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 tomato-production systems in the Central Valley of California was conducted to determine the effects of management practices on soil properties and tomato diseases and to relate disease severity in tomatoes to various 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 severity of Phytophthora root rot caused by Phytophthora parasitica and corky root caused by Pyrenochaeta lycopersici. P. parasitica was detected by leaf bait assay in soil samples from five of the nine conventional farms but not from organic farms, and Phytophthora root rot was observed in three conventional farms only. Corky root was found on most plants in most locations, but incidence and severity were higher in conventional than in organic farms. Corky root-severity values, estimates of P. parasitica populations from the leaf bait assay, and Phytophthora root rot-severity values were grouped into three, two, and two classes, respectively, for use in stepwise and canonical discriminant analyses with 11 variables. The distinction among three classes of corky root severity was consistently associated with three variables: tissue nitrogen, soil nitrate, and nitrogen-mineralization potential. Nitrogen in tomato tissue and nitrate concentration in soil were positively correlated with corky root severity; nitrogen-mineralization potential was negatively correlated with this disease. Six soil variables (clay content, water-stable aggregates, soil-nitrate concentration, organic carbon, electrical conductivity, and soil-water content) contributed most to variability in the presence of P. parasitica in soil. All variables except organic carbon were positively correlated with the presence of P. parasitica in soil. Clay content and water-stable aggregates were also positively associated with Phytophthora root rot, and microbial activity was negatively associated with this disease.