Hypoxylon Canker Affects Stressed Trees

Hypoxylon canker is a fungus that causes cankers and death in hardwood trees. Relatively healthy trees are not invaded by the fungus, but the hypoxylon fungus will readily infect the sapwood of a tree that has been damaged, stressed, or weakened. Natural and man-caused factors that can weaken a tree include defoliation by insects or leaf fungi, saturated soil, fill dirt, soil compaction, excavation in the root zone of the tree, removal of top soil under the tree, disease, herbicide injury, drought, heat, nutrient deficiencies, competition or overcrowding, and other factors. The hypoxylon fungus is considered a weak pathogen in that it is not aggressive enough to invade healthy trees. In addition to the hypoxylon fungus, weakened and stressed trees may become susceptible to a host of other insect and disease pests.
Hypoxylon canker activity usually increases when prolonged drought occurs. When drought stresses trees, the fungus is able to take advantage of these weakened trees. The moisture content of living wood in live, healthy trees is typically high. It is 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.

An early indication that hypoxylon canker may be invading a tree is a noticeable thinning of the crown. This should not be confused with leaf loss due to the autumn season. Also, the crown may 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. The signs of the fungus are:
• (early stages) light to dark reddish brown to olive green colored crusty fungal (stroma) tissue over the cankered area,
• (later stages) grey surface that eventually flakes off after 6 – 12 months to reveal a dark brown to black crusty material that gives a burnt appearance to the tree. These sometimes have the appearance of solidified tar,
• (advanced stages) the signs of the fungus may first appear as small patches a few inches in length, but will eventually merge to form large strips along the trunk and major limbs of the tree

Once Hypoxylon canker is evident, it is usually too late to try to save the tree. Large portions of
the tree will be dead, reducing the desirability as a landscape specimen. In addition, the structural
integrity of the wood is compromised and the tree becomes hazardous. Trees exhibiting signs and
symptoms of Hypoxylon canker should be carefully inspected and considered for removal. Trees that have died from hypoxylon canker and are located in an area where they could fall on structures, roads, fences, powerlines, etc., should be removed as soon as possible. During removal, 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. Caution should be exercised when removing a tree effected by hypoxylon canker.

Probably all oak trees are susceptible to hypoxylon canker. In addition, elm, pecan, hickory, sycamore, maple, beech, and other trees may be infected. The fungus spreads by airborn spores that apparently 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. As mentioned earlier, 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.

There is no known control for hypoxylon canker other than maintaining tree vigor. Apparently 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. Also, infected fire wood is not considered to be 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.