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Effect of Location of Drip Irrigation Emitters and Position of Phytophthora capsici Infections in Roots on Phytophthora Root Rot of Pepper. A. C. Cafe-Filho, Graduate student, Department of Plant Pathology, University of California, Davis 95616. J. M. Duniway. Professor, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 86:1364-1369.. Accepted for publication 10 September 1996. Copyright 1996 The American Phylopathological Society. DOI: 10.1094/Phyto-86-1364.

The effects of the location of drip irrigation emitters and the position of inoculum in roots on Phytophlhora root rot of pepper were studied on cultivars Yolo Wonder B (susceptiblc) and Adra (resistant). In field plots infested with Phylophthora capsici, the location of emitters had major effects on incidence of diseased plants, severity of root symptoms, yield, shoot dry weight, level of soil moisture, and plant leaf water potential. Disease levels were highest with emitters at the soil surface and in the plant row. The subsurface (15 cm deep) position gave the most efficient control in the field without reducing yields in noninfested plots. Disease levels were more severe with cv. Yolo than with cv. Adra. Results were similar in analogous greenhouse tests. In complementary experiments, when zoospores were placed onto tips of roots that were 7 to 10 cm or 14 to 18 cm long, inoculation at different positions in the root system had no effect on the lesion growth rate for either cultivar, but the progress of lesions on 'Yolo' was three to five times faster than on 'Adra'. Moreover, the rate of lesion growth declined with time on 'Adra', but remained constant in 'Yolo', and above-ground symptoms of root rot on 'Yolo' were more severe when inoculum was placed higher in the soil profile. Phytophthora root rot of pepper can be reduced in low rainfall areas by positioning the drip emitters away from plant stems, with a subsurface location giving the best results. The advantages of associating genetic resistance with a well-managed drip system were evident.