Historically, Rhizoctonia solani anastomosis group 3 (AG-3) was regarded as host specific, composed of isolates causing black scurf of potato. Recently, isolates of R. solani AG-3 were reported causing target spot of tobacco in South Africa. In the United States, isolates causing target spot of tobacco were identified as AG-2-2. Our anastomosis tests indicated that isolates of R. solani from diseased tobacco in North Carolina belong to AG-3. Furthermore, the tobacco isolates, like AG-3 from potato, were prototrophic for thiamine, whereas isolates of AG-2-2 were autotrophic for thiamine. Cluster and principal component analyses based on cellular fatty acids indicated that AG-3 isolates from tobacco were distinct from AG-3 isolates from potato [...]
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Janell Stevens Johnk (1993) studied this question.