Devastating oak wilt can be managed if caught early

Across Texas, trees are succumbing to the effects of a fungus that causes a devastating tree disease called oak wilt.

It is a major problem in live oaks, Shumard oaks, Spanish oaks, water oaks, black jack oaks and other members of the red oak family.

Although oak wilt can be fatal, it can be managed with early identification and a precise diagnosis.

Indicators of oak wilt include foliar symptoms, patterns of tree mortality and the presence of fungal mats. Leaves on diseased live oaks often develop yellow veins.

Defoliation may be rapid; damaged leaves are often found beneath the tree. Some live oaks escape infection, others decline over time, but most die within six months after symptoms appear.

Foliar symptoms in red oaks are less distinct. In spring, young leaves simply wilt, fade and turn brown. Mature leaves develop dark green “water soaking” marks, fade and then turn bronze starting at the leaf margins and moving inwards.

Red oaks seldom survive oak wilt and often die within 3-4 weeks after symptoms appear. In summer, diseased red oaks stand out because of their bright fall-colored leaves.

In spring, spore-producing structures, called fungal mats, often form on red oaks that became infected the previous fall.

The mats, which may have a fruity odor, grow in the narrow cracks and hollow areas of the tree bark. These fungal mats typically do not form on red oaks that become infected in spring to summer.

The oak wilt fungus spreads when wood from infected oaks is moved to another location.

Fungal mats on diseased trees, stumps or logs also lure sap-feeding beetles, which spread the disease when they visit other trees.

Also, in dense groups of live oaks, the disease spreads underground up to 150 feet in all directions through interconnected tree roots.

Preventing new infections starts with removing any diseased red oaks, handling firewood properly and painting wounds on healthy oaks.

Trenching (a minimum of 4 feet deep and 100 feet beyond the infection perimeter) can disrupt root connections to reduce spread.

Injections of the appropriate fungicide (propiconazole) to individual high-value trees helps reduce crown loss and extend tree life. These measures do not “cure” oak wilt, but they reduce tree loss.

All wounds on oaks, such as pruning cuts, should be avoided February – June. The least risky times to prune oaks are the coldest and hottest days of the year. Regardless of the season, all cuts to oak trees, stumps or roots should be painted immediately with a wound or latex paint to minimize insect contact and infection. Cover firewood from diseased trees and store it in clear plastic with the edges buried to prevent insects from leaving the wood pile. When replanting in areas where oak wilt is present, select trees less susceptible to the disease, such as chinquapin oak.

For answers to horticulture questions, call the Texas AgriLife Extension, Hood County at 817-579-3280.