Dutch Elm Disease

Within just a few years, entire forests disappear. Dutch elm disease (DED) spreads, and elms die, their healthy green foliage withering and replaced by a sickly yellow. Forest Research, a British government forestry agency, describes this lethal fungal disease as “one of the world’s most serious tree diseases,” which is killing trees by the millions across the United States and the United Kingdom.

The DED fungus spreads through root grafts (natural connections between the roots of separate trees), elm bark beetles, and human activities such as moving or storing elm wood. Freshwounds from pruning can also attract elm back beetles, increasing the chance of infection. A visual symptom of the disease is a wilt or droop on a particular branch. These flagged leaves start to turn brown because the fungus spreads into the vascular system which blocks the tree’s water supply.

First appearing in the Netherlands in 1910, DED wreaked havoc across northern Europe; the first epidemic caused the loss between 10 to 40 percent of elms in European countries. There was a brief relief period in the early 1960s, but a few years later elms again became endangered, starting the second and much more deadly outbreak of the disease, which killed tens of millions of elm trees across the UK. This second outbreak was shown by the UK Forestry Commission research to be “an entirely different, far more aggressive species of DED fungus that had been imported into Britain on elm logs from Canada.”

In 2023, the US Department of Agriculture affirmed that this disease eventually made its way to the U.S. on logs imported from Europe. What used to be one of the largest and long-lived trees in the U.S., the American elm (Ulmus americana) is incredibly susceptible to the disease and was on a sharp decline in the 1970s, according to The Nature Conservancy (TNC). This is particularly due to their period of infection being often at the same time as a thirty-day period of rapid terminal growth.

Only around 1 in 100,000 American elms are immune to the disease. While many American elms died, there are species of elms like U. parvifolia (Lacebark elm) and U. pumila (Siberian elm) which have a high resistance to the disease. However, American elms tend to be the larger trees in their genus. Thus, it is the branches of American elms that rise above smaller fauna in floodwaters to provide homes for ospreys, eagles, barred owls, and other waterfowl to make nests.

The problem of DED is just as big of a concern over the border in Canada; the Red-Tail Land Conservancy states that “most known survivors have just escaped exposure.” The largest surviving urban forest of American elms is located in Winnipeg, Manitoba, made of around 200,000 elms. Each year, the city of Winnipeg spends $3 million fighting off and mitigating the disease. One of these expenses is on the Dursban Turf and Dutch Trig vaccine that helps to combat the disease. Yet, the Winnipeg forest is still losing around 1,400–1,500 trees a year.

Since elm bark beetles are a potent spreader of the disease, in areas like Portland, Oregon, elm pruning has a strict season that coincides with when the beetle isn’t active. An additional precaution Portland takes is that they require that elm wood be burned and buried and forbid it being left inside the same storage rooms as firewood. In some areas, elms have been able to build a resistance to the disease, and there is hope that there will be new generations of American elms that will also develop immunity.

There are also organizations focusing on not just mitigating the disease but also developing new, more disease tolerant versions of elms. The Nature Conservancy (TNC) is working with the United States Forest Service (USFS) to create a genetically diverse elm species that is disease-tolerant. TNC ecologist Dr. Christian Marks has recently formally begun to develop new strains of elms being planted in the Connecticut River Valley. Tens of thousands of elms are being planted, making this the largest elm project in the northeast United States.

Projects like this help to keep endangered species like the American elm able to continue to have a fighting chance against invasive diseases. Elms are not the only trees that are endangered. Chestnut trees are facing a similar disease called chestnut blight, which was introduced to the west coast of the U.S. from Asia in the early 1900s. Many everyday flora are struggling, choking from an invasive species called oriental bittersweet, a vine that smothers plants and uproots trees due to its heavy weight.

All these cases raise a concern that has to do not just with losing specific species but with losing plant diversity in whole habitats. According to the Smithsonian, benefits of tree diversity in forests include “pest and disease resistance, resilience to climate change and increased wildlife habitat.” The museum cites studies that have shown a positive correlation between biodiversity and ecosystem function. Maintaining a level of diversity allows forests to continue to sustain life, allowing for a healthy population of interdependent flora and fauna to flourish.

This biodiversity not only helps the forest dwellers, but also humans. Healthy ecosystems provide the world with around 75% of its freshwater, and over 50% of medicines are derived naturally. In fact, biodiversity loss costs society money. The global economy has lost around $423 billion each year due to invasive alien species, which contribute to 60% of species extinctions, according to a 2025 World Health Organization report. Humankind depends heavily on healthy, functioning environments that can only be effective when there is a stable level of diversity.

Diseases like the Dutch elm disease threaten species into extinction, but these species are also important to the environments they are a part of, which are critical to animals and humankind alike. Organizations like The Nature Conservancy and towns like Portland, Oregon are doing their best to help mitigate the disease and develop new ways for elms to bounce back in numbers and to reestablish themselves as a more prominent part of their ecosystems. Hopefully, in the coming years, there will be more efforts pushing for the conservation of these endangered species threatened by fatal diseases.