Red oak trees in fall

Oak Wilt Disease | Facts and Control

Oak wilt was first described in Wisconsin in 1944, when oak trees were found dying in localized areas. However, the origins of the fungus that causes the disease are not known. Some evidence, such as the existence of some comparatively tolerant native North American species of oak trees, suggests that it could be native to the United States.

Other evidence, such as the fact that the known populations of the fungus that causes Oak Wilt Disease have experienced a significant genetic bottleneck, points to its being an introduced species from a single introduction, rather than a native one. Supporting this is the existence of several highly susceptible North American oak species, indicating that they have had too little time to adapt to, and develop tolerance of, the fungus.

  What Is Oak Wilt Disease

Oak wilt is a fast tree-killing disease of oaks. It's a disease caused by the fungus Ceratocystis fagacearum. The fungus plugs the water-conducting tubes in a tree, and as the transportation of water throughout a tree is stopped, leaves wilt and fall off. The tree most often dies as a result.

  Identification and Symptoms

Oak Wilt Disease Leaf Yellowing The first symptom to become visible is discolouration and die-back in the crown of infected trees. Symptoms in red and white oaks begin to appear within weeks of infection, when yellowing of the l eaves occurs, particularly along the veins. This is followed by ‘scorch’, or browning, typically starting at the tip of the leaf.

There is a clear demarcation line between the dead and live tissues. Within a short time, the affected foliage develops a false autumn color as the tree wilts from the top downwards. Elongated cracks are sometimes present on the trunks of infected, dying red oak species.

  Disease Cycle

Oak Wilt Disease Tree Scorch Oak Wilt Disease fungus, Ceratocystis fagacearum, overwinters as mycelium in infected trees and as fungus pads on dead trees. In the spring the pads produce large numbers of spores that are carried to healthy susceptible trees by insects, which feed on the pads. Sap- and bark-feeding beetles contaminated with the fungus spores introduce the fungus into healthy trees through wounds caused by feeding.

The pathogen spreads rapidly within the xylem vessels and causes foliar symptoms to occur. After the tree is killed the fungus grows throughout the outer wood and also invades the root system. Healthy susceptible oaks growing close to an infected oak can become infected through root grafts. Mycelial mats usually form under the bark a few months after the tree is killed and, through outward pressure against the bark, force it to crack. Insects enter through the bark cracks and feed on the pads.

Mycelium is the vegetative part of a fungus consisting of a mass of branching, thread-like hyphae. Through the mycelium, a fungus absorbs nutrients from its environment.

  Disease Spread

Sap-feeding beetles are key components in the spread of the fungus. Beetles are attracted to enter the tree through the cracks made in the bark by the pressure from the fungal mats beneath. They pick up fungal spores on their bodies, which they then carry into wounds in healthy trees, thereby spreading the disease.

Oak bark beetles are also attracted to stressed oak trees, so they, too, can pick up the spores and spread them in a similar way.

Root grafts are also a significant pathway by which the disease can be spread. Root grafting is the natural underground joining together or inter-twining of the roots of plants close to one another.

  Which Trees Are Susceptible?

Oak Wilt Disease Tree Crack

Oaks in the red oak group (black, red, pin, and others with pointed leaf edges) are most susceptible. After they’re infected, these trees drop their leaves rapidly (usually within a 3-week period), most often beginning in late June throughout August.

Others will lose a portion of their leaves in September, and then rapidly lose all their leaves just after the leaves emerge the following spring.

Oaks in the white oak group (white, swamp white, bur, and others with rounded leaf edges) are less susceptible. Infected trees in this group will drop their leaves on one or more branches for several years in a row. In other words, trees in the white oak group take longer to die and show more chronic symptoms.

  Oak Wilt Disease Prevention

Severing root grafts between healthy and infected oaks is one way of stopping the spread of the disease. A tree care professional and Diggers Hotline should be contacted before this method is attempted. The wood of a newly infected tree that is cut down, needs to be debarked, burned, buried, or covered with a tarp and the sides sealed for a year. This will prevent the beetles from feeding on any fungal mats that may form in the firewood.

There are other oak wilt management strategies available, such as injecting fungicides that help prevent the disease, that a tree care professional, city forester, or county extension agent may be able to offer or suggest to you.

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