Stay Alert for Oak Wilt!

The fungus Bretziella fagacearum causes oak wilt. It grows in the water-conducting vessels of infected trees, clogging them. Also, defensive structures called tyloses are produced by oaks in response to various stresses, including oak wilt infection, which contributes to the blockage. The tree defoliates and dies once the xylem vessels are plugged and no longer transport water.

While no species of oak is immune, the infection spreads rapidly in members of the red oak group. Such trees can die in a matter of weeks, and once a tree is infected, there is no treatment. Oak wilt moves much more slowly in members of the white oak group, usually killing a branch or two a year, and it can take years for infected white oak group trees to succumb.

The red oak group has entire or lobed leaves, and the lobes and apices have bristle tips. The white oak group also has entire, dentate, sinuate, or lobed leaves, but they lack bristle tips. The table below divides native oak species grown in Pennsylvania into those groups.

Red Oak Group

• Scarlet Oak – Quercus coccinea
• Bear Oak – Quercus ilicifolia
• Shingle Oak – Quercus imbricaria
• Pin Oak – Quercus palustris
• Willow Oak – Quercus phellos
• Northern Red Oak – Quercus rubra
• Shumard Oak – Quercus shumardii
• Black Oak – Quercus velutina

White Oak Group

• White Oak – Quercus alba
• Swamp White Oak – Quercus bicolor
• Overcup Oak – Quercus lyrata
• Post Oak – Quercus stellata

Symptoms

In red oaks, oak wilt typically starts at the top or outer portions of the crown and quickly moves downward. Leaves wilt and begin to turn brown from the tips and margins toward the midrib and base. Some will remain green but take on a dull appearance. Intense defoliation accompanies wilting and discoloration. There may be discolored sapwood under the bark of branches with wilted leaves.

In white oaks, oak wilt presents in a more scattered fashion, with a branch here and there showing wilting and discoloration. The leaf symptoms are similar to the red oak group, and there may also be discolored sapwood under the bark.

How Does Oak Wilt Spread?

Oak wilt spreads below ground via root grafts and above ground by insect vectors. Root grafting depends on soil texture, terrain, and the proximity of oaks to one another. Root grafting is most prevalent in sandy soil and flat terrain, but also in heavier soils with hillier terrain. Oaks of the same species within 50 feet of each other will likely have root grafts. Root grafts also occasionally occur between different species of oak. Root grafting is responsible for localized outbreaks of oak wilt. Vascular diseases such as oak wilt are easily transmitted from infected trees to healthy ones through shared vascular tissue in these grafts.

Insects are responsible for spreading oak wilt over slightly longer distances, roughly within a mile of an infected tree, particularly sap beetles (Nitidulidae) and, to a lesser extent, oak bark beetles (Curculionidae). These insects are particularly attracted to fresh wounds – they can arrive at a fresh wound within 10-15 minutes! Wounds must be less than 72 hours old for oak wilt transmission to occur.

They are also attracted to the sporulation mats that form between the bark and wood of red oak group oaks killed by oak wilt (these are rarely, if ever, found on white oak group oaks). These sporulation mats generally form in autumn or spring following defoliation. Mats are elliptic and initially gray or buff-colored and turn black with age. Pressure pads form inside these mats that cause the bark to split, which allows insects access to the sporulation mats. Fungal spores adhere to the insects' bodies as they feed, and they can carry them to healthy trees.

Insect activity is the main reason we do not prune oaks when they are actively growing, especially in spring when the highly susceptible springwood forms. Retired Penn State Extension plant pathology specialist Gary Moorman always recommended pruning oaks from November to mid-April to avoid infection from insect activity.

Recent, unpublished research conducted by Phil Gruszka, director of horticulture and forestry for the Pittsburgh Parks Conservancy, and Danielle Martin, forest pathologist for the USDA Forest Service in Morgantown, West Virginia, looked at oak wilt vectors in three parks in the Pittsburgh area in 2015 and 2016. They discovered that these insects are actually active from March to October, and possibly into November in mild years. Dr. Bruce Fraedrich from Bartlett Tree Research Labs reports that the nitidulid beetles become active between 150 and 175 growing degree-days.

If oaks must be pruned during the growing season (due to storm damage, for example), research conducted by Texas A & M and the Texas Forest Service in 2007 supports the use of wound dressings to make the wounds less attractive to the insect vectors of oak wilt.¹

Managing Oak Wilt in the Landscape

Once a red oak group species has been diagnosed with oak wilt, removal is the only option; propiconazole injections will not arrest the disease in red oak group oaks as they can in studied white oak group species. However, if there is a chance that it could be root-grafted to another oak, those root grafts must be broken before the infected tree is removed. If infected trees are removed before breaking those root grafts, there is a risk that infected sap can "backwash" into the healthy tree.

Use a trencher or vibratory plow to a depth of 3-4 feet to break root grafts. Once the root grafts are broken, still-healthy trees can be injected with propiconazole to protect them. Propiconazole does "cure" oak wilt, but it does provide suppression. The injection should be repeated every other year to maintain a therapeutic level of propiconazole in the treated tree.

Infected trees should be removed as soon as possible after trenching. Prompt removal of infected oaks is important to protect those uninfected trees. Destroy the wood immediately, including the stump, by chipping, burning, burying, or debarking so that it is not attractive to the insects responsible for the spread of oak wilt. Do not stack the wood for firewood or transport logs or firewood with intact bark since insects in the infected wood can leave and carry the fungal spores to healthy trees. This is the primary way that oak wilt spreads over long distances.

Research conducted in Minnesota demonstrated that oak wilt can be arrested in white oak group bur and white oaks with propiconazole injections and pruning of dead branches.²

Citations

¹Camilli, Kim, David N. Appel, and W. Todd Watson, 2007. Studies on Pruning Cuts and Wound Dressings for Oak Wilt Control. Arboriculture & Urban Forestry 33(2):132-139.

²Eggers, Jordan, Jennifer Juzwik, Shawn Bernick, and Lori Mondaunt, 2005. Evaluation of Propiconazole Operational Treatment of Oaks for Oak Wilt Control, Research Note NC-390 of the USDA Forest Service. North Central Research Station, St. Paul, MN.