Oak wilt is caused by the fungus, Bretziella fagacearum. It was first identified in 1944 and its origin is still uncertain. The fungus grows in the vascular tissue of the tree—this cuts off the supply of water and causes the tree to wilt and die. As the fungus grows it spreads below ground through natural root grafts to infect healthy oak trees. Once the fungus has killed a tree it may produce a mat of fungal spores under the bark. When pressure builds in this spore mat it causes the bark to split. Several species of sap beetles are attracted to the strong fruity odor produced by the spore mats. The beetles feed on and tunnel through the mats they get covered with spores. Disease spreads when beetles covered in oak wilt spores transfer these spores to new host trees. The spore mats are especially common in infected red oaks.
Every year oak trees in forests, woodlots and home landscapes die from oak wilt. Oak wilt is an aggressive disease caused by a fungus which grows in the tree’s vascular system. This disease kills thousands of oak trees every year sometimes in as little as two–three months.
Oak wilt spreads several ways and once the disease gets into a tree, it typically spreads quickly throughout the stand or neighborhood to other oak trees. Oak wilt is distributed through much of the eastern US, but there are susceptible hosts throughout North America that may be affected if the disease continues to spread.
The oak wilt pathogen can spread to new hosts in several ways. Oaks tend to grow in stands and when oak roots come in contact with other oak roots they form natural root grafts–connecting individual trees in a network. Natural root grafts between healthy and infected oak trees are a key way the disease is spread on a local scale. Additionally, in some hosts (especially red oaks) the fungus can produce spore mats under the bark of dead and dying trees. These spore mats release infectious spores into the air. Additionally, these spores may be picked up by sap and bark feeding beetles, and spread over longer distances to new, healthy, hosts. When wood containing these infectious spores is moved, for example in infected, unseasoned firewood, it spreads the spores to new locations. Once the spores are moved they can infect new host trees, starting the natural disease cycle in a new location.
Oak wilt is just one of many significant “pests” that can spread to new areas by people. Learn more about human assisted spread of plant pests and pathogens at http://www.dontmovefirewood.org/
All oaks (Quercus spp.) are thought to be susceptible to oak wilt, but different species experience different disease severity. Oaks can be divided into three groups, red, white and live. Oaks in the “red oak group” (for example, red oak, black oak and pin oak), are characterized by lobed leaves with pointed tips and are the most susceptible species, experiencing high mortality when infected. Oaks in the “white oak group” (for example white oak and bur oak), have lobed leaves with round tips and are susceptible but the disease progresses more slowly. Oaks in the “live oak group” (for example, Texas live oak), have oval leaves with pointed to round tips, and while moderately susceptible experience high levels of mortality when infected.
- Red oak group: rapid discoloration of the leaves, wilting and leaf drop.
- Texas live oak: symptoms may progress fast or slow. Leaves develop yellow veins that eventually turn brown and fall off. The canopy gets progressively thinner and trees may die.
- White oak group: symptoms are similar to those in red oaks only the disease progresses more slowly.
Signs of oak wilt may include the presence of spore mats under the bark surface, especially in red oaks. In highly susceptible host species oak wilt can cause rapid death within a single season. Because oak wilt can be confused with many other problems, laboratory testing is needed to confirm diagnosis and ensure you take the appropriate disease management steps.
What to do:
If you suspect an oak could have oak wilt, contact your local extension office to see about sending in a sample. Find your local extension office here http://nifa.usda.gov/partners-and-extension-map.
Once a tree is infected there is no cure, therefore it is important to stop the disease from spreading to healthy oaks. In areas with oak wilt remove infected trees and, where possible, prevent or disrupt root graft formation between healthy and infected oak trees. Avoid pruning oak trees when sap beetles are most active (depends on area but typically spring to summer, April, May and June).
Don’t move oak wood from infected to uninfected areas.
Think you've spotted this pest?
If you think you've found this pest in your landscape contact your local extension office to see about sending in a sample.
Find your local extension office here.