Sunday, August 8, 2021

Oak wilt: No laughing matter

leaf and whole tree symptoms of oak wilt in a red oak treeIt’s normal to tune out all the Chicken Littles (such as yours truly) who run around squawking about this or that invasive forest pest or disease that pose a threat to trees. I mean, how many times can the sky fall, anyway? But the real danger is when we feel so overwhelmed that we throw up our hands. Thinking we can’t make a difference could result in more harm to forests than the pests themselves.

There’s a pithy fable about a child who rescues starfish from the beach after storms, and some busybody informs the kid that they can’t save all the starfish. The child responds by hurling another starfish into the sea and quips “yeah but I saved that one.” Right now we have the chance to help save oaks from a devastating new disease, not by tossing them in the ocean, but by adopting some painless and cost-free practices when pruning or harvesting oaks.

Oak wilt (Bretziella fagacearum), a virulent pathogen first identified in 1944 in Wisconsin, has moved into New York State with a vengeance, mainly in the past decade. This disease, which is of unknown origin, will turn a lush, healthy red oak to a crispy critter in just two weeks. Tree pathogens don’t get much nastier than that – I suppose if it also caused oaks to burst into flames, that would be worse.

Oak wilt spreads through root grafts as well as spore transfer. Underground tree-to-tree spread, while an important pathway near active outbreaks, is less important than airborne transmission. This latter route is where we come in.

Healthy red, black, pin, scarlet, and other “red-type” oaks succumb in a matter of weeks, while “white-type” oaks such as bur and swamp-white oaks don’t bite the dust so spectacularly, taking a year or two to die. After a red-type oak is killed, the pathogen makes mycelial spore pads under the bark, causing small bark splits. A spore-laden ooze, reported to smell like Juicy Fruit gum, is secreted, which attracts insects, the most significant of which are sap beetles in the family Nitidulidae.

Nitidulid beetles feed on sugars from the sapwood of newly cut or wounded hardwoods. Normally, not a problem, unless the beetles have recently wallowed in disease spores at an oak-wilt spore pad. Though spore pads develop only on red-type oaks, all oaks can be infected by a spore-covered beetle if it finds a fresh wound during the beetles’ flight season.

So here’s the big news: Paint is your friend.

From April 1 – July 1, the risk of spreading oak wilt is extreme, and from July – September 30 it’s moderate. Any exposed fresh wound on an oak, whether a stump after a tree removal or a pruning wound, puts them at risk. Rule One is never to cut oaks, or allow them to be accidentally wounded, from March through September. OK, now stop laughing – that’s not a rule.

Although it would be ideal not to prune or harvest oaks all spring and summer, it’s impossible. The work-around is to paint each wound or stump immediately after cutting. Spray paint is easiest, but it can be any cheap, leftover exterior product – whatever you have. But use it right away, as nitidulid beetles can find fresh oak sap in under an hour. On pruning wounds, paint the whole thing (Having spent years trying to convince people not to paint wounds, this is hard for me as an arborist). With stumps, only the sapwood needs to be covered.

It would be fair to ask why a stump should be painted. It’s because depending on how many root grafts are interconnected with that stump, spores deposited on a cut stump could infect many nearby oaks through grafts – roots extend three times the branch length.

Painting oak stumps and pruning wounds between March 1 and September 30 must become standard practice in the forestry, utility-clearing, and tree-care industries, But Jane Q. and John Q. Public have a role to play as well. Demand this practice from any arborist you hire, and put it in the contract if you have a woodlot harvested. If there are oaks in your landscape that you prune yourself, follow the same guidelines.

Oak wilt is not hundreds of miles away from us. It’s as close as the first idiot who brings firewood home from visiting his buddy whose dead tree he helped cut up. (History shows that most such idiots are guys; hence the gender exclusion.) Don’t move firewood long distances!! Here is our chance to hold a very important line. Implementing these strategies will vastly reduce the risk of seeing oaks go the way of the American chestnut. Let’s do our part to prove Chicken Little wrong.

If you suspect oak wilt, please report it to your nearest NYS Department of Environmental Conservation office. See: for more information.

Paul Hetzler, an ISA Certified Arborist and former Cornell Cooperative Extension Educator, is presenting a free program on oak wilt this Thursday, August 12 at 6 PM at the Indian River Lakes Conservancy in Redwood. For details, see

Paul Hetzler has been an ISA-Certified Arborist since 1996 and is a member of NYS Arborists and the Society of American Foresters.

Photo courtesy of Joseph O’Brien, USDA Forest Service/Almanack archive

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Paul Hetzler has been an ISA Certified Arborist since 1996. His work has appeared in the medical journal The Lancet, as well as Highlights for Children Magazine.You can read more of his work at or by picking up a copy of his book Shady Characters: Plant Vampires, Caterpillar Soup, Leprechaun Trees and Other Hilarities of the Natural World

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4 Responses

  1. nathan says:

    we need to find ways of stopping these inported deseases, bugs, funguses, ect. if it requires blocking import of any living material so be it.
    we are loosing so many trees species, black walnut, butternut, elm, chestnuts, spruce, ect , now a variety of oaks…so many food trees, some of the most beautiful woods, graceful and shady trees. once an invasive organism gets here, it is never defeated, never eradicated, mostly millions to billions to fight it and then lost cause…
    Profit of imported products has to be weighed against the cost of mass extinction of native plants, foods and wildlife.

  2. JT says:

    Paul. Interesting article. I was aware of Oak Wilt but have not researched it yet. Has there been any cases in St. Lawrence County that you know of? It happens to be one of the better performing trees in my area in the northern part of the county. I have a quite a few 12 – 18″ dbh trees in my woodlot. No intention of cutting them down, so hopefully that will decrease the chance of getting it. I am just starting to get infected ash trees by the EAB, so I cannot deal with another tree pest.

  3. Charlie Stehlin says:

    There’s also the maple leaf blister which many maples in my neighborhood are consumed with, ugly lesions on their leaves. Personally I think we’re a little too late on the matter of invasive’s and the diseases overcoming our trees. It’s a shame really! We just don’t have the will to change our stinking way of thinking. Look at climate change per example (which is only going to aggravate the problem!) There’s a large school of scholars who just cannot be convinced that what we’re experiencing regards the change in weather patterns all over this wee orb Earth is for real….as if blindness has overcome them even though they have good eyesight. Of course our erected leaders don’t help on these matters. Indeed they’re a major part of the problem.

    • JT says:

      Charlie and Nathan,
      Agree 100% with both of your comments. With globalization run rampant, it is all about the almighty dollar, only looking at the short term gains for investors. We invest so little in protecting the environment compared to GDP which we will end up paying for eventually. Reading the almanac, I see many comments from people like yourself with the same concerns, but you are the few. Most of my circle of friends and co-workers have no idea what is going on. I explain what buckthorn and honeysuckle are, which we have plenty around here, and they are not overly concerned. They think spruce trees and balsam firs are all pine trees. Not their fault, they have never been educated. I think the only way we can get a handle on the problem is to start teaching this stuff in the schools at an early age. Kids should know about the trees, plants and animals in their local environment. Then, when they are older, we would have a much larger pool of people to put pressure on the politicians to enact policies to that help reduce the spread of these invasive species.

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