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.