Dutch elm disease has been reported back in Denver after more than a decade of its absence.
The Denver forestry office discovered the disease last month in the old American elms along 17th Avenue Parkway in South Park Hill.
First detected in the United States in 1928, Dutch elm disease hit the U.S. population hard through the 1930’s. The disease appeared in Denver in 1948. By 1969, it had devastated Denver’s urban elms. Then the disease slowed down for a few decades, last being detected in 2011.
Dutch elm disease is a fungus aided by beetles. There are three vectors of this disease, all of which occur in Colorado: the European bark beetle (Scolytus multistriatus), the banded bark beetle (Scolytus schevyrewi), and the elm bark beetle (Hylurgopinus rufipes).
The European elm bark beetle is a small black beetle with reddish-brown wings. The beetle was inadvertently imported in the early 1900s likely via hardwood from Europe. The banded bark beetle, native to Russia, North China and Central Asia, was found in traps in 2003 in Aurora and there is evidence it had been in the state for the previous decade. The elm bark beetle is the only one native to North America.
Prior to the introduction of Dutch elm disease, the native elm bark beetle was not a major pest. The two non-native species are more aggressive with drought-stressed American elms.
The periodic resurgence of Dutch elm disease could be because once mature elms are gone — due to being killed off by Dutch elm disease or cut down to manage — the beetles have exhausted their main breeding and food source, according to Karim Gharbi, a CSU Extension Service agent for Denver County. The beetle and thereby Dutch elm disease are not seen again until the next generation of trees age.
Another option is the mutation of fungus and change to or from favorable environmental conditions. Dutch elm disease is a fungus, which means its spores need to be spread; it cannot move on its own. Given the Front Range’s cool, wet spring and summer weather, the spread could be affected by temperature and water.
Older elms or stressed ones are the preferred hosts for bark beetles, as are other stressed trees. A female beetle finds a weak spot on the top of an elm and then tunnels a straight chamber, parallel to the grain, to lay eggs. The larvae nestle under the bark over winter. In early spring, the larvae eat channels that spread out from the egg chamber feeding on the phloem and damaging the xylem. These leave the tree with marks under the bark that are centipede-shaped.
The disease spreads if it is already present in the tree. The larvae will eat that and carry the spores in their guts and on their bodies when they leave as adults. This causes the adult beetles to contaminate healthy trees nearby, which is why insecticides are less helpful in control or prevention.
The elm responds by plugging their own xylem channels to try to isolate the fungi, thus starving themselves. Unfortunately, the trees’ response is slower than the movement of the fungus. In this case, the fungus moves into the root system.
Dutch elm disease has been so devastating to elms across the U.S. due to monoculture planting as their roots are often overlapping. creating a natural root graft. Monoculture planting makes pest management harder due to the close proximity. When elms are planted with other trees, as in a native forest area, their distance from each other would lower the risk.
Dutch elm disease will first be visible in the crown, which can make it challenging to observe fast enough. The crown will show a fast die-off especially early in summer. If caught early enough, the tree can be pruned to prevent the spread of the fungus. This mostly slows the decline if caught before it reaches the main trunk. Once infected in the main stem, arborists recommend tree removal within 20 days.
Insecticide use would do more harm than good in this case. It would have to be sprayed in the crown to kill beetles. However, the beetles would be under the bark protected, so it would kill other beneficial insects, too.
Fungicide injections require treatment regularly and may impact other insects. These fungicides can only be done by a trained arborist due to the active ingredients required for Dutch elm disease. Fungicides are not effective at preventing spread through roots and are less effective if there is a present infestation. All in all, chemical management or presentation is expensive and have systemic broader negative impacts.
While there is no cure for Dutch elm disease, do not give up on American elms. They are important cultural icons and native habitats.
Elms are historically and culturally rooted in the American mind. Elms were a popular ornamental tree during the 19th and 20th centuries to line roads and driveways. When cities out west, like Denver, developed at the turn of the 20th century, elms were planted to make the semi-arid West attractive for settling. It was common for the burgeoning urban areas to follow this model of a singular type of tree to line roads or main streets for uniformity and a fast-growing canopy. An iconic example of American elms in Colorado is the Oval on Colorado State University’s campus, home to 99 elms, some dating back to the turn of the century. Many are Valley Forge varieties that are resistant to Dutch elm disease.
American elms provide native habitats for insects, birds and small mammals, all of which complete an important ecosystem, said Gharbi. In a natural native cycle, many of these insects rarely become problems as they are food for birds or predatory wasps. There are common insects that are hosted on American elms like elm sawflies and elm seed bugs or even the elm leaf beetle, which is more of nuisance insect similar to boxelder bugs. American elms are important hosts to pollinators. The spiny elm caterpillar, which as an adult is known as a Mourning Cloak butterfly, and the elm sphinx moth caterpillar both feed on elms.
Additionally, there are many more options now for disease-resistant cultivars. Denver’s forestry office recommends these Dutch elm-disease-resistant varieties of American elms: Colonial Spirit elm; New Harmony American elm; New Horizon elm; Jefferson elm; Lewis & Clark elm; Princeton American elm; and Valley Forge American elm.
There also are species of elms that are Dutch-elm-disease-resistant and not native to the United States. Those include Hybrid Asian elms such as Japanese elm cultivar Discovery or Accolade. Many do grow more slowly, do not reach the same size and have different shapes than the American elms, so read more about each before selecting.
While the loss of more of Denver’s urban elders is devastating, most of us do not need to worry about the news of Dutch elm disease’s reappearance. Instead, shift into disease-resistant varieties and good cultural practices of mixed planting.
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