Visual symptoms of clubroot can be incorrectly attributed to heat stress or to diseases such as blackleg, fusarium wilt or sclerotinia. For that reason, proper diagnosis of clubroot should always include digging up plants to check for gall formation on roots.
- Start at field entrance. Pull up 10 random plants, remove the soil and examine roots for evidence of galls.
- Walk 10 paces into the crop and pull another 10 plants.
- Turn 90 degrees and walk another 10 paces. Sample this way in a zig-zag pattern until 100 plants have been sampled. Record the percentage of infected plants.
- When sampling, do not be selective in choosing plants. Do not look for sick plants. Make sure the plants sampled are random.
When unsure if a field has clubroot infection, then it is appropriate to sample affected plants to first identify the disease. Then use the above technique to quantify the level of infection. The top photo shows canola roots with young clubroot galls. The photo at the bottom of this article shows developed galls. (Photos courtesy Stephen Strelkov.)
Yield losses due to clubroot are about half of the percentage of infected stems. For example, if 10% to 20% of plants are infected, yield loss will likely be around 5% to 10%.
No control products are registered for clubroot in canola. The recommendation is to rotate out of canola for four years in slightly infested fields and seven years in severely infested fields. Control volunteer canola and susceptible weeds (mustard family, dock and hoary cress) in the rotational crops. And consider a clubroot-resistant hybrid the next time canola goes on that field.
For more on identification, prevention and management of clubroot, visit the website www.clubroot.ca.