Clubroot is a soil borne disease that can survive in the
soil for very long periods of time. The spores can go dormant when a host plant
is not present, leading to the need for long crop rotations between brassica
crops to reduce spore load in the soil. Equipment is the main source of disease
spread as the spore-containing
soil gets picked up and carried from field to field on tires and tillage
equipment.
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| Soil on equipment and tires transfers clubroot from field to field. Photo: Canola Council of Canada. |
At
Horticulture Congress in January 2020, Dr. Mary Ruth McDonald from the
University of Guelph spoke about research she is conducting on clubroot
control. One of the points that really stood out was the use of grass cover
crops to help with clubroot control. A grass cover crop will help to keep the
soil in place, therefore reducing the spread of clubroot by equipment. Planting
a grass cover crop on the headlands or at the entry to fields with a history of
clubroot could be beneficial in reducing this spread.
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| The clubroot fungus causes abnormal growth of root tissue, resulting in clubbing. Photo: Rosalie Gillis-Madden, Perennia. |
There is also some evidence that certain species or varieties of grass cover crops could be contributing to the management of clubroot populations by acting as bait crops. Perennial ryegrass has been seen to stimulate the germination of resting clubroot spores, allowing them to be taken up into the root hairs of the grass, but preventing them from completing their lifecycles and being released back into the soil. Dr. McDonald reported the resting spore concentration of bare soil compared to soil that had been planted with grass cover crops. Three varieties of perennial ryegrass (vars. Norlea, All Star, Fiesta), one of smooth bromegrass (common lot), and one of meadow bromegrass (var. Fleet) were tested, resulting in significantly lower resting spore concentrations in the Fiesta perennial ryegrass and smooth bromegrass.
Further research conducted by Sarah Drury, masters student from the University of Guelph under the advisement of Dr. McDonald, tested the effect of several common cover crops on the clubroot resting spore concentration in soil. Under greenhouse conditions, soils were inoculated with resting spores and then seeded with barley, spring wheat, field pea and perennial ryegrass. One treatment was also left fallow. Post-planting spore concentrations were measured and showed a significant reduction in initial levels for the barley treatment. All others were comparable to the initial spore concentrations.
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| Perennial ryegrass has been shown to stimulate germination of resting clubroot spores, in some studies. Photo: Sonny Murray, Perennia. |
Other management strategies include liming to a pH of 7.2 or higher, which inhibits the germination of the resting spores, and crop rotations of at least 4-5 years while controlling alternate weed hosts.