Damping-off of broccoli (Brassica oleracea var. italica) and cauliflower (B. oleracea var. botrytis) seedlings occurred in several greenhouses in Fresno, CA, in 1997. Symptoms included wilting and root and stem rot. Pythium polymastum was consistently isolated from symptomatic tissues placed on corn meal agar amended with 10 ppm pimaricin, 250 ppm ampicillin, 10 ppm rifampicin, and 25 ppm pentachloronitro-benzene. On grass leaves in water, the fungus produced numerous aplerotic oospores in oogonia 43 to 50 μm in diameter (average 46 μm) with spines about 7 μm long. Spherical sporangia were only rarely observed. In the greenhouse, 4-week-old broccoli and cauliflower seedlings were transplanted into potting mix amended with a colonized vermiculite/rye/V8 juice medium to produce approximately 2,500 CFUs per gram of potting medium. Control plants were transplanted into noninfested potting mix. There were six replicate pots per treatment and three plants per pot. After 12 days, the potting mix was gently washed from the roots and the seedlings were dried and weighed. Symptoms on inoculated plants included wilting, severe root rot, black streaks on the lower stems, and death. The fungus was recovered from symptomatic tissues. There were no symptoms on the control plants. Infection by P. polymastum reduced dry weights of surviving broccoli and cauliflower seedlings by 82 and 58%, respectively. Similar results were obtained in a second experiment. This fungus was previously characterized as a pathogen of both cultivated and wild crucifers in Canada (1). This is the first report of P. polymastum in California.
Reference: (1) T. C. Vanterpool. Can. J. Bot. 52:1205, 1974.