VIEW ARTICLE

Ecology and Epidemiology

Cleistothecia, the Source of Primary Inoculum for Grape Powdery Mildew in New York. Roger C. Pearson, Associate professor, Department of Plant Pathology, Cornell University, New York State Agricultural Experiment Station, Geneva 14456; David M. Gadoury, Research associate, Department of Plant Pathology, Cornell University, New York State Agricultural Experiment Station, Geneva 14456. Phytopathology 77:1509-1514. Accepted for publication 7 May 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-1509.

Vineyard surveys of more than 54,000 vines over a 3-yr period failed to provide evidence that Uncinula necator survived winter as mycelium in dormant infected buds. Ascospores of U. necator, but not conidia, were collected by a volumetric spore trap operated continuously in a vineyard for 40 days after bud burst. In spring, the first powdery mildew colonies were consistently found on leaves of shoots (7–30 cm) growing close to exfoliating bark on the head and trunk of the vine. Cleistothecia were found in spring on all plant parts infected during the previous growing season and also in leaf scars and in crevices of exfoliating bark. Most (79–97%) of the cleistothecia borne on leaves, canes, and cluster stems died during winter and spring without releasing ascospores, but 45–75% of the cleistothecia found in bark crevices were viable. When overwintered cleistothecia collected from vineyards at 7- to 14-day intervals were induced to discharge ascospores, about 75–100% of the ascospores were discharged between bud burst and bloom over a 4-yr period. Such ascospores, released from cleistothecia suspended over detached leaves or tissue culture plants, germinated, infected, and gave rise to typical powdery mildew colonies. Cleistothecia appear to be the principal means of overwintering of U. necator in New York vineyards.