Disease threatens summer shade tree

SALEM, Ohio – Sitting outside on a summer’s day enjoying the cool shade of an elm tree is becoming less common due to a fungus threatening the trees’ existence.

This fungus and the disease it causes, Dutch elm disease, is one of the most devastating shade tree diseases in the United States, and the disease has not left Ohio unmarred.

Taking over. The disease, which strikes Dutch and American elms, is transferred by bark beetles that carry the fungus spores from infected trees to healthy ones.

When the fungus enters the trees’ branches, it induces a defensive mechanism and the water vessels swell and produce a gum. This means water cannot nourish the tree and it ends up wilting and dying in a relatively short amount of time.

Ohio’s infestation. The disease is widespread in Ohio and a constant nuisance, according to Pierluigi Bonello, OSU Extension specialist in tree pathology.

“It’s a chronic problem,” he said. “It has been here for a long time and will likely never go away.”

Although the disease was first identified in Belgium in 1918, it didn’t enter this country until 1930. Its initial entry point was Cleveland.

The bark beetles were exported to the United States on elm logs for furniture and bridges.

How to tell. The first sign of Dutch elm disease, Ophiostoma novo-ulmi, is branches that begin to fade and turn yellow. The foliage then wilts and the symptoms generalize to the entire crown of the tree.

The fungus continues to overtake the tree by creeping into the trunk and then roots.

There are no guidelines on how fast the disease moves or how long it takes the tree to die from the time it is infected, Bonello said. However, water-stressed trees are at a higher risk for the disease.

The beetles attack the main stem of diseased trees and mate. In the spring, the new adults emerge from the dead tree with the fungus spores and travel to healthy trees – thus transmitting the disease.

Although a fungicide can be injected into the trunk of the tree to prolong the elm’s life, there is no “cure” to save infected trees, Bonello said.

There are, however, ways to deter infection in surrounding healthy trees, including removing the beetles’ habitat, the bark, so they cannot carry the disease.

Prevention. Bonello offers the following for preventing further infestation:

* Infected trees can be cut down, but the bark must be removed. Because they are bark beetles, they cannot form new colonies if there is no bark. Without the bark, they don’t have a breeding ground.

If the bark is not removed and the tree is chopped into firewood, the beetles can still colonize and find healthy trees, thus spreading the infection.

* The diseased tree can be chipped for mulch. This is the most economical way to dispose of an infected tree; however, the mulch should not be used around other elm trees.

* The tree can be buried.

* Infected trees can be burned.

* A compound called Vapam can be injected into the soil which will chemically sever the root grafts. In addition to beetles, Dutch elm disease can easily transfer tree-to-tree through the roots. Thus, the root grafts must be severed to prevent nearby, healthy trees from becoming infected.

Vapam is a biocide and turns into a gas in the soil. It makes a strip of sterile soil, which then kills the grafts. Once the root tissue is dead, the fungus cannot enter.

This is the least environmentally friendly way to dispose of an infected elm.

* The root grafts can also be severed by a vibratory plow, which few companies have access to. This must be done by a professional.

The plow goes as deep as 60 inches into the soil to cut the root system. A regular trencher will not go deep enough to cut the roots.

* Pruning is possible to rid the elm of infection if the branch is cut back far enough – past where the fungus has spread. However, this is not an effective way to stop the infection because it is impossible to know where in the tree the fungus has traveled.

“Fertilization does not help, in spite of what arborists may say,” Bonello said. “There’s no evidence of this and, in fact, it may make things worse.”

Samples from all over the state are collected at the OSU diagnostic clinic, http://ppdc.osu.edu/.

(You can contact Kristy Alger at 1-800-837-3419, ext. 23, or by e-mail at kalger@farmanddairy.com.)


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