In California, various leafy vegetables are planted at extremely high seeding rates (up to three million seed per acre), grown to the four- to eight-leaf stage, then mechanically clipped and bagged for market as “spring mix” or “baby leaf” salad products. The crops are planted in separate strips, usually four to 16 beds of each, and include Japanese mustard (Brassica campestris subsp. nipposinica), red mustard (Brassica juncea subsp. rugosa), tah tsai (Brassica campestris subsp. narinosa), multiple lettuce (Lactuca sativa) cultivars, spinach (Spinacia oleracea), arugula (Eruca sativa), and Swiss chard (Beta vulgaris subsp. cicla). In 1997 and 1998, severe damping-off disease of Swiss chard was observed in the Salinas Valley (Monterey County). Symptoms on emergent plants consisted of wilting, brown necrosis of crown tissue, and eventual death of seedlings. Because of the high plant density, disease incidence increased rapidly, affecting large numbers of plants. Rhizoctonia solani was isolated consistently from symptomatic plants. Pathogenicity was tested by placing agar plugs of representative isolates adjacent to crowns of potted Swiss chard at the two- to four-leaf stage. Brown stem necrosis and plant collapse occurred within 5 days after inoculation, and R. solani was reisolated. Control plants, inoculated with sterile agar plugs, did not develop disease. Tests were repeated and results were similar. Anastomosis group testing revealed that all five isolates belonged to AG3 (1). For California, this appears to be the first documentation of damping-off of Swiss chard caused by R. solani.
Reference: (1) B. Sneh et al. Identification of Rhizoctonia Species. American Phytopathological Society, St. Paul, MN, 1991.
