Source: Emma Bruder, University of Minnesota Extension, Horticulture Intern, Wright, McLeod, and Meeker Counties, brude083@umn.edu, (612)-394-6302
What is oak wilt?
Oak wilt is a disease caused by the invasive fungus Bretzialla fagacearum, which grows on the outside of the tree and restricts the flow of both water and nutrients. This fungus can be transferred between trees in two ways. The first and more common method is through sap beetles. These insects are attracted to fresh wounds on the tree, and consequently deposit B. fagacearum spores. The other method occurs below ground when the roots of neighboring trees form root grafts with infected trees, allowing for the disease to be exchanged underground.
An infection can be identified through the tree’s appearance. Firstly, an infected tree will present itself through discolored and wilting leaves - the edges of the leaves will turn brown while the center remains green. The fungus will also create spore mats on the trunk of the tree, which appear as gray ovals, often about 2 inches long. Lastly, an infected tree will gradually die. In spring and summer months, this can be easily noted as the dying leaves and branches will stand out against the green of healthy leaves.
Why should I be concerned?
Oak wilt affects all oak species, and is estimated to be present in approximately 40% of Minnesota, including Meeker and Wright Counties. While select trees are able to recover, the large majority of infections are fatal. After a tree is infected, it can take anywhere from 2 months (red oaks) to 20 years (white oaks) for the tree to die. As an infected patch of oaks dies out, an opportunity for other species, such as buckthorn, to invade the area is created.
How can I avoid my trees being infected?
The most important thing to do to avoid an oak wilt infection is to not wound oak trees from April to July, with May and June holding the highest risk. This means no pruning or trimming during this time. If an unavoidable wound occurs, cover the wound immediately with latex-based paint or shellac. Wounds are susceptible for 5 days, so monitor the site and keep it covered.
Another method of preventing oak wilt is to treat uninfected trees with a fungicide. Propiconazole injections will suppress the fungus for 1-2 years, though it will not prevent infections via root grafts.
If you are in the stage of planting oak trees, it is helpful to plant a variety of species. This creates biodiversity, which allows for more stability and resistance against the spread of disease. Additionally, having more distance between your trees will decrease the likelihood of oak wilt transmission. Most spread is found to happen within a 30-foot radius, so a spacing greater than that is advised.
NOTE: Oak wilt can persist in firewood, so avoid using firewood from infected oaks as it can spread to healthy trees!
What should I do if I have an infected tree?
Firstly, you should take photos and report any suspected oak wilt cases to the Department of Natural Resources. You can also contact the University of Minnesota Plant Disease Clinic for a consultation.
To manage the infection and keep it from spreading to nearby oaks, hire a professional to perform root cutting. This process is often performed using a vibratory plow, though trenching machines and other equipment can also work. Once roots are separated, the infected trees should be removed and properly disposed of. Buffers can then be put in place to prevent the spread of future infections.