How to evaluate clubroot losses

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 the 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.

Yield loss due to clubroot is estimated to be about half of the percentage of infected plants. For example, if 10% to 20% of plants are infected, yield loss will likely be 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.

Clubroot in fields seeded to “R” variety

Growers who seeded clubroot-resistant hybrids should know that low-level infection — up to 8% — is normal for these hybrids.

If growers are scouting fields seeded to resistant hybrids and find levels higher than this, a patch of susceptible volunteers is the most likely reason. Volunteer canola densities can easily exceed 10 plants per square foot in canola-on-canola rotations, and will be part of the current crop unless a different herbicide-tolerant system was used.  With canola-wheat-canola rotations, volunteer canola in the next canola crop may persist at levels of about one plant per square foot in certain situations.

If volunteers cannot explain high infection rates in resistant varieties, then growers should contact their seed rep for further diagnosis.

Growers have to pull up canola plants to identify clubroot. The first photo shows galls at early stages of development. The second photo shows developed galls. Photo credit: Stephen Strelkov

Published on Tuesday, August 03, 2010. Updated April 28, 2011