Hypoxylon Canker
Hypoxylon canker, Biscogniauxia atropunctatum, is a fungus that causes stressed oak and other hardwood trees to die. The disease is common across the southern U.S.
Healthy trees are not invaded by the fungus. Instead hypoxylon infects the sapwood of a tree that has been damaged, stressed, or weakened.
Trees can become stressed from defoliation by insects or leaf fungi, saturated soil, fill dirt, soil compaction, excavation in the root zone of the tree, removal of topsoil under the tree, disease, herbicide injury, drought, heat, nutrient deficiencies, competition or overcrowding, and other factors.
In addition to the hypoxylon fungus, weakened and stressed trees may become susceptible to a host of other secondary stressors like insects and diseases.
Hypoxylon canker activity usually increases when prolonged drought occurs. When drought stresses trees, the fungus can take advantage of weakened trees.
Healthy trees have a moisture content of 120 to 160%. It’s difficult for hypoxylon canker to develop in wood that has a normal moisture content.
However, any of the factors listed above could weaken or stress trees causing the moisture content of the wood to reach levels low enough for the hypoxylon fungus to develop. When this happens, the fungus becomes active in the tree and invades and decays the sapwood causing the tree to die. Once hypoxylon actively infects a tree, the tree will likely die.
Symptoms
An early indication that hypoxylon canker is invading trees is a noticeable thinning of the crown. The crown may also exhibit branch dieback.
As the fungus develops, small sections of bark will slough from the trunk and branches and collect at the base of the tree. Where the bark has sloughed off, tan, olive green, or reddish-brown, powdery spores can be seen. Different colors will be seen based on the tree species.
By the time the spores become visible, the tree is dead. In four to eight weeks, these tan areas will turn dark brown to black and become hard. They have the appearance of solidified tar. After several months, the areas will become a silver-gray color.
Once the fungus invades the tree, the sapwood begins to rapidly decay. Dark decay lines can be seen running through the wood.
All oak trees are likely susceptible to hypoxylon canker. In addition, elm, pecan, hickory, sycamore, maple, beech, and other trees may be infected.
The fungus spreads by airborne spores that can infect trees of any age by colonizing the inner bark. The fungus is known to be present in many healthy trees and can survive for long periods of time in the inner bark without invading the sapwood. When a tree is weakened or stressed, the fungus may then invade the sapwood and become one of several factors that ultimately cause the tree to die.
Management
Trees that have died from hypoxylon canker and are in an area where they could fall on structures, roads, fences, powerlines, etc., should be removed as soon as possible.
It is very dangerous to climb trees killed by hypoxylon canker. Because the fungus decays the wood so rapidly, the tree may not support the weight of a climber. Instead, use bucket trucks or other mechanical lift devices.
There is no known control for hypoxylon canker other than maintaining tree vigor. The spores of this fungus are so common in most areas that removing trees infected with hypoxylon canker is of little value in controlling the spread of the disease.
Infected firewood is not considered a source of inoculation. The fungus does not kill groups of trees by spreading from tree to tree.
There is usually little that can be done to avoid naturally occurring stress factors, but many man-caused stress factors can be avoided. During drought periods, supplemental watering is recommended, if the tree is near a water source. Damage to tree roots around construction areas commonly predisposes a tree to infection by hypoxylon canker.