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Relationship Between Amount of Phytophthora parasitica Added to Field Soil and the Development of Root Rot in Processing Tomatoes. D. Neher, 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, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 81:1124-1129. Accepted for publication 2 July 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/Phyto-81-1124.

Field plots in which Phytophthora parasitica was not detected initially were planted to processing tomatoes, and the soil was infested 35 or 45 days after planting in 1987 and 1988, respectively, to give average maximum levels of 12, 34, 728, and 2965 colony-forming units of P. parasitica per gram of soil. Inoculum levels in the four treatments were significantly different when averaged across time between infestation and crop maturity, and disease incidence and severity increased significantly with increasing inoculum levels. For example, final incidences of plants with shoot symptoms were 1.5, 6.4, 14.0, and 24.4% and 0.4, 15.4, 30.2, and 52.3% for the zero, low, intermediate, and high inoculum treatments in 1987 and 1988, respectively. Yield was reduced significantly (20%) only at the highest inoculum level in 1987. However, moderately severe symptoms frequently developed on roots and shoots with low to high levels of inoculum without causing yield losses in both years. Extending furrow irrigations from 4 to 24 h in duration did not significantly affect disease incidence or severity. Crop growth, phenology, and leaf water potentials were not affected significantly by the inoculum or irrigation treatments. The results suggest that development of Phytophthora root rot symptoms on processing tomatoes depended on the inoculum level applied to soil early in the cropping season.

Additional keywords: epidemiology, inoculum level, Lycopersicon esculentum.