Homothallism in Peronospora viciae f.sp. pisi and the effect of temperature on oospore production

Monoconidial cultures derived from seven P. viciae f.sp. pisi isolates, obtained from different countries, were able lo produce oospores, Apparently, these isolates were homothallic. Oospore production of one isolate was studied at 5, 10, 15 and 20°C in systematically colonized shoots, and in local lesions on leaflets, stem parts and pods of the pisum sativum cv. Kelvedon Wonder. The number of oospores produced per gram systemically colonized tissue increased with temperature. In lesions of leaflets and of stem parts, including tendrils, petioles and main stem, most oospores were produced at 20°C. At 10°C, a few oospores were found in stem parts but none in leaflet lesions. At 5° C, no oospores were formed at all. In pods, moe oospores were produced at 15 and 20°C than at 10°C, but the numbers of oospores was smaller than in the other plant parts. Oospores formed at lower temperatures were larger than those formed at higher temperatures. At 20°C, similar oospore densities were found in leaflet lesions of three cultivars widely differing in resistance to downy mildew.