Oak wilt is a disease of oaks (Quercus). Those in the red oak group are most susceptible (black, red, pin, scarlet). White oaks are more or less resistant (bur, overcup, post, white). Other members of the Fagaceae may be affected, including chestnut (Castanea), chinkapin (Castanopsis) and tanoak (Lithocarpus).
The pathogen is Ceratocystis fagacearum. It is an ascomycete similar to Ophiostoma and has both sexual perithecia and an asexual conidial stage. Fruiting occurs on dense fungal mats under the bark of recently killed trees.
Consistent with a wilt disease, symptom expression is exacerbated by hot, dry conditions. On the other hand, the fungus fruits better and vectors are more active under moist, cool conditions. Thus, disease progression is favored by hot, dry conditions, and disease transmission is favored by cool, moist conditions.
A few months after the leaves drop, the fungus produces gray mats of mycelium between the bark and wood at various spots on the trunk. Near the middle of the mat, thick ridges of blackish fungal tissue (pressure pads) raise up to crack open the bark.
On the mats, conidia and, if sexually compatible strains are present, perithecia are produced. Perithecia are black with long necks like Ophiostoma. Both kinds of spores are produced in a sticky matrix that facilitates vectoring by insects.
The fungus puts out a sweet odor that attracts various beetles. The cracking of the bark lets out the odor and lets in the insects, which get contaminated. The insects are sap beetles (nitidulids) as well as oak bark beetles. The two types of insects operate and transmit the fungus differently.
Oak bark beetles visit healthy oaks and make twig cavities where they feed and transmit the fungus. Nitidulids are attracted to fresh wounds on other trees and transmit the fungus to them. They are active primarily May to June.
The fungus grows primarily in the vessels of the sapwood.
Root-to-root spread by grafts is another form of dispersal and trasmission, leading to disease centers. Grafts form most commonly between individuals of the same species and are most common in the red oak group.
Leaves first fade or turn yellow or bronze and begin to wilt. Then leaf tips and margins become brown and necrotic. In some species, veins become necrotic; this is a diagnostic symptom. Symptoms progress inward and downward in the crown, often accompanied by defoliation, until the tree is dead. In the red oak group the disease may kill the tree in a few weeks; in the white oak group it may take several years.
The outer annual ring may show brown discoloration where the fungus is actively disrupting xylem. The symptom is more common in white oaks, though it can occur in red oaks. In a cross-section through a diseased branch the discoloration appears as arcs or spotting; in a tangential cut it appears as brown streaks.
Oak wilt is reported only in the U.S. It was first detected in Wisconsin in 1942 and is now known in 22 states. In the early years it was known primarily in the Midwest; it is now common east into the central Appalachians and south into Texas. Except for Texas and a few spots in the Carolinas, it is largely absent from the Gulf and Atlantic coastal states and their coastal live oak populations.
Management of oak wilt is not easy but it can be done cost-effectively. Approaches include sanitation, protection, and regulation.
Although this disease is apparently native to North America, it was not even described until 1942 and it has become more severe in more areas since then. It is not clear to what extent there has been an increase in the range of the pathogen vs. an increase in severity in more of its old range. Several factors may explain the disease increase: