Every February, I start to get excited about spring and warmer weather just around the corner. During this time of year, we start to think about pre-emergent weed control in lawns, pruning fruit trees, and trimming back perennial plants for new spring growth. This February, I would like to put out a reminder about oak wilt prevention.
Oak wilt is caused by the fungus, Bretziella fagacearum, that invades the water-conducting vessels of the tree (xylem). The tree responds to this invasion by plugging the tissues, which stops the transportation of water to the leaves.
Red oaks like Spanish oak, Shumard Oak, blackjack oak, and water oak are often the culprit for spreading the oak wilt fungus. Fungal mats form beneath the bark of diseased red oaks in the spring, and the nitidulid beetle spreads the fungus from infected trees to fresh wounds on healthy trees. Red oak trees will maintain their leaves, then drop all the leaves at once. The leaves turn bronze, or a branch will turn brown or red. Red oaks rarely survive oak wilt and typically die within four to six weeks after the first signs of infection.
White oaks such as post oak, bur oak, Mexican white oak, Lacey oak, white shin oak, Durand oak, and chinkapin oak are more resistant to oak wilt. Live oaks are susceptible to oak wilt, but the disease affects live oaks most seriously because of their interconnected root system.
Live oaks tend to grow in dense, large groups called motts with interconnected roots. The fungus spreads through the roots of the trees, creating infection centers with patches of dead and dying trees. The infection centers in live oaks can spread an average of 75 feet per year in Texas.
Live oak trees show leaf symptoms of veinal necrosis, tip burn, vein banding, and necrosis on the leaf margins. In common terms, it looks like someone traced the veins with an orange-rust colored marker or dried out the tips and sides of the leaves. The vein banding shows up as very dark green veins and a light color between the veins. Most live oak trees defoliate and die over a three-to-six-month period after the initial appearance of symptoms. Some live oaks take longer to die, and some trees can survive for years in a lengthy decline.
Oak wilt management in Texas depends on three approaches. First, try to prevent new oak wilt infection centers by eliminating diseased red oaks, handling firewood properly, and painting wounds on new oaks. Oaks should not be pruned from February through June to prevent spread of the fungus. The best times to prune is in December and January. All pruning cuts should be painted immediately to prevent exposure to the contaminated insects.
The second approach involves trenching or disrupting the root connections that can spread the disease in an infected area. Finally, injections of the fungicide propinconazole into the roots of individual trees can help extend the life of the tree. This is not a cure, but it can reduce tree loss.
The Texas A&M Forest Service AgriLife Extension has a great website about oak wilt with good photos for identification. Visit https://texasoakwilt.org/ to read more about prevention and treatment of oak wilt. For more information, contact Kate Whitney, Williamson County Extension Horticulturist, at 512-943-3300.
