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Suppression of Corky Root of Tomatoes in Soils from Organic Farms Associated with Soil Microbial Activity and Nitrogen Status of Soil and Tomato Tissue. F. Workneh, Department of Plant Pathology, University of California, Davis 95616; A. H. C. van Bruggen, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 84:688-694. Accepted for publication 15 April 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/Phyto-84-688.

In greenhouse experiments, corky root of tomatoes (Lycopersicon esculentum L.) was less severe in soil from organic farms than in soil from conventional farms when soil collected in the fall was infested with various concentrations of microsclerotia of Pyrenochaeta lycopersici. Maximum disease severity was obtained at inoculum levels of 104 and 105 microsclerotia per milliliter of soil. When soil samples were sterilized by gamma radiation, the increase in disease severity was greater for soils from organic farms than for soils from conventional farms, indicating that biological disease suppression might have been higher in organically managed than in conventionally managed soils. This was supported by a positive correlation between increase in corky root severity after irradiation and soil microbial activity before irradiation. There was no correlation between corky root severity and soil nitrate concentration, but the disease increased at higher ammonium concentrations (in irradiated soil). Fertilization with ammonium nitrate increased corky root severity in organically managed soil but decreased the disease in conventionally managed soil. In a pasteurized conventional soil, however, corky root severity increased with increasing nitrate and ammonium concentrations in soil and with total nitrogen in tomato tissue when ammonium nitrate fertilizer was added. Corky root severity is determined partially by biological disease suppression and partially by nitrogen concentrations in soil and plant tissue.

Additional keywords: farming systems, fluorescein diacetate hydrolysis, nitrogen fertilization.