ABSTRACTReliable methods for oospore production byPhytophthora syringaewere developed using low temperatures and media amended with vegetable oil or made from the leaves of stone fruit trees. All of the fourteen isolates tested individually produced oospores in media after 4 weeks at 5 and 9 C but most did not produce oospores at temperatures above 12 C. Similarly, oospores were observed only in almond leaves kept at 12 C and below. Oospore production was greatly increased when corn oil, linseed oil, or wheat germ oil was added to various base media and the number of oospores produced increased with the amount of wheat germ oil added (up to 48 ml/L medium). The addition ofβ-sitosterol resulted in a slight but significant increase in oospores. Abundant oospores were produced by an almond isolate ofP. syringaein almond leaf medium (75 g fresh leaves per liter medium). Oospores were also produced in media made from the leaves of other stone fruit trees, but not in apple leaf medium.