You, Mr. and Mrs. Homeowner, may have much to do with the spread of Dutch elm disease, which has been spreading rapidly throughout several eastern states during the war years. In Connecticut, the disease is now found in every county.
An outbreak has been discovered in the Province of Quebec, Canada, and a section of the province has been put under plant quarantine.
Specific cures and preventions of the disease are, no doubt, in the making. At present, however, simple sanitation measures constitute the best-known control. And these, to a large extent, are up to the individual homeowner.
Some federal and state control work is going on, but it is impossible to eliminate all local sources of infection by these means.
What are these “sanitation” measures the experts say are up to the individual?
First, let’s take a look at the disease itself and see how it is spread. Then we’ll better understand the reasons behind the recommended measures.
Dutch Elm Disease
Dutch elm disease is caused by a fungus that attacks the water-conducting system of the tree. It obstructs the tubes with a brown gummy substance, so they cannot function normally. This stoppage, in turn, affects the foliage.
In mild cases, the leaves wilt and become discolored. In severe cases, the leaves fall off, resulting in part or all of the tree dying.
If you doubt whether your tree is infected with Dutch elm disease or some other problem, send a sample of the branches that show symptoms to your state agricultural experiment station for identification.
The Dutch elm disease fungus is transmitted from one elm tree to another by the elm bark beetle, which feeds and breeds in the bark of trees of this species.
Diseased wood is particularly attractive to beetles when they are ready to construct their brood galleries.
After they emerge, the young beetles may travel to healthy trees to feed on small branches or twigs. Unfortunately, when they do so, they carry Dutch elm disease spores with them.
Bark Beetles
The bark beetles are the only known agents that transmit Dutch elm disease; therefore, preventing their emergence is the most effective means of protecting trees from infection. The surest method is to remove and destroy all diseased wood.
Since badly diseased trees must come down eventually, removing them before they become breeding places for the bark beetle is only smart. In some cases, however, you needn’t be quite so drastic.
There are two peak periods of bark beetle emergence and, consequently, of Dutch elm disease infection.
The first occurs in the spring and the second in the late summer. The spring or early summer infection is the most serious.
Use Beetle Repellent
If foliage wilts in the spring, the tree should be cut down immediately and burned, debarked, or sprayed with a beetle repellent.
Remove and destroy the bark from the trunk and larger branches in debarking, and burn all the smaller branches.
If a beetle repellent is used, cut off and burn the smaller branches and cut the trunk and larger limbs into lengths that can be handled and sprayed easily.
A good beetle repellent is coal tar creosote; another is a mixture of 12 parts of light fuel oil (26° to 28° Baume) to one part of monochloro naphthalenes; a third is a mixture of 4 parts of light fuel oil to one part of ortho dichlorobenzene.
Cut Elm Wood
The late summer infection may be curbed by more moderate means. Where the disease is confined to a few branches, good results sometimes follow the pruning of dead or diseased limbs. But be sure the cut is well below the last visible brown streaking of the infection.
It is always a potential menace if elm wood is cut for fuel or other purposes and is left where the bark beetles can reach it. It should be stored in beetle-tight sheds or cellars or debarked or sprayed with one of the abovementioned preparations.
If cut elm wood is not to be used, it should be destroyed. Bark beetles do not emerge in the winter, so there is no danger from woodcuts after mid-October. Such wood should, however, be taken care of before mid-May of the next season.
“That’s fine,” the homeowner may say. “But suppose my neighbors and I do all these things. What about bark beetles that may fly in from ‘wild’ elms? Forest trees, for example.”
Scientists working on the disease have an answer for that, too. Ordinarily, they tell us, few bark beetles will travel much more than 1,000′ feet to find new feeding places.
So elms in forests or fields are not much of a menace, and tree men are not too worried about control measures there.
Isolated Elm Trees
You can’t consider yourself safe simply because your elm tree is isolated from diseased trees by distances of more than 1,000’ feet.
It all depends on the number of intervening elms between your healthy tree and the diseased ones and the number of bark beetles at the infected site. If there are 10,000, say, some are bound to try their wings a bit farther than the 1,000-foot limit.
So much for immediate control measures. Now let’s take a look into the future and see what the scientists may have in store for us in the way of new methods of combating the disease.
Controlling Elm Disease
At the Connecticut Agricultural Experiment Station in New Haven, plant pathologists have been working for some years to control Dutch elm disease by internal therapy.
This consists of applying chemicals directly to the tree’s internal system, hoping to kill the fungus or at least slow up its destructive action. Though still in the experimental stage, results thus far look promising.
Dr. George A. Zentmyer, Dr. James G. Horsfall, and Philip P. Wallace, who has been responsible for the work here, report that several organic “medicines” can be applied internally to small elm trees in sufficient amounts to prevent or retard the progress of the vascular fungus involved.
Work with large elms, comparable in size to the valuable shade trees lining the streets of many American towns and cities, has begun this year.
Chemical Therapy
Oxyquinoline Benzoate
One of the most promising chemicals used thus far is oxyquinoline benzoate. This, and other materials, may be applied internally in three ways.
The first method, and the one used in the initial experiments, consists of boring a hole in the tree and inserting a capsule of the “medicine.”
The second means of getting the material into the tree is by boring a hole in the tree, threading in a pipe, and attaching this to a suspended bottle containing the chemical dissolved in water.
The bird method, however, is the most simple. First, the chemicals are watered on the soil and taken up through the root system.
This method will probably be adopted for practical use if and when experimentation shows comparable results on large trees to those in the laboratory and small tree field trials.
Two approaches to the problem of Dutch elm disease control by chemical therapy have been tried. One is by applying materials to diseased trees to cure them.
The other attempts to immunize healthy trees by treating them before they become infected, results thus far seem to indicate that the latter method is the more likely to succeed.
On already diseased trees, results tend to show that oxyquinoline benzoate, or one of the other promising chemicals, will delay the appearance of the symptoms of the disease rather than kill the fungus.
On healthy trees, however, indications to date are that the chemicals will give a fair degree of immunization for at least one season.
Applying Chemicals
What about applying chemicals to the outside of the tree by the more conventional spraying methods?
Scientists worked on this for many years before the internal treatment method entered the picture, but the older approach offered far less promise.
Recently there have been experiments with DDT to control the bark beetle carrier, but no good results have been obtained.
Breeding of Resistant Elms
The breeding of resistant elms is another means of solving the problem that has been attempted. Geneticists in Holland have been working for many years along these lines.
Thus far, the most resistant type they have found is the Christine Bnisman, a seedling selection from Ulmus carpinifolia. Small numbers of this elm are now being grown in experimental plots in this country.
The U. S. Department of Agriculture has also been conducting breeding projects in search of resistant types of elms. The American elm is among the most susceptible species, the English elm somewhat less so, and the Siberian elm may be classed as resistant.
However, the disadvantages of the last mentioned species are its inferior appearance and high breakage rate.
The U.S.D.A. has selected superior American elm seedlings and crossed them with Siberian elm stock in the hope of producing a hybrid combining the best characteristics of each. Thus far, unfortunately, the results have not been very satisfactory.
To sum up what we have said: The field of most promise in the control of Dutch elm disease is chemotherapy or the internal treatment of trees with chemicals.
The day may come, therefore, when you can sprinkle some chemicals around your prized elms each year and be sure they will be safe for the coming season.
As yet, however, that day has not arrived, and in the meantime, sanitation measures by individual homeowners are the surest means of saving our elms.
Infected Areas During 1920s
Dutch elm disease, probably from Holland, entered this country during the 1920s. Since then, it has spread over the northeastern coastal section, from Massachusetts to Maryland and westward into New York State and Pennsylvania.
Secondary infection areas are present in Indiana and Illinois, and there is an isolated infection in the Province of Quebec.
In the generally infected area, 1 percent of the trees have been killed. In regions with the most serious disease, 4 to 5 percent have been killed, and 14 to 15 percent are diseased.
It does not follow, however, that all our elms are doomed, as was the ease with the chestnuts when the blight struck that species.
The Dutch elm disease’s spread rate has not been nearly as rapid. It is undoubtedly true, however, that many prized trees will be lost.
Through community effort, many can be saved. Be your scout for Dutch elm disease, and report suspected trees to the proper authorities. Destroy or, in slight cases, prune thoroughly all trees known to be diseased.
Keep your elms healthy by protecting them against attack by defoliating insects. Clean up Elm woodpiles.
Responsible homeowners cooperating in such a program can greatly minimize the Dutch elm disease threat.
44659 by Amanda Quackenbusr