If you have an American elm and you value it highly, the time is near when you may need to have it sprayed to save it from being destroyed by Dutch elm disease. Chinese elms are immune to the disease.
Since the 1930s millions of American elms have been killed by a fungus that is spread by elm bark beetles. The beetles breed under the bark of dead and dying American elms, and emerge soon after the leaves appear on the trees in the spring.
The beetles fly to nearby elms to feed on the tender barks of twigs, usually in twig crotches. They are likely to carry spores of the fungus on their bodies and when they nibble into the bark the tree becomes infected.
Once within the healthy tree, the fungus grows until it chokes the water-conducting vessels, actually strangling the elm.
Specialists at USDA Agricultural Research Service say the most effective way to protect the elm is to to spray it in the spring to kill the bark beetles before they can cause infection.
It is not a do-it-yourself project unless you have adequate equipment and know-how. The job must be done thoroughly because if even a small area is left unsprayed the tree is vulnerable. The spraying should be done when the wind is not blowing. Otherwise it is difficult to provide adequate coverage.
In 1977, the Environmental Protection Agency approved a new fungicide, Arbortect 20-S, that can help save a newly infected tree so long as the disease isn't too far advanced. The chemical is injected into elm trees - a job for those who have been trained in using it.
In the spring of 1976 a fungicide called Lignasan BLP was approved by EPA for treatment of Dutch elm disease.
Therapeutic (cure) treatment should be made as soon as symptoms of infection appear, usually late June through August. The tree should be treated before 5 percent of the crown shows wilt.
First symptoms usually show up in the leaves on branches where beetles have fed. The leaves wilt, turn yellow and then brown, and fall off prematurely.
The control of Dutch elm disease requires a comprehensive, four-step program. Here are some recommendations:
Since dead or dying elm trees provide breeding sites for beetles which transmit the fungus disease, it is important to remove them. The dead wood should be burned, chipped or debarked.
Don't prune your elms when beetles are active, usually May and August. They will be attracted to the wounds and carry the fungus directly into the tree.
Trees should be sprayed with recommended insecticides to prevent beetles from attacting and infecting healthy trees.
Eliminate root contact between diseased and healthy trees.Digging a trench between the trees will do the job.
Finally, to fight the disease-causing fungus itself, have a trained person apply fungicide according to label directions.