The use of chemicals for curing wilt-infected trees is still in the early experimental stage and is not yet recommended as a practical control measure.
In 1949 Wisconsin workers demonstrated the local spread of oak wilt through natural root grafts. They found that the poisoning of a single healthy tree with sodium arsenite often killed as many as 15 other trees nearby, indicating that their roots were connected.
Recently the results of experiments in Wisconsin explained in part what causes the leaves of diseased trees to wilt. When a tree becomes infected it is stimulated to produce tyloses or swellings in the vessels of the wood. Therefore, the flow of water from the roots to the tree top is restricted and the leaves wilt and die. It is also known that the fungus itself produces a toxin which might be responsible for the actual killing effect on the tree.
In Illinois experiments are being conducted with insects in relation to the spread of oak wilt. Insects of various species are collected from wilt-killed trees and allowed to run over or feed on laboratory cultures of the oak wilt fungus. The insects are then caged on parts of healthy trees to feed on the leaves. A single red oak treated in this way contracted the disease and died. This shows that the disease can be transmitted by an insect.
Controlling the Disease
The spread of oak wilt in local areas may be stopped by preventing the underground movement of the disease from tree to tree through natural root grafts. This can be done by (1) poisoning all healthy trees within 50 feet of diseased trees, (2) cutting a ditch 30 inches deep with a small trenching machine between diseased and healthy trees to sever root connections or (3) severing root connections with a tractor drawn plow on which a knife blade is attached. Unfortunately the use of such heavy equipment is not practical in rocky and hilly areas. Chemicals used for killing trees are sodium arsenite and ammate. Ammate is safe to use but does not kill trees as rapidly as the other poison. In some localities 2,4,5-T used as a trunk spray has given satisfactory results in killing small trees.
If infected trees are left standing mycelial mats with their numerous spores develop under the loosening bark. It is therefore advisable to cut and burn all parts of diseased trees as soon as possible after symptoms appear.
A combination trenching and eradication program was started in the summer of 1950 in the Forest Preserve District of Cook County in Illinois. According to Mr. Noel B. Wysong, Chief Forester, 2 newly wilted trees were found in the Forest Preserve in 1948, 72 trees in 1949, 141 trees in 1950, and 96 trees in 1951. The count for 1952 is not complete but a continued decrease in the number of new infections would indicate good control.
There is no information on resistant species of oak. In very rare cases, however, trees have been observed to recover after showing symptoms in the early spring.
Future Outlook