Phytophthora cactorum can synthesize substances needed for sexual reproduction but requires a stress factor to trigger the process

Phytophthora cactorum grown on basal agarose medium or in liquid basal medium produced oospores after being transferred to water agarose. The numbers of oospores produced under such conditions depended on the age of the culture prior to exposure to nutrient deprivation. When the concentration of basal medium used for cultivation of P. cactorum was increased, the numbers of oospores produced after being transferred to water agarose was also increased. P. cactorum grown on basal agarose medium also produced oospores when its mycelial growth was restricted after reaching the edge of Petri plates. In 5 cm plates oospore formation occurred in the third week, whereas in 9 cm and 14 cm plates oospores appeared in the fourth week. Most oospores were formed near the edge of the plates. The non-saponifiables extracted from mycelia of P. cactorum grown in liquid basal medium were stimulatory to oospore formation by P. cactorum and Phytophthora parasitica, whereas the saponifiables were stimulatory to P. cactorum only. Extracts from culture filtrate and basal medium were not stimulatory to oospore formation by either fungus. When the non-saponifiables were fractionated by Florisil columm chromatography, only fraction 1 was not active. Fractions 2, 3 and 4 were stimulatory to oospore formation by both P. cactorum and P. parasitica. These results support the hypothesis that P. cactorum, and possibly other pythiaceous fungi as well, can synthesize substances needed for sexual reproduction but requires a stress factor to trigger the process.