Diseases of Field Crops: Canola Diseases

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Pub 811: Agronomy Guide >Diseases of Field Crops> Canola Diseases

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Seedling Disease Complex

Incidence: Stand establishment is a major concern in canola production. Poor stand establishment is often due to seedling disease infection by one or more fungi, which is referred to as a "seedling disease complex." The primary fungi involved are Rhizoctonia, Fusarium and Pythium. The problem is greatest under cool conditions.

Appearance: Infection can exhibit many symptoms, including seed decay or rot, pre- and post-emergence damping-off, seedling blight and seedling root rot. These symptoms occur during the first 4 weeks or by the fourth-leaf stage. Seeds may fail to germinate or die shortly after emergence. Seedlings that emerge may appear normal but can have significant root rot. Damping-off occurs when root decay or rot moves up the stem (hypocotyl), causing a girdling or pinching of the stem at or near the soil surface. The stem is weakened and is susceptible to breakage or toppling, where the characteristic reddish-brown lesions are formed. Infected seedlings often wilt or die when stressed because of a reduced (root pruning), constricted or rotted root system, especially under dry conditions. Stands are slow to emerge, thin or patchy. Yields can be reduced when stands are uneven or patchy. Severe plant loss may result in reseeding.

Disease Cycle: These fungi survive in the soil on decaying plant residues. Conditions that cause the developing seed or seedling to grow slowly are ideal for these fungi. The below-ground parts of the seedling harden (become woody) at the two- to four-leaf stage and vigorously growing plants reach this stage quicker. At this stage, the seedlings are able to limit further infection and can regenerate roots quicker than they are lost. Pythium prefers cool, wet soils, whereas Rhizoctonia favours dry, light soils.

Management Strategies: Plant good-quality seed into a firm, moist seedbed when the conditions are suited to promote rapid germination. Fungicide seed treatments will reduce infection and increase stand establishment. Maintain good fertility balance and avoid excess fertilizer, which promotes disease and phytotoxicities. Avoid deep planting of seed.

Blackleg (Leptosphaeria maculans)

Incidence: Blackleg is a fungal disease that occurs in all canola-growing regions of Canada. In Western Canada, two strains (mild and virulent (severe)) of the fungus are found, causing substantial losses in Western Canada. In recent years, blackleg has been increasing in Ontario, especially in winter canola fields. Fortunately, the severe or virulent strain responsible for losses in the West has not been identified, to date, in Ontario.

Appearance: The first symptoms appear on the cotyledons or leaves as round-to-irregular (1-2 cm (3/4 in.)) white to buff lesions that contain numerous small black dots (pycnidia) (see Plate 166). As the season progresses, the fungus may spread to the stem and crown of the plant, producing a canker that can girdle the stem (see Plate 167). Severely infected plants ripen prematurely and have a black to grey discolouration at the base of the stem or crown. In severe cases, infected plants will lodge. Seeds of severely infected plants are small and shrivelled and may be infected with the fungus.

Disease Cycle: The blackleg fungus survives on canola residues (refuse) and on infected plants and seed. The fungus can be spread from field to field on canola refuse or diseased plants. The spores of the fungus are also spread by rain, wind and infected seed.

Management Strategies: Use less susceptible varieties. Most varieties are rated on a 1 to 5 (resistant to highly susceptible) scale. Maintain a good crop rotation that has at least 3 years between canola crops. Fungicide seed treatments will reduce seed-borne infection and minimize the risk of introducing blackleg into new fields. However, the disease can still be spread from field to field on infected plants and refuse.

Plate 166. Blackleg in canola causes round-to-irregular, white-to-buff lesions containing many black dots (pycnidia).

Plate 167. Blackleg spreads to the stem, producing a canker that girdles the lower stem.

White Mould (Sclerotinia sclerotiorum)

This disease also affects dry edible beans, soybeans and sunflowers. See White Mould and Plates 161-165.

Turnip Mosaic Virus

Incidence: Turnip mosaic virus (TuMV) has become a significant problem in some areas where winter canola is grown.

Disease Cycle and Appearance: Infection takes place in the fall and causes leaf mottling (yellow or light green areas surrounded by normal green colour) and wrinkling or puckering of the leaf tissue between the veins (see Plate 168). Spring growth is slow. Severely infected plants are stunted, twisted and generally light-green or yellow. Pods are distorted and a significant proportion of the seeds are poorly filled. The disease appears to be more severe in areas where other cruciferous crops (such as rutabagas) are grown and in fields where pressure from weeds and volunteer cereals is high.

Management Strategies: Volunteer crops of winter canola often have high levels of TuMV infections. Early planting may be helpful in increasing the winter survival of the crop in some areas but appears to also increase the severity of TuMV where the disease is present. Only minor levels of TuMV infection have been observed in spring canola.

Plate 168. Turnip mosaic virus (TuMV) causes leaf mottling and wrinkling or puckering of canola leaves. It can also cause yellowing and stunting.