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More than 10 clubroot pathotypes have been identified in Alberta, and clubroot-resistant (CR) varieties will not protect against all of them. CR varieties with the same source of resistance grown in a short rotation will quickly select for those pathotypes that are virulent to those varieties. With good moisture for clubroot this year, many fields of CR canola are showing high levels of clubroot infection.
Take time now to check suspicious patches for galls. This will help with crop rotation planning, variety selection and containment measures.
Rotating to a canola variety with a different source of resistance is an important management step, but the more important action for that field is to keep it out of canola for at least the next two years. Research in Western Canada shows that three- or four-year rotation is much better than a two-year rotation for clubroot resting spore reduction.
Contain the spread. High moisture tends to bring out the tillage equipment in an effort to dry out the fields. Tillage quickly spreads virulent clubroot spores throughout a field. Moist soil conditions also mean more soil sticking to tires of combines, swathers and trucks. Resting spores have been found in wind-blown dust and likely can move at least short distances in this way, so any practice that favours erosion can favour clubroot spread. A strategy to contain the disease is advised, especially for fields where CR varieties are losing effectiveness.
Containment tips include harvesting infested areas last, cleaning off tires before leaving these areas, avoiding tillage in these areas and possibly grassing them in until spore loads are reduced to a manageable level. When setting aside an area for localized management, include a buffer area about 50% beyond the patch where noticeable symptoms are found.
Reminder: While canola is the primary host crop, tillage in any field will move existing spores around. Clubroot prevention and containment strategies are required for all fields — not just those seeded to canola this year.
Further reading:
A longer article on this topic
Canola Encyclopedia chapter on clubroot