This serious vascular wilt of oaks is caused by the fungus _Ceratocystis
fagacearum_. At least fifty species and varieties of oak are
susceptible. The disease has been most damaging in the Lake States but
is also found in the eastern United States.
[Illustration: Oak wilt symptoms on red oak trees and leaves.]
[Illustration: (cont.)]
Oak wilt symptoms are most noticeable during late spring or early
summer. Red oaks may be killed in as little as three weeks, the lower
branches being affected last. In white oaks, symptoms are usually
confined to a few branches each year and trees may live several years
before death. Leaf symptoms are similar for both red and white oaks.
Leaves turn yellow or brown and become dry progressively from the edge
or tip to the midrib and base. Mature leaves may fall at any symptom
stage from green to brown. Premature leaf shedding is the most
outstanding symptom. A definite characteristic of the disease is the
raising and cracking of the bark due to pressure of mats of the fungus
growing between the bark and wood.
Oak wilt is spread over long distances by insects that pick up spores
while crawling on the mats of infected trees. The disease may also
spread from tree to tree via root graft. Short-distance spread is
controlled by severing all roots of living trees around infected trees
by use of a ditchdigger. Another control is to fell all trees in a
50-foot radius of infected trees; felling and burning of all parts of
infected trees is sometimes done to prevent overland spread.