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Parasitism of Uncinula necator Cleistothecia by the Mycoparasite Ampelomyces quisqualis. S. P. Falk, Department of Plant Pathology, Cornell University, New York State Agricultural Experiment Station, Geneva 14456-0462; D. M. Gadoury(2), P. Cortesi(3), R. C. Pearson(4), and R. C. Seem(5). (2)(3)(4)(5)Department of Plant Pathology, Cornell University, New York State Agricultural Experiment Station, Geneva 14456-0462; (3)Current address: Istituto di Patologia Vegetale, UniversitÓ degli Studi di Milano, 20133 Milano, Italy. Phytopathology 85:794-800. Accepted for publication 3 April 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/Phyto-85-794.

Parasitism of Uncinula necator cleistothecia by the mycoparasite Ampelomyces quisqualis was widespread in the Vitaceae around New York State. Although A. quisqualis did not survive in naturally parasitized U. necator cleistothecia on grape leaves, it did overwinter in parasitized cleistothecia on the bark of grapevines. Although only 1% of the total population of cleistothecia on bark was parasitized, the bark may still be an important site for overwintering of A. quisqualis since the mycoparasite is located adjacent to developing powdery mildew colonies on leaves, analogous to that of healthy cleistothecia, which also overwinter on bark and release primary inoculum to infect emerging grape leaves. In vitro studies of parasitism of U. necator cleistothecia showed that infection occurs only during early stages of development prior to or at the earliest stages of the formation of appendages but before darkening of the cleistothecial wall. When A. quisqualis was applied to grapevines from colonized cotton-wick cultures suspended above vines, parasitism of cleistothecia on leaves increased compared to naturally occurring parasitism, although during a season with high rainfall the level of parasitism was similar by the end of the season. The impact of increased parasitism was a reduction in the number of cleistothecia dispersed from leaves to bark and a reduction (50 to 60%) in the number of cleistothecia overwintering on bark of grapevines. Thus, biological control of grape powdery mildew with A. quisqualis may be further enhanced by a reduction in the level of overwintering inoculum for the next season.