A molecular method is described to aid identification of the obligate
parasite Polymyxa and discriminate between species (P. betae and P. gr
aminis) and isolates. DNA was extracted from zoospores, resting spores
and roots infected with P. betae and P. graminis and compared with th
at from negative control plants that were not inoculated with Polymyxa
but were grown at the same time under the same conditions. The riboso
mal internal transcribed spacers and 5.8S rDNAs were amplified by the
polymerase chain reaction and digested with restriction enzymes to det
ect molecular differences between the species and isolates. There were
differences between P. betae and P. graminis and two subgroups within
P. graminis but so far this has not been correlated with any other bi
ological property.