We have been receiving samples of elms with Dutch elm disease (DED). Several people around the state also called the ISU Plant Disease Clinic about elms that are not doing well. The symptoms described include wilting or flagging of one or more of the upper branches and/or general wilting, yellowing, and browning of leaves. Others have trees that have died a few weeks after they first notice the problem. In the Plant Disease Clinic, we identify the fungus by culturing branch samples that are about Omega inch to 1 inch in diameter and about 6 to 10 inches long and show active wilting with obvious streaking in the wood.
Dutch elm is caused by the fungus Ophiostoma ulmi. It can be spread by bark beetles and through root grafts. Trees infected through root grafts usually show massive wilting in the entire tree crown and may die rapidly. The symptoms usually appear immediately after new leaves emerge in early spring. This fungus blocks the water-conducting vessels within the tree, thus initial symptoms include discoloration and wilting of foliage in affected branches. These leaves turn from dull green to yellow then curl and then turn brown, dry, and brittle. Wilting, shriveling, and browning is observed from late spring until late August or September. Light to dark brown streaks or blue to gray discoloration is observed in the wood immediately beneath the bark.
Sanitation helps prevent the spread of Dutch elm disease. This includes early detection of the symptoms, disruption of root grafts between the healthy and infected trees, and the removal and disposal of all dying and dead elm trees with intact bark.
Debarked logs are o.k. to save, as are chips from DED-infected trees. For the greatest margin of safety, compost the chips in a pile for a few months before using them.
This article originally appeared in the July 11, 1997 issue, p. 110.