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

Abstract

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.

Eradication programs against non-native pests and pathogens of woody plants in Europe: which factors influence their success or failure?

“…The similarity of symptoms to native or previously introduced species can mask the presence of invasive species for long periods, as occurred for Phytophthora cinnamomi and Heterobasidion irregulare, with similar symptoms as Phytophthora × cambivora and Heterobasidion annosum, respectively (Brasier et al 1993;Vettraino et al 2005;Garbelotto et al 2022). For these cases, the development and/or implementation of new genomic biosurveillance tools is critical, so that the taxonomy of the invasive or-ganism can be clarified unequivocally and effective eradication program can be started rapidly (Hamelin and Roe 2020;Luchi et al 2020).…”