Association of Rhizoctonia strains with bare patch disease of wheat in Western Australia

Rhizoctonia-like fungi were isolated from the roots of diseased wheat plants sampled from the centre and periphery of three bare patches, and from apparently healthy plants from outside the patches. Of the isolates recovered, 81% were multinucleate and belonged to R. solani anastomosis group 8, and pectic zymogram group 1-1; the remaining isolates were binucleate Rhizoctonia spp. The multinucleate isolates could be grouped into highly virulent, intermediately virulent, and weakly virulent types. The binucleate isolates were all non-pathogenic. The multinucleate isolates were obtained at a significantly higher frequency from plants within the patches compared with outside the patches, and with the exception of a single isolate, the highly virulent isolates were not found outside the patches. The weakly virulent isolates were present at much lower frequencies than the highly virulent and intermediately virulent forms within the patches. The frequency of occurrence of binucleate isolates did not vary significantly among the locations sampled. None of the multinucleate isolates contained plasmids. Some of the isolates contained a prominent single dsRNA species and one or more minor dsRNA species. The distribution of these dsRNAs was not correlated with pathogenicity.