By Brandon Gallagher Watson
It can probably be claimed, with little objection of hypebole, that introduction of Dutch elm disease (DED) to Europe and North America is the most significant event in the history of urban forestry.
Sounds lofty to say, but think about it; it 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, as the majority of today’s ash population are replacement trees for lost elms.
As the impact of Dutch elm disease is linked to history of urban forestry, its introduction to America is intricately linked to the history of America, and in a way, the American Dream itself. World War I ended and American soldiers returning were in need of housing. This caused a demand for timber required for the homes themselves and the furnishings that would go inside them. American manufacturers began sourcing wood from other countries, including those in Europe.
Meanwhile, in Europe, elm trees had been inexplicably dying for nearly 20 years. The timing of the 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 the dying elms in 1921, which would give rise to the Dutch elm disease moniker. Later it was discovered the fungus originated in Asia where elms had evolved 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 it arrived. Brittan alone lost more than 25 million elms in a just a 30-year span.
Back in America, elms were being planted along pretty much every boulevard they could be planted along. Their durable nature, rapid growth, and the pleasing vase-shape of the mature trees made them a seemingly perfect street tree. Many cities had hundreds of thousands of boulevard elms; in fact, to this day, Elm Street is the fifteenth most popular street name in the United States. The house, the yard, the picket fence, and the stately street tree was the definition of the American Dream.
All was well until 1931, when a furniture company in Cleveland unwittingly bought infected logs from France to build furnishings for all these new homes. Just like the European elms, our American trees had no resistance to the disease. To compound the problem for the trees, the European elm bark beetle was also introduced. This critter was much more efficient at spreading the disease than our native elm bark beetles, and got a head start each year by emerging almost and month and a half earlier.
Within just a few decades, elm trees were dying by the hundreds of thousands in the United States. Cities from the Northeast to Great Lakes experienced catastrophic tree losses. In 1977 alone, the city of Minneapolis tagged a staggering 31,475 publically owned diseased trees. Even if removal crews could work every day of the year, including weekends and holidays, this would require 83 trees [ital>per day<ITAL] P elm.
Early management efforts didn’t do much to give hope to those who feared the worst, as most 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, this was doomed from the start. Nonetheless, spray programs to control the beetles went on for decades with little effect on the spread of the disease.
The breakthrough in protecting and saving the elms came when the focus went 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 won’t be able to germinate — thus the tree cannot get infected. This method has worked at successfully saving tens of thousands of elms during the past three decades, and it is currently considered the industry standard for Dutch elm management.
To understand why certain tree injection techniques and chemicals are recommended and others are not takes just a little understanding of tree, beetle and fungal biology.
First, on the fungus, it is important to know that trees must be treated prior to infection to be effective. Once a tree has the disease, the management becomes much more difficult as it must be physically cut out of the tree by a process known as “tracing.” Tracing has been used to successfully save trees but it is difficult, costly to the client, and not a guarantee that the tree won’t die anyhow. Long story short — treat trees before they get infected.
The other fact important to know is the beetles begin emerging in April, ready to look for new trees to infect. The tree injection season for elm trees cannot begin until the tree has put out new xylem in the form of a new growth ring. A good rule of thumb is about the time American elms drop their seeds — often as late as Memorial Day in northern states. This means we have 6 to 8 weeks at the start of the season — from the beginning of April until Memorial Day — where the trees are susceptible but can’t be treated yet. If treated with thiabendazole, they will be protected for this period in years two and three; but trees treated with annual treatments are still vulnerable during this period.
Lastly, remember that tree injection is an invasive process that requires drilling into the tree for application. Giving the tree time to cope with the wounds associated with tree injection should be a consideration. Macro-infusion treatments call for 1.5 tees per inch of tree diameter, thus a 30-inch-diameter elm would require 45 drill holes. In three years, that tree will have 135 drill holes to CODIT.
With these distinctions in mind, it is clear that a treatment for Dutch elm disease should prevent the fungus from getting established and protect for several years. The latter would protect the trees from the beetles that emerge prior to the start of the treatment season while also greatly reducing the number of drill wounds to the tree. Many treatments were tried during the 1970s and early ‘80s, and finally found success with thiabendazole (Arbotect 20-S), a fungicide commonly used in agriculture for fruit preservation.
When diluted in large volumes of water and injected into elm trees, thiabendazole is able to predictably protect elms against infection for two-and-a-half to three seasons. This is attributed to the chemical’s ability to move into new wood the next season, protecting against the early-season beetles. Annual treatments miss this window while inflicting three times the number of wounds.
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 must still be managed on an individual basis. Many elms still 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 city’s canopy.
Brandon Gallagher Watson is director of communications at Rainbow Treecare Scientific Advancements, and is an ISA Certified Arborist (#MN-4086A).