If your in the Midwest and your oak (Quercus) leaves are now looking like the above photo, it more than likely has oak wilt.
Oak wilt is a disease caused by the fungus Ceratocystis fagacearum, that is either spread by beetles of the nitidulid family (commonly known as sap bugs) or by roots touching underground. The disease clogs the vascular system of the tree causing wilting.
This disease kills red oaks including; red, black, pin, and scarlet varieties. White oaks including; white, bur, and swamp white oaks tend to pull through, although it takes many injectable fungicide treatments and a lot of care must be given.
The symptoms on red and white oaks are different also. Red oaks develop wilting leaves, bronzing, and shedding of the leaves near the top of the tree. Bronzing starts on the edges of the leaves, moving inward to the midrib, along with wilting. Both green and bronzed leaves fall to the ground. Spore mats grow on dead trees in early summer on the sapwood, directly under the bark. There is enough pressure exerted to pop the bark off. The fungus extrudes a fruity odor that attracts the sap beetles for its dispersion.
White oaks develop symptoms within the top of the tree also, but only on a few branches, and not so severe. Streaking of the sapwood is more pronounced on the white oak than the red, and there are no spore mats.
Oak wilt is confused with other problems such as drought, construction stress, borers, and root problems.
These symptoms would include:
- More noticeable during late summer
- Regular size leaves, little wilting
- Leaves browning evenly
- Leaves remain on the tree after discoloring
- Dying trees scattered throughout stand
- More common on stressed sites
- Signs of borers or root disease
Oak Wilt symptoms:
- More noticeable during early summer
- Small leaves, thin crown, wilting
- Edges and tips of leaves bronzing first
- Leaves drop soon after discoloring
- Dying trees found in groups (root grafts)
- Streaking and discoloration of vascular tissues
There is no cure for oak wilt, only management of the disease, which consists of preventing the spread of it.
Other considerations to remember:
- Avoid pruning oaks from April through August as that is the time the spore mats mature and the beetles can infect recently wounded trees
- If a red oak is infected with other non-infected trees around it, sever roots via trenching to 3-4 feet deep to prevent root grafts to healthy trees
- An injectable, systemic fungicide (Alamo) is available from a licensed arborist for white oaks.
- Unfortunately, there is no cure for red oaks, which should be removed and properly disposed of.
© The Naturarian