The best way to keep clubroot from damaging canola yields is to do whatever it takes to keep it out of your fields, Manitoba Agriculture’s plant pathologist told farmers attending Manitoba Ag Days recently.
“For clubroot to occur, the pathogen needs to be present in the field,” Philip Northover said. That seems like stating the obvious, but he said farmers must understand that preventing clubroot’s spread via contaminated soil on equipment, shoes, seed, and even in livestock manure is paramount to containing the threat it poses to the Canadian canola industry.
Plasmdiophora brassicae, better known as clubroot, was first documented in brassica vegetable crops such as cabbage-and broccoli-producing regions of Canada in the 1870s. It first surfaced in canola in 2001, affecting 12 fields in Alberta’s Sturgeon County north of Edmonton. By last summer, it was found in 250 fields in 16 counties across the province, including the Lethbridge area, which was previously believed to be immune due to its drier conditions and high pH soils.
As the pathogen basically hijacks the infected plant’s root system in order to reproduce its spores, creating the characteristic mishapen and bulbuous roots, it can have a severe impact on yields. When scouting fields, it is important to watch for patches of wilted plants.
Canola farmers in affected Alberta counties are facing major changes to their sanitation protocols. It is costing them time as well as increased labour, Northover said.
Northover said although Manitoba farmers aren’t confronted with a major clubroot threat yet, this province’s relatively moist, warm climate make areas such as the Red River Valley vulnerable. Vegetable growers in this province have experienced infestations. Clubroot prefers low pH or acidic soils. Manitoba soils are typically high pH.
But if the spores reach high enough densities they appear capable of overcoming the pH factor to attack crops, he said. It is most often found in fields in which farmers have grown canola back to back or in two-year rotations with cereals.
The pathogen is not a fungus. Rather it is an endoparasitic slime mould, which means it does its damage to plants from the inside out. It needs a host plant in order to reproduce but spores can survive for years in a resting stage.
Spores have been known to live for up to 20 years waiting for the right combination – a host and warm, moist conditions
to pull them out of hibernation. Under the right conditions, they reproduce rapidly in brassica roots.
But he noted the number of viable spores diminishes the longer they remain without a host, meaning extended rotations combined with good sanitation practices may be the best preventive strategy available to producers.
Strangely enough, some seed companies are treating seed with fungicides as a means of reducing clubroot threat. Northover said there is a benefit, but not because the fungicide acts against the clubroot pathogens. Seed treatments help because the process knocks potentially contaminated soil from the seed, he said.