Factors influencing the occurrence of clubroot in semi-resistant populations of Brassica oleracea L.

Abstract

• 1. In breeding for resistanceto clubroot the plants have to be exposed to infection as long as possible. The clubbing of a susceptible variety forms an unreliable measure of the efficiency of the clubroot test.

• 2. Semi-resistant populations may show variability in clubbing by differences in infection conditions.

• 3. A direct effect of temperature, pH and soil moisture on resistance has not been ascertained.

• 4. Between origins of Plasmodiophora brassicae there may be differences caused by unequal spore loads and by physiological specialisation.

• 5. To test sub-species of Brassica oleracea for clubroot resistance use must be made of a physiological race from infested cabbage fields. The plants that are most resistant to it are equally resistant to other origins of the fungus.