Cold weather is here and now is the time to start pruning oak trees with minimal to no risk of spreading oak wilt.
“There have been several days in a row where temperatures were at or near freezing and the risk of oak wilt is very minimal,” says Tivon Feeley DNR Forest Health program leader. This low risk zone for pruning oaks will last until about March first.
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 usually die within the same summer they are 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 in 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 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 avoid pruning and prevent any wounding to oak trees during the growing season — the end of March through the first hard freeze. If a tree is wounded from storm damage or pruning is required during the growing season, treat the wounds immediately with a wound dressing such as acrylic paint. Do not purchase pruning paints/sealants because those products slow the tree’s ability to seal over the wound.