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Don’t let the recent unseasonably warm weather spell confuse you, the time to start pruning your oak trees is fast approaching.

DNR forestry experts say that due to the warm weather conditions, mid-October will be the best time to prune your oak trees. They warn to not prune oak trees between late March and early October to best prevent the spread of oak wilt.

Oak wilt has been present in Iowa for many years and is caused by a fungus. The trees in Iowa most commonly impacted by this disease are species such as red, black, and pin oak, but it can also infect white and bur oak. If black, pin, or red oak are infected by the fungus that causes this disease they will likely die within the same summer they are infected. If infected, white oak and bur oak can often take a number of years before they succumb to this disease.

A healthy tree can be infected by the fungus that causes this disease two different ways. The first is through open wounds during the growing season. The fungus is carried from a diseased tree to a healthy tree with an open wound by a small beetle.

The second form of infection is through root grafts between oak trees of the same species. For example, if a red oak is infected and there is another red oak within 50 to 100 feet there is a good chance that the roots of these trees are grafted and the fungus can move from the diseased tree to the healthy tree.

Symptoms to look for on infected trees usually include leaves turning a bronzed brown along the outer margins of the leaves. These leaves can often still have some green on them as they fall from the tree. The defoliation tends to start at the top of the tree. The best way to prevent the spread of oak wilt is to prevent any wounding to oak trees during the growing season. More information on oak wilt prevention and control can be found by visiting: fs.usda.gov/naspf/sites/default/files/publications/identify_prevent_and_control_oak_wilt_print.pdf.

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