How pruning an oak tree at the right time can head off a deadly new disease

Waiting until the leaves drop in fall is a rule of thumb for pruning shade trees, mainly because you can see bare limbs better, and the job is less "messy" without leaves attached to the cut branches.

But these days there's a much more important reason to prune in fall or winter – particularly oaks. That dormant-season timing helps oaks avoid a deadly disease called "oak wilt" that's been working its way eastward throughout Pennsylvania.

Oak wilt has been detected in 34 of Pennsylvania's 67 counties, up to and including the western doorstep of the Susquehanna River in Juniata, Perry, Cumberland, and Adams counties, according to Jill Rose, a forest pathologist for the state Department of Conservation and Natural Resources' Bureau of Forestry.

"I've been dealing with oak wilt mainly in the northern part of the state and sporadically in the western part," says Rose. "Currently, oak wilt is a rare disease," adds Calvin Norman, a Penn State University oak-wilt researcher and assistant teaching professor of forestry. "But it is becoming more common, and people should be aware of it."

The large red oak family is especially susceptible to the disease, which can kill even a mature oak within two months of infection. There is no cure.

Numerous oak species are native to or common in Pennsylvania, where they populate forests, grow as back-yard shade trees, and serve as arguably best host plant for bird-feeding caterpillars.

Oak wilt's onset can be confused with other more common issues, such as drought stress, leaf loss from spongy-moth caterpillars, and diseases such as anthracnose and bacterial leaf scorch. So it's important to nail down a diagnosis before hitting the panic button.

Besides the speed of infection, oak wilt's tell-tale symptom is that the leaves turn bronze starting from the leaf margins and working inward toward the main vein, says Norman.

This outside-inward discoloration usually starts with a branch or two in the upper crown of the tree, then spreads quickly until much of the tree looks brown and is shedding leaves.

Trouble happens breathtakingly fast in red oak species, especially between May and August when the leaves should be green and healthy.

The red-oak family includes northern red oak (), scarlet oak (), shingle oak (), Shumard oak (), and the long-popular home-landscape pin oak ()

Those summer-dropped leaves can be inspected to see how the browning is taking place. You'll likely find several stages of browning on the fallen leaves to show the browning progression.

Rose says bacterial leaf scorch also can cause leaves to turn brown from the margins inward, but "there is usually a yellow halo between the brown and the green healthy leaf tissue," she adds.

Another difference: In the later stages of an oak-wilt infection, the fungus causing the disease grows black mats under the bark, which creates pressure that often leads to cracks and vertical splits in the bark.

A third diagnostic tool is slicing into a section of dead branch. If it's wilt disease, you'll likely find dark streaks in the inner wood where the branch's vascular system has been plugged by the fungus.

That blockage is what shuts down the tree's water-moving system and leads to leaf wilting and tree death – somewhat akin to clogging of the arteries in a person.

Oak wilt also can target white oaks, although that family is better equipped to resist the fungal blockage. White oaks can limp along with milder symptoms for 10 years, and unlike red oaks, can be treated to slow the disease, both Norman and Rose point out.

The white-oak family includes the widespread native white oak () as well as swamp white oak (), chestnut oak (), and bur oak (

Leaf shape is how to tell red oaks apart from white oaks. White oaks generally have rounded lobes while red oaks tend to have pointed lobes.

Oak wilt spreads in one of two main ways.

The first is via a type of sap beetle that feeds on oak sap.

Rose says species of nitidulid beetles can pick up fungal spores from an infected tree and transfer them when they visit an uninfected one.

"Nitidulid beetles are attracted to wounds on oak trees," says Rose. "Pruning during the 'do-not-prune' window of April 1 to mid-July will increase chances that a nitidulid beetle will transmit the fungus. The population of the beetle drops in mid to late summer, so the chance of transmission is lower then."

Penn State Extension educator Sandy Feather of Allegheny County says the wilt-spreading beetles can show up within 10 or 15 minutes of a wound and travel as far as a mile.

The second main method of disease spread is by underground roots. Oaks often knit their roots together to share nutrients and moisture in a technique called "root grafting."

The downside of this network is that when one tree is infected with disease, the disease can be handed off from one tree's roots to another.

In a forest or oak grove, "this can cause a 'pocket of death' as one tree dies, then the next year all of the trees grafted to it die," says Norman.

That can eventually lead to acres worth of tree death and a fungus that can persist underground for up to five years even after an oak has died.

A third possible method of disease spread is moving spores by equipment, such as chainsaws, pruning saws, and even arborists' tree-climbing spikes. That type of spread can be prevented by disinfecting equipment with a 10 percent bleach solution or similar disinfectant between work on different trees.

Besides avoiding pruning during the growing season, Norman says another way to be vigilant about oak wilt disease is to watch for the wilt-spreading nitidulid beetles between May and August – especially following a summer storm that can blow them into a new area.

The sap-sucking beetles are less than one-half inch in length, elliptical in shape, and reddish brown to black in color. They have three pairs of opposite legs and one pair of antennae.

Oak wilt was first identified in Wisconsin in 1942 and in western Pennsylvania by the 1960s. Feather said it wasn't detected east of the Susquehanna River until 2021 when laboratory tests confirmed isolated cases as far east as the border of Chester and Delaware counties.

The U.S. Department of Agriculture says the disease's origin is unknown, but it's apparently active now only in the U.S.

"We don't have exact numbers on oak wilt infections, but it has become more common, and we've found it in more places than in the past," says Norman. "Some of that may be due to increased vigilance and better identification, some to more spread, and some to climate change improving conditions for its spread, such as more violent storms and more stress on trees."

Norman says the best way to deal with oak wilt is to avoid it in the first place.

"Don't trim, cut, or damage oaks when the beetles that spread oak wilt are flying," he says. "If someone suspects oak wilt, look for multiple leaves that died from the margins in and for rapid canopy death. If the symptoms line up, send a sample into a plant pathology lab."

Penn State's Plant Disease Clinic tests for this disease (and others) at no charge for Pennsylvanians.

Norman says several symptomatic branches of a half-inch to one inch in diameter and six inches long should be sent in a labeled plastic bag and kept cool between cutting and mailing.

"It's not uncommon for fungi to die in hot cars," he says. "Dead fungus in a sample means there is a high likelihood of a false negative."

Rose adds that suspected cases of oak wilt also should be reported to Penn State Extension, while private landowners can get additional guidance from a DCNR service forester.

If oak wilt is confirmed, action should be taken ASAP, Norman says.

That usually means removing red oaks by first cutting a strip around the trunk ("girdling") and treating the exposed band with the herbicide triclopyr, which kills the roots and heads off underground spread.

"Once the tree is dead, it can be cut down," Norman says. "Just cutting a tree down without killing it is not advised because oak roots will push their nutrients to any trees they're grafted to when the trunk is removed."

Penn State research found that giving a dead infected tree several weeks to dry also helped kill the spores and head off above-ground spread.

"Once a tree is felled and allowed to dry, people can use the tree however they'd like," such as for firewood or chipping and mulching, Norman says.

However, even then, wilt-infected wood should be moved off-site, he adds.

"Moving firewood is how so many forest health issues get spread around," Norman says.

Research in Minnesota found that oak wilt disease can be treated in white oaks by pruning dead wood, breaking apart root grafts, and injecting white oaks every two years with the fungicide propiconazole.

Propiconazole injections did not help red oaks, though, and even in white oaks, the fungicide didn't "cure" the disease ... it just suppressed it.

See Minnesota Department of Natural Resources oak wilt management guide and video

Read more on oak wilt disease at Penn State Extension

See Wisconsin's oak-wilt-disease video and management tips