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How Does Dutch Elm Disease Kill Trees: Signs and Symptoms

In the first half of the 20th century, large swathes of elm forests in countries in Europe, the US, and Canada started inexplicably dying. At first, researchers in Europe thought it was caused by the elm bark beetle, but in 1921, two Dutch researchers, working separately, found that the trees in Europe were not dying because of the beetles but because of a fungus spread by the beetles. There was nothing to identify the fungus as particularly Dutch, except that these two phytopathologists lived in the Netherlands. Thus, the fungus was dubbed “Dutch elm disease.” But how does Dutch elm disease kill trees?

These researchers discovered that elm bark beetles spread the fungus that kills the trees—either by Ophiostoma ulmi Nannf or Ophiostoma novo-ulmi Brasier. DED has killed off the majority of the elm trees that previously covered most of the US. Britain alone lost more than 25 million elms in under 30 years. Prior to the elm bark beetle’s arrival in the US, the approximate native range of the American elm tree was from the East Coast to almost the Rocky Mountains, and nearly all the space in between, north to south.

There’s still hope, though. Not all trees die if they fall victim to the infestation. And the silver lining from this pandemic is that the arborists who fought the devastation of DED pioneered what would be known as tree health care, a new industry for the management of urban trees.

Now, the focus in managing DED is not in controlling the beetles that spread the disease. It’s stopping the fungus instead. Here are four ways for you to identify, stop, and perhaps prevent DED where you see it.

Number One: Inspect Your Elm Trees Regularly

An infected tree will plug its tissues to stop the fungus from spreading through the tree with the flow of water. Because of this, leaves and branches will wilt, turn brown, and die out of season. They just aren’t getting the water or nutrients they need to keep living. Eventually, the tree will be so effective at stopping the encroaching fungus that it’ll also die because of the lack of water and nutrients. 

External Signs

Watch the leaves at the top of the tree closely. They’ll be the first to begin to wither and fall off in late spring or summer because the top of the tree is the hardest for the tree to move the water to.

Pay attention to how much you’re watering the tree too. As the summers get hotter, trees need more water anyway. Plus, there’s a recent phenomenon where leaves and sometimes whole branches will fall off a tree. In this case, though, it’s the tree trying to conserve water for itself and getting rid of weaker branches it doesn’t need. When you’re watering your trees, aim the hose, not around the trunk, but farther out, where the water would drip off the ends of the leaves when it rains. This drip line is where the roots of the tree extend to and where your watering will be most effective. Especially in the hot months, you want to water your trees deeply a couple times a week, rather than a small amount every day.

But if you know that your elm tree is getting enough water, it could be that it’s in the early stages of disease. After you notice a few yellow or brown branches at the top of the tree, it could spread to the rest of the crown of the tree and it may begin to lose its leaves. Once your tree is infected, symptoms may begin to show within one to three growing seasons.

Internal Signs

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You can also tell if your elm tree is infected by cutting into the dead branches. Take a look at the wood under the bark. It’s important to cut through the branch at an angle. This will allow you to get a better look. Straight cross-sections of the branch may not show the discoloration under the bark. If your tree is infected, there will be discoloration on the wood. There will be brown streaks in the sapwood of the current year if it was infected recently. You can tell how your tree is doing by how the disease has progressed. If your tree is fighting off the disease, there may be lighter-colored, unstained wood—new growth covering the infected wood.

Also, you can tell if the tree is infected if you see the telltale markings of the elm bark beetle. When the eggs hatch, the larvae chew their way through the tree, leaving a distinctive mark.

Number Two: Clear Out Old Wood That Might Be Infected, and Separate Trees If One Is Infected

If there’s an elm tree nearby that must come down because of the disease, make sure that it’s either completely destroyed or taken away. This will prevent the fungus from contaminating any nearby elm trees. The two most effective methods are fire or complete burial. Many folks consider a felled tree prime material for creating something new—woodwork, outdoor furniture, etc. But if that elm tree was felled on purpose to prevent the spread of DED, don’t reuse the wood. If someone uses that wood for other purposes, they could be acting as an agent of the fungus and spreading it to other trees.

Elm trees also create grafts with each other through their roots, in order to share resources. If one of these trees becomes infected, it’s possible to transfer the disease through these grafts to other healthy trees. Separate these root systems if one elm tree falls to the disease. Otherwise, you may find you’ll have to take down more than one elm tree.

Number Three: Focus on Stopping the Fungus, Not the Beetles

This has been a successful mindset in saving native elm trees, especially in Canada. Pesticides aren’t effective and often raise concerns about affecting the surrounding wildlife more than the beetles themselves. Insecticides can be somewhat effective at killing the adult bark beetles, but it doesn’t always kill the larvae, so you must keep using it, which is not cost-effective or efficient.

Fungicides have proven to be the most effective method, though they can be expensive. If you decide to take this route, make sure you hire an experienced arborist. And if you’ve worked with Mr. Tree before, you know we can provide services during your DED woes. The fungicide will need to be reapplied every couple of seasons, depending on how susceptible your tree is to the fungus.

If your elm tree is susceptible to the disease, it might be a good preventative measure to inject the fungicide even if it hasn’t become diseased. This will help the tree fight off the disease before it becomes a major problem—a vaccination of sorts. Sometimes the disease will also infect only part of the tree. When that happens, inject fungicide into the parts that haven’t yet become infected. This will stop the disease from spreading farther.

Unfortunately, there are several different chemicals used in this procedure, and none of them are completely effective. However, if you inject the tree before it becomes a major problem, and if you watch for new colonies of the fungus, you can keep your tree healthy for longer than if you hadn’t done this preventative maintenance. These injections, which can become expensive quickly, are usually used on elm trees with significance (such as along the Mall in Washington, DC) or with historical worth.

Number Four: Breed Trees for Resistance

The short-term solutions to Dutch elm disease management lie in the fungicide. The long-term management plan lies in breeding elm trees that are resistant to the disease altogether. Asian elm species have moderate to high resistance already and are compatible when breeding into American elm stock. This means the resulting tree has the beautiful shade-tree shape but also a higher resistance to DED. If you have an inclination toward growing an elm tree, ask where the tree came from and whether it has a natural resistance to the fungus.

At Mr. Tree, we aim to provide full-service help for your trees, whether you’re facing Dutch elm disease or end-of-summer clean up. Visit us today to see how we can make your yard something you’re proud of!