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Extension > Garden > Yard and Garden > Trees and Shrubs > Dutch elm disease

Dutch elm disease

Rebecca Koetter, Michelle Grabowski, and Cynthia Ash Kanner

Importance

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. Below is a list showing developed cultivars that have some level of resistance or tolerance to DED and can be planted in Minnesota.

Table 1. Elm Cultivars with Dutch Elm Disease Resistance or Tolerance

Cultivar name Parentage
Accolade™

Hybrid Asian elm

Cathedral

Hybrid elm

Commendation™ Hybrid Asian elm
Danada Charm™ Hybrid Asian elm
Discovery Hybrid Asian elm
Homestead Hybrid elm
Jefferson American elm
New Harmony

American elm

New Horizon

Hybrid elm

Patriot Hybrid Asian elm
Pioneer Hybrid elm
Prairie Expedition® American elm
Prospector Hybrid Asian elm
Princeton American elm
Regal Hybrid elm
Sapporo Autumn Gold Hybrid elm
Trumph™ Hybrid Asian elm
Valley Forge American elm
Vanguard™ Hybrid elm

 

Identification

Biology

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.

Management

Active ingredient Representative trade names*
Propiconazole Alamo
Thiabendazole Arbotect

*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.

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