How to control and stop Oak Wilt

Sap-feeding (nitidulid) beetles are believed to be responsible for much of the long distance spread of oak wilt. (Photo courtesy KXAN)

SAN ANTONIO - Oak Wilt can be a big problem for us in South Texas and the Hill Country. It is one of the most destructive tree diseases in the United States, and it is killing oak trees in central Texas at epidemic proportions.

Oak Wilt is a fungal disease affecting oak trees. Symptoms vary by tree species, but generally consist of leaf discoloration, wilt, defoliation and death.

The fungus spreads from diseased to healthy trees from insects or by connections between tree roots. And as devastating as it can be, it's a very small beetle that is responsible.

I spoke to Charles Martelli. He works at Milberger's Nursery and has forty years of experience. Check out the full live stream video below as he answers questions and explains Oak Wilt.

Prevention plays an important role in the management of oak wilt. You can take an active role in oak wilt prevention by taking the following steps:

  • Avoid pruning or wounding oaks between February 1 and July 1. This is the time of year when oak wilt fungal mats are most likely to form and nitidulid beetles are active. If a nitidulid beetle carries oak wilt spores from a fungal mat to a fresh wound on an uninfected oak tree, the fungus could become established in the disease-free tree. The least hazardous periods for pruning are during the coldest days of midwinter or extended periods of hot weather in mid- to late summer.
  • Sterilize/Sanitize all pruning equipment between trees using denatured methyl alcohol (shellac thinner), isopropyl alcohol, or a general purpose household disinfectant such as Lysol, Listerine, Pine-Sol or related products. Using household bleach is NOT recommended as it can be corrosive to pruning tools as well as people.
  • Immediately paint all wounds on oaks to prevent contact with contaminated beetles. Wounds should be painted, regardless of the time of year they were made, with commercial tree wound dressing or latex paint (color doesn’t matter!). Wounds can be either man made or natural and include freshly-cut stumps and damaged surface roots.
  • Do not transport or buy unseasoned firewood. Fungal mats may form on unseasoned red oak firewood infected with oak wilt making it possible to spread oak wilt to uninfected areas. Seasoned firewood (dried for at least one year) should not present a threat of spreading oak wilt. Also, burning infected wood cannot transmit oak wilt.
  • Promptly remove and either burn or bury all red oaks that are dying or have been recently killed by oak wilt. Generally, this would be oak wilt-infected red oaks that die in the late summer or fall. This will prevent nitidulid beetles from spreading spores from fungal mats that may form on the trees in the fall or the following spring.

There are two main recommendations that are generally given to treat oak wilt infection areas:

1. Stopping the spread through the roots

Measures can be taken to break root connections between live oaks or dense groups of red oaks to reduce or stop root transmission of the oak wilt fungus. The most common technique is to sever roots by trenching at least 4 feet deep with trenching machines, rocksaws or ripper bars. Trenches more than 4 feet deep may be needed to assure control in deeper soils. Correct placement of the trench is critical for successful protection of uninfected trees. There is a delay between colonization of the root system by the fungus and appearance of symptoms in the crown. Therefore, all trees with symptoms should be carefully identified first. Then, the trench should be placed a minimum of 100 feet beyond these symptomatic trees, even though there may be healthy trees at high risk inside the trench. Trees within the 100 foot barrier, including those without symptoms, may be uprooted or cut down and removed to improve the barrier to root transmission. Tree removal should be initiated after trenching, starting with healthy trees adjacent to the trench and gradually working inward to include symptomatic trees.

Oak wilt centers are more easily suppressed when treated early, before they become too large. Untreated trees immediately outside the treated area should be closely monitored for several years. If the pathogen appears to have crossed the barrier, the same measures (new trenching and treatment of trees within the barrier) should be repeated while the diseased site is still small.

2. Fungicide treatment

The fungicide propiconazole (Alamo) can be used as a preventative to reduce oak wilt symptoms in live oaks when applied before infection. Limited success may also be achieved in trees with therapeutic injections during the earliest stages of infection. The fungicide is injected into the tree’s water-conducting vascular system through small holes drilled into the root flare at the base of the tree. Treatment success depends on the health condition of the candidate tree, application rate, and injection technique. Injection should be done only by trained applicators.