Frequency of stump infections by Heterobasidion annosum s.l. and benefits from urea treatments vary with tree species and season in European Alpine forests

The destructive forest pathogen Heterobasidion annosum sensu lato (s.l.) may be controlled either by timing logging operations in periods of the year characterised by a low risk of airborne stump infection or by treating stump surfaces immediately after felling. 1616 stumps of four of the most widely-distributed European host tree species, either untreated or manually and operationally treated with urea, were sampled from 29 pure and mixed forest stands of the western Italian Alps and analysed for the presence of the pathogen. The aims of the study were to (i) compare the overall and seasonal frequency of stump infection of different tree species; (ii) assess the efficacy of treatments on different tree species; (iii) explore if the efficacy of treatment is affected by the season of application, and (iv) estimate the benefits resulting from treatments at different time periods. Overall, the frequency of infection of untreated stumps was significantly higher (P < 0.05) in Norway spruce (Picea abies) than in European larch (Larix decidua), silver fir (Abies alba) and Scots pine (Pinus sylvestris). In general, the frequency of stump infection of each tree species increased throughout the year, reaching a maximum in autumn, possibly in relation to the higher availability of airborne inoculum in that season. Manual operational stump treatment with urea reduced the frequency of infected stumps compared to controls both overall and for every tree species, although the reduction was significant only in the overall analysis and in the analysis performed separately for Norway spruce and larch. Binary logistic regression models point to the general efficacy of treatments regardless of the season of application, although the frequency of infection of treated stumps is predicted to increase slightly in autumn. Despite the efficacy of treatments being slightly reduced in autumn, the predicted reduction of stump infection probability due to treatments, hence the benefits provided by treatments, is expected to vary in magnitude depending on the species but to increase similarly according to a roughly sigmoid curve throughout the year, approaching a maximum in autumn. The information provided by this paper may help forest managers to design profitable and/or sustainable tactics to control the disease.