Some Others

Oak Wilt, a New Threat

Theodore W. Bretz.

Oak wilt is a serious threat to our noble oaks, the lovely trees that since Colonial days have meant much to the economy and development of this country, account for one-third of the hardwood saw timber stand in eastern United States, and are highly valued as ornamentals and as sources of food for wildlife.

Oak wilt has received increasing attention since 1940 because of the damage it has caused in shade oaks and wood-lot and forest oaks in Iowa, Wisconsin, and Minnesota. Since 1947, it has been found in several other Midwestern and Appalachian States from Pennsylvania to North Carolina. It is infectious and destructive, and many foresters, arborists, conservationists, and plant pathologists are concerned about it.

We do not know how long it has been in the United States. An epidemic riving of oak, observed in southern Wisconsin and southern Minnesota 40 years ago, was ascribed to various causes adverse climatic conditions, insects, and other diseases. But investigators of the Wisconsin Agricultural Experiment Station and the Department of Agriculture established the fungus nature of oak wilt in 1942, and it seems probable that the disease may have been responsible for at least some of the earlier mortality of oaks in that area.

Until the summer of 1949, oak wilt was known only in Wisconsin, Minnesota, Iowa, Illinois, and east central Missouri. In 1949, forest pathologists found it in the northern Ozark region of southern Missouri and in northwestern Indiana.

Funds obtained from the Forest Pest Control Act and the Arkansas Resources and Development Commission financed a limited survey by Federal forest pathologists in 1950, mainly in the Ozark Mountains. Low-flying airplanes were used to locate suspected infected trees in the forests. Ground crews then examined the suspected trees and collected specimens for laboratory confirmation of the field diagnosis. As a result of that work, the oak wilt was found to be widely scattered in southern Missouri and in northern Arkansas. Other workers found the disease in a few isolated locations in Kansas, Nebraska, Ohio, and Pennsylvania.

Airplane scouting for oak wilt has proved to be satisfactory, fast, and economical. An aerial observer's most effective survey for wilt is limited to about one-quarter mile. Two observers in a plane can usually survey a strip one-half mile wide through the forest. A third member of a crew can greatly speed the work by keeping the position located at all times on a map and indicating on it the location of suspected trees seen by the observers. Landmarks can also be noted that will be useful to a ground crew who may later visit the area to collect specimens for laboratory culturing.

The development of the aerial-scouting technique to locate oak wilt and the discovery of the disease in the Appalachians brought greatly expanded survey activities in 1951. Federal, State, and private organizations participated in the surveys, and their efforts disclosed the presence of the disease over much of our hardwood area. Besides the States I mentioned, oak wilt was found in Michigan, West Virginia, Kentucky, Tennessee, Virginia, Maryland, and North Carolina.

Oak wilt was not found in any additional States in 1952. It was found in many new locations throughout its range, however, with a notable increase in the number of known infection centers in Ohio and in Pennsylvania west of the Susquehanna River.

Upper left: Discoloration of the inner bark, characteristic of elm phloem necrosis. Upper right: Brown needles on western white pine are a symptom of the pole blight. Lower left: Wilting and discolored leaves are symptoms of deadly oak wilt disease. Lower right: A chestnut blight canker, the foreign destroyer of American chestnuts.

OAK WILT PRODUCES noticeable and characteristic leaf symptoms, which vary somewhat with the species affected. The diseased trees are most conspicuous from mid-.June to mid-September. In species belonging to the red and black oak group, symptoms usually appear first in the top of the tree and at the ends of the lateral branches and progress rapidly downward and inward through the entire crown. The leaves first become dull or pale green, curl upward, and become stiff. They turn yellow or bronze from the apex and margins inward.

The blade tissue next to the petiole is the last to turn brown. Affected leaves may fall from the tree at any stage of symptom development. Defoliation may be slight or nearly complete. Some leaves may remain on the tree to the end of the season and a few may even persist until the following summer. Sometimes sucker growth, in the form of dense clusters of large, succulent leaves, develops on the trunk and larger branches before an infected tree dies. The bark of wilt-killed trees loosens rather rapidly and by the end of the second year following death may begin to shed from the bole.

In white and bur oaks the leaf symptoms are often much more localized. Usually the entire tree does not wilt at once. Individual branches in any part of the crown may develop leaf symptoms, while leaves on the unaffected parts of the tree remain green. The affected leaves may be tan or dark green and look water-soaked. They tend to remain on the branches after they die. The killing of individual branches over a period of years results in stag-heading. A brown or black discoloration in the outer sapwood just under the bark is sometimes present in the twigs and branches of infected trees. When one peels back the bark he can see a diffuse brownish discoloration or longitudinal streaks. In cross section it appears as a brown ring or circle of dark-colored spots just under the bark.

The symptoms are sufficiently distinctive so that a diagnosis can usually be made in the field. We know of no other disease of oak with those Symptoms or effects. For accurate identification, however, particularly in areas in which the disease has not previously been known, laboratory isolation of the causal fungus from diseased specimens is advisable.

The oak wilt fungus is closely related to the fungi that cause the Dutch elm disease and the canker stain disease of the London planetree, and is a near relative of the important blue stain fungi of forest products. It was described and named Chalara quercina by Berch W. Henry in 1944. Later, having discovered new facts regarding its life history, I reclassified and named it Endoconidiophora fagacearum.