Once oak wilt is diagnosed, many people are at a loss as to what to do. In the course of my career here in Central Texas, upon being called onto a property, many people have told me that once live oaks get oak wilt, that they are untreatable and that they should just let them die. This is simply not true.
The most important factor to diagnosing oak wilt is diagnosing it early. If you in any way think that your oaks may be in decline, immediately call me out to inspect them. At this point, I will be able to either eliminate the possibility altogether or, immediately diagnose the disease and start treatment right away.
The longer you wait, the more damage the fungus will do to the tree, and then a successful treatment is less likely. In some cases, even a 20 percent defoliation can be evidence that the disease has progressed too far, and a treatment will be unsuccessful. Generally speaking, when an infected oak is treated, if it takes in enough material, it tends to stay at the canopy density it has when it is treated. This is why you often see live oaks that have been treated looking so thin and defoliated.
We use Alamo fungicide, generally at 20 milliliters per liter, and generally only treat trees once they have been diagnosed with oak wilt, or are in a stand of oaks that are symptomatic. Trees put on new rings of growth every year, creating thicker trunks, roots and branches, and new roots and shoots. Since this is new living tissue, once the Alamo has been injected into the tree the new years subsequent growth does not contain the chemical, and is not protected. We used to recommend treatment initially as a preventive for five-year intervals, with re-application every five years thereafter. Now, we are recommending treatment every two years; but only when the trees are either symptomatic or at high risk of contracting the disease within 12 months after treatment. We use the recommended Texas A&M macro-injection method.
On larger properties such as parks and ranches, or in certain neighborhoods, sometimes trenching can be a good idea. When hundreds or even thousands of oaks are at risk from oak wilt infected trees on property perimeters, often root severing through trenching two feet deep is the best and most cost-effective option for keeping out the disease. This is NOT a panacea, and it will NOT keep beetles from spreading the disease to healthy trees under ideal conditions, but it CAN keep the disease out in certain instances. It is better undertaken where large natural areas and large amounts of trees make Alamo treatment economically unfeasible. In neighborhoods, streets and sewer lines are essentially trenches, and the disease is often confined to city blocks once new beetle infections occur.