Worldwide collections of Pythium arrhenomanes and P. graminicola were
studied morphologically and by examining variation in PCR-amplified rD
NAs. Restriction patterns of the internal transcribed spacer and porti
ons of the nuclear large-subunit rDNA were diagnostic for the two spec
ies. Isolates of P. graminicola shared identical restriction patterns.
Most isolates of P. arrhenomanes exhibited identical restriction patt
erns; however, two genetic subpopulations, each with multiple isolates
from a single geographic location, were detected. A phylogenetic anal
ysis of the molecular data, which included two additional species, ind
icated that P. aphanidermatum was an outgroup and P. myriotylum was mo
re closely related to P. arrhenomanes than was P. graminicola. No rest
riction site variation was detected among the four species in the nucl
ear small-subunit rDNA. Isolates of P. arrhenomanes, including both su
bgroups, differed from the other three species by one restriction site
in the mitochondrial large-subunit rDNA. A mean of less than five ant
heridia per oogonium characterized isolates of P. graminicola, whereas
five or more per oogonium characterized most isolates of P. arrhenoma
nes. The ranges of isolate means for oogonium/oospore diameter overlap
ped between the two species. Discriminant function analysis suggested
that the number of antheridia per oogonium is more important than oogo
nium/oospore diameter in separating the two species. The rDNA results
indicate that P. arrhenomanes and P. graminicola are distinct species.
However, due to similar morphology and intraspecific variation, misid
entifications of isolates of the two species have occurred.