Comparative analyses of phenotypic and ecological traits of North American and European isolates of Heterobasidion annosum

Several isolates from the eastern United States, from Europe, and from the North American population introduced in Italy were inoculated on Scots pine cuttings. Significant differences in lesion size were detected amongst North American (NA) and European (EU) isolates, but not among NA isolates collected in central Italy. These results support the hypothesis that the NA population found in Italy is the result of a bottleneck associated with an introduction event during World War II. In order to compare virulence of NA and EU isolates, seedlings of loblolly pine (a US host) and of Italian stone pine (a European host) were concurrently inoculated with multiple isolates from the USA, from Europe, and of NA origin but found in central Italy. Results indicated that NA and EU isolates have comparable virulence on both hosts. Because NA isolates have been reported to be abundant in coastal pine woods where presence of the indigenous Heterobasidion species is limited, we have attempted to identify differences between the two provenances, other than virulence. By sampling the air for airborne basidiospores in the winter and summer months, we have found that spores of the NA taxon are present all year round, while spores of the EU species are absent in the summer season. We infer that the ability of the exotic taxon to be active all year round has given it an ecological advantage in part responsible for its spread in areas unoccupied by the native taxon.