What drives Dutch Elm Disease?

William Jacobi, Colorado State University, Bugwood.org. Early symptom of DED, yellowing and death of leaves on branches in the growing season.

Jill Thomson, Saskatchewan Perennial Society

Dutch elm disease (DED) is probably one of the best-known tree diseases in North America and Europe.

American elm trees (Ulmus americana) have been planted in many developing cities since the 1900’s, to provide shade for residential areas, and along main streets. They have grown into large, mature trees with a characteristic main trunk that divides into many spreading branches, providing an ideal canopy that shades a significant surrounding area.

Many of us enjoy the cooling effect of the overhead canopy, and this shade will become even more important as our summer temperatures keep climbing. Since the recognition of the destructive nature of DED and its ability to remove mature elms from our landscape, cities have been trying to find suitable replacements for elms, but no other trees are quite as successful at providing shade on city streets.

Dutch elm disease is a very complex disease caused by fungal pathogens,   (Ophiostoma species)that can affect any kind of elm (Ulmus species). It is transmitted by three kinds of elm bark beetle (native elm bark, european elm bark and banded elm bark – in Saskatchewan it is usually either the native or european elm bark beetle that spread the disease), by movement of infected wood, and through natural root connections that occur between trees.

The fungus was first isolated from dying elms in 1921 by a Dutch plant pathologist and another Dutch researcher helped determine that the fungus did cause the disease. Thus, the disease was named Dutch elm disease, although later it was determined that the disease originated in Asia.

The pathogen and beetles were transported to North America in infected wood, probably shipped from Europe after the First World War. DED was first identified in the USA in 1931, and in Ontario in 1946. Many trees died in eastern Canada in the next 40 years.  The disease also spread slowly westward, usually following the stands of native elms (U. americana) that grow along the rivers. It was found in Manitoba in 1975 and Saskatchewan in 1981.

The disease has become established in the native elms of the Qu’Appelle valley, in the Carrot River region and other stands on the eastern side of the province. An isolated elm tree was infected in Regina in 1981 and since then the city has removed more infected trees.  An infected Siberian elm tree (Ulmus pumila) was found in Saskatoon in 2015, it was immediately removed and extra vigilant monitoring of the neighbourhood trees was conducted for about 4 years. Another infected elm occurred in the Westmount area in July 2021 and in July 2023, three infected elms were discovered, two in Sutherland and one in Pleasant Hill.  In August, an additional infected elm was identified in Forest Grove and it is likely about 30 elms will be removed along Central Avenue (City of Saskatoon report, August 28, 2023). 

The disease destroys infected elms quickly, taking one to three years to kill a tree. The first signs of disease are sudden wilting, yellowing and browning of leaves on individual branches (flagging), or the whole tree. When the bark of infected branches is peeled back, brown streaking or mottling is seen on the outer layer of wood.

A laboratory test can confirm the presence of the fungus, which produces spores that travel through and block the conducting tissue of the tree. The fungus is spread from tree to tree by elm bark beetles that make tunnels and lay eggs in dead or dying wood. The larvae develop in infected wood, and when adult beetles emerge they are contaminated with fungal spores that are transmitted to either healthy or infected elms by feeding beetles.

The fungus and beetles have developed a mutually beneficial relationship: adult beetles lay eggs in dead or dying elm wood that is infected with the fungus.  The young beetles that emerge are contaminated with spores of the fungus and initially they feed on healthy trees, transferring the disease to uninfected trees. This ensures dead wood in which the next generations of beetles lay their eggs.

The importance of transmission of the pathogen spores on beetle bodies was recognized when the disease cycle was first studied, and initially control methods targeted the insect vectors. However, this did not prevent the disease and methods to inject pesticides into the trees were used to protect specific elms considered vital to the beauty of city landscapes.

Management programs in most provinces include public education, prevention of movement of firewood, tree maintenance (pruning and removal), sampling trees that are symptomatic, a ban on pruning in the summer when beetles are active, and monitoring of bark beetles to identify types present and population levels.

Prompt removal and destruction of infected trees is considered to be particularly important. Cities have also developed Prevention/Management programs to keep the disease in check because of the aesthetic and economic losses caused by the death of elms.

The city of Saskatoon provides information on prevention of disease transmission, and in August a pamphlet telling us how to “Protect our elm” was distributed with utility bills. The main recommendations, within the city, are:

1. Not to store elm wood

2. Do not prune elms from April 1 to August 31

3. Dispose of elm wood at the landfill (not at compost depots or in green bins).

It is also illegal (hence, very important) not to move elm wood within the province, particularly from home to campsites, and vice versa.

The city will inspect trees that the public suspects may be infected (Saskatoon.ca/dutchelmdisease). Extra vigilance is important in areas were infection has been discovered recently. We can all play a part in controlling this deadly disease.

Jill Thomson is a retired Plant Pathologist who lives in Saskatoon, where she enjoys gardening with her family, including the dogs. This column is provided courtesy of the Saskatchewan Perennial Society (SPS; saskperennial@hotmail.com ). Upcoming event – Fall Plant and Seed Exchange and Plant Sale Sunday September 10th 2:00pm at Saskatoon Forestry and Zoo Hall. Check our website (www.saskperennial.ca) for more details.