In 1998 and 1999, Rhizoctonia solani Kühn and R. cerealis van der Hoeven were isolated from lesions on basal leaf sheaths (2 to 4 cm above the soil level) of 48 wheat plants (Triticum aestivum L. em. Thell.). Plants were collected from 16 locations in Egypt in December 1998 (seedling stage) and March 1999 (adult stage). Disease incidence was approximately 20 and 30% during the 1998 to 1999 and 1999 to 2000 seasons, respectively. Mycelium was white to deep brown, branched at right angles with a septum near the branch, and a slight constriction at the branch base. Multinucleate cells and the production of irregular, brown black sclerotia in culture and on host plants were diagnostic of R. solani (1). Anastomosis groups AG-4 (30 isolates) and AG-1 (6 isolates) of R. solani, distinguished on the basis of hyphal fusion, were recovered. R. cerealis (12 isolates belonging to the anastomosis group CAG-1), which were slow-growing on potato dextrose agar, produced binucleate hyphal cells and yellow sclerotia. The 48 isolates were tested in the greenhouse for pathogenicity using cornmeal and sand inoculum prepared by mixing 3 g of infested cornmeal, 100 g of sand, and 20 ml of distilled water and incubating for 20 days at 28°C. Fifty milliliters of inoculum were added to 500 g of sterile soil in a 15-cm-diameter pot. There were six pots per isolate. Six pots containing noninfested soil were included as checks. Wheat (cv. Giza 160) seeds were disinfested by placing them in 5% of sodium hypochlorite for 5 min, and 10 seeds were planted in each pot. Twenty-five days after planting, an average of five plants survived in the pots treated with R. solani, and seven plants survived in pots treated with R. cerealis. All 10 plants in the noninfested pots survived and were symptomless. The surviving plants treated with R. solani developed a tan cortical rot of the root system and tan zonate lesions on the basal leaf sheaths. The lesions were oriented longitudinally with the stem and were irregular in shape with a white or light tan center surrounded by a wide, dark brown margin. Isolates of R. cerelis caused lesions on the base of the stem of young and mature plants. The lesions, which were larger than those caused by R. solani, were elliptic or “eye” shaped with a pale yellow center and sharply defined narrow border. The fungi were reisolated from the symptomatic tissue.
Reference: (1) J. D. S. Clarkson and M. J. Griffin. Plant Pathol. 26:98, 1977.
