Factors affecting germination of oospores of Peronospora viciae f.sp. pisi in vitro

The effects of host plant exudates, light and temperature on germination of oospores of Peronospora viciae f.sp. pisi in vitro were investigated. Seed and root exudates did not increase percentage germination, whereas light inhibited germination. The first germ tubes appeared after 4, 7, and 14 days of incubation at 15, 10 and 5 °C, respectively. The eventual level of germination was highest and had similar values at 5 and 10 °C. At 20 °C germination was poor and at 25 °C no germination was observed. Oospores placed on membrane filters were incubated on soil. When oospores were retrieved from the membrane filters after six days and placed in water at 10 °C, they germinated within 2 days. On soil significantly less oospores germinated than in water. Germinability of oospores stored in the dark at 5 or 20 °C at 30 or 76% RH was studied over a two-year period. Germinability generally increased over time, but fluctuations were observed indicating the occurrence of secondary dormancy. Time courses of germinability were generally similar for oospores stored at several temperatures and humidities. No effect of light on time course of germinability was found when oospores were exposed to alternating light-dark periods or stored in continuous dark for 140 days. Percentage germination observed in a germination assay was correlated with percentage infection determined in a bioassay.