The invasion of devastative plant diseases has altered entire nations, their customs, and their development. A plant disease, for example, caused the British to become such great tea consumers.
In 1870 in Ceylon, a highly destructive rust disease forced plantation owners to abandon growing coffee and turn to grow tea, a plant that was not susceptible to rust and could grow profitably.
In South America, the invasion of a fungus parasite of the rubber tree, hevea, made rubber production unprofitable and stimulated a search for new growth areas.
As a result, the extensive rubber plantations in Malaya and other southeastern Asiatic countries came into being.
More than a century ago, the blight of white potatoes was primarily responsible for the death of over a quarter of a million people and the mass migration of hundreds of thousands of Irish to the United States.
Four Diseases Of Trees
In the past 50 years, four diseases of trees have altered the character of American forests and the American landscape.
Blight has destroyed vast stands of chestnut. Two diseases, phloem necrosis and Dutch elm disease, have preyed upon the American elm. And last of the four is oak wilt, now spreading through the Midwest.
1. Chestnut Blight Disease
Chestnut blight, also called chestnut bark disease, is the best-known and oldest of the group.
It has caused the rapid disappearance of the American chestnut, one of our best ornamental forests and nut trees.
This disease was believed to be of minor importance when first reported by the late Herman Merkel, who found a few infected trees at the Bronx Zoo in 1904.
However, since then, the blight fungus has practically wiped out the chestnut stands in New England forests and along the eastern slopes of the Allegheny and Blue Ridge mountains, the principal range of this host.
The blight has been found in the chestnut orchards of the Pacific Coast in Washington, Oregon, California, and British Columbia.
Even now, it is spreading rapidly in Italy, where it was first reported in 1938. Chestnut blight has also been found in Switzerland and Yugoslavia.
Today some large chestnuts still stand in the extreme southern and western parts of the tree’s natural range—in Tennessee, Georgia, and South Carolina—but it will be only a matter of time before they, too, will succumb.
The roots of trees infected with chestnut blight remain alive for years after the tops have been killed.
They send sprouts that grow for 8 or 9 years, some even producing nuts before succumbing.
The appearance of such saplings gives some people the mistaken impression that the American chestnut is coming back.
Controlling Chestnut Blight Disease
Although many attempts have been made to control the disease, all have failed. One reason is that the fungus spores are spread by insects, birds, rain, and wind.
Another is that the chestnut blight fungus also infects rather commonly another forest tree, chinquapin.
The fungus has been found growing on red maple, shagbark hickory, staghorn sumac, and dead and dying white, black, post, and chestnut oaks.
Although many suggestions and recommendations have been published, none have proved effective in controlling chestnut blight.
Persistent efforts have been made to find some chemicals that, when injected into the tree, would check the development of cankers.
Numerous claims for such chemicals have been advanced, but I do not know of the existence of any effective material.
The possibility of developing resistant or even immune varieties by selection and breeding has been investigated for some time by amateur and professional plantsmen.
Chestnut Seedlings
At the moment, I have about a dozen two-year-old chestnut seedlings collected by Mr. W. E. Goyne of Mamaroneck, New York, which were grown from nuts from a resistant tree.
Only time will tell whether these seedlings can ward off the disease. The United States Department of Agriculture has been collecting chestnuts from every part of the world.
Among those collected, the Asiatic chestnuts are somewhat resistant to blight and thrive in the natural range of the American chestnuts.
Several crosses between the Asiatic species and the American chestnuts also look promising.
2. Dutch Elm Disease
Dutch elm disease, a fungus disease of another important tree, has been in the limelight more recently.
This disease is believed to have entered the United States in the late 1920s on burled elm logs from Europe.
After killing thousands of trees in the eastern United States, it has spread to the Midwest, the Southwest, and the province of Quebec, Canada.
After more than 20 years of research, no positive control has been developed. The only control is indirect, that is, against the insects (bark beetles), which are the principal disseminators of the fungus.
One way to control the insect carriers is to search and destroy their breeding places. These include all dead, drying, or devitalized elm material such as sick trees, hurricane-damaged ones, broken limbs, and woodpiles of elm and elm fence posts.
European Elm Bark Beetle
For a thorough job, such material must be removed from a relatively large area because the smaller European elm bark beetle, the principal disseminator in this country, frequently flies more than three miles.
The long flights are usually made in search of suitable breeding places, but these beetles may also feed on living trees.
They have been found to carry viable Dutch elm disease fungus spores for more than two miles. The elm bark beetles breed and feed on all species of elm that grow in this country.
Emulsion-Type Spray: Insecticide on Elm Trees
The best way to prevent the movement of these beetles is to apply a residual-type contact insecticide on elm trees. The emulsion-type sprays containing DDT or methoxychlor are the most effective.
Coverage is fully as important as spray concentration, and it can be achieved only with adequate equipment handled by skilled operators under proper weather conditions.
Two applications are necessary, one before the leaves appear and the second about July 1. The pre-foliar (or dormant spray) is far more important because the elms are most susceptible to the disease in spring and early summer.
The excessive use of DDT has led to problems such as an increased population of Putnam scale, mites, and aphids.
However, these pests can be overcome by combining other insecticides and miticides with DDT.
Chemicals Can Control Dutch Elm Disease
As with chestnut blight, several chemicals are claimed to control or prevent Dutch elm disease.
At this time, I am unaware of any that will give control. The latest material to be widely publicized is mercapto acetate, which shows some promise as control when injected into elms.
When used as a foliage spray, this material will prevent the appearance of disease symptoms on affected trees for 3 to 6 weeks.
But I believe it is too early to become optimistic about chemical control for Dutch elm disease.
3. Phloem Necrosis
Phloem necrosis is a disease even more deadly to elms. This disease, now present throughout the Midwest, was first identified as a virus in 1938, following investigations of the death of many elms in Ohio.
The present eastern limit of the disease is a line connecting Marietta, Ohio, Charleston, West Virginia, and Chattanooga, Tennessee.
With the appearance of the disease in the latter city, arborists in the Northeast, where the disease has not yet appeared, are becoming concerned.
Chattanooga is beyond an area of sparse elm growth, which, it had been hoped, would serve as a barrier to the invasion of the disease.
The external symptoms of phloem necrosis are much like those of Dutch elm disease, but the internal symptoms are entirely different.
Phloem necrosis causes a yellow to butterscotch discoloration in the phloem or inner bark but no discoloration in the wood.
On the other hand, Dutch elm disease produces a brown streaking in the wood but no discoloration of the inner bark or phloem.
The elm leafhopper, Scaphoideus inzeolus, is known to be capable of transmitting the phloem necrosis virus.
Two Strains Of The Insect
Actually, there are two strains of this insect in the Central States, but only one can transmit the infectious material.
Currently, the best preventive practice is to use DDT sprays to control the elm leafhopper.
- The Christine Buisman elm, a European selection of the smooth-leaved elm,
- Minus carpinifolia, has proven to be highly resistant to phloem necrosis and the Dutch elm disease.
The United States Department of Agriculture has distributed specimens of this elm to arboretums and nurserymen around the country, and young trees are now available from some nurseries.
4. Oak Wilt Disease
Oak wilt, the newest and most highly publicized tree disease, is also caused by a fungus.
There was a complete story on this disease in this column in March 1954, so I will not devote much space to it now.
At present, oak wilt disease has been found in nineteen states, from Kansas and Nebraska eastward to Pennsylvania and from Minnesota southward to Arkansas. It has not yet been found east of the Susquehanna River.
Very little is known about how the oak wilt fungus spreads. Several species of insects, including the fruit fly and three kinds of bark beetles, are known to be capable of spreading it. They appear to be most effective in disseminating the fungus during May.
Tests have been made recently to determine whether tools used for tree care could transmit the fungus.
It was found that half of the trees received fungus inoculum in chisel wounds, and half of those wounded on the trunk with an artificially contaminated axe or saw developed oak wilt.
Transmission of Fungus
Transmissions from inoculations made in June and July were less successful than those made from February to May.
The fungus was found to be transmitted rather readily on the tools used during some study periods if it was artificially contaminated. But there was no transmission with tools used on wood from diseased trees.
This failure to contaminate such tools would suggest that either the fungus was not present in that portion of the diseased wood in which the tool was used, or if present, it did not adhere to the tools in a manner that would permit successful transmission.
Efficiency Of The Agents In Spreading The Fungus
As I wrote in this column a year and a half ago, I believe that several factors beyond man’s control will determine eventual success or failure in controlling oak wilt.
Among these are the number and efficiency of the agents spreading the fungus. The very obvious absence of any highly efficient means of spreading offers hope.
Wilt will not do to oaks what blight has done to chestnuts or what phloem necrosis virus and Dutch elm disease fungus are doing to elms.
44659 by P. P. Pirone