When most people think of oaks, the image of a mighty, stately, nearly indestructible giant of a tree often comes to mind. While some oak species can live for more than 1000 years, they do have an Achilles heel. A silent killer called oak wilt or "live oak decline" is slowly killing millions of oak trees across many parts of North America. The disease is caused by a fungus called Ceratocystis fagacearum which is spread by sap beetles. So far there is no cure for oak wilt, but there are treatment methods. Red oaks are one of the species most susceptible to the disease, however oak wilt affects live oaks, white oaks, burr oaks and many other oak species.
Can Oak Wilt Be Spread By Using Firewood From Infected Trees?
There are several ways that oak wilt fungus or "live oak decline" can be spread to healthy trees. The most common way that oaks become infected is when a branch or root is cut. Sap beetles, which feed on the sap and spore mats of infected oaks, fly from tree to tree, carrying the fungus to open wounds of healthy oak trees . The lethal fungus, Ceratocystis fagacearum, quickly spreads throughout an infected tree. In most cases death of the tree will occur in about year. Signs of oak wilt include yellowing or dropping leaves and dead branches. If a tree infected with oak wilt spores is felled and cut into firewood, deadly spores can still remain in the wood. Although oak wilt spores in firewood become dormant, they can be reactivated by moisture and infect healthy oak trees near where the wood is stored. Since heat kills the fungus, wood from trees which were killed by oak wilt can be burned in a fireplace without any risk of spreading the disease. It's often tempting to cut down a dead oak tree and use it for firewood, however, because firewood from trees killed by oak wilt poses a danger to healthy trees, using it should always be avoided.
Firewood is not normally a major vector for oak wilt disease. In most cases, trees become infected with oak wilt when people cut a branch, or when a windstorm damages the tree and causes an open wound.
The best way to prevent oak wilt is to immediately spray any new cuts on the tree with pruning paint. This special tree paint contains an insecticide which kills sap beetles. Pruning spray also forms a coating which seals the wound and therefore prevents more sap from being released. There are certain times of the year when sap beetles are most active. Many communities in areas where oak wilt is a problem have enacted laws which restrict tree pruning to certain times of the year. Since sap beetles are generally most active in spring, it is generally safest to prune oak trees in late fall and winter. The Texas AgriLife Extension Service, part of the Texas A&M system, advises that there is "no time of the year when it is totally safe to prune oak trees," and all open wounds should immediately be painted with pruning paint.
Are Older Oak Trees Immune To Oak Wilt?
One might expect that older oak trees would have built up an immunity to oak wilt, however this is not the case. Both older oak trees and younger ones are equally susceptible to oak wilt. Even some of the nation's oldest oaks, such as the 1000 year old "Big Tree" in Lamar Texas, are now endangered by the spread of oak wilt.
Below is a photo of an infected live oak tree in Pedernales Falls State Park, west of Austin Texas. Oak wilt has killed millions of trees in the Texas Hill Country, including some in state parks such as this one. Two separate trees are seen in the photo below. The second tree was most likely infected as the fungus traveled through the interconnected root system.
Other Ways Oak Wilt Is Spread
Many oak species, such as live oaks, feature highly complex root systems, which often intermingle with those of other trees of the same species. We may tend to think of oaks as individual trees, however in many cases a grove of trees shares a common, interconnected root system. These large root systems can even span hundreds of acres in some cases. Oak wilt spreads through this "community root system" at a rate of about 100' per year. The only way to stop the spread of the disease is to use a trenching machine and physically sever the root system of an infected tree from the community root network. Residents of communities that are affected by oak wilt must work together and isolate infected trees from healthy ones by digging trenches. This may seem like a very costly option, but it is often the only alternative if the community is serious about saving their trees.
Is There A Cure For Oak Wilt?
Currently there is no known cure for oak wilt or "live oak decline." Tree specialists can inject a chemical called Alamo into infected oaks to halt the spread of the disease in that particular tree. The tree's root system must still be severed so the fungus does not travel to neighboring oaks. Alamo treatment costs about $35 per inch of tree diameter and success is not always guaranteed.
Where Did Oak Wilt Come From?
Since there were no recorded cases of the disease before the 1950's, scientists believe that oak wilt may have entered North America from the South American continent, or from some unknown area. There is also a possibility that the fungus has always existed in the environment, and because of increased land clearing and development, human activity itself could have helped oak wilt reach "critical mass." Additionally, some scientists suspect that changing weather patterns, possibly caused by global warming, have caused some oak species to become stressed, therefore increasing their chances of becoming infected with oak wilt disease.
If you love the stately old oaks in your community, all is not lost. By following proper safeguards, such as painting tree wounds with pruning paint, avoiding using firewood from oak wilt affected trees, and by isolating infected trees with trenches, the disease can be stopped. If you notice possible signs of oak wilt disease in your neighborhood, such as dying limbs, yellow folaige, etc., do not hesistate to contact your county extension agent, city agency or a qualified tree specialist.