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Disease Note.

Two Diseases of Alfalfa Caused by Rhizoctonia solani AG-1 and AG-4. P. C. Vincelli, Department of Plant Pathology, University of Kentucky, Lexington 40546. L. J. Herr, Department of Plant Pathology, The Ohio State University, Wooster 44691. Plant Dis. 76:1283. Accepted for publication 19 August 1992. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-1283B.

Separate outbreaks of web blight of foliage and stem canker on basal portions of shoots were observed in numerous fields of alfalfa (Medicago sativa L.) during warm, wet conditions in Kentucky during 1991-1992. Isolations consistently yielded Rhizoctonia-like fungi. Isolates from plants with symptoms of web blight (three isolates) and stem canker (four isolates) were characterized as R. solani Kiihn AGI 1B and AG-4, respectively (1). To test pathogenicity of web blight isolates, mycelial fragments were suspended in distilled water and sprayed onto potted 5-mo-old alfalfa cv. Buffalo plants, which were incubated under moist conditions at 30/26 C day/night; web blight was observed after 4 days. For stem canker isolates, colonized, dried wheat grain was sprinkled onto the soil surface of potted plants, which were incubated under moist conditions at 28/22 C; stem cankers were observed after 7 days. For both diseases, R. solani was reisolated from diseased tissues. Both web blight and stem canker have been reported on alfalfa, but AG types of R. solani have not. Although outbreaks of stem canker were observed in a variety of rotation sequences and tillage systems, most of the severe outbreaks occurred in new seedings where the previous crop was a grass. Although we have not documented stand loss after an outbreak of Rhizoctonia stem canker of alfalfa, such a result seems possible, since R. solani AG-4 also infects alfalfa taproots.

Reference: (I) A. Ogoshi. Fitopatol. Bras. 10:371, 1985./p>