Dutch elm disease
M. Grabowski, UMN Extension
Leaves on infected branches turn yellow, wilt and then turn brown
Dutch elm disease (DED) is a fungal disease that will infect all native Minnesota elm trees; however, the disease does not always kill the tree. The success and rate of movement within the tree depends on tree size, time and location of infection in the tree, climatic conditions, and the defensive response of the tree. The fungus that causes Dutch elm disease was first introduced to Minnesota in 1961. The devastating history of Dutch elm disease in Minnesota was recorded by plant pathologist David W. French. Today, the disease can be found in every county in Minnesota yet it is estimated that 1 million elms still remain within communities. Several management strategies have been developed that allow elms to survive if properly cared for.
Pathogen and susceptible plants
Dutch elm disease is caused by 2 closely related fungi, Ophiostoma ulmi and Ophiostoma novo-ulmi. Ophiostoma novo-ulmi is the more aggressive species and is the most common pathogen associated with DED today.
All native species of elm are susceptible to DED. In Minnesota this includes American elm (Ulmus americana), red or slippery elm (U. rubra), and rock elm (U. thomasii). Some cultivars of American elm have a higher tolerance to the disease and may recover if infected. Asiatic elms such as Chinese elm (U. parvifolia), which is not hardy in Minnesota, Japanese elm (U. davidiana var. japonica), and Siberian elm (U. pumila) are resistant, but not immune to the disease. Researchers and plant breeders have developed several hybrid Asian elms and American elms that are resistant or tolerant of DED. Detailed information about elm varieties that grow well in Minnesota can be found in the publication 'Elm Trees- Dutch elm disease resistant varieties'.
- Leaves on one or more branches in the outer crown of the tree turn yellow, wilt and then turn brown.
- Fallen leaves are strewn over lawn unseasonably.
- Symptoms often first appear in late spring and early summer but can occur any time during the growing season or in early spring if infected the fall before.
- Yellowing and wilting of leaves progresses down the infected branch towards the trunk of the tree. The rate of spread down the tree depends on the susceptibility of the tree. Infected trees may die the season they become infected or over a period of several years.
- Brown streaking can be seen along the sapwood of wilted branches of some species when the bark is removed.
- To positively confirm the disease, send a live sample displaying symptoms to the UMN plant disease diagnostic clinic.
Dutch elm disease is a fungal disease that kills elm trees. The fungus cannot move by air or water to infect new trees but rather is carried by beetles or transmitted through grafted (connected) roots. Therefore, knowing the biology of the beetles is as important as the biology of the fungus. The bark beetles responsible for transmission include the native elm bark beetle (Hylurgopinus rufipes), the smaller European elm bark beetle (Scolytus multistriatus) and the banded elm bark beetle (S. schevyrewi).
Adult females of all three species of elm bark beetle lay eggs under the bark of recently dead or dying trees, or in firewood or logs with firmly attached bark. If DED is the cause of the trees decline or demise, the fungal pathogen will be present in the wood. The DED fungus produces sticky spores within tunnels and galleries created by the bark beetles. When the new beetles emerge from infected elms, they carry spores of the fungus on and in their bodies. Smaller European elm bark beetles and banded elm bark beetles feed in twig crotches of healthy trees, which creates a wound and allows the spores to get inside the bark; therefore new infections are seen at small twigs. Several beetles may feed in a single tree resulting in multiple infections scattered throughout the canopy. Native elm bark beetles emerge and feed on larger branches that are 2-10 inches in diameter, consequently main or secondary branches may become directly infected. During the feeding process, or less frequently during the creation of overwintering sites by the native elm bark beetles, spores are introduced into the vascular tissue of a healthy tree and infection occurs.
Once in the trees' vascular system, the fungal spores move up the tree with the flow of water or produce a thread-like growth called mycelium that grows downward towards the root system. In susceptible trees, the fungus is often capable of reaching the root system within the first season and can spread to adjoining elm trees through grafted root systems.
In response to the fungus, the tree produces plug-like structures called tyloses in the water transporting cells of the tree's vascular system in an attempt to stop fungal movement throughout the tree. Unfortunately susceptible trees do not produce tyloses quickly enough to block the fungus. Instead this poorly timed defense response ends up contributing to wilt and decline within the canopy. The brown streaking of the sapwood that can be seen in some species when the bark is peeled back is caused by a combination of the gummed up and damaged vascular system.
- Reduce the number of breeding sites available to the beetles through prompt removal of dead or dying elm wood with intact bark.
- Branches infected with DED should be removed the same year the infection starts.
- Trees with many branches infected with DED should be taken down.
- Wood from DED infected elm trees should be buried, debarked, burned, or chipped.
- Remove infected branches before the disease has moved into the main stem of the tree. All infected branches must be removed at least 5 feet, preferably 10 feet, below the last sign of streaking in the sapwood.
- Dutch elm disease can spread through root grafts from an infected tree to adjacent healthy elms; therefore, if possible, sever root grafts with a vibratory plow before the infected tree is removed in order to prevent this movement.
- Choose Dutch elm disease resistant cultivars for new plantings or as replacement trees (Table 1). Tolerant cultivars are not immune to the disease and may develop wilt if infected. Unlike susceptible trees, however, tolerant elms can block the spread of the pathogen and will not be killed. Infected branches should be pruned out as described above.
- Preventative fungicide injections can be used to protect trees from infection by beetle feeding. Fungicide injections are not very effective in preventing infection through root grafts so it is important that all trees in an area be treated and root grafts severed before removal of an infected tree. Fungicide injections can only be done by a trained arborist and, depending on the chosen fungicide, must be repeated on a 1-3 year cycle.
|Active ingredient||Representative trade names*|
*Trade names are for demonstration purposes only and do not imply endorsement by UMN Extension. Trade names may change over time. Products with the same active ingredient but different trade names should offer disease control as well.