Clubroot: Scouting for pathotype shift

Researchers have identified at least 36 clubroot pathotypes in Western Canada, and roughly half are not controlled by the common clubroot resistance source – often referred to as first generation or “gen 1” resistance. That is why hybrids with clubroot resistance (CR) can still have clubroot galls.

If you find galls on CR canola, other management practices – including longer breaks between canola crops, minimizing soil movement, and controlling volunteer canola – will have to be part of the program. And the next time canola goes on that field, use a different source of resistance.

What are pathotypes? A pathotype is the proportion of the pathogen population that can be differentiated based on its ability to cause disease on a range of hosts. For clubroot in Canada, we use a differential set known as the Canadian Clubroot Differential set (CCD) to differentiate the pathotypes. The CCD contains a number of known clubroot hosts including some canola varieties. Each known pathotype can be identified by the presence or absence of symptoms across the entire differential set when plants are inoculated with the spores of a specific isolate of the pathogen.

How does the pathotype population in a field change? New clubroot sources with different pathotypes could be brought into a field the same way clubroot was originally introduced to the field. Another perhaps more likely way is that clubroot, when it first arrived in a field, included a diverse population of pathotypes. Growing clubroot-resistant varieties with resistance to those major pathotypes will allow minor populations (that are virulent to that particular CR trait) to amplify.

What is happening with clubroot resistance in Alberta?  Current clubroot-resistant (CR) varieties are highly effective against the predominant pathotypes of P. brassicae, and the presence of a CR gene in a plant does not lead to a new pathotype of clubroot being formed. Rather, a portion of the spores that are present in infested soil already have the ability to overcome or “defeat” the resistance gene, as the resistance gene cannot defend against all pathotypes.

That is why steps to keep spore counts low (which include CR varieties but also crop rotation, field sanitation, host-weeds management and scouting) are so important. If relying on CR alone, the more times you plant a variety with the same resistance mechanism in the same field, the faster you can build up a population of P. brassicae spores that can defeat the resistance gene in the plant and cause severe clubroot. It is not that the varieties or genetics have failed, but rather that the pathogen has the ability to adapt and infect. When spores of a different pathotype successfully infect roots and create galls, even small galls, a significant amount of spores for future infection is created in that localized area.

When scouting, if clubroot infection rates (frequency not severity) are greater than five per cent in a random selection of seeded plants (ex. if five per cent of the plants that are randomly selected have clubroot symptoms) or if a patch or patches of clubroot-infected plants are noticed, this could indicate that the clubroot resistance is no longer functional against the pathogen population in the field.

Are you experiencing a pathotype shift? Growers who suspect that a pathotype shift has caused a breakdown of resistance could contact the Canola Council of Canada and ask about a pathotype test. These are not commercially available (the commercial tests just indicate the presence of clubroot DNA) but Alberta researchers are tracking pathotype shift. If interested in submitting a sample, please contact the CCC agronomy specialists using the Canola Watch contact form.

Soil test programs

While these soil test programs will not isolate pathotypes, they will identify the presence of clubroot.

• SaskCanola program. Through the 2020 fall clubroot soil testing program, offered by SaskCanola and the Saskatchewan Ministry of Agriculture, a grower can request a soil sampling bag, collect soil from their field and submit it for testing at no cost. Results will be kept confidential and will only be shared with the rural municipality if they have enacted a clubroot bylaw. All findings of clubroot and the clubroot pathogen will be added to the Saskatchewan Clubroot Distribution map (at an RM level) to raise awareness of the presence of clubroot and to guide proactive biosecurity and management decisions. Because of the value that these test results will provide growers and the industry, SaskCanola will cover the $100 cost of each test. Sign up online.
• Manitoba Canola Growers program. Manitoba Canola Growers are founding supporter of the Pest Surveillance Initiative and are offering free testing to MCGA members. We recognize the valuable role that agronomists play on farms across Manitoba and want to make you aware of this testing service for your customers. Agronomists are welcome and encouraged to submit tests on a farmer’s behalf.  Results will be shared back with both the agronomist and the farmer. Testing information.