Isozyme comparison within and among worldwide sources of three morphologically distinct species of Phytophthora

One hundred and sixty-two isolates representing worldwide sources of Phytophthora cambivora, P. cinnamomi and P. cactorum were compared using 18 isozyme loci separated by starch-gel electrophoresis. Based on isozyme analysis, the three species were clearly separated and each was further subdivided into electrophoretic types (ETs). No evidence suggests a close relationship between P. cambivora and P. cinnamomi. P. cambivora separated into eight ETs. One ET was the most common worldwide; there were four unique ETs from Australia, one from Japan and two from the U.S.A. No distinguishing morphological features were associated with the different ETs. P. cinnamomi also divided into eight ETs. The two ETs, which represented all the A2 isolates used in the study, could be distinguished from the majority of the A1 isolates based on the migration pattern of isocitrate dehydrogenase. Only one exception to this was seen in an isolate which was reported to have switched mating types from A2 to A1. P. cactorum was divided into two ETs, one common and the other represented by two isolates from strawberry from New York. These two isolates also differed in their sporangial dimensions and oospore pigmentation. The potential of cellulose acetate electrophoresis was evaluated as a rapid diagnostic method. Three enzymes — phosphoglucose isomerase, malate dehydrogenase and lactate dehydrogenase — were found to be diagnostic, thus allowing clear differentiation of the three species.