Oak wilt disease symptoms progress differently in red oaks, white oaks, and Texas live oak.

Red oak group
Oak wilt is usually identified in red oaks by the symptoms of rapid leaf discoloration and wilting. Often the initial symptom is a subtle off-green color shift that may be visible in the upper portion of the tree crown. This symptom is apparent in the northern part of the disease range in late June to early July. Shortly after this initial color shift, the leaves begin to wilt from the top of the crown downward. As the disease progresses, individual leaves quickly discolor, taking on a "bronzed" appearance. The discoloration progresses around the margins of the leaf from the tip to the base (Fig. 7B). The progressing discoloration may be interrupted by the leaf veins, as shown in the white oak leaf in Fig. 7A, or may affect the entire upper portion of the leaf, as shown in the red oak leaf in Fig. 7B.

Leaves are cast rapidly as the infection progresses. Commonly, infected trees are almost entirely defoliated within a few weeks of symptom onset. Fallen leaves usually are brown at the tips and margins, and sometimes green at the base and along the lower veins. Trees are often killed in groups or disease "centers," when infection occurs through grafted roots.

Figure 7. Symptoms of oak wilt in A. white oak, B. red oak and C. Texas live oak.
Figure 7. Symptoms of oak wilt in A. white oak, B. red oak and C. Texas live oak.
Occasionally the outer ring of vessels of diseased trees will be plugged with a brown substance that may be visible in cross sections as a ring or a series of dark spots through the outer sapwood, and in tangential cuts as longitudinal streaking of wood exposed after removing the bark. However, this is not always obvious to an untrained observer, especially in the red oaks. The discoloration may be very light or appear as flecks in such sections. Discoloration is most readily seen in tangential cuts on branches.

White oak group
White oaks usually die slowly, one branch at a time, over a period of one to many years. Wilting and death of leaves on individual branches occur in a similar fashion to the disease in red oaks, but usually progresses much more slowly. Affected leaves exhibit a pattern of discoloration similar to that seen in red oaks, with discoloration proceeding from the margins to the base, sometimes interrupted by the leaf veins (Fig. 7A). Brown streaking in the outer growth rings is often readily apparent even to an untrained observer in infected white oaks and bur oaks, but may be missing.

Texas live oak
Texas live oaks can wilt and die rapidly or slowly, depending on the timing of infection and weather conditions, but generally succumb within 1-6 months of infection. Diagnostic leaf symptoms are usually produced somewhere on the tree (especially in spring and fall). Leaves develop yellow ("chlorotic") veins which eventually turn brown ("necrotic"), a symptom termed veinal necrosis (Fig. 7C). Affected leaves fall, and the tree crown progressively thins out until the entire tree is dead. Fallen leaves under the tree may show darker brown veins for months. Sometimes just the tips, margins, or interveinal portions of leaves will turn yellow or brown, but these symptoms are not necessarily the result of oak wilt, and not as useful in diagnosing the disease. A small percentage of Texas live oaks may survive oak wilt infection indefinitely, while suffering varying degrees of crown loss.
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