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Development of an Infection Model for Botrytis Bunch Rot of Grapes Based on Wetness Duration and Temperature. J. C. Broome, Graduate research assistant, Department of Plant Pathology, University of California, Davis, CA 95616; J. T. English(2), J. J. Marois(3), B. A. Latorre(4), and J. C. Aviles(5). (2)Associate professor, University of Missouri, Columbia, MO 65211; (3)Professor, Department of Plant Pathology, University of California, Davis, CA 95616; (4)(5)Professor and research assistant, Facultad de Agronomía, Pontificia Universidad Católica de Chile, Casilla 306-22 Santiago, Chile. Phytopathology 85:97-102. Accepted for publication 25 August 1994. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/Phyto-85-97.

Grape berries were dipped in conidial suspensions of Botrytis cinerea and incubated for 4, 8, 12, 16, or 20 h of wetness at temperatures ranging from 12–30 C. Berries were infected after 4 h of wetness at all temperatures tested. Incidence of berry infection increased with increasing wetness duration at all temperatures. A multiple regression model described the logit of infection as a function of the interaction of wetness duration and temperature (R2=0.75). This model was incorporated into an infield environmental monitoring station and evaluated for two seasons on Thompson Seedless table grapes in the central valley of Chile. Applications of captan (3.2 kg/ha a.i.) or vinclozolin (1 kg/ha a.i.) were made according to the Botrytis model or a standard phenological spray program that consisted of four sprays (at bloom, cluster thinning, veraison, and preharvest) plus additional sprays after major rain events. Disease incidence and severity at harvest were similar whether applications were made according to a standard program (six to nine applications in 1991–1992, four to five in 1992–1993) or according to the Botrytis model (two to four applications in 1991–1992, zero to five in 1992–1993). In some vine-yards, postharvest disease was significantly less when sprays were made according to the Botrytis model recommendations compared with the standard spray program.