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Influence of Frequency and Duration of Furrow Irrigation on the Development of Phytophthora Root Rot and Yield in Processing Tomatoes. J. B. Ristaino, Former graduate research assistant, Department of Plant Pathology, University of California, Davis 95616, Present address: Department of Plant Pathology, Box 7616, North Carolina State University, Raleigh 27695-7616; J. M. Duniway, and J. J. Marois. Professor, and Associate professor, respectively, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 78:1701-1706. Accepted for publication 1 August 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-1701.

Processing tomatoes grown in field plots with soil either infested or uninfested with Phytophthora parasitica were furrow irrigated for 48 hr every 14 days (normal irrigation), for 48 hr every 28 days (less frequent irrigation), or with alternating 48-hr and 24-hr irrigations every 14 days (prolonged irrigation). Disease developed more rapidly and symptom severity was significantly greater on shoots and roots of plants in infested soil that received prolonged irrigations compared with plants that were irrigated less frequently. Midday leaf water potential was reduced significantly as symptom severity increased and, by 90 days after planting, was correlated negatively with fruit yield at harvest (122 days). Disease significantly reduced fruit yield by 68 or 74%, 34 or 60%, and 20 or 43% as compared with uninoculated controls in prolonged, normal, or less frequent irrigation treatments in 1985 or 1986, respectively. Populations of P. parasitica in soil increased from 6 to 17 colony-forming units per gram (cfu/g) of soil after infestation to 67121 cfu/g of soil at harvest. Results clearly show that variations in the frequency and duration of furrow irrigation can have large effects on the development of Phytophthora root rot and yield loss to root rot in processing tomatoes.

Additional keywords: crop loss assessment, epidemiology, Lycopersicon esculentum, plant water relations.