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.