Like the American elm in the 1950s and ash trees today, Minnesota's oak trees are facing potentially significant die-offs due to an exotic and largely invisible killer: oak wilt.

This deadly fungus can kill previously healthy trees within a matter of weeks. Oak wilt is spread by sap-feeding beetles - also known as "picnic beetles" - that introduce the disease to freshly wounded trees. The fungus then quickly moves through the underground root systems that connect entire stands of oak trees, rapidly increasing the impact of one infected tree. Considering the highly contagious nature of oak wilt and the fact that root systems of large oak trees can spread 60 feet or more, all red oaks found within this area may become infected.

Oak wilt kills by interfering with the vessels that transport water throughout the tree. The fungus plugs up those vessels and is fatal. Trees with oak wilt commonly begin dying from the top down, since the top is farthest from the roots and is the first part of the tree to suffer from lack of water. Oaks shedding their leaves in June or July instead of during the late fall can indicate an oak wilt infection. There are at least a dozen copycat disorders that mimic oak wilt symptoms, so it is best to have a laboratory confirm findings and not rely on the visual appearance of the tree alone.

Any wound or pruning cut made during the high risk time (April 1 through July 15) is like ringing the dinner bell for the beetles. They have been known to find a freshly wounded tree in as little as 15 minutes and possibly transmit the infection. Other culprits in the spread of oak wilt are spring storms which break twigs/branches, freshly cut stumps, and movement of firewood.

Trees in the red oak group - those that have pointy-lobed leaves, such as northern red oak, and pin oak- are most susceptible to the disease. White oaks - those with rounded lobes - are much less susceptible, though they occasionally are also infected.

If you would like to reduce the risk of losing entire stands of trees to oak wilt, increase diversity. If you want to plant oak trees, white oak is the best option.

Click here to check the current oak wilt risk status in Minnesota.

Scorched look of a red oak infected with oak wilt.

White oak leaves dropped during the summer show classic drought stress from oak wilt as the tree is unable to move water.