Assessment of biological and molecular variability between and within field isolates of Plasmodiophora brassicae

Plasmodiophora brassicae is an obligate biotroph that causes clubroot, one of the most damaging diseases of crucifers. Differential cultivars and rand om amplified polymorphic DNA markers were used to assess the extent of gene tic diversity among nine single-gall populations of P. brassicae and 37 sin gle-spore isolates (SSI) derived from four of those field samples. Isolates were classified into eight pathotypes, and each isolate was associated wit h a unique molecular genotype. Virulence and DNA polymorphisms were detecte d within and between field isolates, and among SSIs from different pathotyp es, hosts and geographical origins. The relatively high level of genetic di versity among field isolates was similar to that among SSIs derived from a single-club field isolate. Molecular and pathogenicity-based classification s were not clearly correlated, but isolates belonging to pathotype P1 were clustered. Two RAPD markers were specific to pathotype P1. The finding that genetic differences can occur in P. brassicae field isolates will be an im portant consideration in resistance genetic studies and in choosing breedin g strategies to develop durable clubroot resistance.