RELATIONSHIPS AMONG PHYTOPHTHORA ROOT-ROT DEVELOPMENT, P-PARASITICA POPULATIONS IN SOIL, AND YIELD OF TOMATOES UNDER COMMERCIAL FIELD CONDITIONS

In 1989, 1990, and 1991, 6-m-long row segments of tomato cultivar FM6203 were planted adjacent to 10 other cultivars in 12 commercial fields (within 28 km of Davis, California) with histories of Phytophthora root rot. Populations of Phytophthora parasitica at planting ranged from 0 to 3.7 cfu/g of dry soil as determined by dilution plating, and 2-100% of tomato leaf disks used as bait in a soil assay were colonized. Populations increased and fluctuated during the growing season. Aboveground symptoms of disease developed at similar phenological stages, associated with the setting and maturation of fruit, in all cultivars, and the final disease incidence and severity ranged from zero to the maximum possible. Harvestable fruit yield decreased linearly with increased symptom severity for all cultivars and years. Final disease severity was correlated positively with pathogen populations detected by baiting at planting and with increasing soil clay content, and negatively with soil sand content and time after planting at which symptoms first became visible. There were no clear associations between pathogen populations and soil temperature or moisture. The results suggest that there is little difference among processing tomato cultivars in tolerance or yield potential under given levels of disease pressure.