Clubroot in canola is caused by Plasmodiophora brassicae, a parasite that 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.
Why Does it Matter?
- Clubroot is a soil-borne disease that affects all cruciferous crops and, once established, is very difficult to eradicate since the resting spores can survive in the soil for 10 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.
- Clubroot is relatively new to Western Canada. In Alberta, it was first reported in canola in 2003. Subsequent surveys in 2005 to 2009 have found an increasing number of affected fields, especially around the Edmonton area. In 2009, the clubroot pathogen was reported to be present in soil samples from Saskachewan fields but no disease has been seen yet.
- Previously, canola was thought to be only mildly susceptible to clubroot, but in 2004 forty-eight different Canadian canola varieties were found to be highly susceptible.
- Clubroot strains found in Alberta also appear to be more virulent in canola than strains from other regions of Canada.
- Yield loss. Roughly estimated, yield loss from clubroot is about half of the percentage of infected plants. A field with 50% infection will result in a 25% yield loss.
What are the Symptoms?
- 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 shriveling.
- 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.
- Clubroot favours soils that are warm (20o C to 24o C), moist, and acid (pH less than 6.5).
- Examine roots to confirm the presence of clubroot.
What is the Life Cycle?
- In the spring, resting spores in the soil react to chemical signals from canola roots and germinate, producing zoospores that “swim” through water in the soil.
- Zoospores infect root hairs or enter roots through wounds and form an amoeba-like cell that multiplies. The cells join up to form a mass, or plasmodium, of living cell material (protoplasm).
- The plasmodium divides forming secondary zoospores and releasing them into the soil.
- Secondary zoospores are able to penetrate the cortex of the root. Once there, they form a secondary plasmodium, which affects plant hormones and causes root cells to swell and form galls.
- When they are mature, the secondary plasmodia divide and form resting spores in the soil.
- The resting spores of clubroot can move to new locations when carried in infested soil by wind, water run-off and farm machinery. It can also be spread through infected transplants.
How is it Controlled?
- 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 4-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.
How Does 20/20 Seed Labs Inc. Test for Clubroot in Canola?
20/20 Seed Labs Inc. offers a sensitive DNA-based diagnostic test 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. Some details are:
- One-half cup of soil is tested (>100g), the largest sample size tested anywhere using DNA techniques.
- This enhanced test was developed to provide greater assurance of results and is only available at 20/20 Seed Labs Inc.
- The DNA test takes only 1 to 2 days for plant tissue, and 7 to 10 days for soil.
Soil – “Soil testing is the best preventative method available”
- 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 hot-spots 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 2 cup sample of soil. Air dry and send in a Ziploc bag.
- Infected plants are concentrated sources of pathogen, and represent a significant escalation in the amount of inoculum present in a field.
- Scout for suspect plants (planted or volunteer) and submit fresh, frozen or dried roots for testing.
Giving Back For a Solution
A portion of the revenues derived from our clubroot testing program is donated back to the University of Alberta in support of plant pathology research. A solution is important to all of us.