A pathotype system to describe intraspecific variation in pathogenicity ofMeloidogyne chitwoodi

Tests of eight Dutch Meloidogyne chitwoodi isolates to the differential set for host races 1 and 2 in M. chitwoodi provided no evidence for the existe nce of host race 2 in the Netherlands. The data showed deviations from expe cted reactions on the differential hosts; which raised doubts of the useful ness of the host race classification in M. chitwoodi. The term "pathotype" is proposed for groups of isolates of one Meloidogyne sp. that exhibit the same level of pathogenicity on genotypes of one host species. We recommend that the pathotype classification be applied in pathogen-host relationships when several genotypes of a Meloidogyne sp. are tested on several genotype s of one host species. Three pathotypes of M. chitwoodi were identified on Solanum bulbocastanum, suggesting at least two different genetic factors fo r virulence and resistance in the pathogen and the host species, respective ly. The occurrence of several virulence factors in M. chitwoodi will compli cate the successful application of resistance factors from S. bulbocastanum for developing resistant potato cultivars.