The fungus Leptosphaeria maculans causes this disease.
Why Does it Matter?
Blackleg is a serious disease of canola in Canada. In Western Canada, it takes two forms:
- Avirulent blackleg (Leptosphaeria biglobosa) usually appears on leaves and stems in August and causes little damage.
- Virulent blackleg (Leptosphaeria maculans) can cause severe damage, often killing infected plants and seedlings early in the growing season.
In past years the incidence of this disease on the Prairies was highest in Saskatchewan, and parts of Alberta and Manitoba also experienced severe crop losses before an all-out effort was launched to control blackleg through breeding seed for resistance and seed treatments. Today, there are at least four different variants (strains) of virulent blackleg and while resistant canola varieties may have good resistance to some of these variants, they may have less resistance to other strains of virulent blackleg.
What are the Symptoms?
Avirulent Blackleg:
- Symptoms on leaves and stems are similar to those of the virulent strain but much less severe.
- Lesions are shallow, without dark borders and pycnidia (fruiting bodies) are sparse.
- No symptoms are found on pods and seeds.
Virulent Blackleg
On leaves:
- Dirty-white lesions (may be surrounded by a darker border) speckled with tiny black pycnidia.
- When conditions are right the pycnidia will produce an abundance of pinkish ooze, which contains the pycnidiospores.
On stems and crowns:
- Light tan lesions that develop into sunken cankers (surrounded by dark gray to black borders) speckled with black pycnidia.
- Severe crown lesions may expand in size until the plant becomes severed at ground level and topples over.
On pods and seeds:
- Pod lesions are similar to those on leaves – dirty-white and speckled with pycnidia.
- Infected seed may be without symptoms or be shriveled and/or have pycnidia visible on the surface.
What is the Disease Cycle?
- Infected seed as well as infected stubble will result in infected seedlings that develop leaf spots and stem cankers.
- Spores are spread through rain splash and wind from one plant to another.
- Special spores (ascospores) form on over-wintering structures found on stubble that, when mature, can become air-borne and travel long distances.
- The fungus uses a plant’s vascular system to move throughout the plant.
- Spores from leaf spots and stem cankers infect the pods, resulting in infected seed.
How is it Controlled?
- Deep burial of infected stubble to 12 cm (5 inches).
- Good crop rotation practices allow time for infected stubble to decompose. Rotations of at least 3 years, longer if possible, are recommended, particularly in minimum and zero-till systems.
- Use resistant varieties.
- Plant clean, disease-free seed.
- Use seed treatments to control seed-born blackleg.
- Control host plants, such as wild mustard and volunteer canola.
How Does 20/20 Seed Labs Inc. Test for Blackleg in Canola?
In 2003, it was discovered that the fungus responsible virulent blackleg was mutating and producing new strains capable of attacking resistant varieties. It means that testing seed for blackleg, even that of resistant varieties, has become more important than ever.
At 20/20 Seed Labs Inc., our blackleg testing procedure has two parts. First we look for the presence of the fungus that causes blackleg and, if we find it, we then test to see how virulent the strain is.
Blackleg testing:
- 1,000 seeds are surface sterilized to remove contaminants on the seed coat then placed on moistened blotters in a plastic tray.
- The seed is allowed to absorb moisture for 24 hours then it is placed in a freezer for 24 hours to prevent germination. This makes it easier to see infected seed.
- The seed is then incubated for 5 days to allow the fungus to grow.
- A qualified staff member examines the trays for the presence of blackleg-infected seed.
- Infected seeds are removed and set aside for virulence testing.
- This test takes one week to complete if there is no virulence suspected and if required, another 7 to 14 days to determine virulence.
Virulence testing:
- Infected seed is again surface sterilized.
- Seed is placed on agar plates.
- The fungus is allowed to grow for 3 to 7 days.
- A small amount of the fungus is transferred to selective agar. This agar differentiates between the virulent and avirulent strains of the fungus.
- The fungus is allowed to grow for 3 to 7 days.
- A qualified staff member examines the plates and the virulence of the strain is identified.
- This test takes 6 to 14 days to complete.
Please contact us for more information.
