Forest Preserve District of Cook County, Illinois
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Oak Wilt
Nature Bulletin No. 479-A   February 3, 1973
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation

OAK WILT
Our sturdy oaks, symbols of strength, toughness and durability, are menaced by a deadly fungous disease called Oak Wilt, They are being attacked in at least 18 states from Minnesota and Kansas to Pennsylvania and North Carolina. That is alarming to those who recall that, during the first 35 years of this century, all but a few of our native chestnuts were annihilated by Chestnut Blight -- a fungus brought here in young chestnuts from Asia. Since 1930, we have lost untold numbers of fine elms and are desperately battling to save the rest from destruction by Dutch elm disease caused by a European fungus. Now the oaks, most important of all our hardwoods, are similarly threatened.

Oak wilt was first described as a definite disease in 1942, and the fungus was identified in 1944. At that time it was unknown out side of a few locations in southern Wisconsin, northern Illinois and northern Iowa. It may have been present but unnoticed for many years. By 1950 it was become widespread in those regions and had been discovered in Minnesota, Indiana, Ohio, central and southern Illinois, northern Missouri and, to the dismay of foresters and lumbermen, in the Ozark forests of southern Missouri. During 1948, two oaks in our Palos forest preserves were found dying from this disease. The next year there were 72. Careful searches through the woods and from the air in helicopters have discovered greater numbers every year. The maximum was 212 in 1954.

On red and black oaks the first symptom is a wilting and curling of the leaves -- usually near the top of the tree. They gradually become bronze or brown and drop off. The lower branches are affected last but the tree ordinarily loses all its leaves within a week or two and dies during that summer. There is no cure. None recovers. On white and bur oaks, twigs with wilting and dead leaves may be scattered throughout the crown while some branches remain outwardly healthy. Those leaves usually become light brown and may not fall off. The entire tree seldom wilts at once and, becoming more and more "stagheaded", may live several years.

Oak wilt spreads in at least two ways. One is overland -- in "jumps " of from a few hundred yards to a mile or more -- into previously healthy trees. Nobody knows how. The spores of the fungus may be carried by beetles or other insects, by tree-climbing animals such as squirrels, or by woodpeckers and similar birds. That vital question is being feverishly investigated.

The other way is underground from tree to tree. In a dense woodland, the interlacing roots frequently become grafted together where they cross one another -- especially those of the red and black oaks. Normally, such graft unions are beneficial but they become "pipelines" of infection when oak wilt strikes. In our forest preserves, before we cut down and burn every part of each infected tree, we dig a deep narrow trench around it and its neighbors: far enough out so that the fungus cannot be transmitted underground beyond that clump. It is the only control measure known, now.

In the vast forests extending southward and eastward from the Ozarks, oaks comprise about one-third and the most valuable of all hardwood saw-timber.

They support major industries using oak wood for lumber, flooring, heavy timbers, barrels, railroad ties and other purposes. In our Cook County Forest Preserves, in many state, county and municipal parks, in farmers' woodlots, around innumerable resorts and homes -- oaks are the principal trees. It would be a national calamity if we lose them.


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