Cellulose-acetate electrophoresis (CAE) was used to investigate isozyme polymorphisms of 34 Group 1 and Group 2 isolates of Fusarium graminearum. Of the 26 enzyme systems screened, alkaline phosphatase, NADP-dependent glutamate dehydrogenase, peptidase D and phosphoglucomutase were found to be suitable for distinguishing members of Group 1 from members of Group 2. A great deal of similarity in the isozyme patterns among the isolates of a same group, independently of geographic origin, indicated that isolates within a given group are descendant from a same ancestral population. Significant differences in the isozyme patterns between the isolates of Group 1 and Group 2 indicated, however, that the two groups were of different ancestry. Group 1 isolates have since been named F. pseudograminearum. Due to the short run time, minimal need of equipment, small sample volume and staining requirements, CAE generally provides a rapid and accurate method for isozyme analysis of Fusarium species.

Phenotypes of species hybrids created from in vitro fusion of zoospores from Phytophthora nicotianae and P. capsici were characterized and compared. The species hybrids were created as part of a study of sources of genetic variation in populations of the parent species that are pathogenic over a similar range of plants. Four isolates of species hybrids proved to be similar to both P. capsici and P. nicotianae in relation to vegetative and reproductive morphologies. As in a previous study, DNA of P. capsici was detected more readily than that of P. nicotianae in all hybrid isolates. In the present study, DNA of P. nicotianae was detected in three of four hybrids by hybridization of RAPD-PCR products with species-specific DNA from P. nicotianae. By thermal denaturation analyses, DNA melting temperatures and GC contents of parent species and species hybrids were similar. The mean GC content of 47·2% was similar to GC contents reported for other Phytophthora spp. Additionally, the distributions of GC-rich regions of hybrids were more similar to the distribution in P. capsici than in P. nicotianae. By these molecular analyses, the hybrids were shown to be more similar to P. capsici than to P. nicotianae. Even though interspecific somatic fusion is likely to occur rarely under natural conditions, it could contribute to the genetic diversity of heterothallic species of Phytophthora.