Production of sporangia and oospores of Phytophthora fragariae in roots of strawberry plants.
Four cultivars of strawberry with different levels of resistance to P. fragariae were inoculated with encysted zoospores of 4 races of the pathogen. Plants were sampled at 2-d intervals up to 14 d after inoculation at which time roots were excised and placed in nonsterile soil leachate for an additional 2- to 10-d incubation. Production of sporangia by P. fragariae was greatest when roots of susceptible strawberry cultivar Tennessee Beauty were incubated for 8 d at 15°C after inoculation with isolate NC-1 of race Pf-2. An incubation period of 2-4 d in nonsterile soil leachate was necessary to induce max. numbers of sporangia. Increasing the incubation period in nonsterile soil leachate beyond 2-4 d did not increase the number of sporangia produced. Oospore production was observed 4-6 d after inoculation and increased as the incubation period increased to 12 d. Opt. temp. for the production of sporangia of the 4 races of P. fragariae tested were 12-20° 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.