Photo: 20/20 Seed Labs
It’s definitely not good news that nine new strains of clubroot — a disease that can kill canola crops — have been discovered in western Canadian fields by University of Alberta researchers.
Despite the alarming news, the discovery shows how important it is to build a multi-pronged strategy for protecting the crop and not relying solely on canola plants bred to resist the disease, said lead researcher Keisha Hollman.
The discovery of the new pathogens in canola fields tested across Alberta, Saskatchewan and Manitoba in 2017 and 2018 means resistance breakdown first discovered in 2013 is growing.
The research, published in the Canadian Journal of Plant Pathology, brings the number of known clubroot pathotypes to 36, of which 19 can overcome the “first-generation” resistance to clubroot found in most canola varieties grown in Canada.
However, the news is a good opportunity for a refresher on how devastating clubroot can be and what can be done to prevent and manage it.
What is Cluboot?
Clubroot in canola is caused by Plasmodiophora brassicae, a parasite which infects the roots of host plants and produces club-shaped galls that restrict the flow of water and nutrients. Plasmodiophora brassicae is an obligate parasite, meaning it cannot survive without a living host.
Clubroot is a soil-borne disease which affects all cruciferous crops and, once established, is very difficult to eradicate since the resting spores can survive in the soil for up to 20 years. Clubroot resistant canola varieties are now available, but the pool of readily available resistant genetics is limited. The pathogen has already adapted and overcome the clubroot resistance that is present in European canola varieties. There are no registered chemical controls for clubroot in canola.
Underground, Plasmodiophora brassicae infects roots, causing them to develop club-like galls that restrict the flow of nutrients and water to the upper portion of the plant. Above ground, plants may show symptoms of wilting, stunting, yellowing, premature ripening and seed shrivelling. Early season infection may result in plants that look as if they’re heat or drought stressed. Later season infection can make plants look as if they have sclerotinia stem rot, or even fusarium wilt.
How can it be Prevented?
Prevention is critical when it comes to clubroot control. Practice strict sanitation, particularly with equipment, to minimize the spread of clubroot from infected fields to clean fields. Observe a minimum four-year canola rotation. Scout regularly — if you see wilting, stunting, yellowing or premature ripening, check roots to ascertain the cause. To prevent breakdown of resistance, maximize rotations as much as possible when planting resistant varieties into known infected fields.
What Testing Options Exist?
There is a DNA-based diagnostic test available for P. brassicae in plant tissue and soil. Originally developed at the University of Alberta, the test is sensitive enough to detect 1,000 resting spores per gram of soil, or approximately 10 to 100 times lower than the inoculum level at which clubroot symptoms are thought to occur. Essentially, this test provides an early warning of clubroot before economic losses occur.
Survey results have shown that the highest incidence of clubroot occurs at the entrance to fields. Sample in a W-shaped pattern at entrances out to a maximum 150 feet into the field. Low-lying points within the field, homestead garden sites, and soil clumps that may have fallen off machinery are also hotspots for possible pathogen presence. Do not sample randomly across the field.
Clear all loose organic matter from the soil surface and collect the top 5-10 cm of the A-horizon, or less as the depth allows, without taking any of the B-horizon. Submit a minimum two-cup sample of soil. Air dry and send in a Ziploc bag.
With proper management we can mitigate the effects as clubroot as best as possible. It’s a problem that looks to be getting more prevalent, but the tools exist to ensure you manage the risk.