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Characterization of the Crater Disease Strain of Rhizoctonia solani

Crater disease (CD) of wheat is caused by a Rhizoctonia solani strain of ambiguous phylogeny. Anastomosis reactions confirmed placement of CD-causing R. solani in anastomosis group (AG) 6, with results indicating a closer affinity to AG-6 GV than to AG-6 HG. Cultures of CD isolates were initially white to cream, turning a yellowish light brown after 10 days. Concentric rings of dark and light mycelium were evident from an early stage. Mycelium generally was appressed to the agar surface, with sparse aerial growth. A few light-colored, irregularly shaped sclerotia could be discerned after 2 weeks. The mean hyphal diameter of CD-causing R. solani was 7.46 μm (ranging from 5.0 to 10.0 μm), and cells contained a mean number of four (ranging from two to eight) nuclei, compared to a mean hyphal diameter of 8.58 and 8.42 μm and a mean nuclear number of six and four for AG-6 HG and AG-6 GV, respectively. The CD isolates had a slower growth rate (15.3 mm/day) than AG-6 HG (29.1 mm/day) and AG-6 GV (22.6 mm/day) but, like AG-6, were thiamine prototrophic. Conspicuous nodulose swellings were produced by CD-causing R. solani on roots of wheat, and infection resulted in retarded shoot growth. Smaller nodules were evident on bean and soybean roots. Fingerprint patterns generated for the various isolates with four enzymes, HpaII, Sau3AI, TaqI, and CfoI, showed the presence of a unique 610-bp fragment in the pathogen. It is proposed that CD-causing R. solani isolates represent a distinct intersterility group within AG-6 that is more related to subgroup GV than to subgroup HG.