Rhizoctonia solani Kühn anastomosis group (AG) 13 was isolated from asymptomatic root tissue of a corn (Zea mays L.) seedling grown at the Black Belt Branch Experiment Station, Brooksville, MS. Rhizoctonia solani AG-13 was recently reported from cotton grown in Georgia (2). Rhizoctonia solani isolate MS-168 was successfully anastomosed with tester isolate AG-13 (courtesy of D. E. Carling, University of Alaska). The hyphal diameter at the point of anastomosis was reduced, the anastomosis point was obvious, and cell death of adjacent cells was observed. These results were confirmed by D. E. Carling and are consistent with C2 anastomosis hyphal reactions (1). Rhizoctonia solani isolate MS-168 was cultured on potato dextrose agar (PDA) and incubated at 27 ± °C with a 12-h photoperiod for 28 days. Mycelium was buff-brown to beige with diurnal zonation throughout the colony. Aggregates of bulbils developed in the center of the colony that were dark brown, dome shaped, and accompanied by brown exudate. Bulbils were submerged in the medium and scattered across the surface of the colony. The optimal growing temperature of MS-168 was 27°C. Two pathogenicity evaluations were conducted on 170 corn seedlings planted into soilless potting medium. Four-day-old corn seedlings were inoculated with 7-day-old PDA hyphal plugs (2 mm in diameter) of R. solani isolate MS-168 by placing the mycelium side of the hyphal plug in contact with the mesocotyl tissue beneath the soil surface. The hyphal plugs were covered with soil. The nontreated corn seedlings were inoculated with PDA plugs minus the fungus. Corn seedlings were maintained under environmentally controlled conditions at 27 ±2 °C with a 12-h photoperiod and watered to prevent wilting. Disease symptoms on mesocotyl tissue were rated from 1 to 4 in which 1 = no symptoms, 2 = a few pinpoint lesions and diffuse discolored areas, 3 = distinct necrotic lesions, and 4 = girdling lesions (3). Fourteen days postinoculation, treated seedlings had a significantly higher disease rating (1.5) than the nontreated control (1.0). Thirty of eighty-seven corn seedlings inoculated with MS-168 expressed symptoms of discoloration and pinpoint and necrotic lesions on the mesocotyl tissue at the site of inoculation. On the basis of the results of the pathogenicity evaluation, MS-168 can be characterized as weakly virulent on seedling corn when grown under controlled environmental conditions. The identification of R. solani isolate MS-168 (AG-13) from corn in Mississippi broadens the natural distribution of occurrence and host range of this anastomosis group.
References: (1) D. E. Carling. Grouping in Rhizoctonia solani by hyphal anastomosis reactions. Pages 37--47 in: Rhizoctonia Species: Taxonomy, Molecular, Biological, Ecological, Pathology, and Disease Control. B. Sneh et al., eds. Kluwer Academic Publishers, The Netherlands, 1996. (2) D. E. Carling et al. Phytopathology 92:893, 2002. (3) C. S. Rothrock. Report of the cottonseed treatment committee for 1993. Page 216--217 in: Proc. Beltwide Cotton Conf. Natl. Cotton Counc. Am., Memphis, TN, 1994.