The threat posed by the invasive fungal pathogen of pines Heterobasidion irregulare in Europe: observations and predictions

Heterobasidion irregulare is a destructive fungal pathogen of pines native to North America and currently recommended for regulation by EPPO. This basidiomycete was introduced during WWII in central Italy, where the native congener H. annosum is also present. In this talk we review the findings that could help to predict the future trajectory of this alien fungal plant pathogen in Italy and in the European continent. While H. irregulare showed a greater sporulation potential, a faster growth rate in wood and a broader host range than the native H. annosum, precise predictions of future impacts of H. irregulare have been hampered by both the high fragmentation of conifer stands and the low incidence of H. annosum in central Italy. Population genetics studies allowed to predict that as the fragmentation of conifer stands decreases, the spread rate of H. irregulare will increase. Unexpectedly, experiments showed that the presence of the native H. annosum may accelerate and not slow down the spread of H. irregulare. Both results paint a troublesome scenario for areas in central and northern Europe characterized by large contiguous forest stands infested by H. annosum. Are these predicted scenarios likely to be true? In order to answer this question, we have studied the interaction between the two pathogens in central Italy. Field evidence shows not only that H. irregulare is truly invasive with a spread rate of 139 ha/year in a contiguous forest, but also that it is replacing H. annosum (1). Unexpectedly, part of the genome of H. annosum is being replaced by genes from H. irregulare, thanks to gene introgression mediated by interspecific hybridization. In addition, H. annosum genotypes containing H. irregulare genes increase their transmission-traits to levels comparable to those of H. irregulare (2). We conclude that Europe is facing two invasions: one by H. irregulare individuals and one by H. annosum individuals modified by the acquisition of H. irregulare genes. Both invasions are associated with disease that will spread at a faster rate than that caused by native H. annosum, thus increasing the estimated 800 million Euros per year of damage already caused by native Heterobasidion spp. Current ongoing experiments conducted in the framework of the Agritech National Research Center funded by the European Union Next-GenerationEU (PIANO NAZIONALE DI RIPRESA E RESILIENZA (PNRR) – MISSIONE 4 COMPONENTE 2, INVESTIMENTO 1.4 – D.D. 1032 17/06/2022, CN00000022) aimed at assessing the combined effects of rising temperatures and infections by Heterobasidion spp. on key conifer species will allow to refine the predictions on spread and impact of H. irregulare under climate change scenarios.