Distribution and abundance of the introduced ectomycorrhizal fungus Amanita phalloides in North America.

Despite a growing awareness of the global reach of ectomycorrhizal (EM) fungal introductions, little is known about the fate of introduced EM fungi in novel ranges. Using herbarium specimens, species distribution models, and field collections of sporocarps, root tips and extramatrical mycelia, we assessed the distribution and abundance of the European species Amanita phalloides in North America. There are two distinct ranges of the fungus, one along the West Coast (California to British Columbia) and the second on the East Coast (Maryland to Maine). As predicted by a species distribution model, the West Coast range is larger. Amanita phalloides is more frequently found in native forests on the West Coast than on the East Coast. At Point Reyes Peninsula in California, A. phalloides dominates community sporocarp biomass, and is frequent as root tips. In individual soil cores at Point Reyes, root tips of A. phalloides make up 50% of total root tip biomass. Hyphae of A. phalloides are frequent, but make up only 2% of total hyphal biomass. The contrasting patterns of the distribution and abundance of A. phalloides on the East and West Coasts of North America may influence both its future spread and its impacts.

The ectomycorrhizal fungus Amanita phalloides was introduced and is expanding its range on the west coast of North America.

The deadly poisonous Amanita phalloides is common along the west coast of North America. Death cap mushrooms are especially abundant in habitats around the San Francisco Bay, California, but the species grows as far south as Los Angeles County and north to Vancouver Island, Canada. At different times, various authors have considered the species as either native or introduced, and the question of whether A. phalloides is an invasive species remains unanswered. We developed four novel loci and used these in combination with the EF1α and IGS loci to explore the phylogeography of the species. The data provide strong evidence for a European origin of North American populations. Genetic diversity is generally greater in European vs. North American populations, suggestive of a genetic bottleneck; polymorphic sites of at least two loci are only polymorphic within Europe although the number of individuals sampled from Europe was half the number sampled from North America. Endemic alleles are not a feature of North American populations, although alleles unique to different parts of Europe were common and were discovered in Scandinavian, mainland French, and Corsican individuals. Many of these endemic European haplotypes were found together at single sites in California. Early collections of A. phalloides dated prior to 1963 and annotated using sequences of the ITS locus proved to be different species of Amanita. The first Californian collections that we confirmed as A. phalloides were made from the Del Monte Hotel (now the Naval Postgraduate School) in Monterey, and on the campus of the University of California, Berkeley, in 1938 and in 1945. These historical data are used in combination with data on A. phalloides' current distribution to estimate a rate of spread for A. phalloides in California. Many species of ectomycorrhizal (EM) fungi have been introduced across and among continents, but with this evidence, the death cap becomes the only known invasive EM fungus in North America.