Differential effects of cholesterol in mating thalli of Pythium sylvaticum and Phytophthora capsici

Hyphae of male and female isolates of Pythium sylvaticum grown from inoculum blocks with and without 0.01 g/liter cholesterol met and produced sexual unions in water agar. Fewest oospores formed when sterol was omitted from both male and female sides. Sterol added to the male side only did not cause a significant increase in percentage of mature oospores formed in sexual unions, but addition of sterol to the female side resulted in greatly increased numbers of oospores. When two pairs of isolates were mated and cholesterol was added to both sides, a further increase in oospore formation occurred that was significant at the 5% level with one pair and at the 1% level with another pair. These results are viewed as a dosage effect due to the relative amounts of sterol carried in the male and female game-tangia and not to a specific effect on either sex. Rare mature oospores were found in single cultures of the female type isolate of P. sylvaticum. Oospore formation in crosses of isolates N14 and N58 of Phytophthora capsici increased significantly when either isolate was grown from a medium containing 0.03 g/liter cholesterol. These results are viewed as effects of bisexual mating by both Phytophthora isolates.