Factors affecting tests for differential pathogenicity in populations of Plasmodiophora brassicae

Factors affecting the use of the European Clubroot Differential (ECD) series for determining the differential pathogenicity of Plasmodiophora brassicae were studied. Replicate tests made with one collection of P. brassicae did not give consistent results. Collections were shown to be heterogeneous for differential pathogenicity. Selection of components of a collection occurred during passage through a range of differential host cultivars. ECD host 06 was shown not to be uniform. There was a linear relationship between the logarithm of spore concentration in the inoculum used in tests and the incidence of disease, and some ECD hosts required a higher concentration of spores than others to give the same incidence of disease. The implications of these findings for the future use of the ECD series are discussed.