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.