Dutch elm disease, caused by the fungus Ophiostoma novo-ulmi (formerly Ceratocystis ulmi), has devastated enormous numbers of American elms over the past half-century. The fungus originally came from Europe on imported elm logs. This fungus is vectored (i.e. carried from diseased trees to healthy trees) by two species of elm bark beetles: the smaller European elm bark beetle and the native elm bark beetle. It is also spread from plant to plant through root grafts between adjacent trees.
Dutch elm disease symptoms begin as wilting and yellowing of the foliage. Leaves on infected branches turn brown and branches die. Affected trees develop a brown discoloration in the water conducting vessels (xylem) of the wood. This may be seen as a ring of discoloration when a diseased branch is cut or as dark streaks when the bark is peeled back from infected branches. As the disease progresses, major limbs die and eventually the entire tree is killed.
To manage Dutch elm disease, community residents must work together to protect healthy elms and to destroy dead and dying elms in the community so they do not serve as a source of disease. Good sanitation requires that dead and dying elm trees be removed and buried or burned as soon as possible. Eliminate, if possible, all potential elm bark beetle breeding material within 1,000 feet of the trees to be protected. This includes elm branches that are infected, weak, dead, or recently cut. This also means destroying or debarking stumps and elm logs which are being kept for firewood. Sanitation reduces the fungus population as well as that of the bark beetles which vector the disease. Without sanitation, other disease management measures are of little use.
Root grafts need to be mechanically or chemically broken between adjacent trees to prevent underground movement of the fungus from one elm tree to the other.
Control of elm bark beetles is important in preventing the spread of the fungus to healthy trees. Normally this is done in the spring as the leaves are expanding because that is the time when high numbers of the fungus-carrying beetles emerge, although these insects can emerge almost any time during the growing season. Good coverage of insecticide over the entire tree is important to achieve effective insect control.
Injection of trees with fungicides can provide temporary protection from Dutch elm disease.
Therapeutic pruning may be helpful for trees in the very early stages of infection.
If elms are desired in the landscape, choose to plant only disease-resistant elms. In general, native elms are susceptible to Dutch elm disease; elms of European origin vary in their susceptiblity; elms of Asiatic origin are resistant. The following elms are tolerant or resistant: Siberian elm (Ulmus pumila), Chinese elm (U. parvifolia), Scots elm (U. glabra), Buisman elm (U. hollandica cv. 'Christine Buisman'), and Groeneveld elm (U. hollandica cv. 'Groeneveld'). Hybrid clones which are tolerant or resistant include: 'Dynasty', 'Jacan', 'Homestead', 'Pioneer', 'Regal', 'Thompson', 'Sapporo Autumn Gold', and 'Urban Elm'. An American elm selection, 'American Liberty', is also considered tolerant. Recent American elm selections such as 'Princeton', 'Trenton', and 'Valley Forge' are considered resistant.
Contact your local Extension office for fungicide and insecticide recommendations.
