Oak Wilt Symptoms: Live Oak vs Red Oak in San Antonio

The single most useful thing you can do before calling anyone about a sick oak in San Antonio is figure out what kind of oak it is. Oak wilt presents so differently on the two groups that they are almost separate problems, and the urgency is not the same.

First, identify the tree

Live oak - the sprawling, low-branching tree that defines older San Antonio neighborhoods. Small, leathery, oval leaves with smooth edges, dark green on top and paler underneath. It holds its leaves through winter and drops them in a rush in early spring, right as new ones push out. Very common on the North Central side, in the Hill Country towns, and lining streets in Alamo Heights, Terrell Hills, and Olmos Park.

Red oak group - in this area that mostly means Texas red oak (also called Spanish oak), Shumard oak, and blackjack oak. Larger leaves with pointed lobes and bristle tips at the ends. Fully deciduous, with real fall color in a good year. Common in the Hill Country, in Boerne and Fair Oaks Ranch, and planted as landscape trees throughout the metro.

If the leaf has points and bristles, treat it as a red oak. If it is small, smooth-edged, and the tree looks like it has been there a century, treat it as a live oak.

Oak wilt on a live oak: veinal necrosis

The signature symptom on a live oak is called veinal necrosis, and it is specific enough to be close to diagnostic.

Pick a leaf from a declining branch and hold it up. In veinal necrosis, the veins themselves turn yellow or brown while the tissue between the veins stays green. It looks like someone traced the vein pattern with a highlighter. Nothing else in San Antonio does this.

Other live oak signs:

• Leaf drop out of season, especially heavy drop in summer or fall rather than the normal early-spring flush.
• Decline that starts on one side of the canopy and works across.
• Dropped leaves that are still partly green rather than fully brown and crisp.
• Neighboring live oaks showing the same thing in sequence, forming an arc.

Timeline: live oaks usually decline over one to several years. Some hold on a long time. That slower pace is what makes treatment a real conversation on a live oak - there is time to act, and enough healthy tissue left for fungicide injection to do something.

Oak wilt on a red oak: fast and total

Red oaks do not do veinal necrosis. They do something much simpler and much worse.

The leaves go from green to a dull bronze, olive, or brown - often starting at the tips and edges and moving inward - and the tree wilts. Not one branch. The whole canopy, over a period measured in weeks.

Other red oak signs:

• Wilting and browning across the entire crown in a single season, sometimes in under a month.
• Leaves hanging on the tree after they have browned rather than dropping cleanly.
• The tree dies standing, with dead leaves still attached.
• Fungal mats under the bark. Look for a vertical crack or a raised, split area on the trunk or a large limb. Under it you may find a grey-to-tan pad of fungal tissue with a distinctly sweet, fruity, fermented smell.

Timeline: weeks to a few months, and there is essentially no treatment. A red oak showing wilt symptoms is a removal, not a rescue.

Why the red oak is the emergency

A dying live oak is a loss. A dying red oak is a spreader.

Those fungal mats are what attract the sap beetles that carry spores to fresh wounds on healthy trees somewhere else - potentially several properties away. The mats typically form in the cooler, wetter parts of the year, which in Central Texas means late fall through spring.

So the priority order, when both are on a property:

1. Dead or dying red oak with any sign of a crack or mat - remove now. This is the time-sensitive item. Cut it, and do not leave the wood stacked in the yard.
2. Assess the live oaks for veinal necrosis and map the infection center.
3. Decide on trenching and injection based on that map.

Homeowners routinely get this backwards, spending money to inject a prized live oak while a dead Spanish oak sits at the back of the lot growing spore mats.

Things that look like oak wilt but are not

San Antonio summers produce a lot of stressed oaks that are not diseased.

• Drought scorch browns leaf margins and tips uniformly across the whole canopy, not along the veins. It is covered in more depth in drought stress or oak wilt.
• Hypoxylon canker shows as flaking bark with a dark, powdery layer underneath, usually on trees already badly stressed.
• Root damage from construction produces decline on the side where the trenching or grading happened, often a season or two later.
• Normal live oak leaf drop in February and March looks alarming and is not a problem.

The distinguishing question is almost always the same: is the pattern following the veins, and are neighboring oaks going down in sequence? If yes to both, it is not drought.

What to do next

Take photos of a few leaves - held up, backlit, showing the vein pattern - and a photo of the whole tree from a distance that shows how much canopy is affected. Then walk the block and note which other oaks look off. That five minutes of work makes any assessment that follows dramatically more useful, and it is the difference between a quote for one tree and a plan for the property.