PRODUCTION OF SPORANGIA AND OOSPORES OF PHYTOPHTHORA-FRAGARIAE IN ROOTS OF STRAWBERRY PLANTS

Four cultivars of strawberry with different levels of resistance to Phytophthora fragariae were inoculated with encysted zoospores of four races of the pathogen. Plants were sampled at 2-day intervals up to 14 days after inoculation, at which time roots were excised and placed in nonsterile soil leachate for an additional 2- to 10-day incubation. Production of sporangia by P. fragariae was greatest when roots of susceptible strawberry cultivar Tennessee Beauty were incubated for a total of 8 days at 15 C after inoculation with isolate NC-1 of race Pf-2. An incubation period of 2-4 days in nonsterile soil leachate was necessary to induce maximum numbers of sporangia. Increasing the incubation period in nonsterile soil leachate beyond 2-4 days did not increase the number of sporangia produced. Oospore production was observed 4-6 days after inoculation and increased as the incubation period increased to 12 days. Optimum temperatures for the production of sporangia of the four races of P. fragariae tested were 12-20 C depending on the isolate. All races of P. fragariae produced more sporangia on roots of a very susceptible strawberry cultivar than on either moderately susceptible or resistant cultivars.