Assessment of presence and distribution of Armillaria and Heterobasidion root rot fungi in the forest of Vallombrosa (Apennines Mountains, Italy) after severe windstorm damage

One of the main problems for the management and conservation of silver fir stands has long been pathogens causing root rot, in particular Armillaria spp. and Heterobasidion annosum s.l. These opportunistic pathogens are especially threatening now that climate change related stress is increasing tree susceptibility to disease and vulnerability to windstorms. The northern Apennines Mountains (central Italy) are forecast to be one of the areas with the highest temperature increase in the next future. However, no systematic assessment exists of the risk posed by the disturbance due to secondary pathogens in the Apennine forests. In the Nature Reserve of Vallombrosa (northern Apennines), where silver fir forests have been managed and conserved for centuries since the Middle Ages, making it an ideal site for studying these parasites, the high presence of H. annosum was reported already in 1990, while only sporadic observations are available on Armillaria species. The aim of this work was to examine the occurrence of both pathogens, since detailed knowledge about their distribution may assist forest management planning and decision-making. Systematic sampling was undertaken at the intersection of 52 grid points covering the whole forest. Different fungal species from soil and fungal samples (fruiting bodies or rhizomorphs) were identified by combining morphological descriptions with molecular methods. The analyses confirmed the presence of H. abietinum in about 70% of the investigated points. The fungus was detected at two new localities above 1000 metres suggesting a possible expansion of the parasite at upward elevation, which might be associated with climate change. Armillaria was widespread: almost 90% of the samples resulted positive, and four different Armillaria species were successfully identified. The most frequent species were A. cepistipes, whose rhizomorphs were especially abundant, and A. ostoyae, which was often detected just in soil samples. At sites where A. cepistipes was found to coexist with A. gallica, these two species might specialize themselves to necrotrophic and saprotrophic lifestyle, respectively. Besides, there were unexpected findings of A. mellea, supposed to be a residual from the previous rotation of broadleaves.