Root Rot of Hydroponically Grown Spinach Caused by Pythium aphanidermatum and P. dissotocum. M. L. Bates, Research Associate, Environmental Research Laboratory, Department of Plant Pathology, University of Arizona, Tucson 85721. M. E. Stanghellini, Professor, Department of Plant Pathology, University of Arizona, Tucson 85721. Plant Dis. 68:989-991. Accepted for publication 3 May 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/PD-68-989.

Root rot was the limiting factor to commercial production of spinach (Spinacia oleracea) in a greenhouse recirculating hydroponic system in Arizona. Infected plants either died or were severely stunted. Two species of Pythium (P. aphanidermatum and P. dissotocum) were associated with root rot. P. aphanidermatum predominated as the primary causal agent of root rot during the warm summer production months, when nutrient solution temperatures were higher than 23 C, whereas P. dissotocum was the primary or sole causal agent during the winter production months, when nutrient solution temperatures were between 17 and 22 C. Disease symptoms, identical to those observed in the commercial greenhouse, occurred within 6 days after spinach plants were inoculated with either P. aphanidermatum or P. dissotocum. It was also shown that metalaxyl, at concentrations from 1 to 10 μg a.i./ml in the nutrient solution, was effective in controlling root rot of spinach.