Oak wilt and storms do mix!

By Dr. Jennifer Juzwik, Research Plant Pathologist, USDA-FS, NCFES

The severe thunderstorms and accompanying high winds that wracked the Twin Cities Metro Region on May 15th and May 30th left an immediate, dramatic impact on the urban and urban-fringe forests. Landscape and woodland trees were uprooted, major breakage and splitting of many small to large diameter living trees occurred, and untold numbers of smaller limbs were ripped off of otherwise still-standing trees. Clean-up, repair and removal efforts will continue throughout the summer.

Oak WiltFurther impact arising directly from these storms is also expected from the oak wilt fungus ,Ceratocystis fagacearum, in damaged living oaks. Fresh wounds were created at exactly the right time for successful overland spread of the oak wilt fungus to occur. In addition, tree pruning and tree removal activities may have resulted in wounds that provided an open door to oak wilt entry. A large number of new oak wilt infection centers are expected to show up in the Metro Region from late June through early September as a result. Why is this predicted? Let's look at the sequence of events observed this spring that set the stage for oak wilt spread.

First, plenty of spores of the oak wilt fungus were available for acquisition by the insects that transmit the pathogen. The spores are formed on fungal mats that grow between the bark and wood of the oaks (primarily red oaks) killed by oak wilt during the previous summer. This spring, two peaks in spore mat production were documented by USDA-FS researchers. The first occurred the third week of April and the second during the second week of May. Significant numbers of sap-feeding beetles, or nitidulids, were observed on the spore mats during these mat production peaks. The beetles casually acquire oak wilt fungal spores on their bodies as they feed on, burrow through, and lay eggs in these spore mats.

Secondly, wounds penetrating the outer sapwood of oaks were quite attractive to particular species of nitidulids. In studies conducted in the upper Midwest in the 1950's, wounds on red and bur oaks were found to be susceptible to natural oak wilt infections for 24 hours after being made. Three species of nitidulids are particularly attracted to these fresh wounds in east central Minnesota based on recent USDA-FS research in May of this year. Peak numbers of these beetle species were found on 24 and 48 hour old wounds and diminished greatly in wounds older-than 48 hours. The common picnic beetle (Glischrochilus quadrisignatus), commonly thought to be a primary vector, was not one observed on these wounds. The three species found in the wounds were also commonly observed on nearby fresh oak wilt spore mats two weeks prior to the study.

Lastly, the red oaks were in their most susceptible stage for oak wilt infection during May. The large diameter wood vessels formed by red oaks during spring, called springwood, were present in Metro Region red oaks this year beginning in early May.

Conditions for successful overland spread of the oak wilt fungus were perfect following the two major storms. The wood-exposing wounds to living oaks caused by the storms were undoubtedly an attraction to sapfeeding beetles, some of which were likely carrying the oak wilt fungus on their bodies. What, then, can be done to prevent oak wilt infection following spring storms?

The best bet for the landowner with storm damaged oaks is to contact a consulting forester who is familiar with oak wilt. Surfaces of any cuts made during June in an effort to "repair" wounded tree crowns or canopies of oaks should be treated immediately with commercial tree paint or wound dressing. The objective is to provide a physical barrier between any exposed wood cells and any insects that may visit fresh oak wounds. When an entire oak tree is removed prior to July, the surface of the remaining stump should also immediately receive commercial tree paint or wound dressing. This prevents the oak wilt fungus from getting established in the still living stump and then being transmitted via root grafts to adjacent healthy trees. Landowners should also consider regular pruning of their trees (approx. every 4 to 6 years) to lessen the likelihood of tree damage during storms.

Finally, landowners and arborists should also be on the "look-out" this summer for oak wilt symptoms in storm damaged oaks. The outlook for saving adjacent oaks is always best when early detection and diagnosis of oak wilt is followed by immediate disease control actions. For example, spread of the oak wilt fungus from infected trees to nearby healthy oaks through grafted roots can be prevented through severing of the common root system with machinery such as a vibratory plow.

Severe storms are not uncommon during spring in Minnesota and some level of damage to oaks can always be expected, including establishment of new oak wilt centers. However, prolonged and aggravated damage to oak stands caused by oak wilt coming in on storm-wracked trees can be avoided through diligent detection efforts and prompt control actions.

Excerpted from Overstory Summer, 1998 published by the MN Dept. of Agriculture