The invasiveness of a non-native fungal forest pathogen is boosted by the presence of a congeneric native species.
The North American-introduced fungal plant pathogen Heterobasidion irregulare has become invasive in pine stands of central Italy and has broadly hybridized with the native congeneric species H. annosum. In this study, by genotyping Heterobasidion fruiting bodies and mycelia in pine logs inoculated with both fungal species, we showed that H. irregulare developed fruiting bodies at a 1.9-fold higher frequency when spatially overlapping with H. annosum than when by itself. In spite of different fruiting rates, all fruiting bodies were morphologically identical, independently of where they were formed, indicating that increased fruiting rate is likely to increase production of spores. Although all possible nuclear-mitochondrial combinations were identified in hybrids formed in inoculated pine logs, hybrids with nuclei of both species and the H. irregulare mitochondrion were favoured, while hybrids with both nuclei of one species and mitochondria of the other species were less frequent. Based on these results, predictions on the wider invasion of H. irregulare in Europe and recommendations for its containment are formulated.