First Report of Pythium mastophorum on Celery in California. M. R. Vazquez, Department of Plant Pathology, University of California, Davis 95616. R. M. Davis, and A. S. Greathead, Department of Plant Pathology, University of California, Davis 95616. Plant Dis. 80:709. Accepted for publication 25 March 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-0709D.

A stunting and yellowing of celery (Apium graveolens L. var. dulce (Mill.) Pers.) seedlings grown in a soil-less mix (peat/vermiculite, 1:1) occurred in greenhouses in Oxnard, CA, in 1995. Symptoms also included discoloration of the taproots and decayed feeder roots. Infected plants grew slowly, resulting in a delay in transplanting to the field. Pythium mastophorum Drechs, was consistently isolated from infected root tissue on corn meal agar amended with pimaricin, ampicillin, rifampicin, and pentachloronitrobenzene. To complete Koch's postulates, 1-week-old cultures of one isolate of the fungus were blended in water at a rate of approximately one 9-cm-diameter potato dextrose agar plate per liter of water. After the roots of individual 2- lo 4-week-old Celery plants were dipped in the inoculum for 10 s, the plants were transplanted into 10-cm-diameter pots of a peat/vermiculite (1:1) mix. The pots were then irrigated with about 160 ml of the inoculum. Noninoculated plants served as controls. Each treatment included 8 replications. The plants were kept in a greenhouse maintained at 18 to 25°C and irrigated daily. Five weeks later, the plants were lifted from the soil, washed, and weighed. In two separate experiments, the fresh weights of inoculated plants were approximately one-third of the weights of the control plants. Symptoms included stunting, severe discoloration of the tap roots, and loss of most of the feeder roots. Several of the plants died. Control plants remained symptomless. P. mastophorum was reisolated from all the inoculated plants. This is the first report of a rool rot of greenhousc-grown celery caused by P. mastophorum in North America.