Gray Fungus on Tree

Written by erin maurer
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Gray Fungus on Tree
Mushroom-like growth on tree bark may be a sign of a canker infection. (fungus on tree image by rikkidegraz from

Fungus can invade trees and cause large growths that resemble mushrooms or cankers to grow from the bark. Some fungal infections are harmful to trees and need to be treated immediately. Others, like lichens, form a symbiotic relationship with trees and do not cause significant harm. Several types of fungus may appear grey, so it is important to have an infection properly diagnosed by an arborist to determine the proper treatment.

Fungal Infections

Many trees are prone to fungal infections such as verticillium wilt, heart rot, canker and leaf spots, which affect the deciduous trees of North America, according to the North Dakota State University. Verticillium wilt is a fungal infection that starts in the soil. While it primarily affects the growth of leaves, a ring of grey to brown wood may be seen on the inside of trimmed leaves. Heart rot is a fungus that may affect the healthy wood of a tree. Fruiting bodies of fungus, which appear similar to mushrooms, are commonly seen growing around the trunk of a tree infected with heart rot. Canker involves various types of fungi that invade the bark of a tree and cause fungal growths to appear. Some only last for one growing season, while other types return year after year. Leaf spots are localised fungal or bacterial infections that cause spots ranging in colour from white to yellow to black on the leaves of trees. Gray spots on the leaves may be attributed to a leaf-spot causing fungus.

Mold and Lichens

In addition to fungal infections, some moulds and mildews may contribute to grey growths on a tree's trunks and leaves. According to the experts at Bugwood Network of the University of Georgia, container trees are especially susceptible to grey mould, which may cause grey, cotton-like growths to appear on new tree growth. Gray mould is attributed to spores from the B. cinerea fungus. Lichens are grey-green and appear similar to moss growing along tree trunks, wooden fences and other objects. Lichen fungus grows in a mutually beneficial relationship with blue-green bacteria and is not harmful to the trees on which it grows, according to the Tree Boss website.

Susceptible Trees

Some fungi affect most types of deciduous trees with equal fervour. Deciduous trees are defined as those whose leaves change colours and drop annually. Others are more commonly found on specific types of trees. For example, verticillium wilt commonly affects the sugar maple tree. Gray mould can affect most types of trees grown in a nursery but is commonly found in redwood, giant sequoia, western larch and several types of coniferous pine trees. Trees with injuries to the bark are more likely to be invaded by canker-causing fungi.

Control Options

Fungicides are the most common treatment for most grey mould-causing fungi. Applying protective fungicide helps keep fungus at bay, however, additional, cultural treatments are often required. Protecting growing saplings from fertiliser and frost damage makes them less susceptible to grey mould. Identifying the type of fungus causing canker and leaf spot infections is necessary to properly control the disease. Fungicides may be effective, but the chance of saving infected trees multiplies when the specific fungal cause can be identified. Caring for trees and avoiding injuries to the bark and foliage also help prevent fungal infections.

Tree Removal

Verticillium wilt is not a disease easily controlled by fungicides or cultural treatments. Trees infected with this fungus need to be removed and other trees should not be placed in the same ground. Often the fungus lives on in the soil and will infect new trees.

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