County gives tips to protect trees from oak wilt

The Stearns County Soil and Water Conservation District (SWCD) has received grant monies to assist homeowners in managing oak wilt on their properties. The funding has been made available through a partnership between the Minnesota Department of Natural Resources, the USDA Forest Service, and the Minnesota Tree Trust. Grants are available on a first-come, first-served basis, and will cover up to 50 percent of most treatment costs. Homeowners concerned about their oak trees should contact the Stearns County SWCD for more information. The SWCD staff will help verify oak wilt infections and assist with proper treatment. If oak wilt is not already present on your property, preventive measures taken during and after construction will help keep your property free of the disease. If oak wilt is already present, control is best done prior to building. If oak wilt is not controlled, it will continue to spread and kill other oak trees. Oak wilt has been identified in the cities of St. Cloud and St. Augusta, and in Lynden Township. What is oak wilt? Oak wilt is a fungal disease that causes the water-conducting vessels in oak trees to become plugged. Once the vessels are plugged, water movement within the tree stops, causing leaves to wilt and fall from the tree. Red, black and pin oaks are highly susceptible to oak wilt. Once infected, they can die within a few weeks. White and bur oaks are much less susceptible. If infected, they can take months or years to die, or they may even recover. How do trees become infected? Oak wilt spreads in two ways: overland by sap-feeding beetles that carry the fungal spores from infected oaks to fresh wounds on healthy oaks; and underground, from infected oaks to nearby healthy oaks through grafted or interconnected root systems. If oak wilt is not already present on your property or your neighbors’, infection via sap-feeding beetles is your main concern. This is the only known way oak wilt can spread across highways, water, open fields, or other large breaks in oak tree cover. Published studies show that oaks in the upper Midwest are at highest risk of overland infection during spring and early summer. Once a tree becomes infected, the fungus begins spreading to adjacent oaks through grafted roots. In general, any oak within 50 feet of one infected by oak wilt is at risk of root graft infection. The risk of root graft infection is influenced by size of tree, soil type and the presence of natural or human-made barriers. Once an oak becomes infected, the fungus must be contained or it will continue to spread and kill until it runs out of oak trees capable of root grafting with infected ones. Oak wilt does not respect property boundaries. How can oak wilt be prevented? Oak wilt prevention is easy and effective. Do not cut, prune or otherwise wound oaks in the spring and early summer, generally from April 15-July 1. Any activity during this period that cuts or tears through the bark and exposes live wood in oak branches, trunks or roots can place those trees at risk of infection. If an oak is wounded during this period, immediately and thoroughly apply pruning sealer or tree paint over the wound. Torn branches or roots should be cut clean and the cut surface painted. For additional protection, cover treated roots with soil. It is very easy to damage tree roots, trunks and branches during site preparation, road and driveway construction, utility and septic installation and landscaping. A vehicle carelessly backed into a tree can gouge bark sufficiently to expose vulnerable sapwood. It only takes one small wound on one tree to set the stage for oak wilt infection. How can construction damage be prevented? Consider using a consulting arborist for help in protecting your trees. A consultant can assist with the layout of developments and individual building sites. Consultants can also make on-site recommendations to minimize the risk of oak wilt and other construction-related damage to trees. Talk to your builder and contractor about your oak wilt concerns. Make sure your concerns will be communicated to subcontractors and all who deliver materials to the site. Determine how much area is needed for buildings and to maneuver equipment. In general, trees within 15 feet of proposed structures should be removed. If possible, perform all tree removal prior to April 15 or after July 1. If trees are cut from April 15-July 1, immediately apply pruning sealer or paint in a one-inch-wide band around the circumference of the cut surface of any oak stump that is not removed. Erect sturdy, temporary fencing around residual trees and along woods edges, at least 10 feet away from tree trunks. Residual trees will be further protected from soil compaction and/or grade changes if fencing is placed at or beyond the drip line. From April 15-July 1, all equipment operators on site should carry pruning sealer or paint and immediately apply it to exposed oak wood, should accidental wounding occur. Tar should not be used. How can oak wilt be controlled once present? Disrupting root grafts be-tween infected and healthy oaks is the most effective control known. A vibratory (cable) plow or trencher with a minimum five-foot blade is best for severing root grafts. If available, a plow moves faster, cuts at a more consistent depth and creates minimal aboveground disturbance. Proper placement of root graft barriers is essential to control underground spread of oak wilt. Because aboveground symptoms do not reflect the extent of actual infection, proper barrier placement is not simply a matter of cutting between wilted trees and apparently healthy ones. By the time symptoms are visible, oak wilt has already spread throughout the tree and has very likely infected neighboring oaks. Barriers should be constructed only by someone specifically trained to do so. It is important to note that simply cutting down an infected tree does not stop the underground spread of oak wilt. In fact, cutting an infected tree without first placing root graft barriers can accelerate the spread of the fungus into neighboring trees. What should be done with   diseased wood? After all root graft barriers are made, diseased trees should be removed. Reproductive spores of the fungus form under the bark of infected trees. These spore colonies attract sap-feeding beetles to the site, facilitating overland spread of oak wilt. Proper disposal can be one of two methods. Before April 15 of the year following infection, completely debark all wood from the cut trees. Small material can be chipped. Once bark is removed or wood is chipped, spore formation is no longer possible. Alternatively, wood can be cut, split and stacked for firewood; any wood not used by April 15 of the year following infection should be completely covered with clear plastic. Bury the edges of the tarp under soil, or weighted down with rocks or undiseased logs. Leave the tarp on until all bark is loose. if possible, place your covered woodpile in the sun to speed the drying process. How can wooded homesites be kept free of oak wilt? Additional steps can be taken to reduce the risk of oak wilt on your wooded property, even after your home is built. New infections can begin when trees are damaged during spring and early summer storms. Inspect your trees at least once a year, preferably when leaves are down. Look for forked limbs that are beginning to split apart – oak limbs as well as others within striking distance of an oak. Similarly, identify any standing hazardous trees that could injure oaks if they fall. Hazardous trees should be pruned or removed only by a competent arborist. Oaks damaged by storms during the overland infection period should immediately be pruned or cut down and the cut surface treated. There are costs associated with oak wilt prevention and control. But the cost of doing nothing – potential personal injury and property damage from falling dead trees, greater tree removal costs, higher heating and cooling costs, reduced real estate value and aesthetic appeal – may ultimately be far more costly. If you have any questions, contact the
Stearns County SWCD at (320) 251-7800, ext. 3.