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 C D-causing R. solani in anastomosis group (AG) 6, with results indicati ng a closer affinity to AG-6 GV than to AG-6 HG. Cultures of CD isolat es were initially white to cream, turning a yellowish light brown afte r 10 days. Concentric rings of dark and light mycelium were evident fr om an early stage. Mycelium generally was appressed to the agar surfac e with sparse aerial growth. A few light-colored, irregularly shaped s clerotia could be discerned after 2 weeks. The mean hyphal diameter of CD-causing R. solani was 7.46 mu m (ranging from 5.0 to 10.0 mu m), a nd cells contained a mean number of four (ranging from two to eight) n uclei, compared to a mean hyphal diameter of 8.58 and 8.42 mu m and a mean nuclear number of six and four For AG-6 HG and AG-6 GV, respectiv ely. 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 thiami ne prototrophic. Conspicuous nodulose swellings were produced by CD-ca using 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 enzy mes, HpalI, Sau3AI, TaqI, and CfoI, showed the presence of a unique 61 0-bp fragment in the pathogen. It is proposed that CD-causing R. solan i isolates represent a distinct intersterility group within AG-6 that is more related to subgroup GV than to subgroup HG.