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Influence of Temperature and Fungicide on Germination, Growth, and Virulence of Ulocladium cucurbitae on Cucumber. Thomas A. Zitter, Department of Plant Pathology, Cornell University, Ithaca, NY 14853; Louis W. Hsu, Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Phytopathology 82:358-362. Accepted for publication 19 October 1991. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/Phyto-82-358.

Germination of conidia of Ulocladium cucurbitae was greatest at 30 C after 1 h, but germination occurred from 9 to 36 C. Growth of cultures from single conidia was greater at 30 C than at 9, 12, or 36 C. Cultures at 8 C, producing alternarioid-type conidia, produced ulocladioid-type conidia within 4 days of transfer to 21 or 27 C. By 10 days, the ulocladioid type predominated. When starting with ulocladioid conidial types, the conidial population shifted to alternarioid types when cultured at 8 C, but maintained the dominant ulocladioid morphology at 21 and 27 C. In pathogenicity tests using conidial suspensions of the two main types, alternarioid suspensions consistently produced more necrotic lesions and larger diameter lesions on susceptible and resistant cucumber leaves and leaf disks than did ulocladioid-type conidia. Increasing ulocladioid-type conidial concentrations resulted in proportionally more infections, but lesion size remained small. The number of germ tubes per conidium was significantly greater for alternarioid and intermediate types than for ulocladioid types. Sensitivity of conidia to benomyl, chlorothalonil, and iprodione was determined in vitro. Conidia were most sensitive to chlorothalonil, followed by iprodione, and were least sensitive to benomyl. Chlorothalonil and iprodione performed equally well as protective sprays, but were no better than benomyl when used as curative sprays on two of the more susceptible cultivars tested.

Additional keywords: conidium morphology, Cucumis sativus, Ulocladium leaf spot.