Heavy economic damage due to rot pathogen Heterobasidion annosum s.l. in close-to-nature managed Norway spruce forests in the Italian Alps

Root and butt rot pathogens are important fungi that affect the forest carbon sequestration by destroying the root and stem wood of forest trees, by predisposing the forest to windfalls, and by limiting tree growth. Heterobasidion annosum sensu lato is the main pathogen affecting conifers in the Northern Hemisphere. The incidence of natural infection of Heterobasidion spp. and economic damage were recorded in Norway spruce (Picea abies) forests in the Central-Eastern Italian Alps. About 1 900 trees were sampled from 63 transects in pure stands of Norway spruce or mixed stands dominated by Norway spruce. Results showed that the pathogen was present in most of the transects, though with different incidence. All three species of Heterobasidion were found in the study, but only one or two species were found in any given transect. The percentage of trees infected in each transect differed remarkably, ranging from 39 to 100%. The average percentage of infected stump surface differed between transects, ranging from about 10 to 49%. This study shows the recurrent and heavy presence of H. annosum in Alpine forests and the surprisingly heavy damage caused by this fungus. The ecological role of this pathogen and its economic impact are discussed