Double-stranded RNA elements in the root rot fungus Heterobasidion annosum

This thesis comprises studies concerning the potential of using double-stranded RNA (dsRNA) mycoviruses as biological control agents against Heterobasidion annosum, a pathogen that causes root and butt rot in conifers. A large number of H. annosum isolates of various origin have been screened for the presence of dsRNA and it was found that dsRNA elements are present in a low proportion of isolates from most forest sites affected by H. unnosum. The highest proportion, up to 25%, of dsRNA infected isolates were found in localities heavily infected with H. unnosum. Sequence information indicates that the dsRNA elements in H. annosum belong to the virus family Partitiviridae. Studies on transmission of dsRNA in H. annosum indicated that horizontal transmission between vegetative cells occurs readily in this species and that anastomoses that allow dsRNA to transfer occur between both homo- and heterokaryotic mycelia, and also between isolates from different intersterility groups. Vertical transmission into spores was less efficient, particularly into conidiospores. It was concluded that conidia are not important for the spread of dsRNA in H. annosum, while basidiospores transmit dsFNA efficiently enough to be of importance for the spread of dsRNA. There was little difference in virulence between dsRNA infected and dsRNA free H. annosum isolates of the same genotype in virulence tests on ten-day-old seedlings of pine and spruce. Moreover, the virulence of the fungus was not affected when dsRNA was transferred from isolates of the S intersterility group to a P isolate. In conclusion, the present work indicates that partitivirus-like dsRNA elements cannot be used as biocontrol agents against H. annosum, since they do not cause hypovirulence in this species. However, the ease with which dsRNA was transmitted between incompatible isolates shows promise that deleterious mycoviruses have a potential as biocontrol agents in H. annosum, although the low transmission rate of dsRNA into conidia poses a potential problem concerning application of such a biocontrol ag