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When Phytophthora colocasiae (A2) was used as a hormone producer and P. parasitica (A1) as a hormone receptor, no sex organs of P. parasitica were observed in matings lasting for 7 h, but the amount of hormone produced was sufficient to stimulate the production of 341 oospores cm−2 6 d later. The maximum induction of sex organs was reached in matings lasting 48 h. Hormone production was inhibited by light, but the effect of light on oospore development was small. Temperatures of 10 and 15°C inhibited the growth and hormone production by P. colocasiae, and prevented P. parasitica from forming new sex organs after stimulation by hormone. The effect of temperature on hormone production differed from that on oospore formation. Hormone production was poor at 30°C, but oospore development was good at this temperature.