Variation in Pathogenicity Among South African Isolates of Phytophthora cinnamomi

Phytophthora cinnamomi isolates from South Africa were evaluated for differences in growth rate in vitro and levels of pathogenicity towards Eucalyptus smithii in the field. Inoculations were conducted in the field in summer and winter in two subsequent years at two locations in South Africa using 59 P. cinnamomi isolates. The isolates differed significantly in growth rate in vitro, as well as in levels of pathogenicity to E. smithii in the field. Growth rate in vitro was significantly influenced by interactions with culture age, geographic origin and genetic background as determined using isozymes. Levels of pathogenicity in the field were influenced by season of inoculation and average minimum temperatures at trial sites. The host from which P. cinnamomi isolates were originally obtained did not significantly affect levels of pathogenicity in the field. Culture age had a significant negative effect on growth rate in vitro and pathogenicity in the field. Significant differences in levels of pathogenicity could be found for different multilocus isozyme genotypes. Geographic origin and mating type of P. cinnamomi isolates had no significant effect on levels of pathogenicity in the field. A positive correlation was found between growth rate in vitro and levels of pathogenicity in the field. Levels of variation for pathogenicity within A1 mating type isolates were significantly lower than for A2 mating type isolates. Results of this study provide valuable information on selection of P. cinnamomi isolates for future resistance/tolerance screening assays of Eucalyptus germplasm in South Africa.