Disease Note.

Iprodione-Resistant Botrytis cinerea in Virginia Vineyards. A.B. A.M. Baudoin, Department of Plant Pathology, Physiology and Weed Science, Virginia Polytechnic Institute and State University, Blacksburg 24061. Plant Dis. 78:102. Accepted for publication 18 October 1993. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0102C.

Fifty isolates of Botrytis cinerea Pers.:Fr. obtained from 14 vineyards (Vitis vinifera L. and interspecific hybrids) in Virginia during August-October 1992 were tested for resistance to benomyl and iprodione by comparing radial growth on fungicide-amended and nonamended potato-dextrose agar (PDA). Benomyl resistance (little or no growth reduction at 5 µg/ml of benomyl) was detected in nine of the 14 vineyards and in 66% of all isolates. Iprodione resistance (growth on 2.5 µg/ml of iprodione at least 20% of that on nonamended medium) was detected in two vineyards in northern Virginia, approximately 30 km from each other; all seven of the isolates obtained from these vineyards had ultralow-level resistance (1), as evidenced by an EC50 ranging from 1.0 to 1.8 µg a.i./ml (compared with 0.15-0.25 µg/ml for sensitive isolates) and a fibrillose (as opposed to dense) colony margin on PDA or malt extract agar amended with 40 g/L of NaCI (1). Additional isolates obtained from dead wood and tendril samples in March 1993 were resistant to iprodione (all nine samples from one vineyard and 12 of 13 samples from the second) as well as to benomyl. Dipping table grapes in 300-2,400 µg/ml of iprodione before spray-inoculating them with B. cinerea conidia (5 × 104 per milliliter provided good protection against sensitive isolates but poor protection against resistant isolates, as judged by percent berries infected and percent surface affected. The two vineyards in which iprodione-resistant isolates were found had received one, three, and four and four, three, and three iprodione sprays in 1990, 1991, and 1992, respectively. This is the first report of dicarboxamide resistance in B. cinerea on grape in the United States.