Natural infection of Sitka spruce thinning stumps in Britain by spores of Heterobasidion annosum and long-term survival of the fungus

Summary On 48 sites throughout Scotland and north England freshly cut stumps were either inoculated with basidiospores of H. annosum or allowed to become infected naturally. Sites were in first rotation plantations growing in high rainfall or low rainfall areas on either peat or mineral soils. After 2 years, infection varied greatly between sites. It was generally much higher following inoculation, but otherwise little of the variation can be explained. Overall, only 10.5 per cent of stumps became infected naturally; colonies were generally small and almost half the stumps contained only homokaryons. Colonization took place principally in the heartwood but stumps sampled 8 years after inoculation showed that in contrast to previous findings this did not prejudice long-term survival. The cross-sectional area of each stump occupied by H. annosum at two years was also not a good predictor of survival. There was a marked decline in survival of H. annosum in stumps sampled after 8 years, which implied a corresponding decline in the overall level of incidence to only 2.8 per cent across all sites. Even in those stumps in which the fungus survived, it failed to colonize two-thirds of the available roots. These results reinforce the conclusions of earlier work that there is a low risk of serious disease on peat soils in the uplands. It is suggested that for Sitka spruce stumps on wet sites, the risk of successful colonization is so low, particularly on peaty soils, that H. annosum may not become permanently established in stands on these site types, even if ambient spore loads increase. By contrast, on mineral soils in low rainfall areas, the risk of a build-up of inoculum in unprotected thinning stumps and the transfer of infection to residual trees in the stand is much higher.