Effects of irrigation management on Phytophthora capsici root rots of bell pepper, squash and tomato

When field soil was infested with Phytophthora capsici, root rot in squash was progressively reduced with decreasing frequencies of furrow irrigation. Yields in infested soil irrigated infrequently did not differ from yields in noninfested controls, but were 2.5 times larger than in infested soil irrigated often. In noninfested plots, neither yield or plant water potential were affected by irrigation schedules, and plants irrigated less extracted water from deeper soil layers.; The same trend was found in two out of three field experiments with pepper. Evidence is presented that the disease variability between years was related to temperature effects on pepper development, and less frequent irrigations clearly reduced disease development in pepper genotypes with partial resistance to P. capsici.; The effects of the location of drip lines and of the position of P. capsici inoculum on roots were studied on susceptible and resistant peppers. In the field, disease was highest with drip lines at the soil surface, close to plant rows. Subsurface irrigation gave the most efficient control, without reducing yields in noninfested plots. Two greenhouse experiments confirmed these results. In a growth chamber at 22/25{dollar}spcirc{dollar}C, rates of lesion growth on resistant pepper were 20 to 30% of the rate on susceptible pepper, and declined with time.; Dispersal of P. capsici and P. parasitica in furrow irrigation water was studied in long rows of pepper, tomato and squash. After three irrigations, both pathogens were detected in the water up to 70 m from initial inoculum. Numbers of infected tomato fruit increased with distance downstream from the initial inoculum, suggesting accumulation of secondary inoculum with time and repeated irrigations. Conversely, disease gradients on roots usually peaked at the source and decreased rapidly with distances downstream. Infection upstream was negligible. Disease levels in squash were highly variable, confined to locations near the source in some rows while developing to severe levels in others. Squash parts in contact with irrigation water may have served as routes of invasion by P. capsici. Transport of inoculum from water through soil to the roots was evidently limited.