| Some mature samples of Uncinula necator cleistothecia were observed and its morphology was studied in detail. The percentage of viable cleistothecia were monitored during overwintering. The effect of weather factors on release, germination, and the pathogenicity of ascospores were studied.Results indicated that:1. Generally the mature concavo-convex cleistothecia are scattered on the surface of leaves, but sometimes they are clustered together. The diameter of cleistothecia is in a range of 76.8~145μm. The number of appendages per cleistothecium is 7~26, which is about 224.9~628.5μm in length, and 4.45~9.6μm in width. The upper part of appendage is transparent, its base is dark brown, and the top curls up into one or two and a half circles. Each cleistothecium contains 3~6 asci, which are oval or near oval or near spheroid in shape and have a short handle in general, of a size in a range of (36.8~71.8)μm×(19.6~54.9)μm. The number of ascospore per ascus varies from 3 to 7, and they are colorless and oblong.2. The viability of cleistothecia during the fall and the winter months decrease faster in the vineyard condition than in the laboratory. In the vineyard condition, there were approximately 71.4% of the cleistothecia were viable on 25 November 2007; however, viability declined to 59.1% on December 10, 40.5 on January 12, 32.2% on March 10.3. The optimum temperature for ascospore release is 15℃. The optimum temperature and relative humidity range from 20°to 25℃and 80% to 100% respectively. In laboratory condition, there is no direct effect caused by light.4. The matured ascospores were pathogenic on healthy leaves indicating their probable role as a inoculum source.
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