The death cap is an alien in America

The death cap is common along the west coast of North America, especially abundant around San Francisco Bay, with records reaching south to Los Angeles County and north to Vancouver Island. For decades authors divided into those who considered it native and those who saw a newcomer. Genetics settled it.

In 2009 Anne Pringle and colleagues developed four new loci and combined them with the EF1α and IGS markers to explore the species phylogeography. The data pointed firmly to a European origin for North American populations. Genetic diversity is markedly greater in Europe than in America, consistent with a bottleneck. North American populations have no endemic alleles, whereas alleles unique to different parts of Europe — Scandinavian, mainland French and Corsican — are common there, and in California several were found together at single sites.

There is an interesting archival thread. Earlier Californian collections dated before 1963, once annotated with ITS sequences, turned out to be other Amanita species. The first confirmed records come from Monterey in 1938 and from the University of California, Berkeley campus in 1945. Those made it possible to estimate a rate of spread.

The authors state their conclusion firmly: although many ectomycorrhizal fungi have been moved between continents, the death cap remains the only documented invasive ectomycorrhizal fungus in North America.

The conservation significance is twofold. A mycorrhizal fungus travels with the roots of nursery stock, so the route of introduction is horticulture and urban planting rather than wind. And because this is a lethally poisonous species, its spread is a public-health matter as much as an ecological one.

Introduced with plants, then also spread by spores

Alien status rests on historical vouchers, distribution and genetics together, not simply an absence of old reports. Pringle and colleagues demonstrated a European origin for North American Amanita phalloides populations and an expanding West Coast range. Newer genotyping clarified the mechanism: almost every site contained many small, distinct genets, indicating frequent establishment from sexual spores. One Californian genet nevertheless extended nearly 10 m, and the same individuals were recovered in 2004, 2014 and 2015. Transport with live plants is a plausible route for the original introduction, but “it spreads not by wind” is wrong. These studies establish origin, spread and local dominance; they do not yet quantify net displacement of native fungi or ecosystem damage.