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

A worldwide collection of 60 isolates of Pythium ultimum and 37 asexual (sexually sterile) isolates presumed to be P. ultimum were compared by isozyme analysis, morphological characters, and their growth rate at different temperatures. Isolates were separated into 10 multilocus genotypes as defined by nine isozyme loci. Five genotypes (U1-U5) comprising 85 isolates differed from each other merely by homo- or hetero-zygous states at one and (or) two loci. Four isolates in genotype U6 produced zoospores and were presumed to be var. sporangiiferum. They differed at one locus from genotypes U1-U5. Genotypes U7, U8, U9, and U10 differed from U1-U5 at one, two, three, and five loci, respectively. Sexual isolates occurred in genotypes U1-U3 and U6-U10, and asexual isolates in genotypes U2-U6. Only minor differences were detected in morphology in six isolates (genotypes U8-U10). A single isolate (genotype U10) differed in growth rate from all others. Isozyme analysis is an effective means of distinguishing P. ultimum from nine other species included for comparative purposes and for identifying asexual isolates that cannot be positively identified by morphology.