Introduction
Oak wilt is an aggressive tree disease that affects many species of oak. The causal agent of oak wilt is the fungus Ceratocytis fagacearum. Ceratocytis fagacearum is believed to have been introduced to the United States from Central or South America in the early 1900’s. Oak wilt was first identified in North America in 1944, and has since become one of the most widespread tree diseases in the eastern United States. Symptoms vary by tree species, but generally consist of leaf discoloration, a wilting of foliage, defoliation, and tree mortality. The oak wilt fungus is spread by insects, or through root grafts. Oak wilt is responsible for killing thousands of oaks in forests, woodlots, and landscapes each year.
Distribution
Oak wilt is common in the central and eastern United States, where conditions are favorable for spore production and beetle activity in spring and early summer. The disease may also be observed in pockets of Texas throughout late fall and winter.
Hosts
Oak wilt affects all oak species, but impacts the two main groups differently. Oaks in North and Central America are sorted into two groups based on their characteristics. Those in the white oak group have rounded leaf edges, and pores that are clogged with tyloses. Those in the red oak group have pointed leaf edges, and enlarged, open pores. Due to their open pores, oaks in the red oak group are more susceptible to oak wilt. When infected, they generally succumb to the disease within a year. Oaks in the white oak group are less susceptible to the disease, and can persist for years following infection. Some oaks in the white oak group, such as bur oak, are more easily infected than others, but still exhibit a greater resistance to the disease than oaks in the red oak group.
Disease Cycle
The oak wilt fungus may be transmitted in two ways: through the transfer of spores by insect vectors, or by the movement of the fungus through root grafts. If conditions are favorable after infected oaks have wilted, spores develop on fungal mats that form under the bark of the tree. These fungal mats produce asexual spores called endoconidia, which are barrel-shaped, and form in chains. If compatible mating types are present, the fungal mats will produce sexual spores called ascospores in fruiting structures called perithecia. Certain species of sap beetles in the family Nitidulidae are attracted to the fruity aroma of the fungal mats, as well as the bleeding sap of wounded oak trees. The beetles migrate to the mats to feed and propagate. As the beetles navigate across the diseased trees, they become laden with spores, which they transport to healthy trees. Oak bark beetles can also transmit the fungus. They breed beneath the bark of infected trees. After depositing their eggs, adults emerge carrying spores, and migrate to healthy oaks to feed. The fungus spreads to the healthy tree, creating a new infection. When the larvae hatch and develop into adults, they also carry spores, which they transmit to healthy trees as they feed.
Infections occur most frequently via root grafts. Trees within fifty feet of a diseased tree are vulnerable to infection. Spores that have been produced within the diseased tree spread through the vascular tissue. Most root grafts form between oaks of the same species. Red oak roots graft more often than white oak roots. Root grafts between red and white oaks are rare.
Symptoms in Red Oak
Infections on red oaks are most common in spring, when new vessel wood is being formed. Symptoms may occur as early as May. On red oaks, wilting progresses from the top of the tree downwards. Leaves turn dull green or bronze, and appear water-soaked. As the disease progresses, leaves turn yellow or brown, and curl around the midrib before dropping from the tree. Leaf damage begins from the tip and outer edges, and advances toward the midrib and base. Leaves fall in all stages of discoloration. Seemingly healthy leaves may be shed from infected branches. Discoloration of the vascular tissue also occurs, with brown streaks or spots appearing under the bark in the sapwood. Oak wilt progresses rapidly, with some red oaks succumbing to the infection within one to two months. Most diseased red oaks die within a year of infection.
Symptoms in White Oak
Symptoms are more variable in white oaks, although foliar symptoms tend to be similar. In white oaks that exhibit an increased susceptibility to the disease, symptoms resemble those of red oaks. The disease progresses rapidly, with many trees succumbing to infection within one growing season. This is particularly common in bur oaks. Most other white oaks decline more gradually. If the fungus persists, a few branches may be killed during the first year. Over subsequent years, the tree will decline, and may eventually die. Some white oaks can recover from the disease, but may continue to harbor the pathogen.
Symptoms in Southern Live Oak
In pockets of Texas, oak wilt can become prevalent. Southern live oaks, also called Texas live oaks, are especially vulnerable to infection. When infected, southern live oaks wilt and die. Disease progression varies depending on the timing of the infection, and the weather conditions. If conditions are favorable, infected southern live oaks may succumb to the disease within one to six months.
On infected Texas live oaks, leaves develop clorotic veins, a condition referred to veinal necrosis. Necrosis often spreads to the leaf tips, margins, and interveinal portions of the leaves. As the disease advances, the leaves turn brown, and eventually fall, with the tree crown becoming increasingly sparse. Fallen leaves may feature darker brown veins that persist for months.
Prevention & Management
- There is no permanent treatment for oak wilt. However, there are a number of preventative methods one can use to reduce the probability of infection.
- Susceptible oaks should be inspected for symptoms of oak wilt. If fungal mats, or beetles are present, remove the tree to prevent additional infections.
- Infected trees should be promptly removed, with the diseased wood safely disposed of. Logs from infected trees should not be moved to unaffected areas.
- To prevent beetle transmission, avoid pruning oaks during spring in the east and Midwest, and from February to June in Texas. Pruning wounds can attract beetles carrying the oak wilt fungus.
- Prevent mechanical and environmental injuries to susceptible oaks, especially when conditions are favorable for infection.
- Creating mechanical and chemical barriers between diseased and healthy trees can prevent the oak wilt fungus from spreading through root grafts. Trenchers or vibrating plows can be used to disrupt root grafts down to a depth of two to four feet.
- Soil fumigants may also be applied to sever root connections. Soil fumigants are restricted-use pesticides. They are primarily used in locations where vibratory plowing or trenching is not possible. Soil fumigants are often volatile, and can be difficult to manage. As such, they should only be handled by a licensed pesticide applicator. In locations that necessitate the use of a soil fumigant, holes are drilled into the soil at prescribed intervals, and the pesticide is applied therein. Once administered, the pesticide is absorbed into the soil where it kills any roots in a localized area.
- Fungicides have been developed that may be effective in preventing oak wilt when injected into healthy oak trees. A single treatment can protect red oaks from infection for up to two years. In southern live oaks, fungicidal treatments have prevented mortality, though foliar thinning generally still occurs.
Photo courtesy of M. Grabowski, University of Minnesota
