Dutch Elm Disease

Treaty of Penn with Indians (Benjamin West, 1772, oil on canvas) shows Penn treating with the Delaware under under the great elm at Shackamaxon

Elm trees (genus Ulmus, family Ulmaceae) are deciduous and semi-deciduous trees which come in all sorts of shapes and sizes.  They first evolved 40 million years ago in central Asia, and since then they have spread around the entire northern hemisphere, even crossing the equator in the rain forests of Indonesia–and that is just the natural distribution of these trees. During the eighteenth and nineteenth century elm-loving gardeners took the plants elsewhere on the globe, particularly Australia, which is now renowned for its avenues of elms. But the pinnacle of elms was in America: the American elm is a particularly large and beautiful member of the family capable of growing 50 meters (130 feet) high and 40 meters (120 feet) wide in almost any soil.  The American elm’s tapering curve caused avenues of the trees to look like gothic vaults. Many of North America’s streets used to be huge 100 foot tall glowing green cathedrals made of living elm.

Elm-lined streets before and after Dutch Elm disease

The use of the past tense has probably warned you that there is a horror movie twist to this story. Alas, in 1928, a shipment of logs from The Netherlands destined for use as veneer in the Ohio furniture industry arrived in America.  The logs carried elm bark beetles which in turn carried ascomycete microfungi. It was the American beginning of Dutch elm disease, a blight which later wiped out millions of trees. Various different species of bark beetle are capable of transmitting closely related microfungi (namely Ophiostoma ulmi, Ophiostoma himal-ulmi, and the virulent Ophiostoma novo-ulmi).  These fungi seem to be able to hybridize with one another and form new strains…with new strengths.  The fungi apparently hail from China, but their province is unclear—at some point the trail goes cold (although Chinese species of elm seem resistant to the blight). Whatever the case, European and American elm trees were not ready for the spores.  Even with heavy use of pesticide, fungicide, and aggressive quarantines, the great elm populations of Western Europe and North America dwindled to a fraction of what they were in the beginning of the twentieth century.

Life Cycle of Ophiostoma ulmi

Dutch elm disease is its own special horror if you are a lover of trees (which I imagine you are if you are reading this blog), but there is a darker terror lurking behind the blight.  Dutch elm disease is one of the most famous blights of our era in North America so far, but it is by no means the only one.  Merriam Webster defines a blight as “(a) a disease or injury of plants marked by the formation of lesions, withering, and death of parts; or (b) an organism (as an insect or a fungus) that causes blights.” These blights are spreading and multiplying.  There is an oak blight, a bean blight, and a tomato blight. There are invasive tent caterpillars, mites, and galls. Although blights are technically defined as destroyers of plants, there are worrisome parallels in the world of animals. Frogs around the world have been dwindling from an exotic amphibian fungus. Bats in the northeast cannot hibernate thanks to a different fungus and they expire of energy loss.  And the fungi are not the only worrying players.  Everyone has followed the mass die-offs of honeybees from mites and who knows what else. The mixed-up super-fast dynamics of our human world mean that all sorts of critters, weeds, bugs, bacteria, protists, spores, and viruses end up traveling all over the place.  How long till a Dutch elm disease type plague strikes some life form we hold even more dearly?

Before I creep anyone out too much by writing in this vein, it is instructive to look back at the fossil record of elms. This is not the first great die-back for the trees.  According to pollen samples taken from ancient bogs, six thousand years ago, during the mid-Holocene period, all elm trees suddenly died back close to extinction in northwest Europe.  To a lesser extent the same thing happened again 3000 years ago.  It is possible that elm tree already establishing immunities to the current blights.  As I mentioned before, most Asian species are at least somewhat resistant to the fungus, and certain individuals and cultivars of once-common European and American Elms are gradually being discovered.  Our grandchildren might once again live on streets that are green cathedrals…

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