Choosing our battles: Dutch elm disease is a fight we can win
As the fight against our latest invasive pest, emerald ash borer, is now being waged in Winnipeg it may be easy to forget about our older and more familiar foe – Dutch elm disease. Over 8,000 Winnipeg trees were marked for removal during the 2017 surveillance season and we anticipate the same for 2018.
Not to detract from the latest news of emerald ash borer, it’s still important not to lose sight of the battles we can win. In the case of Dutch elm disease, all is not lost. It’s a disease that can be successfully managed, and elms can remain a permanent feature on our landscape.
A quick review of DED
Dutch elm disease (DED) is caused by an invasive fungus from Asia. Elm bark beetles are unwitting carriers of the sticky fungal spores as they travel from one elm tree to the next, and the fungus quickly spreads into the tree’s vascular system, blocking the vital tissues that transport water and nutrients. The result is sudden wilting and premature leaf drop in the canopy, and the disease can kill a tree within a single season.
Keeping up with the disease
The dreaded “orange dot” has become an all-too-familiar sight on our streets, but boulevard trees account for only a small fraction of all the elms we lose. In fact, 80 per cent of elms lost to DED are located on private property and these can be some of the most difficult trees to remove. Private trees can be wedged against homes, garages, sheds, fences, or have powerlines overhead – in some cases, all of the above may apply at once! These cases require special attention and co-ordination with utility companies, and are often the trees that end up at the bottom of the removal schedule. Elms along river banks can also be a difficult reach and are usually only accessible in the winter when removal crews can take advantage of the frozen rivers. To date, a total of about 670 trees from 2015 and 2016 are awaiting removal in Winnipeg. It’s largely accepted that these lingering elms are responsible for the creation of new DED “hot spots” the following year and contribute to the rising infection rate.
Turning the tide
It has become clear that the rapid removal of infected elms is one of the best strategies for keeping the disease under control. It’s not feasible to remove all 8,000 elms at once, but those trees which harbour the most elm bark beetles would be top candidates for a rapid removal program. The University of Winnipeg is preparing for its second summer of research to try and determine just that – how to quickly identify which trees should be removed first. In the meantime, the city of Winnipeg is implementing a three-year plan to get on top of the DED removal backlog, with the goal of getting the 2015, 2016 and about 60 per cent of the 2017 marked elms out by the end of 2018.
The battle isn’t all about removals though – fungicide injections have been used in Winnipeg for years, helping homeowners preserve highvalue elms on their properties. Tree planting is another crucial part of the DED management equation. The city of Winnipeg plants about 2,000 trees per year, but it’s up to homeowners to plant trees on private property. We know that tree species diversity helps make our urban canopy more resistant to DED and other pests. Fortunately, we have better tree selection than ever before. Hackberry, buckeye, butternut, alder, Amur cork tree (and more!) are showing up in private yards, breaking up the elm and ash monocultures of decades past.
Are we still planting elms? Of course! Our native American elm is one of the hardiest trees in our region and grows exceptionally well in our urban environment (gardeners know they grow pretty much anywhere – even eaves troughs). Not many trees can boast a life expectancy of a hundred years. Elms can live for decades before succumbing to disease. Many have been part of our neighborhoods for over a century and are still standing despite the spread of DED.
It’s crucial that we don’t let down our guard and give up on our elms. To “win” the battle with DED is not to completely eradicate it, but to manage it knowing that invasive species are simply part of our new reality, and adjusting our expectations for our trees accordingly.
The University of Winnipeg dissects diseased elms to learn more about the beetles within.
The familiar orange tag of a diseased elm.