VARIABLES ASSOCIATED WITH CORKY ROOT AND PHYTOPHTHORA ROOT-ROT OF TOMATOES IN ORGANIC AND CONVENTIONAL FARMS

In 1989 and 1990, a comparative study of organic and conventional toma to-production systems in the Central Valley of California was conducte d to determine the effects of management practices on soil properties and tomato diseases and to relate disease severity in tomatoes to vari ous soil and plant variables. Twenty sampling locations were randomly selected at each of nine (1989) and 18 (1990) farms to measure 10 soil variables, plant biomass and nitrogen content, and incidence and seve rity of Phytophthora root rot caused by Phytophthora parasitica and co rky root caused by Pyrenochaeta lycopersici. P. parasitica was detecte d by leaf bait assay in soil samples from five of the nine conventiona l farms but not from organic farms, and Phytophthora root rot was obse rved in three conventional farms only. Corky root was found on most pl ants in most locations, but incidence and severity were higher in conv entional than in organic farms. Corky root-severity values, estimates of P. parasitica populations from the leaf bait assay, and Phytophthor a root rot-severity values were grouped into three, two, and two class es, respectively, for use in stepwise and canonical discriminant analy ses with 11 variables. The distinction among three classes of corky ro ot severity was consistently associated with three variables: tissue n itrogen, soil nitrate, and nitrogen-mineralization potential. Nitrogen in tomato tissue and nitrate concentration in soil were positively co rrelated with corky root severity; nitrogen-mineralization potential w as negatively correlated with this disease. Six soil variables (clay c ontent, water-stable aggregates, soil-nitrate concentration, organic c arbon, electrical conductivity, and soil-water content) contributed mo st to variability in the presence of P. parasitica in soil. All variab les except organic carbon were positively correlated with the presence of P. parasitica in soil. Clay content and water-stable aggregates we re also positively associated with Phytophthora root rot, and microbia l activity was negatively associated with this disease.