Oak Wilt Prevention Pruning in Saint Paul, MN

Oak wilt prevention pruning means doing all routine work on oaks in the months when the disease cannot get in. Oak wilt is a fungus that clogs an oak's water-conducting tissue. In Saint Paul it is mainly a problem for red oaks and is concentrated in the Highwood and Battle Creek area, according to Saint Paul Forestry, but oaks grow across the city's bluffs, parks and older neighborhoods. Sap beetles carry the fungus from infected trees to fresh wounds, so a pruning cut made in May can do more harm than the limb it removed.

The calendar

Saint Paul Forestry advises against pruning oaks from April through October, and Forestry prunes its own oaks only in winter. The University of Minnesota Extension rates April through July as the high-risk period, August through October as low, and November through February as safe. Our routine oak pruning, from deadwood removal to clearance and thinning, goes on the dormant-season schedule. If you call in June about an oak, we will look at it, write the quote and put it on the winter list.

When an oak is wounded in summer

Storms do not wait for winter. If wind breaks an oak limb in the high-risk months, Extension advises sealing the wound immediately with latex paint or shellac. We clean up the break with a proper cut and seal it the same visit, and leave any other pruning on that tree for winter.

Red oaks and white oaks

The difference matters. In red oaks, which include northern red, northern pin and black oak, Extension notes that complete wilting, leaf loss and death can occur in as little as four weeks after a branch infection. White oaks, including white, bur and swamp white oak, decline more slowly, often over two to five years or longer. Leaves on an infected red oak turn brown from the edges inward and drop while partly green, usually starting at the top of the crown. Other problems can look similar, so a suspected case is worth confirming.

How it spreads underground

Most new oak wilt infections move through root grafts between neighboring oaks. In a Twin Cities study cited by Extension, most spread happened within 30 feet, with some trees infected up to 50 feet from the nearest diseased oak. Root grafts form between different oak species too. On a block of mature oaks, that means one infected tree puts the neighbors at risk. Stopping an expanding center usually takes a root barrier, cutting the root connections with a vibratory plow, before the infected tree is removed.

Treatment and wilted wood

Injection with the fungicide propiconazole can protect healthy oaks for up to two years, according to Extension, and white oaks with early symptoms affecting less than 30% of the crown can be protected too. It does not stop root spread, and symptomatic red oaks are not recommended for treatment. Oaks that wilted during the growing season should be felled in fall or winter. The wood should be debarked, or split into firewood and stacked on site, then covered with clear plastic tarp by late March with the edges buried. Chips and bark can stay on site, but logs from recently wilted trees should not be moved.