VARIATION IN PLASMODIOPHORA BRASSICAE WORON

Three races of Plasmodiophora brassicae were distinguished by their different capacities to form clubs on different hosts; one failed to club turnip and swede and the others were identified by their effects on different turnip and swede varieties. In contrast to their effects on clubbing, these races caused equal numbers of root-hair infections with zoosporangia, on all crucifers tested. It is evidently the later stages in the life cycle of the pathogen that cannot develop in clubroot-resistant hosts. Some partially resistant varieties became clubbed when spore concentrations were increased above the level necessary to cause complete infection of susceptible varieties. Varieties susceptible to a particular race of P. brassicae do not seem to fall into any taxonomic group within the Cruciferae. Susceptible and resistant varieties may belong to the same species or may occur throughout a number of species and genera.