Taxonomic classification of asexual isolates ofPythium ultimum based on cultural characteristics and mitochondrial DNA restriction patterns

Taxonomic characteristics of 8 isolates ofPythium ultimum and 11 isolates ofPythium in the spherical hyphal swelling (HS) group of Van der Platts-Niterink were compared. Isolates in the two groups had identical temperature growth responses and morphological features of hyphal swellings. All isolates were pathogenic on sugar beet. Attempts to cross HS isolates among themselves and with opposite mating types ofP. heterothallicum andP. sylvaticum failed. HS isolates were not induced to form antheridia when paired with isolates ofP. ultimum using a polycarbonate membrane sandwich technique. In single culture, some HS isolates formed low numbers of spherical structures encompassed by swollen hyphal masses resembling antheridia. Comparisons of restriction banding patterns ofHindIII-digested mitochondrial DNA revealed that all but 1 isolate ofP. ultimum were identical; however, the variable isolate shared 80% of the bands in common with the others. Seven of 11 HS isolates had banding patterns identical to the predominantP. ultimum pattern and 1 isolate shared 96% comigrating bands. On the basis of the number of shared characteristics, these isolates appear to beP. ultimum which have lost the ability to reproduce sexually.