Clubroot, caused by the soil-borne pathogen Plasmodiophora brassicae can cause yellowing, stunting, wilting and club-like roots on susceptible Brassica species including broccoli, cabbage, canola and cauliflower. Clubroot causes an estimated yield loss of 10–15% in Brassica crops worldwide and in severely infested fields a 30–100% yield loss can occur. There are different races of clubroot known as pathotypes and the resistance of many cultivars is pathotype dependent.

Clubroot is classified as neither a fungi nor bacteria, but shares many attributes of both. The resting spores of clubroot are very resilient, and have the ability to remain dormant in the soil for up to 18 years. The resting spores are activated when there is adequate soil moisture, temperatures are above 12°C and root exudates from a potential host roots stimulate the spores to germinate and release primary zoospores. These zoospores have two whiplash tails that allow them to swim short distances in water films to Brassica root hairs. Once a zoospore contacts a root hair, it infects the cell, divides, and goes on to cause secondary infection within the root cortex.

Spores responsible for secondary infection in the root cortex disrupt the host cell metabolism. They cause the host cells to expand and cell division to increase resulting in the formation of clubs. While it takes only one spore to cause an infection, many spores are responsible for the secondary infection and the formation of clubs. Clubbing of susceptible host roots is typical when concentrations of spores reach over 1000 spores per gram of soil. At the end of the season, clubbed roots break down in the soil and release millions of new spores that will have the ability to infect next year's crop.

The spores are extremely difficult to eradicate, as they have been found over one meter deep within the soil profile. There are few cost-effective management strategies currently available at this time. Liming to raise the pH to ≥7.2 has been found to be effective in some situations, but not all. Resistant cultivars of some broccoli, Brussels sprouts, canola and cabbage are available, but most Brassica lines are susceptible. The best management strategy is avoidance.

To limit the pathogen's spread, do not share equipment from fields where clubroot is suspected and avoid all forms of soil movement. High-pressure washing has found to be effective at removing spores from equipment, but takes time and is labour intensive. When working in fields with clubroot, always work from the least infested areas to those that are more infested.

The distribution of clubroot throughout Ontario is unknown and there is even more uncertainty surrounding the specific pathotypes that are present. We are currently organizing a clubroot survey to determine the distribution of this pathogen throughout Southwestern Ontario. If you suspect you have clubroot in your field, we are requesting soil samples or clubbed roots to determine if clubroot is present in your field and if so, what pathotype(s) is/are present.

We would greatly appreciate your help in this study and in return we will be able to make better decisions with regards to cultivar resistance as well as limit the spread of the pathogen to other areas of Ontario. Previous studies have shown sampling at the field entrance has the best outcome, as the entrance typically has the highest amount of resting spores. Five litres of soil or clubbed roots from three plants are requested for the pathotype testing. If you have any questions about soil sampling or would prefer if we took the soil samples, please contact Travis Cranmer at travis.cranmer@ontario.ca or (519) 826-4963.

Figure 1. Pak choy with severe clubbing, six weeks after seeding.
Stunted cabbage with severe clubroot, ten weeks after seeding.
Figure 2. Stunted cabbage with severe clubroot, ten weeks after seeding.