Clubroot is caused by the fungus Plasmodiophora brassicae, which may persist in the soil as spores for many years. Clubroot infects dock, red clover, ryegrass, strawberries and orchardgrass in addition to members of the crucifer family.
The pathogen, which is not seed-borne, is usually introduced by planting infected transplants. The fungus invades healthy tissue of susceptible plants and causes abnormal cell division, which produces enlarged, distorted roots. Nutrient and water absorption are limited in the misshapen roots. Swollen roots often crack, spread new spores into the soil and leave the plants exposed to invasion by soft rot organisms. Cool, wet, acidic soils favor pathogen propagation and infection. Movement of infected soil or water through irrigation, movement of machinery or by soil erosion, can then spread spores between fields.
The first symptom of clubroot on aboveground plant parts is temporary leaf wilting during warm days with subsequent recovery at night. Plants infected while young eventually wilt and die, while those infected later often fail to form a head of marketable size.
There is no economical way of eliminating the clubroot organism from fields once infested. Therefore, it is essential to prevent the introduction of the pathogen.
Never plant infected transplants into a clean field. Avoid southern field-grown transplants.
Prevention and Management
- Grow your own transplants in soiless media, pasteurized soil-based media, in well drained, uninfested, or chemically sterilized seedbeds.
- Choose well-drained sites, free of the clubroot pathogen for crop production.
- Rotations, which include cereals and other non-host crops for 7 or more years, have been helpful at minimizing the incidence of clubroot in some areas.
- Alkaline soils limit clubroot infection. Commercial control is often achieved by raising the pH of a field above 7.2 with a combination of crushed lime-stone and hydrated lime. Raise the pH above 6.8 by applying crushed limestone six months prior to planting crucifers. Then apply 1,500 pounds of hydrated lime at least six weeks before planting to elevate the pH to 7.2 or more.
- The addition of pentachloronitrobenzene (Terrachlor 75WP), fluazinam (Omega 500F) or cyazofamid (Ranman) to the transplant water helps prevent cracked roots from being infected by soft rot organisms.
By: Jude Boucher, Integrated Pest Management, University of Connecticut, Storrs, CT
Reprinted from: Grower, Vol. 91-8
Reviewed and updated: Jude Boucher, UConn IPM, 2012
This information was developed for conditions in the Northeast. Use in other geographical areas may be inappropriate.
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