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Before we get into a discussion on clubroot (Plamsodiophora brassicae) pathotypes, know this: Key management steps of minimizing soil movement, early deployment of resistant varieties, careful scouting to detect early infestations, controlling host weeds and extended crop rotation remain of utmost importance.
In time, when molecular markers become available and we better understand field populations of these pathotypes, we will be able to enhance recommendations to make more refined choices when it comes to clubroot resistant (CR) canola varieties. But until that time comes, the following pathotypes discussion, though valuable, will not change the application of these key management steps outlines in paragraph 1. However, if pathotype population shifts mean that current CR varieties do not work, more intense management may be needed, such as grassing heavily infested areas and even liming.
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 happened 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 have the ability to overcome or “defeat” the resistance gene, as the resistance gene cannot defend against all pathotype.
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 scouting, if clubroot infection rates (frequency not severity) are greater than 5% in a random selection of seeded plants (ex. if 5% 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.
Further reading:
Canola Encyclopedia: Managing clubroot
Clubroot management: Don’t wait until you see dead patches
Understanding clubroot resistance
Terminology of genetic resistance and loss of resistance
Grow CR varieties as soon as clubroot is in the area