Dutch elm disease

How does Dutch elm disease survive and spread?

Dutch elm disease is caused by two closely related fungi, Ophiostoma ulmi and Ophiostoma novo-ulmi. Ophiostoma novo-ulmi is more aggressive and is the most common pathogen associated with DED today.

The fungus that causes Dutch elm disease is an invasive species and was first introduced to Minnesota in 1961. Plant pathologist David W. French recorded the devastating history of Dutch elm disease in Minnesota. Today, the disease can be found in every county in Minnesota yet it is estimated that 1 million elms still remain within communities.

How does Dutch elm disease spread?

Carried by bark beetles

• The native elm bark beetle (Hylurgopinus rufipes), the smaller European elm bark beetle (Scolytus multistriatus), and the banded elm bark beetle (S. schevyrewi) can all carry spores of the DED fungus from one tree to another.

• 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.

• Larvae feed on the tree's inner bark and sapwood, creating galleries and tunnels as they feed.

• If the tree is infected with or was killed by DED, the DED fungus will be present in the wood. Sticky spores of the DED fungus are produced within tunnels and galleries created by the bark beetles.

• When the new beetles emerge as adults from infected elms, they carry fungus spores on and in their bodies.

• Newly emerged beetles fly to healthy trees to feed.

• As the beetles chew through the bark, spores on the beetle’s body are knocked off in the process. These spores start new DED infections.

  • Smaller European elm bark beetles and banded elm bark beetles feed in twig crotches of healthy trees. Therefore new infections are seen in small twigs.

  • Native elm bark beetles feed on larger branches that are 2-10 inches in diameter. These infections occur on main or secondary branches.

  • Several beetles may feed in a single tree resulting in multiple infections scattered throughout the canopy.

  • Occasionally native elm bark beetles introduce DED into the lower branches when burrowing to create an overwintering site.

Movement within the tree

• Once in the tree's vascular system, the fungal spores are carried up the tree with the flow of water.

• 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 through the tree.

• Unfortunately, susceptible trees do not produce tyloses quickly enough to block the fungus. Instead, this poorly timed defense response can cause wilt and decline within the canopy.

Movement through root grafts

• The DED fungus produces a thread-like growth called mycelium that grows downward towards the root system.

• In susceptible trees, the fungus can reach the root system in the first season of infection.

• Neighboring elm trees will form root grafts, that allow water and nutrients to flow from one tree to another.

  • Root grafts commonly occur between neighboring trees of the same species.

  • Root grafts occasionally occur between neighboring trees from different species.

• The DED fungus can move through root grafts to infect neighboring trees.

• Infection that begins through a root graft often moves very quickly through the tree.

How to manage Dutch elm disease

• Reduce the number of breeding sites available to the beetles by promptly removing 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. If possible, sever root grafts with a vibratory plow before removing the infected tree to prevent this movement.

• Choose Dutch elm disease-resistant cultivars for new plantings or as replacement trees.

  • Tolerant cultivars are not immune to the disease and may develop wilt if infected.

  • Unlike susceptible trees, tolerant elms can block the pathogen spread 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 the removal of an infected tree.

  • Fungicide injections can only be done by a trained arborist. Depending on the chosen fungicide, the treatment must be repeated every 1-3 years.

  • Fungicides with the active ingredients thiabendazole and propiconazole are effective against DED.