The best way to prevent the spread of oak wilt is to not prune any oak tree during the growing season. For that reason, the Iowa Department of Natural Resources (DNR) suggests to not start any pruning until at least the first frost.
“The risk of transmitting oak wilt through a pruning wound this time of year is low,” says Tivon Feeley, with the Iowa DNR’s forest health program. “However, to reduce the risk even further, it is best to wait until the first hard frost to start pruning.”
Oak wilt is caused by a fungus and has been present in Iowa for many years. A healthy oak tree can be infected by the fungus that causes this disease two different ways. The first is through open wounds during the growing season, when 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.
The trees in Iowa most commonly impacted by this disease are species such as red, black and pin oaks, but white and bur oaks can be infected as well. If black, pin or red oaks are infected by the fungus that causes this disease they usually die within the same year they are infected. White oak and bur oak can often take a number of years before they succumb to this disease.
Feeley says 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.
Feeley recommends that 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. Those products slow the tree’s ability to seal over the wound, he said.
More information on oak wilt prevention and control can be found here http://na.fs.fed.us/pubs/howtos/ht_oakwilt/identify_prevent_and_control_oak_wilt_print.pdf.