It was recently reported that North American (NA) individuals of the forest pathogen Heterobasidion annosum were found in a single pine stand near Rome, in association with the movement of US troops during World War II. Here, we report on some aspects of the invasion biology of this pathogen in Italian coastal pinewoods, and on its interaction with native (EU) Heterobasidion populations. Spores of Heterobasidion were sampled using woody traps in pine stands along 280 km of coast around Rome. DNA of single-spore colonies was characterized by two sets of nuclear and one set of mitochondrial taxon-specific polymerase chain reaction primers. NA spores were found not only in a single site, but in many locations over a wide geographic area. Invasion occurred at an estimated rate of 1.3 km/year through invasion corridors provided by single trees, and not necessarily by sizable patches of forests. Within the 100-km long range of expansion, the NA taxon was dominant in all pure pine stands. Because abundance of the EU taxon is low and identical among stands within and outside the area invaded by NA individuals, we infer that the exotic population has invaded habitats mostly unoccupied by the native species. Discrepancy between a mitochondrial and a nuclear marker occurred in 3.8% of spores from one site, a mixed oak-pine forest where both taxa were equally represented. Combined phylogenetic analyses on nuclear and mitochondrial loci confirmed these isolates were recombinant. The finding of hybrids indicates that genetic interaction between NA and EU Heterobasidion taxa is occurring as a result of their current sympatry.

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The fungal forest pathogen Heterobasidion annosum has been introduced from North America into Italy and is now associated with high mortality of Italian stone pines. Due to the presence of a closely related native H. annosum taxon, this pathosystem presents an unusual opportunity to test specific ecological and evolutionary factors influencing fungal invasions. Comparative inoculation experiments on Scots pine cuttings and on seedlings of European and North American pines failed to identify significant increased pathogenicity of North American genotypes on European hosts congruent with lack of host-pathogen co-evolution. However, spore trappings indicate that while reproductive potential of native H. annosum was significantly reduced in the dry season, that of the invasive taxon was consistently high regardless of season. Ecological differences between the native and exotic taxon may therefore facilitate this invasion. Understanding which factors enhance this emerging forest disease is important both for biotic invasion theory and for disease control.

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