Oak wilt in San Antonio

What oak wilt is

Oak wilt is a fungal disease that blocks an oak's water-conducting tissue. It spreads in two main ways:

• Through connected roots. Live oaks often grow in groups with roots grafted together, so the disease can move from tree to tree underground, sometimes spreading across a neighborhood over time.
• By beetles. Sap-feeding beetles carry spores from fungal mats on infected red oaks to fresh wounds on healthy oaks. That's why fresh pruning cuts are dangerous in spring.

Which trees are at risk

Red oaks, including Texas red oak and Shumard oak, are highly susceptible and often die within weeks of symptoms. Live oaks are the most common victims around San Antonio because of their root connections. They may die over months, and some survive with treatment. White oaks are more resistant.

Signs to look for

• On live oaks: leaves with yellow or brown veins (veinal necrosis), and leaf drop out of season.
• On red oaks: leaves turning dull green, bronze or brown from the tips in spring or early summer.
• Several nearby oaks declining one after another.

Other problems can look similar, so get a diagnosis from a certified arborist or Texas A&M Forest Service before acting.

Prevention

• Avoid pruning or wounding oaks from February through June. That's the high-risk season for beetle spread.
• Paint every fresh oak wound immediately, any time of year, including storm breaks and mower scrapes.
• Be careful with firewood. Firewood from diseased red oaks can carry the fungus. Don't bring unknown oak firewood home, and cover or dispose of suspect wood properly.

Treatment options

Arborists may recommend trenching to cut root connections and stop underground spread, and fungicide injections (propiconazole) to protect high-value live oaks. Treatment works best before a tree is heavily affected. Diseased red oaks are usually removed and handled carefully.