Rows of stately elm trees, rising to the height of cathedrals, once proudly lined boulevards across North America. Then, in 1930, a furniture company in Cleveland imported elm logs from England infected with a pathogen known as Dutch elm disease. Soon, those majestic leafy elms began to die. Millions of proud elm trees have fallen to the disease, stripping cities of their presence across the continent.
Some elm survive, however. Through exemplary vigilance, Alberta and British Columbia have almost completely succeeded in keeping out the disease. Efforts in Alberta led by a non-profit called the Society to Prevent Dutch Elm Disease (STOPDED) mean that the province today counts about 600,000 healthy American elms – among the most impressive collection of elms on the continent. Edmonton alone boasts about 80,000 elm trees. Alberta estimates its elms’ value at $2 billion.
Last summer, the City of Lethbridge, home to extensive canopies of elm trees, found two elms infected with the disease. Lethbridge promptly removed the trees to stamp out the illness. Alberta has now banned elm-pruning from April through September, to protect elm health.
The ban on pruning targets what researchers know about the elm disease transmission system. Dutch elm disease is a fungus that grows just under the bark of infected trees. The fungus can then spread to galleries in a tree where beetles breed – either a native elm bark beetle, or the European elm bark beetle. In spring young beetles emerge and fly to other elm trees to eat their bark, carrying and spreading the disease with them. The fungus fills the tree’s xylem vessels – the pathways that transport sap through a tree. To prevent the spread of the fungus, an infected elm plugs its pathways with gums, which then stops sap flow, and the tree dies.
Beetles love to feast on fresh tree wounds, so summer elm pruning could attract an infected bark beetle. That’s why Alberta permits pruning of elm trees only between October 1 and March 31 when beetles are dormant. The province also bans imports of elm firewood from Saskatchewan.
Winnipeg counts the most elm trees of any city in North America. Dutch elm disease has killed tens of thousands of the city’s elms, but elm lovers hold out hope. Trees Winnipeg has declared 2021 the Year of the Elm Tree. To reduce spread, Winnipeg strives to quickly remove infected trees, since a single dead elm can produce tens of thousands of contaminated beetles. Ensuring quick removal of affected branches can save an elm. Trees Winnipeg asks locals to chip the wood of infected elm trees into mulch – or take the wood to the dump.
The disease also spreads through the roots of infected trees to healthy trees; one ambitious intervention to protect elms is to dig root graft barrier trenches, between one and 1.5 metres deep.
Arborists typically install trenches midway between the infected tree and the healthy tree. One must know where the fungus is located in the diseased tree before digging the trench. If the disease stain has reached ground level in the infected tree, a second trench may be necessary.
Trees Winnipeg suggests injections as another option to protect healthy, high-value elm trees. Fungicide treatments work best when applied to healthy trees and must be repeated every few years, depending on the product.
Some fungicides require annual application. BioForest, a company that works to advance tree health in Canada, offers a fungicide, Arbotect 20-S as a preventative treatment against Dutch elm disease. BioForest calls Arbotect 20-S “the only scientifically proven fungicide that provides multi-year protection from Dutch elm disease.”, and it provides up to three years of protection once administered.
Arborists apply the fungicide through a macro-infusion injection system: they introduce a large volume of solution directly into the root flares of the tree, which ensures complete and even distribution of the solution throughout the tree’s canopy. The fungicide cannot save a tree that is already infected by the disease however.
One application of Arbotect 20-S protects the entire tree for up to three years, offering significant cost savings, says BioForest.
Another simple, benevolent act can go a long way towards ensuring a long life for your local elm trees. With climate change, Canadian cities face increasingly hot summers and frequent stretches of drought. Every elm tree, whether on your property or the city’s road allowance, will enjoy a drink. “During the spring and especially the summer months,” says Trees Winnipeg, “watering your boulevard trees will go a long way in helping them fight off any disease and pests.”