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When does oak wilt season begin?

March 17, 2021
Two mature oak trees grow side-by-side, one with brown leaves due to oak wilt.

After the first couple of warm sunny days in March, we start to get the emails. “Is it still okay to prune my oak trees, or should I be concerned about oak wilt?”

I’m always happy to respond to these emails. It shows that the homeowners and arborists that write in are concerned about keeping their trees healthy.

Oak wilt is caused by a fungus and is responsible for killing large numbers of oaks every year in Minnesota. It is currently found in central and southeastern Minnesota. Oak wilt is most severe in the red oak family, including northern red oak and northern pin oak.

Don't prune in spring and summer

We recommend not pruning oaks in the spring and summer because sap-feeding beetles that carry the oak wilt fungus are attracted to the chemicals produced by fresh wounds. If wounded oak trees are visited by these fungus-contaminated beetles, oak wilt spreads. Oak wilt can also be spread belowground through root grafting.

The high-risk season of oak wilt begins when the weather warms up enough to make the sap-feeding beetles active. We update the oak wilt risk status on this website when we're confident the status has changed.

Our general recommendation is to not prune oaks after April 1. While this date is a good rule of thumb, the actual date for the start of the high-risk period of oak wilt is slightly different every year.

Our decision to set the oak wilt high-risk period to high is based on several factors:

Have there been consecutive days with temperatures above 60 degrees?

Temperature is a useful predictor of the phenology, or seasonal timing, of insects.  More specifically, the number of degree days that have accumulated in a season can be used to determine when the sap-feeding beetles become active.

We use the University of Wisconsin’s Ag Weather Data as a tool to quantify how warm spring has been, using the weather in the Twin Cities as a guide.

Is there snow cover?

If there is snow cover, sap-feeding beetles will not likely emerge. Many of you might remember the record snowfall totals in April 2018. In that year the high-risk period didn’t begin until April 25. Snow cover was across much of the state well into the middle of April.

What do the experts say?

We rely a lot on scientists and specialists to provide input on when to call the high-risk period.

  • Jennifer Juzwik is a research plant pathologist with the USDA Forest Service-Northern Research Station who always provides suggestions on the high-risk period.
  • Others across the state, such as the forest health specialists in the Minnesota Department of Natural Resources, are great resources that provide updates on conditions in their area.

From our records, here is the calendar date when the high-risk period of oak wilt began in recent years:

  • 2020: April 6
  • 2019: April 4
  • 2018: April 25
  • 2017: March 31
  • 2016: April 1

Oak wilt is a serious tree disease that can potentially kill a large number of Minnesota’s oaks.  Read more about oak wilt and learn how to keep your oaks healthy.

Matt Russell, Extension forestry specialist

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