Seed decision: What clubroot resistance to choose?

Canola growers should use clubroot-resistant (CR) canola varieties and extend their rotation to canola once every three years. But which CR variety should they use?

The list of CR canola varieties available in Western Canada is posted on the Canola Encyclopedia, under the clubroot.ca. Varieties with asterisks (**) beside the name have CR resistance different from or in addition to the first generation resistance.

First generation resistance will provide very good protection in most fields. Varieties with new sources of resistance should be used and rotated in fields where original sources have been used repeatedly on fields with high spore loads or if pathotypes are breaking the original sources of resistance. This can be identified though scouting.

We know that a clubroot infestation will contain more than one pathotype and that no known CR is resistant to all of them. Therefore, growing any CR variety selects for pathotypes not controlled by that resistance source. The shift in pathotype populations can happen fairly quickly, especially under high spore loads. When growers see evidence of this shift (i.e. clubroot detected even with a CR variety) it is time to grow a different source of resistance.

Naming system for clubroot resistance

Canada has settled on a pathotype naming system that includes a number and a letter. The number aligns with the original system (Williams differential set) and the letter indicates pathotypes within each Williams designation. Pathotype 3H is the predominant pathotype in most clubroot populations and first generation resistance works well to protect against this pathotype. The risk is that repeat use of first generation resistance selects for pathotypes that overcome that source of resistance. That is why two-year breaks between canola crops along with scouting are important steps in clubroot management. (These are discussed below.) More pathotypes are discovered every year and the use of CR in high-spore, high-risk situations will eventually select for pathotypes that overcome all sources of resistance. Genetics may not be an option for farms in that situation.

What else should you do?

Under low spore pressure and with a minimum two-year break between clubroot-host crops, all the genetics should stand up quite well, including original sources of resistance. That is why keeping spore levels low should be a top management goal. Growers can do this by following all steps in this clubroot management recipe:

• Vigilantly scout all canola fields for symptoms, even if growing a CR variety.
• Keep a minimum 2-year break between canola crops. This crop rotation is crucial in the stewardship of genetic resistance. With a 2-year break between clubroot hosts, we see a rapid decline in living resting spores.
• Seed CR varieties and understand if/when to deploy different sources of CR. Planting CR varieties before the disease arrives and gets established will give you a better chance of keeping the resting spore load at a manageable level and maintaining effective resistance. Rotation of resistance could also be important to maintain resistance efficacy, and avoid a “shift” in the population, as described earlier in this article. With repeated use of varieties with the same resistant traits under high spore loads, virulent races can multiply and effectiveness of resistant will be seriously compromised.
• Limit activities that can introduce foreign soil or cause erosion. Minimum tillage and equipment sanitation (as simple as knocking off visible dirt before leaving a field) will greatly reduce the risk of moving infested soil around. Note that wet soil conditions increase the amount of soil that clings to equipment.
• Control host weeds. Common weeds that can host clubroot include stinkweed, shepherd’s purse, flixweed, all mustards and volunteer canola. They need to be controlled within three weeks of emergence to prevent a new batch of spores being produced.
• Isolate field entrances and hot spots. Use patch management strategies to reduce spore loads, such as grassing the affected area, which will also limit soil movement. Patches that are visibly worse than the remainder of your field often have millions more spores per gram of soil than elsewhere and are often the first place where clubroot resistance breaks down. Removing these hot spots from cultivation for a few extra years significantly reduces spread and risk of resistance breakdown. Consider liming as an additional patch management technique, increasing the pH to 7.3 can significant reduce gall formation. As part of patch management, having separate field entrances and exits could reduce the amount of infested soil leaving the field on machinery.

When combined, like ingredients in a recipe, these practices will help growers to limit clubroot damage in canola, allowing for the continued success and profitability of canola production across the Prairies.

Compare varieties

Twelve CR varieties were included in Canola Performance Trials in 2019. You can find results in the PDF booklet posted at canolaperformancetrials.ca. If CR is the most important trait, growers will want to short-list their varieties for 2020 based on which ones have the CR trait. From that short list, they can then pick varieties with yield, days to maturity and any other traits they’re looking for.

Clubroot maps

The latest Saskatchewan map, including 2019 survey results, is posted. SaskCanola made a significant investment in covering the cost of soil tests to help understand the spread of clubroot in Saskatchewan.

All provincial maps are posted at clubroot.ca. The section will be updated as new maps are released.