Oak wilt is an aggressive disease that affects many species of oak. It is caused by the fungal pathogen, Ceratocystis fagacearum. Red oaks are seriously threatened by this disease and die within weeks, whereas white oaks are the lesser susceptible species and can take one or more years to die . Oak trees have a tendency to intertwine their roots, creating an underground pathway for the disease to spread from one oak tree to another. The disease is also spread by insects, injury and storm damage. Pruning during the spring and summer can also facilitate spread, as these insects are attracted to tree wounds during this time of year.
Detection and Leaf Symptoms
Differs in each group.
Red oak group
(Northern red oak, black oak, pin oak, scarlet oak)
The main period of infection is in the spring, when new vessel wood is being formed. In NJ, symptoms in red oak occur in early July. Fugal mats develop under the bark of the tree and can usually be detected by a “sweet” smell and bark cracking. Oak wilt is usually identified in red oaks by rapid leaf discoloration and wilting. Often the initial symptom is a subtle off-green color shift that may be visible in the upper portion of the tree crown. Shortly after this initial color shift, the leaves begin to wilt from the top of the crown downward. As the disease progresses, individual leaves quickly discolor, taking on a scorched appearance, where the margins begin to brown but the base of the leaf remains green. Commonly, infected trees will drop all of its leaves within a matter of weeks and die. Red oak trees with oak wilt will have symptomatic leaf scorch in early July, and by the end of July or early August, all the leaves will drop from the tree
Fungal mat on a red oak. Photo credit: USDA Forest Service
|Cracks. Photo credit: Joseph Obrien
| Red oak wilt. Photo credit: Paul A. Mistretta
White oak group
(whiteoak, swamp white oak, chestnut oak)
Fungus mats seen in red oaks seldom appear on white oaks. While it takes longer for white oaks to succumb to oak wilt, they will die a year or more after initial infection. The leaves of white oaks also have a scorched appearance through the summer and drop leaves intermittently. If the tree is cut down, the tree might exhibit see brown streaks in the outer growth rings. White oaks frequently have discolored infected annual rings when diseased.
White oak wilt. Photo credit: Fred Baker
Is it Oak Wilt, Oak Decline or Bacterial Leaf Scorch?
Oak decline is a slow-acting decline of oak trees due to factors such as climate, site quality and advancing tree age. No single cause is responsible for the decline. Oak wilt, on the other hand, usually occurs in discrete pockets, or over a fairly large area, but do not typically spread outward from an initial infection center. Trees with oak decline may die over a period of years, or may ultimately survive a decline episode with only dead branches in the crown. Although bacterial leaf scorch effected trees have similar looking leaf scorch to oak wilt affected trees, BLS symptomatic leaf scorch occurs during a 2-3-week period in August and oak wilt symptomatic leaf scorch occurs in July. With bacterial leaf scorch some leaves may drop prematurely, however, the tree will not lose all of its leaves in its entirety, whereas those affected with oak wilt will drop all their leaves by the end of July/early August. It can also take multiple years for a tree to succumb to bacterial leaf scorch. Trees affected by bacterial leaf scorch usually exhibit a slow decline over 5 or more years before tree morality occurs, which is unlike oak wilt, where tree mortality occurs in a matter of weeks. Therefore, the two main differences between oak wilt effected trees and bacterial leaf scorch affected trees is timing of leaf scorch (oak wilt occurs in July and bacterial leaf scorch occurs in August) and onset of tree mortality (oak wilt trees will die in a matter of weeks, and bacterial leaf scorch trees will die in 5 or more years).
Bacterial Leaf Scorch. Photo credit: John Hartman
The oak wilt fungus moves from tree to tree in two ways: underground through the root grafts or above ground through the spread of nitidulid beetles, also known as sap beetles, carry spores of the fungus from spore mats on infected trees to wounds on healthy trees, causing infection and death of the tree. Most new tree infections occur when the fungus moves from an infected tree to a nearby healthy tree through connected root systems in a process called local spread. The roots of trees in each oak group commonly graft to roots of other trees in the same group, forming a continuous underground network. When one tree in a group becomes infected and dies, the fungus spreads through the connected root systems, killing more trees and creating an infection center.
Currently no known chemical treatment is capable of “curing” the disease. There are fungicides that are effective at protecting trees from oak wilt, however this treatment needs to be applied every year and can be expensive. Treatment must occur prior to infection. Contact a professional, licensed pesticide applicator for assistance. Trees that are infected with or have died from oak wilt, should be removed and properly treated to prevent development of spore mats. These treatments include debarking, chipping or splitting, and drying the wood. Covering dead wood with plastic, burying the edges for six months, and then air drying for a similar time will kill the fungus and any associated insects. Trees that die in summer should be removed and treated before the following spring, which is when new spore mats can develop. If the wood is sufficiently dried, however, spore mats will not develop. In addition, the roots from the affected tree must be cut to prevent the spread to adjacent oak trees, usually through the use of a trenching or cutting tool that cuts roots 5 feet below ground. Because the fungus can survive in the soil for more than 5 years, it is advised that oak trees are not planted or supported on the affected site for at least 7-10 years after tree removal.
Surrounding trees need to be removed radially from the initial source tree and roots should be cut to prevent the underground movement of the fungus. Cut trees should also be disposed of properly as it can harbor the fungus up to a year after cutting. Early detection and rapid response is the best option to reduce the spread of oak wilt.
Any oak pruning or activities that may damage the bark on non-affected oak trees should be limited to the fall or winter months when the transmission of the fungal spores from insects is minimal. These insects can carry fungal spores from an oak wilt affected tree and are also attracted to fresh wounds (pruning) in non-affected trees, spreading the disease from tree to tree.
A word of caution: Removing a diseased tree that is still living may actually facilitate the spread of the disease by accelerating the movement of the fungus into adjacent trees that are grafted to it by the roots. To avoid this problem, disrupt interconnected roots before removing living diseased trees.
Source: How To Identify, Prevent, and Control Oak Wilt, United States Department of Agriculture, Forest Service, www.na.fs.fed.us
Source: Oak Wilt: A Threat to Red Oaks and White Oaks Species, David L. Roberts, PhD, Michigan State University
Who to contact:
For more information on Oak Wilt please call the NJ Forest Service, Forest Health Program at 609-292-2532.