Disease Note.

First Report of Pythium uncinulatum on Romaine Lettuce in California. R. M. Davis, Department of Plant Pathology, University of California, Davis 95616, and J. L. Aquiar, Cooperative Extension, Indio, Calif. 92201.. C. Q. Winterbottom, Department of Plant Pathology, University of California, Davis 95616, and J. L. Aquiar, Cooperative Extension, Indio, Calif. 92201. Plant Dis. 79:642. Accepted for publication 18 May 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0642C.

Stunted and yellowed Romaine lettuce (Lactuca sativa L. var. langifo-lia Lam.) was observed in two fields (about 40 ha each) in the Coachella Valley of California in the winter of 1993. Scattered diseased plants resulted in an estimated 20 to 30% loss in yield. Symptoms included yellowing of the outer leaves and a dark yellow to brown discoloration of the tap roots. Feeder roots were sometimes sparse. Pythium uncinulatum Plaats-Niterink & Blok was consistently isolated from infected root tissue on corn meal agar amended with pimaricin, ampicillin, rifampicin, and pentachloronitrobenzene (PCNB). To complete Koch's postulates, 4-week-old inoculum of one isolate of the fungus, produced at 25°C on vermiculite amended with V8 juice and oats, was mixed into steamed U.C. mix at a rate of approximately 30 cc per liter of mix. Individual 2-to 4-week-old Romaine lettuce plants were transplanted into the infested mix in 12-cm-diameter pots in a greenhouse maintained at 18 to 25°C. Noninoculated plants served as controls. Each treatment included 10 to 20 replications. Eight weeks later the plants were lifted from the soil, washed, and weighed. In three separate experiments, the fresh weights of inoculated plants were about half of the weights of the control plants. Symptoms included a slight discoloration of the leaves and a general browning of the tap roots. None of the plants died. Control plants remained symptomless. Pythium uncinulatum was reisolated from all the inoculated plants. This is the first report of a root rot of lettuce caused by P. uncinulatum in North America. The first report of the occurrence of this fungus outside Europe (I) was on asymptomatic, field-grown lettuce in Arizona (2).