Clubroot
Beginner
Scientific Name
Plasmodiophora brassicae
Identification
- Wilting, stunting and yellowing of plants in patches in the field
- When the plants are pulled, a characteristic “clubbing” of the root tissue is evident
Often Confused With
Nematodes
Period of Activity
Clubroot is present in the soil in most areas of the province. Once in a field, the resting spores persist for many years. Disease severity increases with soil acidity, temperature and moisture content. Optimal temperature for development and spore germination range from 19.5- 23°C (67- 73°F) and 16- 21°C (61- 70°F) respectively. At the same time, moisture levels above 50- 70% of the maximum water-holding capacity of the soil are needed for infection.
Scouting Notes
While walking fields, look for wilted and/or stunted plants. Symptomatic plants often occur in patches and may or may not be in wet areas. Dig around or pull the plant to look for the cause of the damage. Record the percentage of the field impacted by clubroot.
Thresholds
None established.
Advanced
Scientific Name
Plasmodiophora brassicae
Clubroot is a soil-borne disease. This fungus affects all members of the crucifer family commercially grown in Ontario, though some species do not show symptoms. It also attacks more than 200 species of plants, not all belonging to the mustard family.
The disease is present in the soil in most areas of the province and the resting spores persist for many years. Once present in the soil, special management guidelines must be followed if crucifers are to be grown in that field again.
Identification
If symptoms are present, the first noticeable indication of clubroot is usually the wilting of plants in patches. These plants may or may not be in wet areas. When the plants are pulled a characteristic “clubbing” of the root tissue is evident. This may occur on any of the underground parts of the plant.
Often Confused With
Nematodes
Biology
The disease is spread from field to field by the movement of infected plants and contaminated soil, water, manure, or on equipment. The fungus enters the root through fine root hairs or wounds. Once in the root, it forms and releases spores causing root tissue decay. There are two types of spores. The first type swims in the water phase of the soil and infect other roots. The other type is a “resting” spore which can survive in the soil for more than 10 years until suitable temperature and moisture conditions arise and susceptible crops are present.
Clubbing results from abnormal growth of the root tissue caused by the invasion of the clubroot fungus. Severely distorted roots have difficulty absorbing water and minerals from the soil. Affected plants will be stunted, with the lower leaves yellow and wilted.
Period of Activity
Clubroot is present in the soil in most areas of the province. Once in a field, the resting spores persist for many years. Disease severity increases with soil acidity, temperature and moisture content. Optimal temperature for development and spore germination range from 19.5- 23°C (67- 73°F) and 16- 21°C (61- 70°F) respectively. At the same time, moisture levels above 50- 70% of the maximum water-holding capacity of the soil are needed for infection. However, clubroot can occur in alkaline soils when inoculum levels are high, soil moisture is greater than 70% of field capacity, and temperatures are favourable.
Scouting Notes
While walking fields, look for wilted and/or stunted plants. Symptomatic plants often occur in patches and may or may not be in wet areas. Dig around or pull the plant to look for the cause of the damage. Record the percentage of the field impacted by clubroot.
Thresholds
None established.
Management Notes
- There are four primary factors governing clubroot development – soil pH, soil moisture, spore load (how much of the fungus is present), and calcium/magnesium levels. pH is the single most important factor. In Ontario, clubroot is most frequently found where the soil pH is less than 7.2. However, excessive, prolonged moisture and very high spore loads will cause disease even at pH levels above 7.2.
- Growing crucifers in well-drained, warm soil, eradication of related weed hosts, fungicidal soil drenches, and lime application to maintain pH above 7.2 are, when combined, the most successful methods of control.
- Agricultural limestone takes a minimum of one year to effectively change the pH. For faster activity a minimum of 1700 kg/ha hydrated lime can be added (regardless of soil pH) at least six weeks prior to field transplanting. Hydrated lime changes soil pH only temporarily. Agricultural limestone is effective for several years.
- Use clubroot-free locations for outdoor seed beds. Do not add hydrated lime in seedbeds, as it may mask the presence of the fungus, and allow it to move with the transplants to the field, where subsequent infection may occur if soil pH is favourable.
- Clubroot spores need free water to move about in the soil. These conditions can arise in heavy soil with high water-holding capacity and also in sandy soils that do not drain well.
- In muck soil, any crop in the ground for more than 6 weeks is at risk of becoming infected with clubroot.
- Hardpan layers under the soil surface can create a favourable environment for the fungus in light soils. Avoiding soil compaction is an important consideration.
- When transplanting, discard all plants in a lot if clubroot is found on any seedling. Others may be infected and not yet show symptoms. If transplanted, they will infest that field with the fungus.
- Rotation with non-crucifers is the most effective way to prevent an increase in spore load.
- With sufficient calcium, it is more difficult for the fungus to penetrate the root tissue. In this instance, calcium acts on the root by building stronger cell walls. Calcium also inhibits the germination of the resting spores.
- The elimination of cruciferous weeds will prevent the build-up of the clubroot fungus during rotation.
