Received for publication January 2, 1979. Vermont Agricultural Experiment Station Journal Article No. 404. This research was supported in part by a grant from the Horticultural Research Institute, Washington, D.C.
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Present address: Department of Plant Pathology, University of Massachusetts, Amherst, MA 01003, and Department of Horticulture, Coastal Plain Experiment Station, Tifton, GA 31794, respectively.