Preventing the Spread of Disease

PREVENT OAK WILT: PRUNE OAKS IN THE WINTER MONTHS

It is cold out, but now is a great time to be pruning oak trees. If you have oak trees on your property the best way to prevent infection by the disease called oak wilt is to only prune your trees during the winter months. This disease has been present in Iowa for many years, and it 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 summer they are infected. White oak and bur oak can often take a number of years before they succumb to this disease after infection.

Basically, 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 (April- September). 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.

Infected black, red, and pin oak can start showing disease symptoms by June, and infected white and bur oak may not start showing symptoms until mid- to late summer. 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. There is often a sharp line of separation between discolored and green tissue.

If a black, red, or pin oak is infected by the fungus that causes this disease there is no way to stop the tree from dieing, but trenching to cut root grafts can be used to limit the spread of the fungus to neighboring trees. The best way to prevent the spread of oak wilt is to not prune any oak tree between March and October 1. If a tree is wounded or pruned during this time treat the wounds immediately with a non tar-based wound dressing such as latex paint. So, if you need to do some pruning on your oaks, dress warmly and get outside during the winter months to complete the work.

The photos at top show a Red Oak tree that was pruned in the summer of 2008 and is infected with oak wilt fungus.

Article used with permission from the Iowa DNR.  View the original document here.