Oak Wilt Alert

The following is from Master Gardener Gene Frye about the revised “no prune” period for Iowa oak trees: 

  ISU Extension Foresters have just expanded the recommended “no-prune” period for Iowa oak trees to extend from March 1 through October 31 in order to minimize the chances of contracting the oak wilt disease.  The previous recommendation for not pruning was from mid-March through August.

 Oak wilt, a fungus disease, is fatal to all oaks in the red oak family within one year of the onset of the first symptoms, but white oaks may live for several years, and some even recover from the disease, but with substantial damage. 

 The main method of contracting oak wilt is through pruning or other wounds made during the growing season, with the resulting sap attracting certain insects carrying the oak wilt fungus on their bodies.  These insects bore into the wound carrying the fungus spores with them. 

 The other method of spreading oak wilt is through root grafts of nearby oaks of the same species.  Root grafts allow trees to exchange body fluids, so to speak.  Since the roots of most trees extend out to about the height of the tree, trees nearer than about one hundred feet of an infected tree of the same species can also acquire the disease.  Red oaks will not root graft with white oaks. 

If oaks must be pruned during the no-prune period, for example to manage storm damage, the pruning wounds should immediately be covered with a mixture of an outside white latex paint diluted one-to-one with water.  This is one of the few circumstances in which Extension Foresters recommend use of wound dressings.

 The first symptoms of oak wilt are a general drying and falling of leaves, usually during June or July.  In the case of red oaks, the whole tree is usually affected, but on white oaks, only individual branches show symptoms the first year of infection, followed the next years by more branches being affected.

  Positive diagnosis of oak wilt requires a laboratory test, but a reasonably reliable diagnosis can be made by finding longitudinal brown streaks just underneath the bark on affected branches.  Unfortunately, there are several other ailments that can appear to be oak wilt. Oak wilt usually appears in clumps, and is not nearly as devastating as chestnut blight or Dutch elm disease.  Infestations can usually be contained by application of good management practices. 

For more detailed information on oak wilt, see Extension publication SUL 15, “Oak Wilt—Identification and Management,” which is available at County Extension Offices for $1.00 or can be downloaded free of charge on the Internet at:  www.extension.iastate.edu/store  

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1 Response so far »

  1. 1

    Keith Brown said,

    It is interesting to read about oak wilt in your area. The precautions you take in Michigan are similar to the techniques we use in Texas, but time periods vary a little and here in Austin most of our oaks are live oaks which graft roots, so 95% of our problem is underground. The one point on oak wilt that I like to make is to warn people that they need to make sure that the information they are reading is local. So many times I see people reading internet articles about a tree problem in a different state and then try to bring the principals to their trees. Regional changes make a big difference in tree and disease behavior. I’m glad to see you are referring people to your local extension agent. Great Post!!


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