Dutch Elm Disease (DED) is a vascular disease which has killed countless planted and natural stands of native American or White elm trees, Ulmus americana L.
The Siberian elm, U. pumila L., an introduced tree, does not appear to be as susceptible to DED as the American elm.In fact, there has only been one confirmed case in Winnipeg of DED in a Siberian elm.
HISTORICAL DISTRIBUTION OF DUTCH ELM DISEASE IN NORTH AMERICA (1930 – 2000)
Map Courtesy Sunday Oghiahke, University of Manitoba Entomology Department
HISTORY OF DED
DED is the most destructive disease of elms in North America. It has devastated elm populations around the world. DED was first identified in the Netherlands and northern France in 1919.The disease was brought to Europe from the Dutch East Indies in Southeast Asia during the late 19th century.
The first infections in North America were observed in Ohio in 1930. In Canada, the disease was found in Richeleau County, Quebec in 1944. Since then, DED has spread to almost every area in both Canada and the U.S. where there are ranges of native and planted elms. DED entered Manitoba via the United States from diseased firewood that was transported across the border
Before the occurrence of DED in Manitoba, it is estimated that the natural elm population in the central and southern portions of the province was approximately 20 million trees. Also since the early 1900's over five million elms were planted in cities, towns and in rural shelter belts.
Although DED was rampant in eastern Canada, it was not until 1975 that the disease was found in Winnipeg. A handful of diseased elms were first sighted in Wildwood Park, a lovely enclave on the banks of the Red River. Municipal and provincial officials sprang into action and initiated a cost-shared, integrated program of control which currently includes surveillance, sanitation, pruning, removal, therapeutic injections, basal treatments, education, community action, research, reforestation and preventative programs.
In 1980, Manitoba enacted the Dutch Elm Disease Act, a legal framework for the administration of the DED control program.
By 1990, the DED program was a fully integrated pest management effort that encompassed 50 towns, cities and municipalities across an area of approximately 100,000 square kilometers.
Over the years, both the province and the city have spent a lot of money to manage DED losses. In Winnipeg, over $2 million each year is spent on DED control. The annual loss of elms to DED has been kept to a level of less than 2% of the total inventory.This figure is considered acceptable to maintain a large elm population. More recently the loss rate has crept up to 3%. We are currently working with the Province of Manitoba and City of Winnipeg to reassess the DED program and revise the management practices.
The symptoms of the disease first appear in the leaves and can be observed from June to mid-August.The first symptom is the sudden wilting or drooping of the leaves on one or more of the major branches in the crown of the tree.This stage is subtle and difficult to identify by the untrained observer. With experience, it becomes easier to pick out the earliest phase of DED
The next stage is termed flagging as the leaves first turn yellow, then brown and shrivel.
As the disease advances, and it can do so at a rapid rate, the affected branch or branches die and more of the tree becomes infected. Most discoloured leaves will fall from the tree although some will remain on the tree into winter.
If infected later in the summer, the leaves will turn yellow and droop. Premature leaf drop often occurs. These infections are difficult to distinguish from early fall colouration which can occur as early as mid-August.
Elms that are infected later in the season may not show any symptoms but will have smaller than normal leaves the next spring. The tree will suddenly wilt and die rapidly in early summer.
Symptoms of the disease can also be seen under the bark of infected branches. Dark brown streaks in wood can be seen by removing the bark. Occasionally, the elm is affected by other diseases such as Verticillium wilt and Dothiorella wilt, which can mimic the symptoms of DED If this is suspected, it may be necessary to test the sample in the laboratory to determine which disease is responsible for the symptoms.
DED is caused by the fungus Ophiostoma ulmi which produces a toxic substance, to which the tree reacts, blocking the water conducting cells of the tree. The cells lose their normal function, resulting in wilting leaves and ultimately, the death of the tree.
DED is fatal to both large and small trees. The time it takes to kill the tree varies, depending upon age and growing conditions. Although younger trees seem to be somewhat less vulnerable than older trees, when a younger vigorous tree becomes infected, it can die within a few weeks. Older, slower growing trees may live for a year or two but eventually succumb to the disease.
DED is spread by elm bark beetles, primarily the Native Elm Bark Beetle. (The European Elm Bark Beetle also carries the disease but presently is not a concern in Manitoba.)
The tiny beetles (1/8 in. or 3 mm long), dark brown or black, carry the disease spores on their bodies from infected trees to healthy elms on which they feed.
The adult beetle breeds in recently dead or dying elm wood. Breeding takes place between the bark and the wood where the female constructs brood galleries (channels) in a V-shaped formation at right angles to the grain of the wood.
Eggs laid along the brood galleries hatch and the larvae feed, producing their own galleries. Whenfully developed, the larvae pupate and emerge as adults. If this breeding takes place in diseased wood, the new adult beetle may emerge with the very sticky DED spores attached. These spores can cause new infections if introduced into the water conducting tissues of healthy elm trees when the beetle feeds in the upper crown of the tree.
The life cycle of beetles varies: adults overwinter in the base of healthy elm trees. Larvae overwinter in brood trees and wood. The over wintering adults emerge in mid-April to mid-May and feed briefly before selecting brood material. This feeding stage is responsible for the majority of DED infections.
Signs of beetle activity can be seen in spring or fall as red boring dust on the lower trunk bark of healthy elms.
To a lesser extent, DED may be spread by underground root grafts between trees up to 40 feet or 13 meters apart.