Light‐dependent Oospore Germination of Phytophthora cactorum in the Presence of Susceptible Host Plant

Abstract Oospores of Phytophthora cactorum were produced in cleared V‐8 broth supplemented with 30 mg/l β‐sitosterol in the dark at 20–22°C for 2–3 months. Environmental conditions during oospore formation or maturation such as light, low temperature and also ageing could not compensate the light requirement for oospore germination. Dark grown oospores did not infect hypocotyl of susceptible safflower seedlings, an indicator plant for the pathogen, if they were not photoactivated 3–15 days or longer prior to infection. If oospores are the sole source of survival of P. cactorum in soil, then the collar region of the plant would be the main site of infection. Thus any treatment which reduce the chance of oospore germination on the soil surface may support management of this disease.