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.
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.
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.
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
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.
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.
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.
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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