Influence of Selected Fungicide Regimes on Frequency of Dicarboximide-Resistant and Dicarboximide-Sensitive Strains of Botrytis cinerea. Rita J. Vali, Senior Research Technician, Department of Plant Pathology, The Pennsylvania State University, University Park 16802. Gary W. Moorman, Associate Professor, Department of Plant Pathology, The Pennsylvania State University, University Park 16802. Plant Dis. 76:919-924. Accepted for publication 6 April 1992. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-0919.

Isolates of Botrytis cinerea from the field, sensitive and resistant to the dicarboximide fungicide vinclozolin, were used in evaluating fungicide regimes for managing vinclozolin-resistant populations. On geranium, mixed populations, composed of a strain resistant to and a strain sensitive to dicarboximide, were repeatedly subjected to eight fungicide regimes. Disease control and effect on the frequency of vinclozolin resistance were evaluated using a geranium leaf-disk assay in which disease incidence and the percentage of conidia resistant to vinclozolin were quantified after each exposure to fungicide. The percentage of conidia resistant to vinclozolin increased from an initial frequency of 0.2 to 100% after two applications of vinclozolin alone, with a concomitant increase in disease incidence. The incidence of disease when chlorothalonil and cupric hydroxide were applied alone was approximately 30 and 50%, respectively, and the frequency of vinclozolin resistance remained low. Half-strength mixtures of vinclozolin with either chlorothalonil or cupric hydroxide slightly delayed the increase in frequency of resistance. Full-strength mixtures or alternations of vinclozolin with either chlorothalonil or cupric hydroxide failed to delay the increase in the percentage of conidia resistant to vinclozolin. Disease control under these regimes was characteristic of that obtained with chlorothalonil or cupric hydroxide alone, making the inclusion of vinclozolin superfluous. In the alternating regimes, the level of resistance to vinclozolin did not decrease when chlorothalonil or cupric hydroxide were applied, suggesting that the populations resistant to vinclozolin were stable under the conditions of this study. The stability of the resistant strain in the absence of vinclozolin may partially be due to the similarity in sporulation and germination between the resistant and the sensitive strains.