News release: Dead leaves, dying branches – is it oak wilt or lingering effects of drought?

May 23, 2024

Knowing the difference between oak wilt and other stressors can help people save money and care for their trees

It’s no secret that Minnesotans love their trees and want to keep them healthy – but sometimes concern leads down the wrong the path.

According to the Minnesota Department of Natural Resources, in 2023 many people misdiagnosed deadly, invasive oak wilt when the culprit for most ailing oaks was general oak decline fueled by the severe drought seen in many parts of Minnesota at that time.

“Symptoms like rapid death or dying branches can be attributed to both oak wilt and oak decline spurred by severe drought,” said Brian Schwingle, DNR Forest Health Program coordinator. “If in doubt, send a sample to a lab for a definitive oak wilt test before committing to expensive treatment.”

Oak wilt is an invasive fungal disease that kills all of Minnesota’s oak species. In recent years, the disease has expanded from east-central and southeast Minnesota to northern forests in Crow Wing, Cass and Pine counties. Treatment can be costly, but prevention is effective. Oak wilt is most apparent on red oaks, where it causes near-total leaf drop over one or two months. For white oaks and bur oaks, oak wilt symptoms often resemble general decline, which can make diagnosis tricky.

Oak decline can be thought of as death by a thousand cuts. Decline most often results in sections of the tree canopy dying and holding onto dead leaves for months. Severe decline can result in trees dying in just one summer, although something is usually stressing these trees for years prior to dying - like old age and soil compaction. Other common factors that provoke decline on stressed trees include severe drought and native pests like twolined chestnut borer and Armillaria root disease.

Last year, less than 18% of oak samples submitted to the Plant Disease Clinic at University of Minnesota were positive for oak wilt. Anyone can submit a sample to be tested for a fee. It is worth the time and upfront cost to confirm if a tree has oak wilt before implementing oak wilt management.

Most of the current oak decline and dieback across the state can be attributed to the recent years of drought.

“The good news is much of Minnesota is out of drought status, and that will help oaks recover,” said Schwingle. “However, drought impacts linger in trees, so even if this year is wet we’ll still see stressed, declining and dying oaks. The best thing people can do is to water their yard trees during dry conditions.”

Tips for mulching and watering can be found on the DNR’s tree care webpage.

Other steps people can take to help stressed trees include avoiding unnecessary pruning, refraining from using fertilizer when it isn’t necessary and preventing soil compaction around the roots. If people are in an area with oak wilt risk, they should not prune between April to July, when risk of spread is high.

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