What Are the Causes of Oak Wilt?

Oak wilt is a fungal disease common in oak trees, especially red oak.

A large number of oak trees are damaged or killed by oak wilt each year. Oak wilt is a rapidly progressing fungal disease first discovered in the United States in 1944. To date, oak wilt has never been found in any other countries. Red oak trees are the most susceptible to oak wilt, with many infected trees dying within three weeks of infection.

Cause

Oak wilt is caused by the fungus "Ceratocystis fagacearum," which often enters oak trees through pruning injuries or wounds from hail or storms. When oak trees become infected with this fungus, they release a material within their vessels called tyloses. This changes the way the oak tree receives and uses water. As the fungus continues to spread, it penetrates the internal wood and the outer layer of bark. Spore mats are often visible on oak trees with oak wilt, and these spores are carried on the air to infect other trees in close proximity. Spore mats are most common during the spring and winter months, when the weather is damp and cool.

Hosts

White oaks are considered resistant to oak wilt, but some trees under stress may become infected. White oaks infected with oak wilt typically take several years to die. Red oaks are most susceptible to oak wilt, with many infected trees dying rapidly after infection. According to the United States Department of Agriculture, northern oak trees most often killed by oak wilt include black oak, white oak, northern red oak, northern pin oak and bur oak. Some southern species are susceptible as well, including shumard oak, scrub live oak, Texas live oak, Spanish oak, Southern red oak and blackjack oak.

Effects

Oak wilt causes infected oak trees to develop leaf discoloration. After discoloration occurs, the oak tree leaves at the top of the tree become bronzed and leaves wilt. As oak wilt infection spreads, infected trees lose their leaves and closely resemble oak trees suffering from drought stress. Within the span of two or three weeks, oak trees infected with oak wilt are completely defoliated and dead.

Control

It is important to properly identify oak wilt, as a number of conditions that may resemble oak wilt infection exists. Insects such as tent caterpillars and spider mites often infest oak trees and cause symptoms similar to oak wilt. Diseases such as oak anthracnose and leaf blister also produce similar symptoms. Pruning injuries, chemical injuries or poor soil are all causes of reduced vigor and death in oak trees and may be easily confused with oak wilt. The only way to properly identify oak wilt is through examining diseased tissues during a complete fungal analysis. Oak trees infected with oak wilt usually die, and little can be done to save them. The life of the tree may be prolonged with fungicides injections, but the result is not certain. Take measures to avoid the spread of disease by implementing proper sanitation and severing the root system of infected trees.

References

• USDA: How To Identify, Prevent And Control Oak Wilt
• NCRS: Texas Forestry Technical Note TX-2; Oak Wilt