Quick comparison: Blackleg, clubroot, sclerotinia stem rot

April 4, 2018 - Issue 4

On phones, this table is best viewed in landscape/horizontal mode. If that doesn’t work, click here for the PDF: Big 3 canola diseases: Quick comparison

Blackleg Clubroot Sclerotinia stem rot
Susceptible host plants Canola and other brassicas Canola and other brassicas, including some common weeds. Broadleaf crops, including canola, soybeans, sunflower, pulses.
Key distinguishing features Lesions with specks of picnidia inside form on leaves. Infection spreads to the base of the stem, where it forms cankers. Clipping open stems will reveal blackened tissue inside. Moderate cases will cause yield loss, even if the plant doesn’t die. Severe cases will completely blacken the stem, cutting off all flow of nutrients and killing the plant. Galls form on roots. In serious cases, large tuberous galls will restrict flow of nutrients and water up and down the plant, killing the plant. Lesions form on leaves and stems. Eventually the stems rot, then turn white and brittle. Plants die prematurely and seed set is greatly reduced. Black sclerotia form inside (and sometimes outside) damaged stems.
How it rests waiting for the next host crop On pieces of infected canola stubble As each gall breaks down, it releases billions of spores into the soil. As sclerotia bodies in the soil
How it spreads Pseudothecia and pycnidia on infected canola residue release spores that continue the infection cycle. Pycnidiospores from pycnidia travel only a few metres. Smaller ascospores, shot into the air by pseudothecia, can travel farther. Spores move with the soil. When soil moves, clubroot moves. Soil moving through fields and from field to field on machinery is the most common vector for spread of the disease. Spores infect host roots, continuing the cycle. Sclerotia produce apothecia that release spores. Spores can carry for kilometres on the wind, but most will come from within fields or from connecting fields. When release coincides with canola flowering and moist/humid conditions, infection can occur.
Benefit of crop rotation 2- or 3-year break between canola can almost eliminate risk because host stubble deteriorates. 2- or 3-year break between canola can reduce viable spores by 90%, but this can still be enough spores for widespread damage if spore counts start off high. (1,000,000 per gram of soil.) Rotation benefit for sclerotinia is less than it is for blackleg and clubroot because many crops are susceptible, sclerotia are widespread and released spores carry to nearby fields. However, higher severity will be observed with shorter rotations of susceptible crops.
Genetic resistance available Yes Yes Improving tolerance in selected varieties
Genetic resistance groups identified Yes No, but coming soon? No
How to use R varieties as part of a disease management program. Always use R or MR varieties, and if blackleg is getting worse, switch to a different R-gene. Use R varieties in areas where clubroot is found or suspected. This will help keep soil spore loads low and limit yield loss for the long term. Sclerotinia-tolerant varieties can reduce disease in high-risk situations, but fungicide may still be required.
Effective fungicides available Yes, for early-season infection. No Yes, for application at flowering

How to word the blackleg R-gene labels? The team behind the new blackleg resistance-gene labeling system (read more about it) would like your help in choosing pathologically correct terms that also resonate well with producers/agronomists. Please take this 5-question survey to share your preferences.

Further reading:

Blackleg section of Canola Encyclopedia
Clubroot section of Canola Encyclopedia
Sclerotinia stem rot section of Canola Encyclopedia
Sclerotinia stem rot risk calculator (Manitoba Agriculture version)
Blackleg calculator (BASF version)
VIDEO: Blackleg in canola; Understanding blackleg resistance and management tools

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