Characterization of a New Anastomosis Group (AG-9) ofRhizoctonia solani

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Abstract
Isolates of Rhizoctonia solani, anastomosis group-9 (AG-9), fruited on 2% V-8 juice agar. Metabasidia were barrel shaped, not constricted about the middle, 11.6-22.3 .times. 6.9-10.8 .mu., and somewhat wider than supporting hyphae. Sterigmata were 9.2-63.9 .mu.m long and numbered 1-5 per basidium. Adventitious septation in sterigmata was rare. Basidiospores were 7.7-11.6 .times. 4.6-7.7 .mu.m, smooth, thin walled, hyaline, and prominently apiculate. Spores tended to be unilaterally flattened and widest at the distal end. Repetition of basidiospores was not observed. Morphological characteristics of the perfect state indicate that isolates of AG-9 are properly classified as Thanatephorus cucumeris. Mycelium of isolates of AG-9 growing on potato-dextrose agar was whitish to light tan initially, becoming dark brown with age. Sclerotia, generally lighter in color than the mature mycelium, were scattered randomly over the agar surface. Concentric rings were produced by most isolates. In a growth chamber study, isolates of AG-9 were not pathogenic on established plants representing nine species. However, field recovery of isolates on AG-9 from root lesions on lettuce and carrot and from lesions on the subterranean and aerial stems of potato indicate that they may be mildly parasitic. Most isolates of AG-9 were thiamine-autotrophic, but some were auxotrophic. Thus, AG-9 differs from other anastomosis groups of R. solani, where thiamine requirement is reported to be a groupwide characteristic.