EFFECTS OF THE FREQUENCY OF FURROW IRRIGATION ON ROOT AND FRUIT ROTS OF SQUASH CAUSED BY PHYTOPHTHORA-CAPSICI

Squash (Cucurbita pepo var. melopepo cv. Early Summer Crookneck) was grown from seed in field plots initially free of Phytophthora capsici at Davis, California. Thirty to forty days after seeding, the soil adjacent to the plants in half of the plots was infested with P. capsici propagules. Furrow irrigation was applied uniformly before the soil was infested and every 7, 14, or 21 days thereafter, for both infested and noninfested plots. Disease progress was significantly slower, onset of disease was delayed, and final severity of symptoms on shoots and roots was significantly reduced with decreasing frequency of irrigation. Yields in infested soil irrigated every 14 and 21 days did not differ from the yields obtained in the corresponding noninfested controls. In contrast, the yield in infested soil irrigated every 7 days was only 40% of the yield obtained in the corresponding controls. Yield losses due to the direct infection of fruit were limited to 20% by weight, and the results suggested that root and shoot symptoms had to become severe for yield loss to be significant. In the absence of the pathogen, irrigation frequency had no significant effect on yield or plant water potential, and plants irrigated less often extracted water from deeper soil layers. The results imply that less frequent furrow irrigation is an effective way to reduce losses due to P. capsici in squash fields in California.