Isozyme variation, morphology, and growth response to temperature in Pythium irregulare

Isozyme-based genetic diversity, morphological characters, and growth rate at different temperatures were compared in a worldwide collection of 125 isolates presumed to be Pythium irregulare Buisman. The isozyme data was analysed with previously published data for Pythium ultimum Trow and Pythium sylvaticum Campbell & Hendrix. UPGMA cluster analysis yielded a dendrogram with four distinct groups: P. ultimum, P. sylvaticum, and two for P. irregulare. Putative P. irregulare isolates were separated into 33 multilocus genotypes defined by 11 isozyme loci: group A contained 116 isolates in 25 genotypes, and group B, 8 isolates in 7 genotypes. One genotype with a single isolate was determined as P. sylvaticum. Based on the isozyme analysis, group B was considered a distinct taxonomic entity from group A, but lacked any unique morphological character. There was a wide range in oogonium and oospore sizes among different isolates of P. irregulare, with those in group B generally being larger. Some isolates in grou...