Clubroot: When genetic resistance no longer works…

April 2, 2015 - Issue 5

…The best management option left is the four-year rotation. Discovery of different clubroot pathotypes in central Alberta will change the rotation plans for some growers. No current varieties have strong resistance to these pathotypes.

If growers have used resistant canola two or three times already on fields that were moderately to highly infested with clubroot, these fields are at high risk for having one or more of the different pathotypes. Resistance breakdown is a numbers game, and with the number of spores a serious clubroot patch contributes to the soil, most bets would be placed on clubroot’s ability to adapt. Longer rotation is necessary to slow the pathogen shift that is occurring in these fields.

Canola every two years instead of every four years gives science only half the time to work on solutions. A four-year rotation is a good disease management standard — especially when genetic resistance is not available.

Update on clubroot pathotypes: Stephen Strelkov with the University of Alberta tested soil samples from 27 fields in Alberta that were seeded to resistant varieties in 2014 and showed more than expected levels of gall formation. Of those 27 fields, 16 have clubroot pathotypes that suggest the clubroot population in those fields had shifted to overcome the resistance trait in the variety grown.

Preliminary testing indicates there may be more than one different pathotype, as scientists are not able to confirm all are 5x. (5x is the first pathotype identified capable of overcoming Canadian clubroot resistance.) Further testing is underway to determine this.

These fields are at very high risk of developing high spore loads of a new pathotype that current resistance won’t control. The patches currently range from 10 acres to a few square feet, but can spread quickly without immediate rotation action, reduced tillage and equipment sanitation. Even with a four-year break, these fields could still be at risk of increasing spore loads leading to an economic level of infection. What the four-year break provides, in the case of these fields with the new pathotypes and those with heavy spore loads, is time for a solution to come forward — hopefully.

This experience should encourage continued vigilance in growers who have been fortunate enough to have kept clubroot out or at low levels on their farm, and also remind them how important sanitation is and will continue to be.

With these different pathotype(s) now present, the following management steps bear repeating. Here are ways, in addition to rotation, that will reduce the risk of selecting for the different virulent pathotypes in areas known to have clubroot:

—Keep a close watch. Look for unexpected levels of clubroot in fields seeded to resistant varieties. This is a sign that pathotype shift has reduced the effectiveness of the resistance trait in your fields.
— Sanitation. Few growers are taking time to disinfect (final misting with bleach, EcoClear, or HyperOx) equipment because of the time it takes. But growers in these higher-risk zones must at least knock the dirt off equipment before leaving fields. At least 90% of clubroot spores that move from field to field can be stopped by scraping off dirt.
—Avoid tillage as it moves a lot of clubroot-infested soil around the field and from field to field.
—Control host weeds and canola volunteers that provide a bridge for clubroot galls and spore build up even when canola is not produced on a field.
—Continue using resistant varieties and look for new genetics in the future. Never use susceptible varieties on clubroot-infested soil or in areas with a high prevalence of clubroot.

Growers who do not yet have clubroot in their fields are encouraged to use the following practices:

—Scout vigilantly for signs of clubroot.
—Avoid the introduction of clubroot by cleaning and monitoring equipment entering fields.
—Use cleaned and treated seed only.
—Use resistant varieties if in or near an infested area.
—Control volunteers and host weeds.

Podcast on clubroot

Canola Watch