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, an d from apparently healthy plants from outside the patches. Of the isol ates recovered, 81% were multinucleate and belonged to R. solani anast omosis group 8, and pectic zymogram group 1-1; the remaining isolates were binucleate Rhizoctonia spp. The multinucleate isolates could be g rouped into highly virulent, intermediately virulent, and weakly virul ent types. The binucleate isolates were all non-pathogenic. The multin ucleate isolates were obtained at a significantly higher frequency fro m plants within the patches compared with outside the patches, and wit h the exception of a single isolate, the highly virulent isolates were not found outside the patches. The weakly virulent isolates were pres ent at much lower frequencies than the highly virulent and intermediat ely virulent forms within the patches. The frequency of occurrence of binucleate isolates did not vary significantly among the locations sam pled. None of the multinucleate isolates contained plasmids. Some of t he isolates contained a prominent single dsRNA species and one or more minor dsRNA species. The distribution of these dsRNAs was not correla ted with pathogenicity.