The primary sign of oak wilt is the wilting of leaves and defoliation. Trees affected by oak wilt begin to wilt one branch at a time, usually from the top down. Affected leaves begin to discolor and rapidly fall from the tree. Other stress factors affecting oaks typically do not cause leaf drop, so this can be one way to differentiate oak wilt–infected trees. Leaves drop in all stages of discoloration. Even entirely green leaves may fall from affected branches. Some affected branches hold green leaves longer than others—sometimes until autumn. Therefore, the crowns of trees with oak wilt are seldom as uniformly brown as those of trees suffering from other forms of stress. But in dry years, the appearance of trees with oak wilt may be confused with that of trees with drought symptoms or oak decline, another disorder of oak.
Streaks in wood
A twig cut rom a rapidly wilting oak wilt?infected tree will often show a ring of dots where tyloses (thin cellular membranes that inflate, clogging the vessels) have blocked the water-conducting cells of the tree. Similarly, shaving the bark from a twig will often reveal yellow, bluish, or brownish streaks in the wood immediately beneath the bark.
From the air
Established oak wilt infections sites tend to have a bull's-eye appearance when viewed from the air. One or more dead trees can be found at the center of the pocket surrounded by a ring of dying oaks.