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Incidence of Dicarboximide Fungicide Resistance in Botrytis cinerea Monitored in Two Greenhouses. G. W. Moorman, Department of Plant Pathology, The Pennsylvania State University, 111 Buckhout Laboratory, University Park 16802.. R. J. Lease. Department of Plant Pathology, The Pennsylvania State University, 111 Buckhout Laboratory, University Park 16802. Plant Dis. 79:319. Accepted for publication 24 January 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0319C.

Two commercial greenhouses where a wide variety of potted flowering plants and foliage was grown were visited every 3 mo from March 1992 through December 1993 to collect 48-50 separate samples of Botrytis cinerea Pers :Fr. conidia from both in each sampling. Dicarboximide resistance in each population was determined from the percentage of spore germination on freshly prepared potato dextrose agar (PDA) amended with 20 ?g vinclozolin per milliliter compared with germination on fungicide-free PDA. Resistance was prevalent in greenhouse A even though dicarboximides had never been used in that facility. The average incidence of resistance in March, June, September, and December 1992 and 1993 was 34, 28, 27, 13, 56, 52, 49, and 56%, respectively. In greenhouse B, the incidence of resistance was 76, 98, 100, 89, 74, 49, 60, and 63%, respectively. The sharp increase in resistance to 98 and 100% of the population occurred in greenhouse B following general applications of dicarboximides. The decline to 49% occurred 6 mo after dicarboximide use ceased. The resurgence in resistance in B to 60% followed the introduction of new plant material into greenhouse B from other growers' greenhouses. Although the incidence in both greenhouses fluctuated greatly over the monitoring period, the percentage of samples containing at least one resistant conidium remained relatively constant, varying from 80 to 100% of the samples from greenhouse A and from 95 to 100% of those from greenhouse B. While there was no indication of a fungicide control failure in the crops treated, the scattered distribution and persistence of B cinerea isolates resistant to dicarboximides poses a significant threat to the successful use of dicarboximides in these greenhouses.