Oak wilt reaches Crow Wing County, putting Minnesota's northern trees at risk

The edges of the leaves of an old oak tree near Brainerd turned brown last summer when they should have been at their healthiest. This spring, as everything around the tree was blooming, its leaves fell, creating a telltale carpet of dead leaves with brown edges and green centers. Samples sent to a lab in the Twin Cities recently confirmed what arborists suspected: Oak wilt has made it to Crow Wing County.

It's the farthest north the disease, always fatal to most oak species, has been found. The spread now covers about a third of the state, putting one of Minnesota's most important trees at risk.

One of the striking things about oak wilt, caused by an invasive fungus, is how quickly it attacks a tree once it's there, said Rachael Dube, forest health specialist for the Department of Natural Resources (DNR). Most red oaks die within two months of infection, she said. Some are killed in just a couple of weeks.

"While it's devastating to individual trees, what we're very concerned about over time is the ecological impact of losing oak trees," Dube said.

Oaks are one of the most prevalent big trees in Minnesota. They're stately, beautiful, popular in yards and parks throughout the state. They're relied upon economically as the No. 1 harvested hardwood in Minnesota. The tough, long-lived trees are expected to do well even as the climate continues to change.

"Entire ecosystems depend on them functioning," Dube said. "The trees themselves are habitat. So many wildlife species depend on acorns for food."

Increasingly endangered species that once thrived in oak savanna, such as the redheaded woodpecker, rely on old hollowed out oaks for shelter.

"Things like invasive buckthorn tend to proliferate as soon as oaks are gone," she said. "So a lot of this concern is really about what comes after."

The disease attacks trees in small pockets, just a few acres at a time.

The saving grace is that, left on its own, oak wilt spreads very slowly. It's been in Minnesota since 1945, but it still has yet to reach much of the state. Its two main ways of natural spread are not very efficient.

Spores can attach to native beetles that carry the fungus to new trees. Fortunately, those beetles tend to travel no more than half a mile in their lifetimes.

It can also spread through the trees' roots. Even when an infected dead oak is cut down, the fungus can survive in its roots underground for years. When other oaks are close enough, their root systems will become entwined and the fungus will slowly spread from root to root.

It's unclear how oak wilt made it to Crow Wing County. It's possible it came from firewood, especially given how popular Brainerd area lakes are, she said. But the disease has been knocking on the county's door for some time and it may have spread into the area through beetles.

The key to protecting oaks is to not prune, trim or otherwise harm them during months the beetles are most active — from April to July. The beetles that carry the spores aren't usually able to harm a mature healthy oak, Dube said. But if they can find a way into the tree through a gaping wound or a pruned branch, they'll spread oak wilt easily.

"Really, prevention is our best way and only way to slow the spread," Dube said. "It's crucial that people just don't do anything to harm an oak during those few months when the pathogen is producing spores and when the beetle is abundant and flying around."

Greg Stanley • 612-673-4882