Residual Efficacy of Fungicides Used in the Management of Botrytis cinerea on Greenhouse-Grown Geraniums. Gary W. Moorman, Department of Plant Pathology, The Pennsylvania State University, 210 Buckhout Laboratory, University Park 16802. Roxanne J. Lease, Department of Plant Pathology, The Pennsylvania State University, 210 Buckhout Laboratory, University Park 16802. Plant Dis. 76:374-376. Accepted for publication 5 November 1991. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-0374.

Fungicides were applied singly or in mixtures to seed geraniums (Pelargonium × hortorum ‘Red Elite’). Immediately after sprays dried (day 0) and weekly for 3 wk, 1-cm-diameter disks excised from treated leaves were inoculated with spores from a benomyl-resistant, vinclozolin-sensitive strain of Botrytis cinerea. After 10 days of incubation, the number of leaf disks exhibiting the browning characteristic of Botrytis blight was recorded. Of the materials applied singly, vinclozolin provided the best protection, followed by chlorothalonil and mancozeb. Zineb, dicloran, and cupric hydroxide allowed more than 50% of the treated leaf disks to become infected. The relative performance of fungicides used singly was similar whether plants were watered directly in the pots or irrigated overhead during the experiment. Two, three, or four-component mixtures of vinclozolin, chlorothalonil, cupric hydroxide, and mancozeb at reduced rates were more effective and had longer residual efficacy than the fungicides applied singly. Because reduced-rate mixtures controlled B. cinerea as well as full-strength mixtures, the cost of applying a mixture can be lowered by using reduced rates without sacrificing residual efficacy. To reduce the exposure of B. cinerea populations to vinclozolin and, thereby, lessen the selection pressure toward vinclozolin resistance, results indicate that the reduced-rate, four-component mixture of vinclozolin, chlorothalonil, cupric hydroxide, and mancozeb or vinclozolin-free, reduced-rate, two- or three-component mixtures of chlorothalonil, cupric hydroxide, and mancozeb provided good protection of geranium leaf tissue for 21 days.