Abstract
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.