Crater Disease of Wheat Caused by Rhizoctonia solani AG-6. D. E. Carling, Agricultural and Forestry Experiment Station, University of Alaska Fairbanks, 533 East Fireweed, Palmer 99645 . Linda Meyer, Department of Microbiology and Plant Pathology, University of Pretoria, Pretoria 0002, South Africa; and K. A. Brainard, Agricultural and Forestry Experiment Station, University of Alaska Fairbanks, 533 East Fire-weed, Palmer 99645. Plant Dis. 80:1429. Accepted for publication 5 September 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-1429A.

Crater disease of wheat (Triticum aestivum L.) in South Africa and patchy stunting of cereals in Tanzania are caused by strains of Rhizoctonia solani Kuhn (2). A similar strain of R. solani has been isolated from the roots of umbrella-thorn (Acacia tortilis (Forsk.) Hayne subsp, hetera-cantha (Burch) Brenan), an indigenous tree. Until now, the anastomosis group (AG) affinity of strains of R. solani causing crater disease and patchy stunting has not been known. Fourteen isolates collected from the Springbok Flats (South Africa) and one from the Hanang wheat complex (Tanzania), were characterized by anastomosis reaction. Each isolate had been recovered from diseased wheat plants and each, upon inoculation onto healthy wheat plants, produced symptoms of crater disease. All isolates were multinucleate and all produced a category 2 (C2) anastomosis reaction (1) when paired with three different tester isolates representing AG-6. One additional isolate, collected from the roots of A tortilis subsp. heteracantha growing in the Springbok Flats, was also found to be a member of AG-6. The isolates from A. tortilis subsp, heteracantha and Tanzania, plus one isolate selected from the Springbok Flats collection, were paired with tester isolates representing all other AGs of R. solani. Since some isolates of AG-6 are known to produce a limited or "bridging" anastomosis reaction with some isolates of AG-8, tester isolates representing all five zymogram groups of AG-8 were included. The isolate from A. tortilis subsp, heteracantha produced no anastomosis reaction with testers of any AG other than AG-6, but bridging reactions were observed between AG-1 IA testers and the isolates from Tanzania and the Springbok Flats. The existence of these bridging reactions is the first reported instance of bridging between isolates of AG-1 and any other AG; however, a relationship between isolates that cause crater disease and isolates of AG-1 was suggested in an earlier molecular study. This is the first report identifying R. solani AG-6 as the causal agent of crater disease of wheat. In addition, this is the first reported evidence that isolates of AG-6, heretofore described as avirulent, are capable of causing a plant disease.