Dutch Elm Disease
Ophiostoma novo-ulmi
A lethal fungal wilt of elm trees, spread by bark beetles and through connected roots, that has killed tens of millions of American elms and reshaped streets and forests across the country. The fungus plugs the water-conducting vessels, so infected trees wilt and die, often within a single season for a susceptible elm.
🔎 How to spot it
The first symptom is flagging: leaves on one or more upper branches suddenly yellow, wilt, curl, and turn brown in late spring or summer while the rest of the tree is still green. Peeling the bark from a wilting branch reveals dark brown or purplish streaks running lengthwise in the outer wood, and a cut across an infected twig shows a broken ring or dots of brown staining in the most recent growth ring. Whole-crown wilt and rapid thinning follow.
🥀 Damage it causes
The fungus colonizes and clogs the vessels that carry water up the tree, so the crown wilts and dies, sometimes branch by branch over a season or two and sometimes all at once. Susceptible American and European elms are usually killed, and once the fungus moves through shared root grafts it can spread to neighboring elms. The loss of large, mature elms is the most striking damage.
🔬 What causes it
Dutch elm disease is caused by the fungi Ophiostoma novo-ulmi and the older, less aggressive Ophiostoma ulmi, carried from tree to tree by elm bark beetles that breed in dying elm wood and by natural root grafts between closely spaced elms. Beetles pick up sticky spores in infected wood and introduce the fungus when they feed in healthy twig crotches. Stressed and wounded trees are colonized most readily.
🛡️ Prevent it
Promptly remove and destroy dead, dying, and beetle-infested elm wood by chipping, burning, or burying it so it cannot breed the bark beetles that spread the fungus, and do not store elm firewood with the bark on. Where elms grow close together, sever the connecting roots by trenching between them to stop underground spread. Plant disease-resistant elm cultivars or other species to rebuild lost canopy.
🧯 If it is already here
High-value elms can be protected before infection with professionally applied fungicide injections, and a tree caught very early with only a branch or two flagging can sometimes be saved by prompt therapeutic injection and removal of the affected limbs well below the staining. Once a large part of the crown is wilting the tree cannot be saved and should be removed. Quick sanitation of infected wood protects the remaining elms.
💡 Good to know
Dutch elm disease responds best to early action, so scout elms through the growing season and react at the first flagging branch. The combination of beetle spread above ground and root-graft spread below means saving a neighborhood of elms takes coordinated sanitation and root cutting, not just treating one tree. Resistant elm cultivars now make it possible to plant elms again.
For educational and informational purposes only. Disease management advice is general guidance drawn from university cooperative extension sources; always identify a problem positively and read and follow the label on any product before use, especially around food crops, children, and pets.