Effects of seasonality and climate on the sporulation of single Heterobasidion annosum sensu stricto and H. parviporum fruiting bodies in Norway spruce stands of Latvia

The root and butt rot fungi Heterobasidion annosum sensu stricto (s.s.) and H. parviporum are reported as some of the most widespread and destructive pathogens of conifer forests in Europe. By using passive spore traps, in this study conducted over 3 years in Latvian Norway spruce mixed forest stands, we explored the temporal dynamics of sporulation of H. annosum s.s. and H. parviporum overall and at the individual fruiting body scale and we assessed the effects of both season and climatic conditions on sporulation rate. Data suggested an overwhelming importance of the Heterobasidion genotype in determining spore loads. Indeed, we detected a single highly sporulating fruiting body overwhelmingly contributing to the overall airspora regardless of the time period. Peaks in spore release were observed in late summer and early fall for most fruiting bodies. Results clearly proved a prevalent role played by some climatic variables, including average temperatures and the relative humidity 2-4 weeks before spore trapping. In contrast, seasonality explained less variability in sporulation. We fitted, tested, and validated models able to predict the sporulation of Heterobasidion spp. based either on climatic variables, or seasonality. However, for risk prediction, we recommend to use models based on short-term climatic variables rather than seasonality.