A Primer on Oak Wilt


Oak wilt is one of the most widely-known issues affecting oaks in the state of Wisconsin, and for good reason. The fact that this pathogen can kill mature trees in less than one growing season demands attention. The cycle of this pathogen begins when a tree is wounded and sap begins to flow at the wound site. Since one of the main components of sap is sugar produced by photosynthesis, the wound attracts a sugar-seeking beetle called the Nitadulid beetle or “Picnic Beetle”, nicknamed as such due to its preference for sweets. The Oak Wilt fungus lives in the beetle’s mouthparts, which spreads the fungus while feeding. Picnic beetle feeding occurs at fresh wound sites during the growing season, with the largest amount of activity in April-July in Dane County. (Take note that the dates can fluctuate significantly and are highly dependent on temperature.)

An infected tree can spread the pathogen secondarily through root grafts, where tree roots of the same species fuse and share resources underground. For this reason, oak wilt can spread through a network of connected trees in neighborhoods and forests alike at alarming rates.

Red Oaks vs White Oaks

The two main groups in Soutern Wisconsin are the red oak group (red oak, black oak and pin oak) and the white oak group (white oak, bur oak and swamp white oaks). When it comes to succeptibiity to oak wilt, the red oak group are more vulnerable compared to white oaks. The main reason for this is due to the tendancy of red oaks to form root grafts to neighboring reds. White oaks are less succeptible and can be treated for oak wilt if it’s diagnosed early enough. Chickapin oaks, though native to only a small part of the state, are the most suceptible member of the white oak group and should be monitored closely for symptoms.

The main tool for management of oak wilt is preventive treatment with fungicide injections. The main factor for determining whether to treat preventatively is proximity to other red oaks. If you live in an area where you have few neighbors and those neighbors do not have red oaks then you may decide not to treat your reds. However, if you live in an area where you have other red oaks nearby, preventative treatment is strongly reccomended. The important distinction is that if a tree in the white oak group becomes infected, it can live with the disease as long as it recieves regular oak wilt treatment after being positively identified. The case is not the same for a red oak, which cannot be saved once it is positive for the oak wilt pathogen.

You may not know where to start and that’s ok. Give Tree Health Management a call and one of our arborists can help you determine what kind of trees you have, and how to care for them.

We are your arborists after all!

*image credit Wisconsin DNR

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