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Two northern red oaks brush their knobby twigs against the sky. The right-hand tree was infected with oak wilt and its leaves died and fell off in the course of a single month. The left-hand tree was girdled and treated with herbicide during the growing season in order to kill any connected roots that may harbor and spread the oak wilt fungus. Photo by Emily Stone.

Snowshoes crunched loudly on the hard crust as we made our way toward a cluster flagged trees. After inspecting the furrowed bark of a large northern red oak, Paul Cigan, a forest health specialist with the Wisconsin Department of Natural Resources, pulled out his hatchet and started slicing off chunks of bark. Holding the rich, reddish-brown underside up to his nose, he inhaled deeply. “That’s it,” he exclaimed, “that’s the fruity smell I was telling you about. It’s the smell of oak wilt.” 

I’m all about scratch-and-sniff in nature, so when he offered me the piece, I had no reservations about holding it up to my nose as well. It did, in fact, smell quite a bit like bananas. Slightly fermented bananas, to be exact; like when I thaw a brown one from the freezer for baking banana bread. 
This pleasant smell has a worrisome source, though. Oak wilt is a fungal pathogen that kills trees in a single season. While it’s already widespread in southern Wisconsin, it has only just arrived here in the north. And we have a lot of big, beautiful oaks. Paul had watched this one die last summer, after he spotted its reddening leaves during an aerial survey. 

Local foresters have marked the oak-wilt-infected tree with red paint. Neighboring oaks that are close enough to have grafted their roots must also be removed, and they are marked with blue paint. While it’s sad to see potentially healthy trees killed pre-emptively, it is an essential part of oak wilt management. Prevention is preferred. Photo by Emily Stone.

Hoping to show me an example of the characteristic fungal mat that appears under the bark of an oak-wilt-killed tree, Paul peeled off even more bark. He didn’t find the fungal mat, but he did hand me a bark chunk with a white larva still cradled in its tunnel. “It’s two-lined chestnut borer,” he declared, and we examined the meandering galleries the borer and its buddies had chewed into the tree’s cambium. Chestnut borers only attack already-stressed trees, and while their damage looks a little bit like oak wilt, they take anywhere from two to several years to kill the tree. Rapid death by oak wilt counts as severe stress, so that’s why these little pests are here. 

Still hoping to show me what the fungal mat of oak wilt looks like as it bulges underneath the bark, Paul and I headed off to another infection site in Washburn County. I’d been anticipating a nice long walk in the woods on this scouting trip, but instead we pulled into the maintenance road leading to a cellphone tower. Being on town land, the infection of this oak tree was noted quickly by local foresters. The oak-wilt-killed tree was flagged with bright pink tape and spray-painted with an ominous red circle. Oaks in a surrounding cluster were marked with blue paint. 

It’s a common story. Early last summer, roadwork led to inconspicuous scuffs on the base of the tree’s trunk. When sweet tree juices began oozing out of the wounds, the scent lured in hungry beetles. Those sap-feeding beetles had already eaten a meal of banana-scented oak wilt fungus somewhere else in the forest, and had picked up fungal spores on their bodies. They transferred those spores onto the new oak’s fresh wound.

The fungus worked quickly to invade the tree’s water conducting system. While white oaks seem to be able to mount a defense and exhibit a degree of resistance to the disease, the red oaks that are dominant up here don’t stand a chance. The oak’s leaves wilted from the crown down, and within a month the tree was dead.

Despite hacking away at the bark, Paul still couldn’t produce a fungal mat to show me. “That’s probably a good thing,” he commented. Oak wilt isn’t supposed to be active yet this early in the spring, and that gives foresters, loggers, and landowners a window of freedom. From August through March, the fungus isn’t in reproductive mode. No fungal mats are giving off spores, the beetles don’t fly in the winter, and the trees aren’t growing vulnerable new cells. Even if an oak is injured, it won’t become infected.

Paul Cigan, a forest health specialist with the Wisconsin Department of Natural Resources, slices bark off an oak-wilt-killed tree in search of the characteristic fungal mat. Photo by Emily Stone.

During the fungi’s active time, however, from April through July, how you treat your oak trees can mean the difference between life and death for them. Any sort of wound, whether it’s a scrape from a bulldozer, a pruning cut, a logged stump, or even a broken twig can be the entry point for oak wilt into your forest, and your neighbor’s forest, too. Using wound-sealer to cover injuries immediately can help. (Beetles can find a new wound in 15 minutes or less!) Paul mentioned it several times through the day: the only thing that will prevent oak wilt from becoming a widespread problem in our forests up here is responsible landowners and preventative care.

Luckily, there are many things we can do to prevent oak wilt. In contrast to the beetle that spreads Dutch elm disease, the beetle that spreads oak wilt can’t chew its own holes. We can be careful not to make holes for them. When caught early by observant land managers, infections can be contained. 
Beetles aren’t the only way that oak wilt spreads, though. The fungus can travel through the tree’s roots, pass through root grafts with nearby oaks, and kill them, too. The blue-painted trees on this site were already dead, but not from the fungus. They’d been girdled and painted with herbicide. While it sounds drastic, this is the most reliable method to make sure that there aren’t infected root grafts that will spread the fungus below ground. “Collateral damage,” Paul called it with a sigh. It’s better that these few trees die than every oak in the forest. They are still salvageable as lumber, and even the fungus-killed trees can be used for firewood if you quarantine the logs under plastic for a year so that beetles can’t access them and spread their fungal spores.

Oaks are a major component in our forests, and they are important ecologically, economically, and aesthetically. Last summer, for the first time, an infected oak was detected and treated just southeast of Seeley, Wisconsin—a main hub on the Birkie Trail. While mentioning this, the truth came out. Paul’s a big cross-country skier, and his dedication to the control of this disease is partly fueled by his love for the forests surrounding our beloved American Birkebeiner Trails. Skiing is a personal challenge, but preventing oak wilt will have to be a team effort. 

Special Note: Emily’s book, Natural Connections: Exploring Northwoods Nature through Science and Your Senses is here! Order your copy at http://cablemuseum.org/natural-connections-book/. 
For 50 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI! Our new exhibit: “Bee Amazed!” opens on May 1, 2018.