The alien invasive forest pathogen Heterobasidion irregulare is replacing the native Heterobasidion annosum

Invasions by alien pathogens are a major threat to forest conservation. The North American fungal pathogen of conifers Heterobasidion irregulare, inadvertently introduced in Central Italy in the 1940s, has been spreading causing high mortality of Italian stone pine (Pinus pinea). While invading newfound niches, H. irregulare has established itself in the current range of the native congener H. annosum. The aims of this study were to determine whether in time: (I) H. irregulare populations may be increasing in size; (II) H. irregulare may be replacing H. annosum, rather than simply coexisting with it; and, (III) H. annosum may disappear in forests infested by H. irregulare. The presence, abundance and distribution of H. annosum and H. irregulare were assessed through an aerobiological assay replicated ten years apart in a forest in which both species have been coexisting. Replacement index (RI), Markov chains and geometric progressions were used to model the interspecific interaction between the two species and to assess the invasiveness of H. irregulare. Results showed that, in 10 years, the incidence of H. annosum dropped from 39.4 to 6.1%, while that of H. irregulare increased from 57.6 to 81.8%, with the alien pathogen replacing the native species (RI = 84.6%) and spreading at a maximum rate of 139 ha/year. Although our models show that the extinction of H. annosum may be unlikely, the ability of H. irregulare to replace it suggests the alien pathogen may also readily colonize those parts of Europe where H. annosum is more abundant than in Central Italy.