Gardening Articles: Landscaping :: Trees, Shrubs, & Vines

Changing the Rules Because of Oak Wilt

by Susan Littlefield


For years gardeners were counseled to paint over tree wounds and pruning cuts to seal them. This seemed like a logical course of action, like putting a bandage over a scraped knee. It wasn't until the 1970s that this practice was assessed scientifically, when it was found that sealing tree wounds didn't promote healing and, in some cases, even increased the chances of decay. Gardeners were then advised to leave pruning cuts and other tree wounds unsealed. That advice still holds, but now with one important exception -- when pruning oaks in areas where oak wilt is a threat.

Oak wilt is one of the most serious tree diseases in the eastern U.S. A fungal disease first identified in 1944, it attacks many species of oaks and kills thousands of trees every year. Researchers are not sure if the disease-causing fungus is native or imported, but either way, it's an aggressive pathogen. Trees in the red-black oak group are especially susceptible, but those in the white and live oak groups are also at risk.

The disease is currently found mainly in the Midwest and Texas, but occurs as far east as western Pennsylvania and eastern Virginia, and has even been found in pockets in North and South Carolina (areas in red on the map). The most common way for the oak wilt fungus to infect trees is through wounds, and it's thought that the increase in home construction in oak woods, with the resulting increase in damage to trees, has added to the spread of the disease in recent years.

The oak wilt fungus forms what are called spore mats on infected trees. These mats emit an odor that attracts certain species of sap beetles, which then pick up spores on their bodies. If these beetles then visit a fresh wound leaking sap on an uninfected tree, they can deposit spores and spread the disease. And this is why the recommendation on using wound dressings on oaks has changed. Sealing cuts and wounds will make them less likely to attract the beetles that spread the disease.

The most favorable time for new infections to get started varies regionally. Spring and early summer are the time when spore mats are formed and beetles are flying, usually from about April 15 to July 1 in the Great Lakes region, with the period gradually lengthening as you go southward, extending from February through June in Texas. So it's important to avoid pruning trees during the critical period, if possible. In northern areas, pruning cuts made anytime except during the winter dormant season should be coated immediately with latex paint to seal them. From Missouri south to Texas, cuts made at any time of the year should be sealed.

If you are involved in a construction project or other activity that could potentially wound trees, do your best to protect plants from damage. And watch out for weed whacker and lawn mower damage to tree trunks!

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