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First Report of Rhizoctonia solani AG-7 on Cotton in Mississippi

During a field study of the soilborne mycobiota on cotton (Gossypium hirsutum L.) roots, Rhizoctonia solani Kühn AG-7 was isolated from dark brown lesions present on the tap, feeder, and secondary roots onto potato-dextrose agar (PDA) (34 g of medium per liter of distilled water). Isolate identification was confirmed using tester R. solani AG-7 isolates (Carling, University of Alaska) for comparison during anastomosis pairing. To confirm pathogenicity, six AG-7 isolates (65, 66, 67, 68, 69, 70) obtained from cotton roots were tested in a plant growth incubator (18--24°C) by mixing 2.5 ml of 2-week-old cornmeal sand inoculum (3 g cornmeal, 100 g sand, and 20 ml distilled water) with 500 ml of autoclaved soil into each of five replicate polystyrene pots (15 cm wide × 20 cm long) per isolate. Five control pots containing noninfested soil were added for comparison. Five seed of cv. Deltapine 50 were sown into each pot. For all six isolates, mean stand counts (ranging from 0 and 1.3 plants per pot) were significantly lower (P ≤ 0.05) compared with those of plants in noninfested pots (3.8 plants per pot) at 21 days after planting. Furthermore, when the roots of surviving seedlings were evaluated for disease severity, brown, discolored lesions were observed at the base of stems and on roots in infested pots for all six isolates. Six AG-7 cultures of the pathogen were reisolated from symptomatic tissues onto PDA. The experiment was repeated with similar results. This is the first report of AG-7 occurring in Mississippi.