Yield Loss in Hydroponically Grown Lettuce Attributed to Subclinical Infection of Feeder Rootlets by Pythium dissotocum. M. E. Stanghellini, Professor, Department of Plant Pathology, University of Arizona, Tucson 85721. W. C. Kronland, Research Associate, Department of Plant Pathology, University of Arizona, Tucson 85721. Plant Dis. 70:1053-1056. Accepted for publication 15 July 1986. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/PD-70-1053.

Pythium dissotocum, reported for the first time as a root pathogen of hydroponically grown lettuce, was responsible for significant yield reductions (35–54 and 12–17% reductions at 18 and 28 C, respectively) in the absence of visible root or foliar symptoms. The fungus was isolated from 92% of the rootlets assayed and occupied about 75% of the total root length assayed. Microscopic examination of infected roots revealed haustorialike fungal structures within healthy-appearing epidermal cells. P. dissotocum, in addition to P. uncinulatum, P. irregulare, P. sylvaticum, P. violae, P. catenulatum, and P. rostratum, was also consistently isolated from healthy-appearing feeder rootlets collected from field-grown head lettuce plants.