Oak wilt

What is oak wilt?

Oak wilt is a deadly disease of oaks. A fungus grows in the water-carrying vessels of the wood and blocks them. The tree can no longer move water to its leaves, so they wilt, turn brown and fall.

Trees in the red oak group (northern red, black, scarlet, pin, shingle and Shumard oaks) are hit hardest. University of Minnesota Extension says a red oak can die in as little as four weeks after a branch is infected. White oaks (white, bur, swamp white) resist better. They may lose a few branches a year and can take years to die, and some survive.

The disease spreads two main ways. Underground, roots of nearby oaks grow together (root grafts), and the fungus passes through them to neighboring oaks. Above ground, small sap beetles carry spores from fungal mats on dead trees to fresh wounds on healthy oaks. People also move it long distances in oak firewood and logs that still have bark.

Plants it infects: northern red oak, black oak, scarlet oak, pin oak, northern pin oak, shingle oak, Shumard oak, white oak, bur oak, swamp white oak, chestnut.

Symptoms and signs

• Leaves turn brown from the tips and edges inward, while the area near the midvein stays green longest
• Red oaks wilt from the top and outer crown down, drop leaves fast, and often die by late summer
• White oaks lose one or a few branches at a time over several years
• Dark streaks in the sapwood just under the bark of wilting branches
• On dead red oaks, black fungal mats (pressure pads) form under the bark and crack it open
• Groups of dead or dying oaks next to each other, because the fungus moves through joined roots

Oak wilt identification: what to look for

• Symptoms: In red oaks, leaves wilt and turn brown to red-brown from the tips and edges inward. Wilting starts at the top or outer crown. Heavy leaf drop follows, and trees often die by late summer.
• Symptoms: In white oaks, symptoms show up on scattered single branches. Decline is slow and may pause for a year or more.
• Damage: Peeling bark from an affected branch may show dark brown to black streaks in the outer wood.
• Signs: Black fungal mats form between the bark and wood of red oaks that died the year before. The bark above them cracks. Mats are rare on white oaks.
• Timing: Symptoms are easiest to see from May to August. After mid-September, normal fall stress can look similar.

Disease cycle

The fungus lives in the wood of infected oaks. In red oaks that died the year before, it forms spore mats under the bark, mostly in spring and fall. Sap beetles feed on the mats, pick up spores and fly to fresh wounds on healthy oaks. Penn State says a wound must be less than 72 hours old to be infected this way. The fungus also travels through root grafts to nearby oaks, sometimes between different oak species. Once inside, it spreads through the water-carrying wood. Untreated infected logs can carry the fungus for up to one growing season.

When to look: Look for symptoms from May through August. The highest risk of new beetle-spread infections is spring into early summer. Penn State lists May to mid-July as the riskiest window, and University of Minnesota rates April through July as high risk.

Oak wilt look-alikes

A few things get mixed up with it:

• Twolined chestnut borer damage (Agrilus bilineatus): Penn State says attacked trees show scattered branch death starting in late summer, with more branches dying as the attack goes on.
• Bacterial leaf scorch (Xylella fastidiosa): Leaf browning can look like oak wilt. A lab test tells them apart.
• Hypoxylon canker: Leaves ashy black fungal patches on the trunk, a thin crown and small leaves. Penn State says oak wilt trees show none of these.

How to manage oak wilt

Cultural control

• Do not prune, wound or cut oaks during the high-risk season. A Penn State article recommends pruning oaks from November to mid-April.
• If an oak must be cut in the growing season (for example after a storm), cover the wound right away. Minnesota Extension names latex-based paint or shellac.
• Never use climbing spikes on oaks (Purdue).
• Cut root grafts between sick and healthy oaks with a trencher or vibratory plow. Penn State gives a depth of 3 to 4 feet. Minnesota says a plow with a 5-foot blade is common.
• Remove infected red oaks, ideally in winter, and grind out the stumps (Purdue). Debark the wood, or burn it as soon as possible, and do not move it off the site.
• Before moving to the next tree, brush sawdust and chips off saws and spray them with a bleach solution (Penn State). Purdue notes that tools have never been shown to spread the disease.

Chemical control

There is no cure for red oaks that are already wilting. University extension sources say professional applicators can inject the fungicide propiconazole into healthy oaks near an infection to help protect them. Purdue says red oaks can be injected every other year. Penn State says it suppresses the disease but does not cure it. It can also help white and bur oaks if treatment starts before about 30% of the crown is lost. The injection does not stop spread through roots, so root grafts still need to be cut. In forests, Penn State notes infected trees are sometimes killed with the herbicide triclopyr instead. Use only products labeled for this use; the label is the law. Tree injection should be done by an ISA Certified Arborist who holds a pesticide applicator license (see pesticide applicator license guide).

Use only products labeled for this disease and this site; the label is the law. For trees, work with an ISA Certified Arborist. If you hire someone to apply pesticides, make sure they hold your state’s pesticide applicator license; the rules for each state are in our pesticide applicator license guide.

Prevention

• Schedule oak pruning and any cutting that could wound oaks for the dormant season.
• Avoid bark and root wounds on oaks from equipment during spring and early summer, when beetle risk is highest.
• Do not move oak firewood or logs from wilted trees. Burn or use it on site.
• Split firewood from infected oaks small and stack it loosely so it dries before the next spring.
• Plant a diverse mix of tree species, not just oaks (Minnesota Extension).

When to call a pro

Call an ISA Certified Arborist as soon as a red oak starts browning in summer or several oaks die close together. An arborist can take lab samples, plan root-graft cuts, and decide which nearby trees to inject or remove. Penn State Extension says suspected cases should be confirmed by a lab and names the Penn State Plant Disease Clinic as a good option.

Related guides: Tree database: mature size and planting distance · Pennsylvania tree identification · Firewood rules by state · All pests and diseases

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