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Dispersal of Phytophthora parasitica in Tomato Fields by Furrow Irrigation. D. Neher, Department of Plant Pathology, University of California, Davis 95616. J. M. Duniway, Department of Plant Pathology, University of California, Davis 95616. Plant Dis. 76:582-586. Accepted for publication 17 January 1992. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-0582.

Dispersal of Phytophthora parasitica from point sources of inoculum buried in irrigation furrows in field plots of processing tomatoes (Lycopersicon esculentum) was monitored during five successive irrigations. During the fourth and fifth irrigations, leaf-disk baits floated on the surface of irrigation water became colonized by P. parasitica at distances 2, 4, 8, 16, and 32 m downstream from infestation sites, but no leaf disks became colonized 2 m upstream in infested furrows or at any distance in noninfested furrows. Tomato seedlings placed into furrows 50 m downstream from infestation sites became infected during all five irrigations. At the end of the season, incidence of buckeye rot on tomato fruit, caused by P. parasitica, increased linearly with logarithmic increases in distance from 0 to 68 m downstream from original points of inoculum. Symptoms of root rot on tomato plants were severe and, with one exception, final populations of P. parasitica in the soil under furrows were high only at or very near infestation sites. Shoot symptoms were visible only at an original point of infestation and after the fifth irrigation. Dispersal of the pathogen in irrigation water was most important in development of buckeye rot on fruit. Although the pathogen was also dispersed to roots, the severity of symptoms on roots at distances ≥2 m was generally below that necessary for development of shoot symptoms or yield loss.