Dispersal of Phytophthora capsici and P. parasitica in furrow‐irrigated rows of bell pepper, tomato and squash

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Abstract
Dispersal of Phytophthora capsici and P. parasitica from point sources buried near the upper end of 74 m‐long irrigation furrows was studied with three annual host crops. Furrows next to tomato were inoculated with either P. capsici or P. parasitica, while furrows next to pepper and squash were inoculated with P. capsici only. Irrigation was carried out on a 14‐day cycle. Dispersal of each fungus in the water was monitored by transplants in the furrows and the incidence of infected tomato fruit along furrows. Disease gradients on roots and shoots of all hosts were also measured. Repeated irrigations dispersed P. capsici and P. parasitica up to 70 m from the source. Fruit infection increased with increasing distances downstream, suggesting an accumulation of secondary inoculum with the repeated flow of water. Conversely, gradients of disease severity on roots of tomato and pepper peaked at the source and rapidly decreased to low levels up to 32 m downstream. Fruit and root infection upstream from source was negligible. Root and crown rot in squash was highly variable, being confined to locations near the initial inoculum in some units while developing to severe levels several meters downstream in other units. Unlike the other hosts, squash petioles were in contact with the irrigation water and may have served as routes of invasion by P. capsici. Transport of inoculum from the furrow water to the roots was limited. P. capsici isolates were more virulent on tomato roots and caused a higher incidence of buckeye rot on tomato fruit than did isolates of P. parasitica.