Dutch elm disease (DED) devastated elms throughout Europe and much of North America in the second half of the 20th century. It derives its name "Dutch" from the first description of the disease and its cause in the 1920s by Dutch botanists Bea Schwarz and Christina Johanna Buisman. Owing to its geographical isolation and effective quarantine enforcement, Australia has so far remained unaffected by DED, as have the provinces of Alberta and British Columbia in western Canada.

DED is caused by a microfungus transmitted by two species of Scolytus elm-bark beetles, which act as vectors. The disease affects all species of elms native to North America and Europe, but many Asiatic species have evolved antifungal genes and are resistant. Fungal spores, introduced into wounds in the tree caused by the beetles, invade the xylem or vascular system. The tree responds by producing tyloses, effectively blocking the flow from roots to leaves. Woodland trees in North America are not quite as susceptible to the disease because they usually lack the root grafting of the urban elms and are somewhat more isolated from each other. In France, inoculation with the fungus of over 300 clones of the European species failed to find a single variety that possessed of any significant resistance.

The first, less aggressive strain of the disease fungus, Ophiostoma ulmi, arrived in Europe from Asia in 1910, and was accidentally introduced to North America in 1928. It was steadily weakened by viruses in Europe and had all but disappeared by the 1940s. However, the disease had a much greater and longer-lasting impact in North America, owing to the greater susceptibility of the American elm, Ulmus americana, which masked the emergence of the second, far more virulent strain of the disease Ophiostoma novo-ulmi. It appeared in the United States sometime in the 1940s, and was originally believed to be a mutation of O. ulmi. Limited gene flow from O. ulmi to O. novo-ulmi was probably responsible for the creation of the North American subspecies O. novo-ulmi subsp. americana. It was first recognized in Britain in the early 1970s, believed to have been introduced via a cargo of Canadian rock elm destined for the boatbuilding industry, and rapidly eradicated most of the mature elms from western Europe. A second subspecies, O. novo-ulmi subsp. novo-ulmi, caused similar devastation in Eastern Europe and Central This subspecies, which was introduced to North America, and like O. ulmi, is thought to have originated in Asia. The two subspecies have now hybridized in Europe where their ranges have overlapped. The hypothesis that O. novo-ulmi arose from a hybrid of the original O. ulmi and another strain endemic to the Himalayas, Ophiostoma himal-ulmi, is now discredited.

No sign indicates the current pandemic is waning, and no evidence has been found of a susceptibility of the fungus to a disease of its own caused by d-factors: naturally occurring virus-like agents that severely debilitated the original O. ulmi and reduced its sporulation.