In vitro test of virulence in the progeny of a Heterobasidion interspecific cross

Annosum root rot, caused by Heterobasidion annosum s.l., is the economically most devastating disease of coniferous forests in the northern hemisphere. We have analysed virulence of the progeny isolates of a H. annosum interspecific cross between one S and one P homokaryotic isolate from North America. Virulence was measured as mortality rate among 29 pine (Pinus sylvestris) or 58 spruce (Picea abies) seedlings per homokaryotic progeny isolate. The assay showed a wide range of virulence among the 97 progeny isolates on pine and segregated as a continuous character. The heritability of virulence on pine was estimated to be 0.088 in this study. A strong correlation between virulence on pine and spruce was also found, although the range of virulence between isolates on spruce was smaller than that on pine. No correlation between high virulence and either fast radial growth rate on malt extract agar or high wood decay capacity was found.