A beetle-borne fungus wiped out most of the West's mature elms
Elms once lined the streets of Europe and North America and filled English hedgerows at densities above 1,000 trees per square kilometre. Then Dutch elm disease, a microfungus carried by bark beetles, killed most mature European and American elms. Australia, thanks to isolation and strict quarantine, still has its elms.
Elms make up the genus Ulmus, deciduous or semi-deciduous trees found across most of the Northern Hemisphere, from North America and Europe to Iran, Libya and Indonesia. Their flowers lack petals and are pollinated by wind. The fruit is a light, round winged seed, green with chlorophyll so it can photosynthesise before the leaves unfold; British elm seeds run about 50,000 to the pound. Leaves are typically lopsided at the base. Distantly, elms are relatives of figs, hops, nettles and cannabis.
Nobody agrees how many species exist, with estimates from 30 to 40, because elms hybridise readily and some populations spread by suckers as seed-sterile microspecies. The ecologist Oliver Rackham called Ulmus the most critical genus in British flora and quipped that its species exist more in the human mind than in the genes. Those who argue over elm identification even have a name: pteleologists, from the Greek for elm. Asia holds the greatest diversity, with around two dozen species.
The disease is named after the Dutch botanists Bea Schwarz and Christina Johanna Buisman, who described it in the 1920s. Spores enter wounds made by Scolytus beetles and invade the water-conducting tissue; the tree plugs its own vessels in response and starves its leaves. A milder strain reached Europe from Asia in 1910 and North America in 1928, then faded as viruses weakened it. A far more aggressive species arrived in Britain in the early 1970s, probably in Canadian rock elm destined for boatbuilders, and swiftly killed most mature elms of western Europe. Many Asian elms carry resistance genes.
People had planted elms as shade trees for centuries. A ball-headed field elm was grown in Persia since time immemorial, and in North America the American elm was prized for fast growth, strong wood and tolerance of pollution. From about 1850 to 1920 the contorted, weeping Camperdown elm was the fashionable small tree for gardens. Breeders have since developed disease-resistant cultivars to bring the elm back to streets and forests.
Source: Elm
