AbstractWhen oospores of 2 single zoospore isolates of P. fragariae from inoculated roots of the strawberry clone VS1 were extracted and placed on agar under various conditions, < 15% of one isolate germinated and germination was unaffected by age, while 40-70% of young oospores < 4 wk old of the second isolate germinated. Spore germination of the second isolate fell with age, the fall being accelerated if the roots containing the spores had been held at temp. > 15°C. Prolonged exposure to 30°C in roots or on agar caused loss of oospore viability. Germination was opt. at 15°C. Although slower at 10°C and 3°C and absent at -1°C it was rapidly recovered when spores from these temp. were transferred to 15°C. Temp. > 15°C progressively reduced the numbers of germinated spores and although more germinated when transferred to 15°C the increase in their numbers was much slower than for spores held at lower temp. Low water potential created by adding NaCl, KCl, mannitol and sucrose to agar slowed germination but had little effect on the final numbers of active spores although germ tube formation was suppressed at water potentials of -1.5 MPa. Spores germinated at pH 4-8, opt. for germ tube formation being pH 5.5-7.