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Prevalence and Pathogenicity of Anastomosis Groups of Rhizoctonia solani from Wheat and Sugar Beet in Texas. C. M. RUSH, Department of Plant Pathology, Texas Agricultural Experiment Station, Bushland 79012. D. E. CARLING, Agricultural and Forestry Experiment Station, University of Alaska Fairbanks, Palmer 99645; R. M. HARVESON, Texas Agricultural Experiment Station, Bushland 79012; and J. T. MATHIESON, DowElanco, Greenfield, IN 46140. Plant Dis. 78:349-352. Accepted for publication 22 November 1993. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0349.

Ninety-eight isolates of Rhizocionia spp., primarily R. solani, were isolated from wheat and sugar beets grown in the Texas Panhandle and typed for anastomosis group (AG). Of the 46 isolates from mature beet, 89% were AG2-2; of the 45 isolates from wheat, 95% were AG4; and most of the isolates obtained from beet seedlings were either AG4 or AG5. Two isolates of binucleate Rhizoctonia sp. also were recovered, one from mature sugar beet and one from beet seedlings. Randomly selected isolates from each AG were capable of colonizing wheat, corn, cotton, and sorghum residue saprophytically; and optimum temperature for growth of most isolates was between 20 and 30C. In pathogenicity studies, isolates of AG2-2 and AG4 reduced emergence and final stand of sugar beet seedlings, and isolates of AG2-2 caused severe root rot on mature sugar beet. On wheat, none of the isolates reduced emergence, but isolates of AG4 and AG5 caused significant postemergence root rot. Although some isolates of AG2-2, AG4, and AG5 reduced emergence and caused root discoloration on seedlings of corn, cotton, and sorghum, none were highly virulent on these crops. Both isolates of binucleate Rhizocionia sp. were either avirulent or caused only slight root discoloration. Since AG4, the predominant AG of R. solani on wheat, was highly virulent to sugar beet seedlings, wheat preceding sugar beets in rotation is not advised.