It can probably be claimed, with little hypebole, that the introduction of Dutch elm disease (DED) in Europe and North America is the most significant event in the history of urban forestry.
Sounds lofty to say, but think about it: Dutch elm disease has affected everything from the way we view monoculture street plantings to our understanding of invasive pests. It forever altered urban forestry policy and law, and certainly changed the public’s awareness of street tree management.
Arborists trying to combat the devastation of this disease pioneered the profession of ‘tree health care’, opening up an entirely new industry for managing the care of urban trees. Even our current emerald ash borer pandemic is directly linked to Dutch elm disease because the majority of today’s ash populations are replacement trees for lost elms.
As the impact of Dutch elm disease is linked to the history of urban forestry, its introduction to America is intricately linked to the history of America itself, and, in a way, the American Dream. World War I had ended and American soldiers returning were in need of housing. This caused a demand for timber need to build both the homes themselves and the furnishings that would go inside them. American manufacturers began sourcing wood from other countries, including those in Europe.
Elms Dying in Europe
Meanwhile, in Europe, elm trees had been inexplicably dying for nearly 20 years. The timing of the tree deaths around The Great War and the way a seemingly healthy tree would up and die in midsummer led many to assume the deaths were related to nerve gas used by combatants.
A young phytopathologist from the Netherlands named Bea Schwartz first isolated a fungus from dying elms in 1921, which would give rise to the Dutch elm disease moniker. Another Dutch researcher, Christine Buisman, would also be instrumental in showing the disease was, in fact, caused by this fungus.
Later, it was discovered the fungus responsible for Dutch elm disease originated in Asia where elms had developed a resistance to the fungus over several millions of years. The elms in Europe had no defense and thus were dying by the millions after the disease arrived. Britain alone lost more than 25 million elms in a just a 30-year span.
Dutch Elm’s Induction into North America
Meanwhile, back in North America, it was well into 1931 when a furniture company in Cleveland, Ohio unwittingly bought infected logs from France. Just like the European elms, American trees had no resistance to the disease. To compound the problem for the trees, the European elm bark beetle was also introduced to North America. This insect was much more efficient at spreading Dutch elm disease than native elm bark beetles, and got a head start by emerging almost a month and a half earlier each year.
Within just a few decades, elm trees were dying by the hundreds of thousands in the U.S. Cities from the Northeast to the Great Lakes regions experienced catastrophic tree losses. In 1977 alone, the City of Minneapolis tagged a staggering 31,475 publicly owned diseased trees. Even if removal crews could work every day of the year, including weekends and holidays, this would require 83 trees per day to be removed and disposed of.
Keep in mind these were not small trees: most were 50+ years old with 35” (89cm) diameter trunks and quarter-acre canopies. The amount of effort this required and the affect on the visual value of the city cannot be overstated. Many thought this would bring about the extinction of the American elm.
Dutch Elm Disease in Canada
According to reports, Dutch elm disease reached eastern Canada during the Second World War, and spread to Ontario in 1967; Manitoba in 1975; and Saskatchewan in 1981. In Toronto, 80% of the elm trees have been lost to Dutch elm disease; many more fell victim in Ottawa, Montreal and other cities during the 1970s and 1980s. Quebec City still has about 21,000 elms, thanks to a prevention program initiated in 1981.
Alberta and British Columbia are the only provinces that are currently free of Dutch elm disease. (In an isolated case, an elm tree in southeastern Alberta was found to be diseased in 1998 and was immediately destroyed.) Today, Alberta still has a large number of elms unaffected by Dutch elm disease. Many streets and parks in Edmonton and Calgary are still lined with healthy, mature trees. Aggressive measures are being taken to prevent the spread of the disease into Alberta, as well as to other parts of Canada.
Early Dutch Elm Disease Management Efforts Fail
Early management efforts didn’t do much to give hope to those who feared the worst, as most efforts were ineffective. These early attempts focused on killing the beetles with insecticides but – given the small size of the insects and the large size of the trees – these attempts were doomed from the start. Nonetheless, spray programs to control the beetles went on for decades with little effect on the spread of the disease.
Canada’s Dutch Elm Management Breakthrough
The breakthrough in protecting and saving elms from Dutch elm disease in Canada came when the focus shifted from stopping the beetles to stopping the fungus. If enough fungicide could be injected into the tree’s vascular system then a spore introduced by a beetle wouldn’t be able to germinate, so the tree could not get infected. This method has worked at successfully saving ten of thousands of elms over the past three decades and is currently considered the industry standard for Dutch elm management.
There’s Still Work to Be Done
Management of Dutch elm disease has come a long way since the days of spraying DDT from helicopters to kill beetles, but it still requires action on the part of homeowners and municipalities. Quick recognition and removal of diseased trees is key to the overall management, but individual trees still must be managed one by one.
Many elms continue to thrive in our urban forests and those that do tend to be high value, or significant, trees. Dutch elm disease has certainly shaped the history of urban forestry, and not necessarily all in a bad way. It has taught many valuable lessons, including reminding us of the value and fragility of our cities’ tree canopies.