Clubroot is Becoming Commonplace

Clubroot is no longer hiding. Canola producers in Northeastern Ontario have reported symptoms over the past 5 to 8 years and there were certainly fields with yield loss in the past few years that generated concern. But in 2024 above-ground symptoms were more obvious in Temiskaming District. Patches in fields matured early or died prior to pod fill. There were whole fields with essentially zero yield. Thousands of acres are affected to some degree. This story will play out across other Ontario spring and winter canola production areas where clubroot already exists and as it continues to spread.  

In Western Canada, clubroot commonly appears in a patch near the field entrance. We do see that in some Ontario fields, but often the first time we see it is in long strips in a field following a watercourse or the path of equipment. Spores may have been deposited near the field entrance first, but in many fields we have not seen an indication of clubroot until there is yield loss.

There are certainly cases where it would have been found a few years prior to devastating yield losses if we had scouted diligently and checked plant roots in high-risk areas across all fields. When spore counts in the soil are low, clubroot will infect plant roots and form small galls on otherwise large and healthy root systems (Figure 1), and the plants mature normally without any aboveground symptoms. Spore counts in the soil rise rapidly when there are canola plants available for infection. It takes about 10,000 spores per gram of soil to infect a plant, but one infected plant will return billions of spores to the soil. After infection in a susceptible canola crop spore counts will have climbed dramatically. Spore counts may increase again the next season if there is volunteer canola or other Brassica weeds (mustard family weeds such as pennycress, shepherd’s purse, wild mustard). The very next canola crop could then have high levels of infection and die prematurely. Wet areas of the field usually have the worst clubroot infection, and a wet spring promotes early infection and therefore a greater degree of infection. Lack of healthy root tissue from early or severe infection is ultimately why the plant dies (Figure 2).  

Figure 1. Canola plants with healthy above-ground tissue and small galls on the roots, indicating the field contains clubroot spores. Sept 2024 Temiskaming District.
Figure 2. Canola plants with severely galled roots and no healthy root tissue that will die before maturing. Left photo – July 2017 in Temiskaming District. Right photo – a volunteer plant, Sept 2024 in Temiskaming District.

There are also examples of fields with clubroot where it is absolutely shocking that clubs can be found. When an old pasture field is ploughed down and planted to canola, how could there be clubs in that first season of production? The answer may lie in the ditches. There are many fields in the Blanche River Valley with clubroot, for example. When there is excess water, it flows freely in ditches and across fields, potentially moving relatively long distances across the landscape. It pools in the corner of fields for days or weeks until conditions are drier. In this type of scenario, the soil is essentially shared and so too are the clubroot spores. A producer in this area may have long crops rotations and have their own equipment which reduces risk of infected soil being tracked onto their farm, but they can’t exactly stop the water movement across the landscape.

Soil moves in many ways, and it can carry clubroot spores. Moving small amounts of soil is lower risk, such as soil tracked by animals. Higher volumes of soil movement are a greater risk, including:

  • shared equipment – tiling equipment, fertilizer spreaders, sprayers, combines
  • environmental factors – significant drifts of soil e.g. when muck soils are dry and blowing, flooded creeks, runoff into ditches
  • intentionally transferring soil – construction activities, removing muck/peat off fields
  • recreation – ATVs and dirt bikes

It is also easy to forget that it’s not just fields with canola in them that we need to worry about. This disease is present in the soil, which means when we plough down a corn silage field after harvest or run our ATV across a soybean field the risk of moving clubroot is the same as when there is canola in those fields.

If clubroot is found in your fields, research from the Prairies and in Ontario indicates that a combined approach can be highly successful for managing clubroot. First, growers need to reduce the levels of resting spores in the soil to the point where canola can be grown again. This is accomplished using crop rotation. Then grow only canola varieties with resistance to clubroot. These have ‘first generation’ or ‘second generation’ resistance, referring partly to when the resistant varieties were developed. In general, varieties with first generation resistance have a single resistance gene.  Most of the clubroot in northern Ontario can overcome first generation resistance, so using varieties with second generation resistance is recommended.

Approach for clubroot management:

  1. Have a minimum 2 -year break from canola and other brassica crops, such as camelina. Longer rotations may be better. During those break years, be sure to remove volunteer canola and susceptible weeds in the mustard family within a few weeks of emergence. Rotate to wheat, barley or grass cover crops which have been shown to reduce the amount of clubroot surviving in soils or moving between fields.
  2. After the break, grow a clubroot-resistant variety. For Northern Ontario, this should be a variety with ‘second generation’ resistance, based on testing at the University of Guelph.
  3. Plant canola as early as possible. There are many reasons for choosing when to plant, but getting the canola crop growing before the soil is warm enough for clubroot infection can be helpful.
  4. Scout fields diligently so you know when and where clubroot appears. Always check wet areas even if, for example, you know the canola looks poor because of a problem with the tile – that’s where clubroot will set it.

The canola group led by Prof. Mary Ruth McDonald at the University of Guelph can do testing to see if the clubroot organism in a field can overcome first generation resistance. They are interested in working with crop consultants and growers to test clubbed roots from more fields. 

Contact Dr. McDonald: email mrmcdona@uoguelph.ca or Meghan Moran at meghan.moran@ontario.ca.

This article was written with support from Dr. Mary Ruth McDonald, University of Guelph, and Dr. Bruce Gossen, Agriculture and AgriFood Canada.