Effect of culture origin on chemical stimulation of sexual reproduction in Phytophthora and Pythium

Lecithin washed with NaCl solution was stimulatory to oospore formation of Phytophthora boehmeriae, P. sojae, P. cactorum, and Pythium aphanidermatum. Commercial glycerides of palmitic acid with 99% purity were found to contain a substance inhibitory to oospore formation. Purification with florisil column chromatography was more effective in removing the inhibitory substance than thin-layer chromatography or NaCl washing. After purification, monopalmitin, dipalmitin and tripalmitin were found to be stimulatory to the sexual reproduction of Phytophthora for the first time. These compounds were stimulatory to oospore formation of P. boehmeriae and P. sojae but not P. medicaginis, and were stimulatory to isolate S317 of P. sojae but not isolate CN-1 of the same species. Pythium aphanidermatum cultured on basal medium grew sparsely but steadily and produced oospores after being transferred to nutrient-free water agarose. Extracts from mycelia of P. boehmeriae, P. cactorum, and P. sojae grown in liquid basal medium were stimulatory to their own sexual reproduction. Results support the hypothesis that pythiaceous fungi can synthesize substances needed for sexual reproduction but require a stress factor to trigger the process.