Problem type: Disease
Name of problem: Dutch Elm Disease
Plant name(s): American and European elm species; Other elm species are susceptible
Symptoms / Characteristics:
Symptoms usually begin on one branch high in the tree. The familiar symptoms associated with leaf wilt and branch dieback, are secondary symptoms. This can be noticed from mid June to mid July. Leaves on these branches begin to wilt and curl, later shriveling and turning brown and will usually remain on the tree. Later on in the season, leaves on one or more branches will begin to yellow and droop and may drop prematurely. The disease will progress to other branches in the crown of the tree over the season causing wilt and dieback throughout the crown. All branches may die killing a tree within a few weeks and others trees may survive several years.
Initial symptoms are inside the trunk of the tree. The water transporting system of the tree is affected, resulting in lack of water to the crown and death of foliage. If bark is removed, brownish-red streaks following the grain of the wood will be evident. The cut end of a branch will have brown staining in the outer ring of wood.
Dutch elm disease is caused by the fungus Ceratocystis ulmi and is spread in two ways. Spores produced by the fungus can be carried by either the native elm bark beetle or the European elm bark beetle. These beetles feed on the wood, excavating galleries in the bark. Galleries found under the bark indicate beetle activity. The fungus produces spores in the beetle galleries and as the beetles emerge, the spores stick to the beetle where it is carried to a new site.
Another way of infection is through the root systems of closely associated trees. If the roots of two trees have become naturally grafted together (grown into one another), the fungus can travel from one root system to the other. It can then quickly ascend the trunk and infect the main stem, killing most, if not the whole tree.
Control / Preventions:
Sanitation is a primary factor in controlling Dutch elm disease. Dead and dying wood should be removed as soon as possible. All infected plant material should be disposed of properly according to the Dutch elm disease act. Do not store elm wood for firewood, as this is a major source of breeding material for bark beetles. American elms should not be pruned between April 1 and July 31 unless special authorization is granted.
Keeping elm trees healthy will eliminate sites for beetles to breed, as they like to feed on recently dead wood. Good tree maintenance including watering, fertilizing, insect control, and especially pruning will help ensure trees are kept healthy and vigorous. Disinfect pruning tools between cuts on trees that are showing signs of the disease. If root grafts may be present between a healthy and a diseased tree, root systems may need to be separated by trenching. Consult trained personnel for proper techniques and equipment and check with local utilities before digging.
Systemic fungicides can be used to control the fungus. This involves injecting chemicals directly into the water conducting system of the tree, which are then translocated throughout the tree. Injections are very costly and are only carried out on high value trees by trained personnel. There is no guarantee that this type of treatment will be effective.
Careful monitoring of trees, basic knowledge of the disease and the elm bark beetle and good maintenance practices are important tools in preventing the spread of Dutch elm disease. Infected branches and trees can then be identified quickly and dealt with accordingly.