A continental catastrophe
Dutch Elm Disease (DED) is a vascular wilt disease responsible for the death of tens of millions of elm trees across Europe and North America. The name is misleading; it does not originate in the Netherlands, but rather refers to the pioneering work of Dutch phytopathologists Bea Schwarz and Christine Buisman, who first identified the fungus in 1921. The pathogen itself is believed to be native to Asia, where local elms show a high degree of resistance.
The disease is caused by microfungi of the genus Ophiostoma. The first pandemic, which began in Europe around 1910 and reached North America by 1928, was caused by Ophiostoma ulmi. A second, far more destructive pandemic began in the 1940s, caused by the more aggressive species Ophiostoma novo-ulmi. This species arrived in the UK from Canada in the late 1960s on infected elm logs and quickly spread, killing over 25 million trees. In the United States, the disease had killed an estimated 77 million elms by 1989.
The fungus relies on insect vectors for transmission. The primary carriers are elm bark beetles, particularly the European Scolytus multistriatus and the native North American Hylurgopinus rufipes. These beetles breed in dying or dead elm wood. When new adult beetles emerge, they carry fungal spores on their bodies and fly to healthy elms to feed on the tender bark in twig crotches. During this feeding, the spores are introduced into the tree's vascular system, specifically the xylem.
A self-inflicted wound
Once inside the xylem, the network of vessels that transports water from the roots to the leaves, the fungus spreads rapidly. The tree's own defense mechanisms are turned against it. To compartmentalize the invader, the elm produces gums and bladder-like cellular growths called tyloses. These structures are intended to block the fungus, but they also fatally block the tree's own water-conducting vessels.
Starved of water, the leaves on an infected branch begin to wilt and die, a symptom known as "flagging." This can happen rapidly, often in a single summer. Cutting into an infected branch reveals characteristic dark brown or purple streaks in the outer wood, evidence of the fungal invasion. The disease can also spread between closely planted trees through natural root grafts, creating a domino effect in urban avenues and forests. The American elm (Ulmus americana) proved especially vulnerable due to its genetic uniformity and the common practice of planting it in monocultures along city streets.