The Threat of Dutch Elm Disease

GardenLine | Trees and Shrubs | The Threat of Dutch Elm Disease

Russell Friesen

This is the first in a series or articles addressing the issue of Dutch elm disease.

American elm is one of the largest, most impressive, and most abundant landscape trees in Saskatchewan. When fully mature, a single American elm may tower to 30 meters with the broad arching branches spreading to cover most of an average city lot. The distinctive vase-like branching pattern of these trees also permits them to form graceful gothic vaults over the streets they line. The trees are relatively fast growing, they thrive on a wide range of soils, tolerate soil compaction, root disturbance, drought and extreme winter cold. With this impressive list of features, it is no wonder that American elm became the most widely-planted street tree in North America over the past century.

As with most things which seem too good to be true, American elm faces one major drawback. A well-established 100-year-old American elm can be killed in as little as two weeks if it is attacked by Dutch elm disease.

The fungus responsible for Dutch elm disease is thought to have originated in Asia. This site of origin is proposed because Asian species of elm demonstrate at least a moderate level of resistance to the fungus which causes the disease. Having co- existed for many generations with the fungus, susceptible Asian elms were slowly eliminated from the population and replaced by seedlings with greater and greater resistance. In this way, the elms of Asia established a natural equilibrium with the disease. Dutch elm disease only became an epidemic in the early part of this century when the fungus responsible for the disease was somehow transported from Asia to Europe. Once in Europe, where the elms had no previous exposure to the disease, an epidemic was at hand. The disease's arrival in Europe was marked by the sudden decline and death of millions of elms in the first two decades of this century. In 1920, the fungus responsible for the mysterious death of Europe's elm trees was positively identified by a scientist working in HollandØleading to the name Dutch elm disease.

After having decimated the elms of Europe, the fungus responsible for the disease was accidentally transported to North America in the early 1930's, when diseased elm logs were imported to America. Like the elms of Europe, the native American elm had no pre-exposure to the fungus, and was therefore highly susceptible to invasion. Soon after its arrival, decimation of elms in Eastern North America was begun. Since that time, the disease has proceeded along a steady westward course across the North American continent wherever elm trees grow naturally, or in cultivation. In the time since the first diseased logs arrived in this continent, over 40 million elm trees in the United States have been killed, with the worst devastation being found in those cities which ignored the problem. Cities such as Des Moines, Iowa where little preventative action was taken lost a quarter of a million trees in six years. It was not uncommon for a city to lose 90% of its elms in as little as ten years leaving tree-lined streets suddenly transformed into empty wind-swept places. Property values dropped and municipal budgets ballooned to cover the cost of wholesale tree removals. Of all the lessons learned during the spread of this disease, the single most valuable lesson was that a proactive line of defense, focused on maintaining tree health and preventing the spread of infection was the only effective means of preventing total catastrophic loss of the elms. Such a line of defense has also proven to be the least costly overall.

How does the fungus get to your tree?

The fungus responsible for Dutch elm disease would be quite innocuous if it were not for its association with the elm bark beetle. Left to itself, the fungus would survive in a few dead trees and would be relatively harmless, like many other disease organisms. Unfortunately for the elm trees, however, North American and European elm bark beetles travel from infected trees where they deposit their eggs to healthy trees to feed. By flying from tree to tree, these beetles deliver spores of the fungus in a very effective manner.

The Prospects for Saskatchewan

Saskatchewan falls in the extreme western most region of North America where the American elm can be found growing wild. Until recently, our native elms were the last non-diseased stands of their species in the world. With the westward spread of the disease, these trees are now affected. In the last several years, hundreds of elms in the Qu'Appelle Valley, Souris River and Cumberland House regions have been infected and destroyed. As well, the disease has been identified in trees in and around the city of Regina.

The province of Saskatchewan has declared Dutch elm disease to be a pest under the Pest Control Act. Under this legislation, owners of diseased elm trees are compelled to remove and dispose of the wood by burying it at an approved landfill or burning the wood on site. Under this legislation it is illegal to posses elm firewood diseased or otherwise unless it has been kiln dried or has had the bark removed. Elm wood can not be transported from any site unless it is being taken directly to an approved landfill. Further, it is illegal to prune elm trees between the dates of April 15th and July 30th as the bark beetles which play a critical role in the transmission of the disease are attracted to pruning wounds during this time. Pest control legislation is enforced at the municipal level. Each municipality is to appoint a pest control officer. It is the duty of this officer to ensure that diseased elm trees are removed. What can the concerned public do about Dutch elm disease?

The first step in any effective control program is an accurate assessment of the risks. The starting point therefore is a determination of the number and location of elm trees in the province. Almost every town in Saskatchewan has elm trees, but we have no way of knowing how many trees there are, or exactly where they are located.

Tree health is a second major factor. Trees which are unhealthy for any reason provide ideal breeding grounds for the beetles, and excellent home for the fungus to become established. This means that all unhealthy elms must be pruned to reduce the number of sites of possible infection. Proper preventive care involves pruning not only for the removal of dead wood but also to improve the trees structure and improving light penetration in the interior branches. Citizens can check with their local government to ensure that a pest control officer has been appointed for their municipality. In Saskatchewan, the cities have appointed pest control officers but many rural municipalities, even those where Dutch elm disease is already a problem no such individual has been appointed.

Public participation in "Elm Watch" programs is a third line of defense in early detection of diseased trees. The Saskatoon-based group SOS Elms is endeavouring to organize communities throughout the province.

For more information on Dutch elm disease contact your local municipal parks department, SOS Elms (343-7497), SPRR Forestry Branch (953-2221).

Sustainable horticultural information, offered free of charge to the public with the support of the University of Saskatchewan Extension Division, the Department of Plant Sciences and the Provincial Government.