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Effects of Irrigation on Buckeye Rot of Tomato Fruit Caused by Phytophthora parasitica. M. W. Hoy, Postgraduate research plant pathologist, Department of Plant Pathology, University of California, Davis 95616; J. M. Ogawa(2), and J. M. Duniway(3). (2)(3)Professors, respectively, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 74:474-478. Accepted for publication 17 November 1983. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-474.

The influence of irrigation frequency on the incidence of buckeye rot of green tomato fruit was studied in a field plot infested with Phytophthora parasitica. The incidence of fruit infections during the final days before harvest in 1980 was significantly higher after a 4-day interval between irrigations than after a 25-day interval between irrigations. In 1982, irrigations were applied every 4, 8, 16, or 32 days throughout the last 64 days of crop growth. The rate and incidence of infection of fruit sampled during the final irrigation increased significantly as the interval between prior irrigations was decreased. Furthermore, the final incidence of infection on fruit grown in the field increased from 24 to 84% as irrigation frequency increased from once every 32 to once every 4 days. Yield of healthy fruit declined as irrigation frequency increased from every 16 to every 4 days. When soil samples collected 3 days after an irrigation were flooded in the laboratory, 80% of cherry tomatoes floated over the samples were infected by P. parasitica within 6 hr. In contrast, infection was less than 10% for fruit floated for 16 hr over soil collected from furrows that had not been irrigated for 28 days. The results suggest that the inoculum of P. parasitica, probably in the form of zoospores, is formed more rapidly and abundantly when previous irrigations have been frequent and have not allowed the soil to dry extensively.