Dutch Elm Disease, a severe disease that attacks elm trees. Some elm species, including the American elm, are more seriously affected than others. A hybrid species called the urban elm has been developed that is resistant to the disease. The disease is caused by a fungus carried from tree to tree by the European bark beetle and the American bark beetle. The fungus contains a toxin that causes the tree's vascular system to close off the movement of water to the branches. Eventually the wood under the bark of the branches turns brown and the leaves wilt, turn yellow, and fall prematurely from lack of water and minerals. This disease can also be spread when trees are planted so close together that their roots touch.

Some diseased trees live for one to three years, but others die in as little as four weeks. There is no known cure, but the spread of the fungus within a diseased tree can sometimes be halted by injecting fungus-killing bacteria into the tree. The injections are also used to protect healthy trees. Diseased trees that have not been treated, or that have been treated without success, should be removed promptly.

The bark beetles are difficult to control. The most effective insecticides are DDT and methoxychlor, but the use of DDT has been banned because of its toxic side effects and methoxychlor is too expensive to be used on a large scale.

Dutch elm disease was first discovered in 1919 in the Netherlands. It was unkown in the United States until 1930, when it appeared in Ohio. Since that time it has killed hundreds of thousands of elm trees from New England to Texas.

The fungus that causes the disease is Ceratocystis ulmiof the class Ascomycetes. The European bark beetle is Scolytus multistriatus; the American bark beetle, Hylurgopinus rufipes. Both belong to the family Scolytidae.