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Understanding clubroot

Clubroot is a relatively new disease that has affected parts of the canola growing area of Western Canada. In canola, it was initially found in a couple of fields in the St. Albert area of Alberta in 2003, but since then it has spread to many municipalities in central Alberta surrounding Edmonton, a couple of municipalities in southern Alberta, one in eastern Alberta near Lloydminster as well as a couple of municipalities in north central Saskatchewan.

Clubroot is a soil borne disease that causes swollen galls to appear on the roots. This cuts off the supply of water and nutrients to the plant, causing premature ripening and death of the plant. It affects many cruciferous crops such as canola, mustard, cabbage, broccoli, radishes and turnips as well as cruciferous weeds such as wild mustard, stinkweed, shepherd’s purse and volunteer canola.

Yield losses are typically estimated to be 50 per cent of the percentage of affected plants, similar to estimated losses from sclerotinia. However, just like other diseases, the effect on yield varies depending on when the plants are infected and the severity of infection. Yield losses can range up to nearly 100 per cent under severe disease pressure. The disease does well under conditions of good moisture, warm soils and lower soil pH. Because soil moisture is a main driver, clay soils and poorly drained soils such as solonetzic soils are prime candidates for the development and spread of the disease. Although soil pH is a factor in the speed of increase of the disease, it does not mean that the disease will not develop on higher pH soils. It just means that on higher pH soils, it will be slower to develop into major infestations.

Resting spores

The reason clubroot is so serious is that its resting spores are long lived (up to 20 years) with a typical half-life of about four years. This means that, once the disease is established in a field, major infestations can take a long time to allow disease inoculum to reach manageable levels.

Because the resting spores are soil borne, the disease typically spreads through soil movement. Clubroot surveys conducted in Alberta indicate that almost all new cases of the disease are found near the field entrance, which means that soil movement on equipment is a principal carrier of the disease. Other methods of spread include water and wind erosion that move soil from field to field and within fields as well as tillage which can spread infections from patches to throughout a field. In theory, any method of soil movement can move the disease, including animals and hunters, but the relative risk is directly proportional to the amount of soil moved.

To make sure that clubroot does not take a major chunk out of your profits, try as much as possible to not have clubroot establish on your fields. If you do get it, do as much as possible to keep the disease at manageable levels. †

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