CLUBROOT of CRUCIFERS                          Bulletin #5096

Introduction

Clubroot is a serious disease of crucifers caused by the primitive fungus, Plasmodiophora brassicae. It is worldwide in occurrence having been spread by the activities of man.

Symptoms

The first symptom usually observed is a wilting of the foliage especially on hot days. Wilting is the result of impaired root function which reduces the capacity of the plant to take up water. The diseased roots are also less able to absorb nutrients and so the foliage may appear yellowish and the plant may be stunted. Although these above-ground symptoms could be due to other causes (e.g. cabbage maggot), an abnormal enlargement of the roots is diagnostic for this disease. The fungus penetrates the roots directly or through wounds and induces the root cells to enlarge. The resulting enlarged roots appear spindle-shaped, knobby, spherical, or club-shaped. All roots including the underground stem may be affected.

Environmental Conditions

Soils that are cool and wet favor the disease and its survival. Especially important is the pH of the soil. Germination of the fungus spores is inhibited in alkaline soils. Conversely, acid conditions favor the disease.

Survival and Dispersal

Once the soil is infested by the clubroot fungus, the fungus will remain viable for years. It can live for 10 years in the absence of its host and, because it infects cruciferous weeds, it is likely to be present even longer. The infected roots eventually disintegrate in the soil releasing masses of spores. These long-lived spores can be transported in any manner that transports soil through the field. Soil can be carried on shoes, tools, farm equipment, runoff, splashing water, wind, etc. Although the disease is not seed born, it is readily carried on infected transplants.

Management

1) A four year rotation for cole crops is currently recommended unless club root is detected, then opt for a seven year rotation. Because the spores are so long-lived, rotation may only be marginally effective. 2) Maintain the soil pH at least at 6.8, preferably above 7.2. This high pH may adversely affect some rotation crops. Add at least 1500 lb/acre of hydrated lime when planting in clubroot infested soils that are pH 7.0 and above. Lower pH may require more hydrated lime. Apply the lime at least 6 weeks before setting out the crop. 3) Grow or buy clean transplants to avoid the initial infestation. Seedbeds should be well-drained and disease-free. 4) PCNB (Terraclor 75WP) can be applied as a solution in the transplant water (2-6 lb/100 gal; 1 cup of solution per plant) or applied at the rate of 30lb/acre.

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