The Oak may be our mightiest tree and a symbol of strength, but it has one great weakness: the dreaded fungal disease known as oak wilt. Oak wilt has been decimating Oaks for decades across the Eastern U.S., from the Upper Midwest down through Texas. Nearly always fatal, oak wilt can kill a grove of mature Oak trees in mere months, sometimes weeks.
What is oak wilt and how does oak wilt kill trees?
Oak wilt is caused by a fungus that invades and plugs the water-conducting tissues of a tree. The infected tree tries to save itself and stop the spread of the fungus by plugging up its own water-uptake vessels. This reaction prevents water and nutrients from reaching its leaves, which wilt, turn reddish brown, and drop, starting at the top and spreading through the crown. Sooner or later, the tree dies.
Which Oaks get oak wilt?
All Oak species (Quercus spp.) are vulnerable to oak wilt, although the disease has not yet reached the western U.S. There are three main groups of oaks: red, white, and live. Red oaks are the most vulnerable to oak wilt, usually dying within weeks of infection.
Red oaks include Shumard Oak (Q. shumardii), Texas Red Oak (Q. texana), Spanish Oak (Q. buckleyi), Northern Pin Oak (Q. ellipsoidalis), Northern Red Oak (Q. rubra), and Blackjack Oak (Q. marilandica).
White oaks have some resistance to the fungus, and it may take years for oak wilt to kill them. These include Post Oak (Q. stellata), Lacey Oak (Q. laceyi), Bur Oak (Q. macrocarpa), Mexican White Oak (Q. polymorpha), and Chinquapin Oak (Q. muehlenbergii).
Live oaks are particularly vulnerable because of their tendency to form interconnected roots with other live oaks, allowing the fungus to pass underground from infected to healthy trees. Live oaks include Escarpment Live Oak (Q. fusiformis) and Southern Live Oak (Q. virginiana).
How does oak wilt start?
The fungus that causes oak wilt can infect a healthy tree in three ways: via sap-feeding beetles, through interconnected roots, and via transport of firewood cut from infected trees.
Sap beetles are attracted to the fresh, oozing wounds on trees as a food source and find them very quickly by flying from one tree to another. Whether such wounds are caused by intentional pruning, friction between branches that have grown close enough to touch, or accidental damage from mowers, weed trimmers, or other equipment, they allow the introduction of the fatal fungus. Sap beetles carry fungal spores on their bodies from infected trees to healthy trees as they feed on the sap.
Does oak wilt spread?
Trees can also be infected underground when their roots grow together, allowing the fungus to be transmitted from an infected tree to a healthy one. Live oaks are particularly vulnerable because they often grow in clusters and their roots naturally graft together.
Finally, transporting firewood from infected trees can cause the spread of oak wilt by releasing fungal spores or by transporting beetles that are carrying spores on their bodies.
How to prevent oak wilt
Because oak wilt is nearly always fatal, prevention is vitally important for keeping your oaks healthy. Monitor oak wilt reports online to see if oak wilt has been found in your city or neighborhood. Be especially careful about when and how you prune your Oaks, and make certain that anyone you hire to trim your trees knows and follows oak wilt prevention practices.
Don’t prune in spring or summer
Sap-feeding beetles are most active in spring and summer. Avoid any oak tree trimming during that time frame. In Texas and the Southeast, do not prune Oaks from February through June. In the Midwest and northern states, do not prune Oaks from April through July. Winter is the safest time to prune.
Paint cuts and wounds ASAP
If pruning is absolutely necessary, for example in spring or summer due to an injured branch, storm damage, or some other can’t-wait scenario, then cuts should be painted immediately, or within 15 minutes max. If you wait until the pruning job is done, it can be too late, as the beetles that transmit the fungus are expert at tracking down wounds. Black or brown latex house paint works fine. Don’t bother painting cuts in winter.
Using the same saw to prune multiple trees can accidentally infect healthy trees. To prevent spreading the fungus, sanitize your saw between trees by spraying it thoroughly with a bleach solution or Lysol spray. If you hire a tree-trimming service, make sure they do this too.
Avoid infected firewood
Firewood that was recently cut should be avoided. Only buy seasoned firewood that has been dried for 12 months. Otherwise, cover firewood tightly with clear plastic, burying the edges of the plastic with soil to prevent insect movement out of the wood and into your healthy trees.
How to treat oak wilt
Dig a trench
If oak wilt has been diagnosed on your property or a neighbor’s, trenching is one way to potentially save your uninfected trees. Trenching severs root connections between trees. It’s expensive and not guaranteed, but it may be worth the cost to try to save heritage or otherwise important trees. Hire an arborist experienced in oak wilt prevention to do this kind of work.
Inject with fungicide
Oaks near infected trees can be injected with a fungicide as a preventive measure. It’s not usually effective for trees already showing signs of infection, but it can be appropriate for asymptomatic trees.