Dutch Elm Disease
U.S. Department of Agriculture, Forest Service. 1979. A guide to common insects and diseases of forest trees in the northeastern United States. Northeast. Area State and Private Forestry, Forest Insect and Disease Management., Broomall, PA. p. 123, illus.(USDA Forest Service, Northeast Area State and Private Forestry Publication. NA-FR-4)
Dutch elm disease, caused by the fungus Ceratocystis ulmi, affects all elm species, although Asiatic elms are somewhat resistant. American elm is more susceptible than the other native or European species. Dutch elm disease causes a wilting or shriveling and drying out of the leaves and shoots. Affected branches are killed quickly and the leaves drop. One or more branches may be affected, or disease symptoms may develop throughout the entire crown at once. Infected trees usually die within a few weeks, but some die slowly, branch by branch, over a period of years. Inside the bark of affected trees, the outer layer of wood becomes mottled, streaked, or uniformly discolored. A cross section of the stem reveals brown spots or a dark outer annual ring.
The adults of two species of elm bark beetle, the smaller European elm bark beetle, Scolytus multistriatus, and the native elm bark beetle, Hylurgopinus rufipes, carry the Dutch elm disease fungus. The introduced European elm bark beetle is a more abundant and effective carrier of the disease fungus, although the native elm bark beetle inhabits a larger area. In the United States, there are usually two beetle generations each year. Beetles lay eggs either in weakened living trees or in dead elm wood with bark that is still tight and uncracked. The larvae feed just beneath the bark.
The fungus and the beetles (larvae) both overwinter in weakened and dead elm. In spring the adult beetles emerge from the wood, often with the sticky fungus spores clinging to them. The beetles introduce fungus into wounds they make as they feed in twig crotches or on the bark of young branches of healthy trees. The fungus soon fruits within the tree's vessels, and spores are carried rapidly by the tree's transport system to other parts of the tree. The fungus also can be spread through root grafts between an infected tree and an adjacent healthy one. Elm trees infected in the spring or early summer usually die within a few weeks. Those infected in late summer may not die until they begin to leaf out the next spring.