Oak wilt is a vascular disease of red, pine, white and bur oaks. It is primarily transmitted through root grafts between oak trees. Over land infections are made by picnic beetles feeding on oak wilt fungus mats that are then attracted to fresh wounds on trees. Oak wilt tends to occur in new infections areas where tree damage has occurred due to construction injury, around new buildings or in restructured landscapes. It can also appear in new areas after storm damage has caused wounding in susceptible trees.
Once oak wilt is on site, it can move between adjacent trees through root grafts. Thus, damage to one tree can create an infection center that can infect all the trees surrounding it. Early detection of diseased trees is crucial to mitigating the extent of the damage from the fungus.
Oak wilt infections of red and pin oak trees are terminal. The fungus moves rapidly through the xylem system of these trees and decline and death can occur within weeks to months. Red and pin oaks that die from oak wilt often retain the dead foliage since death occurred so fast the leaves were unable to naturally senesce (Figure 1). While the infection is moving through the tree it is also moving through the roots to other red and pin oaks that may have grafted their roots to the primary tree (Figure 2).
Mitigation of disease spread is through disrupting the fungus spread through the root system to other trees. This can be accomplished mechanically by cutting roots are chemically with a fungicide by treating the adjacent trees before they become infected.
On bur and white oak trees the pathogen infects the trees in the same manner through root grafts and over land spread through picnic beetles landing on wounded trees. However, once a bur or white oak is infected the pathogen moves slower through the xylem tissue. This slower movement provides an opportunity to treat infected trees and prevent both the death of the infected tree and the spread of the pathogen to other trees through root grafts.
Timing of fungicide treatments for oak wilt infected bur and white oaks is a concern. Trees with significant infection and decline have compromised xylem systems (Figure 3). If the xylem tissues are too compromised the fungicide concentrations get too high and can cause damage and death of the tree. Alternately, if concentrations are reduced to account for the compromised xylem systems, not enough product may be taken up by the tree to manage the active fungal pathogen. Therefore, initial diagnosis of infection is critical to effectively manage the disease in infected bur and white oaks.
In the field, oak wilt symptoms are initiated by foliage at the ends of branches looking scorched and curled (Figure 4). Stems from these areas of suspicion can then be examined for staining of the xylem tissue. The xylem stains are created as a plant response to the infection. The dark brown stains are created by the tree producing tyloses that are meant to close and plug up the xylem to prevent the fungus from spreading (Figure 5). Unfortunately, oak wilt infections are not slowed by this plant response.
In addition to examining the stain in the tissues, suspected infections can also be cultured in the laboratory. Until recently, this was the primary way of confirming infection. However, it was possible to get false negatives due to the plant tissue drying out or other methodological errors. Fortunately, the USFS Northeastern Research Station teamed up with the University of Minnesota to develop a rapid oak wilt DNA analysis test that is now available through the University’s Plant Disease Clinic. For around $100 the new test can determine if tree tissues have the oak wilt fungus DNA present. It is quick, reliable and the risk of false negatives is greatly diminished.
The professional plant health experts at Ostvig Tree Care are trained on diagnosing oak wilt and other tree diseases on oaks in the Twin Cities. If you have any concerns on the health of your oaks, they’d be happy to work with you and use all the technical resources available through the University of Minnesota to diagnose and manage any problems.
Dr. John Lloyd, Ph.D. of Plant Health Doctors, LLC contributes to the Ostvig Tree Care website blog. He is also available through Ostvig Tree Care to answer questions on tree care and plant health issues. Feel free to send any questions you may have about this blog or any other tree issues to firstname.lastname@example.org. Questions and responses will be addressed individually. Ostvig Tree Care reserves the right to selectively publish submitted questions and answers on the Ostvig Tree care website and other media platforms.