Variation in pathogenicity among South African isolates of Phytophthora cinnamomi

Phytophthora cinnamomi isolates from South Africa were evaluated for differ ences in growth rate in vitro and levels of pathogenicity towards Eucalyptu s 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 5 9 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 cu lture age, geographic origin and genetic background as determined using iso zymes. Levels of pathogenicity in the field were influenced by season of in oculation and average minimum temperatures at trial sites. The host from wh ich P. cinnamomi isolates were originally obtained did not significantly af fect levels of pathogenicity in the field. Culture age had a significant ne gative effect on growth rate in vitro and pathogenicity in the field. Signi ficant differences in levels of pathogenicity could be found for different multilocus isozyme genotypes. Geographic origin and mating type of P. cinna momi isolates had no significant effect on levels of pathogenicity in the f ield. A positive correlation was found between growth rate in vitro and lev els of pathogenicity in the field. Levels of variation for pathogenicity wi thin A1 mating type isolates were significantly lower than for A2 mating ty pe isolates. Results of this study provide valuable information on selectio n of P. cinnamomi isolates for future resistance/tolerance screening assays of Eucalyptus germplasm in South Africa.