, the severity of stem rot symptoms and the resulting effect on yield will vary according to temperature, rainfall, crop density and especially the stage of crop growth at the time of infection.

Yield losses reflect yield reduction per infected plant and the percentage of infected plants in a crop. In general, when conditions for the disease are favourable and infections occur throughout the flowering period, yield reduction per infected plant can equal 50% or more. Yield losses can be attributed to:

• smaller and fewer seeds
• premature ripening
• shattered pods
• loss of smaller, shrunken seeds during combining

A formula has been developed to estimate the bushel loss due to sclerotinia infection (loss/infected plant X infected plants X yield potential). For example, if the yield loss per plant is 50%, 25% infected plants and a yield potential of 2,240 kg/ha (40 bu/ac), the loss would be 0.5 x 0.25 x 2,240 (40) = 280 kg/ha (5 bu/ac).

However, if infections were delayed until late bloom or if dry weather set in after early bloom, yield reduction per infected plant could be as little as 10%. Using the example of a crop with 25% infected plants, the loss would be 0.1 x 0.25 x 2,240 kg/ha (40 bu/ac) = 56 kg/ha (1 bu/ac). Use this calculation as part of the process of deciding whether to spray to control the disease.

Symptoms

Two to three weeks after infection, soft watery lesions or areas of very light brown discolouration become obvious on the leaves, main stems and branches. Lesions expand, become greyish white, and may have faint concentric markings (Figure 15). Plants with girdled stems wilt, ripen prematurely and become conspicuously straw-coloured in a crop that is otherwise still green. The stems of infected plants eventually bleach and tend to shred and break (Figure 16). Infected plants may produce fewer pods per plant, fewer seeds per pod or small, shrivelled seeds that blow out the back of the combine. The extent of damage depends on whether the main stems or branches are infected and at what stage during flowering infection occurs. Severely infected crops frequently lodge, shatter at swathing and are difficult to swath (Figure 17).

Figure 15. Sclerotinia Lesion on Canola Stem

Photo by Beth Hoar

Figure 16. Sclerotinia on Canola Stem

Photo by Beth Hoar

Figure 17. Sclerotinia Infected Stems in Lodged Canola

Photo by Phil Thomas

When the bleached stems of diseased plants are split open, a white mouldy growth and hard, black resting bodies (sclerotia) are visible (Figure 18). Sclerotia vary in size and shape. They may be small and round like a canola seed or up to 2 cm (0.8") long and cylindrical, ovoid or irregular in shape. Under moist conditions, sclerotia and the white mouldy growth may also occur on the surface of infected areas of the plant. At harvest the sclerotia are either threshed out with the seed or left in the field.

Figure 18. Sclerotia Inside Canola Stem

Photo by Beth Hoar

Disease Cycle

The sclerotinia disease cycle is shown in Figure 19.

Figure 19. Sclerotinia Disease Cycle

The stem rot fungus overwinters as sclerotia in the soil, in stubble at the soil surface and mixed with seed. Sclerotia are storage organs that can remain viable in the field for five years or more. Studies done at the University of Saskatchewan in Saskatoon, SK showed that sclerotia are present in virtually all soils of western Canada at numbers sufficient to cause devastating infection levels. Each year some sclerotia will germinate when conditions are suitable but others will remain dormant. Germination produces either mycelium (microscopic filaments), which may infect plants in direct contact with sclerotia, or spore-producing apothecia (small golf-tee shaped structures) as shown in Figure 20.

Figure 20. Apothecia

Photo by Phil Thomas

Most infections in canola result from airborne spores produced by apothecia at the soil surface. However, for sclerotia to germinate and produce apothecia, they require prolonged moist soil conditions (at least 10 days above the wilting point) and moderate temperatures of 15 to 25°C. Normally, such conditions do not occur until the crop canopy closes and permanently shades the soil surface. Since this is typically at the late rosette stage, with the 10- day delay, apothecia appear as flowering starts. Only sclerotia in the top few centimetres of soil will produce functional apothecia, since the apothecial stalks are rarely longer than 5 cm (2"). Deeply buried sclerotia will not produce apothecia but can remain dormant. If brought near the surface by cultivation, they may germinate.

A single sclerotium can produce up to 15 apothecia, either at one time or over a period of weeks. Apothecia only grow from sclerotia and not from any plant tissue or residue in the soil. Occasionally apothecia emerge from inside plant tissue from sclerotia trapped inside undecomposed stubble. In western Canada, apothecia normally begin to appear in June but most develop during flowering. Apothecia can continue to develop until late September but the critical period for causing damaging infections is from early to full bloom. Research has shown that plant infection rarely developed before the plants were in the mid-flowering stage and that apothecia rarely appeared in the field before plants were in bud.

Apothecia (Figure 20) are typically 5 to 15 mm (0.2 to 0.6") in diameter. The upper end of this range is about the size of a small fingernail or slightly smaller than a dime. Apothecia produce millions of microscopic spores that are released into moving air currents. Even the lightest breeze easily carries the spores across a field or into adjacent fields, possibly as far as several kilometres (miles). Honeybees can also carry spores.

Spores cannot infect the leaves and stems directly-they must first grow in dead petals or other organic material adhering to leaves and stems. The petals provide the food source necessary for the spores to germinate, grow and eventually penetrate the plant. Infection usually occurs in the leaf axils or at points of stem branching where the airborne or petal-borne spores land and droplets of water can be frequently found (Figure 21). Moist conditions from rainfall or heavy dew, which may keep leaves and stems wet for two to three days, are also necessary for infection. Spores can remain alive for up to 21 days.

Figure 21. Start of Sclerotinia Infection from Canola Petal in Leaf Axil

Photo by Beth Hoar

When leaves are infected the fungus can spread down into the stem. The fungus grows and invades healthy stem tissue when conditions are favourable. A dense canopy provides better conditions for symptom development. A study at Outlook, SK found the incidence of sclerotinia was directly related to the degree of lodging (Figure 22).

Figure 22. Relationship Between Lodging and Sclerotinia Infection of Irrigated Canola

Heavy stands tend to lodge, and stem rot will spread from plant to plant by direct contact, especially if wet weather delays swathing. Spread will also occur in wet swaths. The fungus eventually forms new sclerotia in diseased plants that are returned to the soil at harvest, completing the disease cycle.

Management

Rotation

Allow a rotation of at least four years between susceptible crops. While crop rotation is less effective in controlling sclerotinia (Figure 23) (due to wind-blown spores from nearby fields), research at AAFC Brandon, MB Research Centre shows that a longer rotation reduces the overall risk of higher levels of infection through reduced primary inoculum (apothecia). Sclerotia can survive up to five to 10 years in the soil. However, over time they either germinate or are destroyed by soil microbial activity.

Figure 23. Impact of Rotation on Canola Sclerotinia Disease

The stem rot fungus has a worldwide host range of over 400 species, mostly in broadleaf plant families. Sunflower, safflower and occasionally flax crops are quite susceptible to this disease. Mustard, field peas, beans, carrots, crambe, potatoes, lentils, soybeans, fababeans, clovers and alfalfa are also susceptible to some degree. Weed hosts that can produce sclerotia after infection include:

• chickweed
• stinkweed
• hemp-nettle
• thistles
• shepherd's purse
• narrow-leafed hawk's-beard
• false ragweed
• wild mustard

Cereals and grasses are not susceptible, and can help reduce viable sclerotia in the soil through decay and germination in the absence of susceptible hosts.

Avoid seeding canola adjacent to a field which had a heavily infected crop the previous year. Control of susceptible weeds and volunteer plants in cereal crops helps reduce sclerotia levels.

Seeding

Since crop density is an important factor, use seeding rates lighter than or up to the recommended seeding rate. Use clean pedigreed seed, free of sclerotia. Growers report that fall seeded canola tends to have less sclerotinia infection than canola sown in early to mid-May.

Tillage

Research in Ontario has shown that no-tillage combined with a good rotation results in reduced numbers of apothecia. Burying the black sclerotia 8 to 10 cm (3 to 4") deep will prevent the development of apothecia from reaching the soil surface. Burial of canola residue speeds up sclerotia breakdown and reduces the spread of the disease. However, later tillage brings sclerotia back up near the soil surface. In order to keep sclerotia buried, use minimum and shallow tillage for cereals sown in fields where infected crop stubble has been worked down and buried. The amount of disease control is usually proportional to the effectiveness of residue burial. Under conditions favourable for disease spread, sclerotinia can spread even if less than 10% of crop residues are left unburied.

Harvest

Growers can apply fungicide for sclerotinia control and achieve good to excellent results in the standing crop. Unfortunately, this disease may progress rapidly in the swath in wet years particularly in B. napus cultivars. Do not swath canola if rain is forecast, particularly if the crop is immature (green) when cut. In wet compacted swaths, particularly on the turns, sclerotinia rot progresses rapidly. The disease can be detected by a rotten "egg like" smell coming from the swaths. This problem is obviously more prevalent in the wetter regions of western Canada. The heavier and more compact the swath, the greater the likelihood for sclerotinia to rot the swath before combining.

Steps to follow to control sclerotinia in the swath:

• direct combine the crop
• do not swath immature stands (at least 30% of the seed must be ripe)
• do not swath if rain is in the immediate forecast
• avoid compacting swaths
• use a high cut to allow for better drying
• avoid heavy swaths on the turns

Up to one-third of a canola crop may be lost in the swath due to sclerotinia rot. Additionally, this damages and reduces the quality of the seed and increases the number of sclerotes.

Variety

All canola varieties with flower petals are susceptible to sclerotinia stem rot. However, apetalous varieties are tolerant to sclerotinia as they produce flowers without petals. Since flower petals are the initial food source for sclerotinia spores their absence greatly reduces, but not necessarily eliminates the risk of disease development. In short season growing areas, B. rapa varieties tend to have lighter canopies and generally lower infection levels.

Sclerotinia Forecasting

Forecasting systems have been developed for stem rot in canola that uses either petal testing, a checklist or environmental risk maps based on environmental conditions. The forecasting risk maps for Alberta, Saskatchewan and Manitoba can be accessed on the Canola Council's Web site.

While no forecast system is 100% accurate they do provide practical direction in making a decision to control the disease.

Factors Involved in Sclerotinia Forecasting

Many factors influence a forecasting system and its relationship to the actual incidence of disease. Most predictive models evaluate several environmental and crop variables such as:

• field cropping history
• field disease history
• apothecia presence
• rainfall
• soil moisture
• weather forecast
• canopy density

Other important variables affecting the relationship and incidence of the disease include:

• changing inoculum levels during flowering
• heat units
• daily and weather related inoculum fluctuations
• light penetration
• leaf area index
• crop height

Field and nearby field cropping and disease history are an indirect means of measuring the potential for presence of spores. While sclerotia within the field are considered the main source of spores, those produced in nearby fields and blown into the crop can also be important in disease development.

Scouting for Apothecia

The presence of apothecia is a good indicator of the potential for spore production. However, scouting for apothecia may be quite difficult, as sclerotia are not found in high numbers in most infected soils. Therefore, the sampling site or the individual carrying out the sampling may influence accurate estimates of apothecia numbers. Also each apothecia produces large numbers of spores so relatively few apothecia are needed to cause a localized high level of disease. Scout low wet spots in the field that provide better conditions for sclerotia to germinate, nearby fields that had canola or field peas with some level of sclerotinia infection in the past two to three years and/or other dense canola or other crops. Scout any crops, including heavy cereal stands that are dense enough to produce conditions favourable for sclerotia germination and with a history of sclerotinia in the past. Low wet spots and a dense canopy protect the sensitive apothecia from desiccation and allow them to release infectious spores over a longer period.

Weather

Rainfall and soil moisture are necessary for sclerotia germination, spore production, and spore germination and growth. Ideal canola growing weather is also ideal for sclerotinia. Soil moisture is more of an indication of sclerotia germination and, therefore, the potential for spore production rather than that of disease development. Frequently water from heavy dews dripping off the plant is enough moisture for sclerotia germination.

Weather forecasts can improve the reliability of the disease forecast because sudden weather changes can cause infestations to occur unexpectedly or high-risk fields may show limited disease development. Hot, dry windy weather can greatly reduce the risk of sclerotinia infection.

Canopy Density

A dense canopy decreases evaporation and prolongs the period of favourable moisture conditions within the canopy. Both soil moisture and canopy density are indirect measures of moisture availability in a crop canopy. As a crop develops, the canopy shades the soil surface and inhibits the evaporation of water. If soil moisture is high, relative humidity levels within the canopy will be higher than that above the crop canopy. If soil moisture is low, however, there will not be enough available moisture to increase the relative humidity within the canopy for infection to develop.

Petal Test

The petal test method developed by the University of Saskatchewan is based on the fact that the amount of disease that develops in the field is determined by both the number of spore-carrying petals and by weather conditions. Rainfall, humidity and temperature affect how many plants actually become diseased. Prediction of stem rot risk is based on the numbers of spores present during flowering.

The petal test involves:

• collecting canola petals, starting at early flower
• placing petals in plates containing a culture medium specifically designed to encourage growth of the sclerotinia fungus
• scoring canola flower petals in the culture plates by identifying colonies of sclerotinia
• calculating the percentage of petals that were infected and estimating the risk of disease from the results

The average percentage of infected flower petals can be used to estimate the probable percentage of diseased plants that could develop in the field being evaluated. Use the chart in Figure 24 as a guide to estimate probable per cent yield loss.

Figure 24. An Estimation of Potential Yield Loss Based on Average % Canola Petals Infected with Sclerotinia Spores at Early Bloom

When only the average per cent infected petals at early bloom are considered it is easy to flow through the chart to estimate the probable per cent yield loss.

Canopy density influences the potential disease infection levels to some extent. Heavy stands with dense foliage are likely to develop more disease for a given level of petal infection than light, open stands. The effects of canopy density on disease risk are shown by the transverse arrows at the top of the chart in Figure 24. For example, a crop with low percentage (0 to 45) infected petals may be at a moderate disease risk if the stand is moderately heavy. Conversely, a crop with high percentage (90 to 100) infected petals may only be at moderate disease risk if the crop is open and light.

Weather also influences disease potential, as shown by the transverse arrows in the middle of the chart. Changes in the weather as the crop goes from early to full flower can increase or decrease levels of petal infection and change disease risk. If conditions become drier after early flower, disease risk will decrease because of fewer infected petals. Therefore, less disease would be expected than initially predicted. Conversely, if conditions are dry at early flower but become wetter, disease risk may increase. Because of the weather influence on petal infection and disease risk, the petal test kit manufacturers recommend that a second or even third petal test be carried out if weather conditions change appreciably after the first test.

Unfortunately, the relationships of disease with changes in petal infection are complex and there are no simple rules to follow. The per cent infected petals may be low at early flower because of low levels of apothecia or because dry weather conditions have been unsuitable for spore production. Then prediction for both disease risk and yield loss would be low and spraying would not be recommended. However, if conditions become wetter and a second petal test is carried out, the per cent infected petals, disease risk and yield loss estimates may increase. Disease risk predictions based entirely on later tests may be overestimates because sclerotinia has less time to damage the plants and cause yield loss. In this case, it may be economically viable to spray the fungicide at a reduced rate.

Research at the University of Guelph in Guelph, ON shows that measuring soil moisture levels a week previous to and the week of petal collection helps increase the accuracy of the petal test. A soil that is just above the wilting point will have enough soil moisture to germinate sclerotia.

Checklist to Forecast Sclerotinia

The following sclerotinia checklist developed in Sweden can be a reasonably accurate and reliable predictive forecast system that assists growers in deciding when to apply a foliar fungicide. Growing conditions in Sweden are similar to the higher rainfall areas of western Canada.

Fill out the checklist (Table 3) and assess the crop shortly after first flower. First flower occurs when 75% of the canola plants have three open flowers on the main stem. Usually this occurs during the last week of June or the first week of July.

Read each question and circle the point value assigned to the answer chosen. Count up the points for each question and enter the total.

A second evaluation at 10 to 15% flowering would likely increase the accuracy of the forecast.

Table 3. Sclerotinia Stem Rot Checklist (circle the risk points that apply to your field)

Risk Factor	Possible Answers	Risk Points
Number of Years Since Canola Crop	More than six years	0
	Three to six years	5
	One to two years	10
Disease Incidence in Last Host Crop	None	0
	Low (1 to 10%)	5
	Moderate (11 to 30%)	10
	High (31 to 100%)	15
Crop Density	Low	0
	Normal	5
	High	10
Rain in the Last Two Weeks	Less than 10 mm (0.4")	0
	10 to 30 mm (0.4 to 1.2")	5
	More than 30 mm (1.2")	10
Weather Forecast	High pressure	0
	Variable	10
	Low pressure	15
Regional Risk for Apothecia Development	None found	0
	Low numbers	10
	High numbers	15
Total Risk Points

Swedish researchers who developed this model (through evaluation of 800 fields over a 10-year period) found that with a total of less than 40 risk points, the risk of heavy infection (disease incidence exceeding 25%) was low (Figure 25).

Figure 25. Frequency Distribution for Fields with More than 25% Infected Plants and 25% or Less Infected Plants

A threshold value of 40 risk points accurately identified 75% of the fields that need spraying for sclerotinia but also 16% of the fields that did not need a fungicide. A threshold of 35 risk points increased the accuracy to almost 90% of the fields need spraying but also included more than 20% of the fields that did not need spraying. Therefore, if the risk points are 40 or higher, it is likely worth spraying. If the risk points are less than 40, it is not likely worth spraying.

Bloom Stage Identification

If you plan to use a fungicide for sclerotinia control you must decide when to spray. Sample several plants over the field and assess the number of open flowers. One way to check for bloom stage is to find the main stem, pull off the secondary branches, and count only the open flowers on the main stem. Generally, it takes a crop from two to four days to move from first flower to 10% bloom (Table 4).

Table 4. Identification of Flowering Stages of Canola

Flowering Stage	B. napus Canola (flowers - main stem)	B. rapa Canola (flowers - main stem)
10%	10	6 to 7
20%	14 to 16	10 to 12
30%	20	14 to 16

At 30% bloom, a field of canola is said to be in full bloom-when the maximum number of flowers are open at one time (Figure 26 and 27).

Figure 26. Canola Field at 30% Bloom

Photo Courtesy of BASF

Figure 27. Canola Plant at 30% Bloom

Photo Courtesy of BASF

The optimum time to spray is before the majority of petals begin to drop off and the pods set. Put another way, spray the crop at its most yellow, the day when the maximum number of flowers are open. The objective of the fungicide application is to cover as many petals as possible while ensuring that some chemical also penetrates into the canopy to help protect potential infection sites (such as leaf axils and bases). The chemical is only active on those petals that are present at the time of spraying. The chemical will not protect petals that emerge after spraying, but some chemical coverage within the canopy may help to restrict infection.

If sprayed at the optimum time, the maximum number of fungicide-covered petals will fall into the canola canopy (lower leaf axils, leaves and shoots). Infection of the canola plant will only take place from sclerotinia-infected petals. When the petals fall into lower leaf axils the presence of one or more petals carrying fungicide will likely prevent sclerotinia infection.

For growers without the equipment to spray for sclerotinia, due to the very short time of four to eight days from first flower to 20 to 30% bloom, book a custom applicator as early as possible.

Foliar Fungicide

Where required, the use of fungicides not only increases yields but also reduces dockage due to sclerotia contamination of the seed and small, shrivelled seed. Since the cost of spraying a fungicide is high and sclerotinia disease incidence varies greatly among years, regions and fields, systematic spraying is not profitable. By applying a fungicide only when necessary, yield losses due to heavy infestations as well as unnecessary fungicide applications can be avoided. Sclerotinia stem rot can be effectively controlled by a single protective fungicide treatment during full flower (growth stage 63 to 65) as shown from disease control trials in northwest central Alberta in Table 5.

Table 5. Control of Sclerotinia with Aerial Application of Fungicide in Canola (1981-1982)

Species and Fungicide	Sprayed Average % Infection	Sprayed Yield (bu/ac)		Unsprayed Average % Infection	Unsprayed Yield (bu/ac)		Yield Increase (bu/ac)
		kg/ha	bu/ac		kg/ha	bu/ac	kg/ha	bu/ac
Brassica napus
Fungicide 1	3	2,425	43.3	56	1,294	23.1	1,131	20.2
Fungicide 1	7	1,305	23.3	70	812	14.5	493	8.8
Fungicide 1	11	1,658	29.6	76	902	16.1	756	13.5
Fungicide 1	6	2,184	39.0	19	1,977	35.3	207	3.7
Fungicide 2	10	1,288	23.0	70	812	14.5	476	8.5
Fungicide 2	4	2,128	38.0	19	1,982	35.4	151	2.7
Fungicide 2	3	1,753	31.3	31	1,394	24.9	358	6.4
Brassica rapa
Fungicide 1	2	1,613	28.8	18	1,770	31.6	157	-2.8
Fungicide 1	8	2,397	42.8	44	1,736	31.0	661	11.8
Fungicide 2	1	1,731	30.9	18	1,770	31.6	39	-.0.7
Fungicide 2	2	2,072	37.0	44	1,736	31.0	336	6.0

Threshold disease levels are those where the cost of spraying equals the yield increase due to fungicide application. Research results indicate that it may be economically justifiable to apply a fungicide when field scouting (checklist or petal test) indicates that disease levels will reach 15 to 25% in B. napus and over 30% in B. rapa by crop maturity. Base threshold levels on current dollar return per bushel and application costs of the fungicide per acre.

Alternaria Black Spot (Grey Leaf Spot)

Alternaria black spot is caused by the fungi Alternaria brassicae, A. alternata and A. raphani. Black spot is one of the most common diseases of canola in western Canada. Although Alternaria is present every year on the Canadian prairies, the severity of this disease varies considerably from year to year and area to area based largely on the moisture and temperature situation. The disease can infect all growth stages of the canola plant.

Canola plants vary in their susceptibility to black spot attack throughout their development. Aging plants are more susceptible than young or intermediate-aged plants. Therefore, black spot epidemics intensify at flowering and reach their maximum intensity in ripening plants. A heavy infection on leaves and stems reduces photosynthetic potential of the plant, but is generally not damaging early in the season. However, when the disease spreads extensively onto green pods it may increase the green seed count and seed chlorophyll content, and cause premature pod shatter, shrivelled seed, lower 1,000-kernel weight and reduce oil content.

Seeds may become infected following the development of the disease on pods, adversely affecting seed quality and germination. Infected seeds are either killed or damaged and are generally non-viable. An AAFC Saskatoon, SK Research Centre study in 1989 looking at nearly 400 seed samples from Alberta and Saskatchewan found fairly high levels of infected seed (Figure 28).

Figure 28. Incidence of Alternaria Fungi in Seed Samples of B. rapa Varieties from Saskatchewan and Alberta (1989)

The incidence of A. brassicae was higher and A. alternata lower in Alberta seed samples due to higher August rainfall in that province in 1989. Further studies also found infected seeds in B. napus varieties but at much lower levels than those of B. rapa. Severely infected seed lots have shown reductions in germination of 12 to 30%.

The greatest yield loss from Alternaria black spot comes from pod shatter. Pod shatter results from uneven drying of the disease-infected pods causing them to split, particularly under dry, windy conditions in the swath. A 1988 University of Alberta disease survey estimated an average 30% yield loss in B. rapa crops from black spot in central Alberta. B. napus varieties usually suffer less damage than B. rapa varieties. However, recent research has reported substantial yield losses in B. napus under conditions favourable for disease development.

Symptoms

Alternaria black spot disease causes symptoms in the seedling stage on cotyledons and in later growth stages on leaves, leaf petiole, stem, flowers, pods and seeds. Infected seed may rot in the ground or produce seedlings with dark spots on the cotyledons. Some infected seedlings may also exhibit damping-off or wirestem disease. All above ground plant parts are susceptible to infection.

The disease first appears on the cotyledons in the form of small light brown lesions that soon turn black due to the appearance of spore masses (under humid conditions) and act as a source of infection for other parts of the plant. The initial infections on lower leaves develop distinct brown to blackish lesions or spots with yellow halos around them (Figure 29). The lesions vary in size (1 to 20 mm or 0.04 to 0.8") and colour, depending on environmental conditions. They may be entirely grey under moist conditions, and grey with a purplish or black border, or entirely black under less favourable conditions. These lesions can multiply rapidly and later spread to the upper leaves, stem and pods.

Figure 29. Alternaria Lesions on Canola Leaf

Photo by Beth Hoar

On severely infected leaves several lesions unite to cause defoliation under humid conditions. Stem and pod lesions first appear as small brown or black dots that may develop into conspicuous spots or longer and wider lesions of various shapes (Figure 30). The spots or lesions may be entirely black or dark-bordered with a greyish white centre. In severe outbreaks, the upper part of the stems and pods wither. Pods may show sunken, dark brown to black circular lesions (Figure 31). Deep lesions on pods cause infection in the seed. Diseased seed just beneath the black spots on the pods can be small, shrivelled, grey to brown. Grey or black seeds that are dark green inside are likely to have been damaged by black spot and are unlikely to change colour in the swath. Infected pods may ripen prematurely and shatter while the crop is standing or in the swath (Figure 32).

Figure 30. Alternaria on Canola Stems

Photo by Beth Hoar

Figure 31. Alternaria on Canola Pods

Photo by Beth Hoar

Figure 32. Canola Pods Shattering due to Alternaria

Photo by Phil Thomas

Standard disease area diagrams developed by pathologists at the University of Alberta in Edmonton, AB allow estimation of the intermediate levels of disease severity by comparing a diseased plant with diagrams showing both more and less disease (Figure 33). To calculate the disease severity, the leaf and/or pod to be assessed are matched to one of the diagrams of the black areas or areas damaged shown (representing 1%, 5%, 10%, 20% 30%, and 50%) for each leaf and/or pod covered by the actual lesions (Figure 33).

Figure 33. Leaves (a) and Pods (b) of Canola Showing Alternaria Infection Levels

Disease Cycle

Black spot fungi overwinter on infected crop residue, on cruciferous weeds and to a lesser extent on/in seed (Figure 34).

Figure 34. Alternaria Black Spot Disease Cycle

Spring infections on plants begin directly from infected seed or from spores produced on crop residue or from infections on cruciferous weeds. Many cruciferous plants are hosts on which the fungi can survive from year to year. These include tame mustard, flixweed, hedge mustard, tumbling mustard and stinkweed. Spores, after landing on susceptible plant tissue, remain intact until exposed to moisture from dew or rain, then germinate, penetrate and cause lesions within a few days. The leaf lesions or spots are important because they produce more wind-borne or rain-splashed spores that may cause more infection on the same or neighbouring plants.

Humid conditions and moderate temperatures favour the disease. Cooler temperatures (10 to 15°C) on days with rain and wind promote abundant spore production especially on days where leaves remain wet over longer periods. Wind spreads spores throughout the crop canopy. The cycle continues throughout the season under favourable conditions. Black spot epidemics intensify at flowering when heavy crop canopies promote humid conditions and reach their maximum intensity in ripening plants. High concentrations of airborne spores are needed to initiate development of severe epidemics on canola leaves and pods. Early lodging and cool wet weather in the podding stage become critical factors that lead to major black spot outbreaks. Lodged canopies remain wet longer promoting greater spore production. Hot and dry conditions can interrupt black spot epidemics as the absence of moisture greatly reduces spore production. Black spot disease can be quite variable across a field with more serious infection levels on lower slopes than on upper slopes. Seeds may become infected following development of lesions on the pods. At harvest time, spores produced on stems, branches and pods may also infect the seed in the combine.

Management

Rotation

Crop rotation has a limited effectiveness in controlling black spot disease. Frequently, fields that have not had canola grown for several years experience black spot, causing considerable damage despite the rotation. Black spot spores are excellent wind travellers and can spread to areas remote from the location of production. Three years of noncruciferous crops between canola crops will help reduce airborne spores from crop residue. Control cruciferous weeds and volunteer canola during the rotation.

Tillage

Incorporate diseased stubble into the soil if canola is to be grown on an adjacent field the following year. Buried stubble cannot release black spot spores. However, the amount of disease control is usually proportional to the effectiveness of residue burial. Unfortunately, under conditions favourable for disease spread, Alternaria black spot can spread even if less than 10% of crop residues are left unburied.

Varieties

While all varieties of canola are susceptible to black spot disease to various degrees, growers may be able to lessen the risk of serious losses by sowing B. napus varieties which are less susceptible than B. rapa. B. napus varieties have more leaf wax which provides more tolerance to black spot disease.

Seed

Plant well-cleaned, high germination pedigreed seed. Use seed free of small, shrunken, and infected seed to reduce the seed-borne level of Alternaria. In the laboratory, Alternaria can be observed in the germination test for canola. It is identified by greyish-white mycelium that covers an infected seed or has spread to a nearby seedling. Presence of Alternaria is often noted on the seed lot certificate.

Fungicide Seed Treatments

Registered seed treatments are effective in reducing seedborne inoculum and increasing seed germination. However, planting treated seed will not safeguard against black spot since Alternaria also overwinters on plant residue and the spores are spread by wind.

Foliar Fungicides

Aerial fungicide application applied at 95% flowering provides economical control of this disease. Research studies on the control of black spot disease by AAFC Saskatoon, SK Research Centre and Alberta Agriculture Food and Rural Development showed yield increases ranging from 9 to 36% where a fungicide was applied at late flowering (Table 6).

Table 6. Canola Yield Increases with Fungicide Control of Alternaria

Location/Year	Species	% Disease Incidence No Fungicide	% Disease Incidence Fungicide Treated	% Yield Increase over the Check
Medstead, SK/95	B. rapa	7.5	2.9	36
Medstead/96	B. rapa	7.1	2.4	22
Canwood, SK/96	B. rapa	4.0	0.2	13
Lake Lenore, SK/96	B. rapa	21.4	2.8	36
Olds, AB/89	B. rapa	20.2	5.6	15.9
Innisfail, AB/88	B. rapa	29.1	17.6	9.2
Ellerslie, AB/98	B. rapa	20	8.8	17.2
Ellerslie/99	B. rapa	30	20	14.4
Ellerslie/98	B. rapa	15	13.8	12.7
Ellerslie/99	B. rapa	23.8	13.8	11.9
Ellerslie/98	B. napus	10	0	16.5
Ellerslie/99	B. napus	22.5	11.8	11.9

In addition, fungicide control of black spot disease also increased seed germination, reduced green seed count from 8 to 4% and increased thousand seed weight relative to the untreated controls in the research.

Harvest

Early swathing of badly infected crops may reduce serious losses from shattering. Swath fields infected with black spot as straight cutting extends the time period over which shatter damage from black spot occurs. Any practice that allows the crop to dry more quickly following a rain or dew may reduce the impact of this disease. Adequate balanced fertility and lower plant populations will help reduce early lodging. Adjust swathers to produce open rather than heavy swaths. Avoid the use of wide swathers in heavy stands and swathing in the rain. A lighter swath exposed to good airflow and rapid drying is less at risk.

Plant Stress

Factors that cause plant stress are known to increase the plant's susceptibility to black spot disease. A nutrient deficiency can cause stress. Follow soil test recommendations to ensure adequate fertility of both macro and micronutrients. Sulphur deficient plants tend to be more susceptible to black spot disease. Apply insecticides when warranted since insect injury predisposes plants to black spot disease.

Seed

Seed from southern and drier areas of the prairies generally carries less seed-borne infection. Pedigreed seed growers can take steps to reduce infected seed levels. Extended exposure of crops to wet weather after ripening can result in high levels of seed-borne Alternaria and reduce seed germination. Harvest seed fields as soon as the seed is properly matured and weather permits, using a grain dryer if necessary. To improve the quality of seed lots use proper cleaning procedures to remove small, shrivelled seeds that tend to contain higher levels of Alternaria and have lower germination. Do not use seed lots with high levels of Alternaria for seed since the overall vigour of the seed may be adversely affected.

White Leaf Spot and Gray Stem

This disease is caused by the fungus Pseudocercosporella capsellae. White leaf spot and gray stem are widespread throughout the canola growing areas of western Canada. Although this disease can be found in most fields when the crop is ripening, it usually develops too late in the growing season to affect crop yields significantly.

Symptoms

White leaf spot appears in the summer. Severe leaf spotting can result in premature leaf loss. As the crop ripens, large purple to grey-speckled stem and pod lesions develop. At harvest, some plants may be completely discoloured and frequently the entire field turns a purple or grey colour. Disease symptoms may appear earlier in the season on crops under stress from lack of moisture, insufficient nitrogen or severe competition from weeds.

Figure 35. Gray Stem

Photo by Beth Hoar

Disease Cycle

The fungus overwinters as thick-walled mycelium on crop residue and produces wind-borne spores in the spring that infect canola plants. Following infection in early summer, white to buff-coloured spots develop on lower leaves. These lesions produce wind-borne spores that cause the rapid spread of the disease in the ripening crop. The disease is usually not seed-borne. The disease has a wide host range among cruciferous weeds, including shepherd's purse, hare's ear mustard and ball mustard.

Management

Crop rotation and control of volunteer canola and cruciferous weeds help reduce infection. Good crop production practices that reduce stress due to weed competition and nutrient deficiency help delay disease development.

Aster Yellows

Aster yellows disease is caused by a phytoplasma (previously called mycoplasma-like organism) and is spread by aster leafhoppers. This phytoplasma has an extremely wide host range, and can infect about 300 species of plants. Generally, in most canola fields it infects a very low percentage of plants. However, in years favouring the spread of the aster leafhopper some fields have been observed with up to 15% or more infected plants. The severity of this disease cannot be predicted from the number of aster leafhoppers present due to many different factors such as weather conditions, numbers of infected leafhoppers and rainfall amounts.

Symptoms

Plants infected at an early growth stage fail to produce flowers and set pods. In western Canada, most plants are infected when leafhopper populations are at a maximum in mid-June to the end of June. Infected plants develop distorted leaf-like flower structures that remain green and fail to ripen. Plants show a bunched appearance at the tips of branches where blue-green, sterile, hollow bladders are formed in place of normal pods (Figure 36). Normal appearing pods may be present on the lower portions of infected plants but fail to set seed. These undeveloped pods remain on the plant but are usually quite small.

Figure 36. Aster Yellows

Photo by Phil Thomas

Disease Cycle

The disease is spread primarily by the aster leafhopper (Macrosteles quadrilinetus). The insect is olive-green or straw coloured with six dark coloured spots on the forehead. The abdomen is charcoal and the wings are opaque. Adult leafhoppers cannot survive winters in Canada. They may overwinter as eggs that take two weeks and five nymphal stages to develop into first generation adults which appear in late May to early July depending on the location. Adults may also migrate from the U.S., usually arriving in early to mid-June. Because leafhopper movements are regulated by wind and weather patterns, migrations are not consistent from year to year. Both local and long-distance migration influences the incidence and severity of the aster leafhopper infections and aster yellows.

Aster leafhoppers become infected by feeding on infected host plants. Leafhoppers must feed on a susceptible plant for a sufficient time to inoculate themselves with aster yellows. It has been estimated that about 2 to 5% of the leafhoppers may carry the disease. Migrant leafhoppers may also arrive infected with aster yellows. After the leafhopper is infected it takes two to three weeks for the leafhopper to become capable of transmitting the disease. Leafhopper feeding on canola plants is not economically damaging, but in the feeding process the plants may be infected with aster yellows. Once in the plant the disease multiplies rapidly and symptoms may appear in one week in younger plants and two to three weeks in older plants.

Management

There is no practical means of controlling this disease in canola. Infected host plants act as reservoirs of aster yellows and influence the level of disease in the next season. Certain weeds in cultivated crops may influence the incidence of the disease. Host crops can be winter wheat, wheat, oats, barley, rye, canola, potatoes, flax, sunflowers, fababeans, tomatoes, sage, celery, lettuce, and carrots. Weed species include knotweed, plantain, lamb's-quarters, sow thistle, ragweed, quackgrass, pineapple weed, wild asters, wild carrot and stinkweed. Control of biennial and perennial host weeds in the field and surrounding area may reduce the incidence of this disease.

Clubroot

Clubroot is caused by a fungus Plasmodiophora brassicae. In Canada, clubroot is a serious problem in cabbage, cauliflower and rutabaga crops, chiefly in eastern Canada and the coastal regions of British Columbia. The disease has been identified in Alberta and Manitoba but has not appeared in canola in the prairie provinces. Where the disease is a problem in other crucifers, especially in eastern Canada, be alert for disease symptoms.

Symptoms

Galls appear on the roots of infected plants, ranging from tiny nodules to large, club-shaped outgrowths that may involve most of the root system. The galls are at first firm and white but become soft and greyish brown as they mature and decay. Severely affected plants are stunted and wilt under moisture stress because much of the taproot is destroyed.

Disease Cycle

Resting spores of the fungus can survive in soil for many years. In the presence of susceptible roots, the spores germinate and release tiny organisms that swim in free water to the surface of the rootlets, penetrate and form a fungal colony inside the root cells. The fungal colony causes cells to enlarge and divide rapidly, resulting in the characteristic galls. Late in the season, resting spores develop in the infected roots and are released into the soil as the galls decay. Fields become infected mainly by the movement of soil on cultivation equipment and by seedling transplants. The fungus exists in many races to which some cultivated crucifers, for example cabbage, have been bred for resistance.

Management

Infected fields must be kept free of susceptible crops for many years because of the long-lived resting spores. Do not move cultivating equipment from infected to non-infected areas before thoroughly cleaning the equipment. Liming may reduce disease severity on acidic soils.

Downey Mildew

Downey mildew is caused by the fungus Peronspora parasitica, a close relative of white rust. Downey mildew occurs in almost constant association with staghead as part of the white rust/downey mildew disease complex. Downey mildew also appears alone on infected seedlings and leaves of the rosette stage of B. rapa during periods of cool, wet weather in the spring. The fungus is soil-borne and seedborne and may persist in the soil for five to 10 years. The fungus appears as a white mealy growth on the lower surface of leaves (Figure 37) and on green stagheads caused by the white rust fungus (Figure 40). The upper leaf surface over the infected area usually turns yellow. The disease cycle is similar to that of staghead. The control recommendations are the same as for staghead. Use a crop rotation with noncruciferous crops making certain to control volunteer canola, stinkweed and wild mustard.

Figure 37. Downey Mildew on Underside of Canola Leaf

Photo by Phil Thomas

Fusarium Wilt

Fusarium wilt, a new fungal disease of canola, has been observed in the Peace River and northeast agricultural regions of Alberta and in Manitoba.

Two soil-borne fungi, Fusarium avenaceum and Fusarium oxysporum, have been identified as pathogens for this disease. These fungi were isolated from infected plant tissue. Severe wilt symptoms were produced when healthy canola plants were inoculated with the isolates-40 to 90% of the plants died, depending on the isolate.

Fusarium wilt of canola was first seen in Alberta in 1999, the first report in North America. It was observed in several fields in the Peace River region of Alberta in August of 1999. In one field near Ft. Vermilion, AB estimated yield loss was 30%. The disease was also reported in the northeast region of Alberta, in July of 1999. A mean wilt incidence of 7% was observed in fields in the Two Hills, AB area. Incidence in individual fields ranged from 0 to 29%. For one field near Andrew, AB, estimated yield loss was 18%. Fusarium wilt is not as common as other canola diseases but it can be a problem. Some varieties of canola may be more susceptible than others to this disease.

Symptoms

• Yellow or reddish-brown streaks, often occurring only on one side of the stem or on the branches (Figure 38). Some plants may have an orange discolouration at the base of the stem. Plants with minor infection may also ripen prematurely and tend to shatter.
• Chlorosis and necrosis of stems, vascular discolouration, poor seed set and premature desiccation (Figure 39).
• Premature death in severely infected plants. Stems and/or branches turn brown, but plants remain upright with roots intact (Figure 40). No visible lesions are visible on stems or roots. Plants are often stunted and have small pods with no seeds.

Figure 38. Fusarium Wilt on Canola Stem

Photo by Beth Hoar

Figure 39. Fusarium Wilt on Canola Pods

Photo by Beth Hoar

Figure 40. Canola Field with Severe Infection of Fusarium Wilt

Photo by Phil Thomas

White Rust (Staghead)

The fungus Albugo candida causes this disease. White rust is a common disease of B. rapa cultivars throughout western Canada. Some cultivars have resistance to one of two races that have been identified from B. rapa but are susceptible to the other. Cultivars with resistance to both races are being developed. Cultivars of B. napus are resistant to the current races of A. candida from B. rapa and from mustard. Yield losses in excess of 20% have been recorded on susceptible cultivars when severely infected. White rust can cause disease problems on mustard (B. juncea) and occurs on cruciferous weeds. Races are generally fairly host or species specific (for example white rust from the weed shepherd's purse does not infect B. rapa and vice versa).

Symptoms

White to cream-coloured masses or pustules of "white rust" appear on the underside of leaves from the seedling stage onward. Following infection of the stems and pods, raised green blisters form that turn white during wet weather. The most conspicuous symptom is the presence of swollen, twisted and distorted inflorescences called "stagheads" that become brown, hard and dry as they mature (Figure 41).

Figure 41. Staghead Symptoms on Canola that is also Infected with Downey Mildew

Photo by Beth Hoar

Disease Cycle

The fungus overwinters as resting spores in decaying infected plant tissues (mainly stagheads) or as a seed contaminant. These spores may remain dormant in soil or on seed for a number of years. In the spring, some of the spores germinate and infect the cotyledons and leaves of young susceptible plants. These infections develop and white pustules are formed on the underside of leaves or on stems. The pustules release chalk-like, airborne spores that can spread the disease to other parts of the plant or to nearby plants to cause secondary infections on leaves, stems or flower buds. Stagheads develop from infected flower buds. At harvest, stagheads may be broken during threshing resulting in contamination of the seed with resting spores.

Management

Grow resistant cultivars, use certified seed and use a crop rotation with at least three years between canola crops.

Canola Disease Management

Major disease damage can only occur when the following conditions are satisfied:

• the crop is susceptible
• the disease is present
• the environmental conditions are favourable for the disease
• the disease has sufficient time to develop to cause damage

Disease management strategies must counter one or more of these factors to reduce or prevent initial disease populations, and to slow their rates of increase. Because disease control is only part of the total production system, disease management must integrate with other crop production factors.

A major component of disease management is proper field scouting. Scouting provides the information on diseases present, their severity and potential crop loss if untreated. Scout fields for disease weekly, from crop emergence until maturity. The information gathered can be used to justify the use of fungicide application for disease control. It will also prevent the unnecessary use and expense of fungicide application or ensure it is applied when it will give the greatest economic return.

To scout fields for disease:

• Review the field history. Identify any fields that have had chronic disease problems. Check for these diseases when scouting.
• Scout weekly. Start checking the field before emergence. If stand establishment problems occur, take corrective action immediately, before the seedbed moisture is depleted. Reseeding a crop destroyed by a seedling disease is usually successful if the soil temperature and moisture conditions are favourable for vigorous seedling growth. After emergence check the most disease prone locations first. Foliar diseases tend to be worst in lush heavy growth areas.
• Scouting requires examination of five to 10 randomly selected sites that represent the majority of plants within the field. Closely examine the leaves, stems and roots of each plant selected. The use of a magnifying glass or hand lens may be particularly useful.
• Identify all diseases present as soon as possible. Some diseases increase quickly (e.g. Alternaria) and must be dealt with promptly. Where symptoms are in question consult with a crop specialist for further information on disease identification. Provide one or two plants (root, stem and soil).
• Keep good field records. Map the areas where diseased plants are found to determine where to apply treatments, to monitor any disease increase and to assist in planning future crop rotations. The map will help if the same crop is grown in this field in future years.

Include the following practices in a disease management program:

• Follow a crop rotation in which canola is grown not more than once every four years. Cereals and grasses are good rotational crops because they are not susceptible to diseases of crucifers. Also, volunteer canola and cruciferous weeds can be easily controlled in these crops using herbicides. If canola is grown in rotation with pulses, make sure the canola directly follows the pulse crop. Pulses in the rotation provide benefits that under most conditions outweigh any risk from sclerotinia. Crop rotation by itself rarely gives complete disease control. Under favourable conditions, spores can blow in from adjacent fields. Crop rotation will delay the occurrence of some diseases in a field and in many cases this delay is sufficient to allow the crop to escape serious damage during its most susceptible growth stage.
• Select a variety first on its suitability to the frostfree period of the area and second on its agronomic performance and disease reaction. Grow B. napus varieties where possible because they are less susceptible to the common diseases of canola.
• Select seed from relatively disease-free fields. Avoid seed from areas with virulent blackleg. This will prevent its introduction through infected seed.
• Use direct seeding or reduced tillage, as research has shown that significant crop losses from diseases are not substantially increased. Under conventional tillage begin seedbed preparation in the fall, unless the field is subject to appreciable wind and water erosion. Fall incorporation of herbicides and fertilizers reduces the number of tillage operations to be done in the spring, and conserves seedbed moisture.
• Sow canola shallowly into firm, moist, weed-free, warm soil. Use seedbed practices that establish an optimum seedbed favourable for rapid germination and emergence of seedlings, and less conducive to the development of seedling diseases. Do not place harmful quantities of fertilizer with the seed. Ensure that plants receive balanced fertility as nutrient imbalances can predispose the crop to pathogen attack. A vigorously growing, well-fertilized crop can compensate for pathogen attack depending on the disease.
• Treat seed with a recommended combination fungicide-insecticide seed treatment to provide protection during the emergence period from seedling diseases and flea beetle injury.
• Begin crop scouting as soon as the crop is sown.
• In areas where sclerotinia stem rot has been a problem, evaluate canola fields and adjacent cereal crops for the potential risk of the disease. If the crop is high-risk, apply a fungicide.
• Examine the crop throughout the season for the presence of diseases that may affect crops to be sown in adjacent fields the following year. Grey leaf spot (Alternaria black spot), sclerotinia stem rot and blackleg are potentially serious diseases that can be spread from field to field by airborne spores. Cultural practices, such as ploughing infected stubble, may reduce the build-up of those diseases in the following crop year. Avoid seeding canola adjacent to fields that were heavily diseased the previous season.
• New fungicides are continually being developed for improved disease control. Be certain to apply fungicide at the optimum time and in the proper concentration. Follow the label instructions. Since registrations changes from year to year, obtain the current provincial fungicide guidelines annually. Always check annual publications for current recommendations.
• Seek out assistance from public and private crop specialists to help in identifying and managing canola diseases.