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 group A had well developed oogonial spines, but others were essentially spineless, whereas all those in group B were spineless. Both groups A and B contained isolates with distinctly aplerotic oospores and others with essentially plerotic oospores. Antheridial number and shape were highly variable both within and among isolates in the two groups. Growth rate over a range of temperatures varied among isolates in both groups and was not a reliable taxonomic criterion. The irregular shape of oogonia and, when present, oogonial spines were the only reliable characters for distinguishing P. irregulare isolates from other taxa.