SUPPRESSION OF CORKY ROOT OF TOMATOES IN SOILS FROM ORGANIC FARMS ASSOCIATED WITH SOIL MICROBIAL ACTIVITY AND NITROGEN STATUS OF SOIL AND TOMATO TISSUE

In greenhouse experiments, corky root of tomatoes (Lycopersicon escule ntum L.) was less severe in soil from organic farms than in soil from conventional farms when soil collected in the fall was infested with v arious concentrations of microsclerotia of Pyrenochaeta lycopersici. M aximum disease severity was obtained at inoculum levels of 10(4) and 1 0(5) microsclerotia per milliliter of soil. When soil samples were ste rilized by gamma radiation, the increase in disease severity was great er for soils from organic farms than for soils from conventional farms , indicating that biological disease suppression might have been highe r in organically managed than in conventionally managed soils. This wa s supported by a positive correlation between increase in corky root s everity after irradiation and soil microbial activity before irradiati on. There was no correlation between corky root severity and soil nitr ate concentration, but the disease increased at higher ammonium concen trations (in irradiated soil). Fertilization with ammonium nitrate inc reased corky root severity in organically managed soil but decreased t he disease in conventionally managed soil. In a pasteurized convention al soil, however, corky root severity increased with increasing nitrat e and ammonium concentrations in soil and with total nitrogen in tomat o tissue when ammonium nitrate fertilizer was added. Corky root severi ty is determined partially by biological disease suppression and parti ally by nitrogen concentrations in soil and plant tissue.