Hypoxylon canker is harmful to oaks
A few weeks ago we discussed pinewood nematode and how it is devastating our pine trees. This week we will discuss Hypoxylon canker.
One of the greatest losses that a homeowner can experience is the loss of a tree. Not only is a tree an aesthetic part of the yard, but it also provides shade for the homeowner as well as shelter for birds and other animals. Trees in the landscape are often focal points so that the entire family can enjoy the wonders of nature. Therefore, it is tragic that although it takes years for a tree to develop, it requires only a short period of time for a disease or diseases to kill it.
Although there are early records of Hypoxylon canker of oaks in Oklahoma, not much attention was given to its potential until the spring of 1979. Reports about the seriousness of this disease were first received from Pushmataha County in southeastern Oklahoma. The disease is severe to very severe in several areas in Oklahoma County. The Bethany and Edmond areas are the heaviest affected areas.
The Hypoxylon canker fungus infects most species of oaks and has been diagnosed from numerous habitats, including forest sites, trees in pastures, recently developed home sites, and established residential areas.
The casual organism of oak Hypoxylon canker is a fungus, Hypoxylon atropunctatum. Unfortunately, little is known about this organism that attacks and kills trees. However, it is known that trees that have been stressed or weakened from drought or have had their root systems injured are much more susceptible to this disease than healthy trees.
Research indicates that the organism enters branches through wounds. The fungus then grows through the sapwood and causes decay. The first outward symptoms that may be evident are yellowing and wilting of leaves and death of top branches. The fungus is capable of spreading up to three feet above and below the point of entry within one growing season. Researchers have been able to isolate the fungus from seedling oak trees which showed no symptoms of infection. This would indicate that the fungus may be active in the trees for a number of years before disease symptoms are noticed. When the trees are weakened, particularly by drought, or injured, the disease is capable of overcoming resistance of the host, and the tree dies.
The disease progresses through the branches, causing dieback symptoms. Another early symptom is a lightening of the bark. These silver areas look as if they area has been sprayed with bleach.
There is no effective control for this disease, which is due in part to lack of recognition of the early stages of the infection. Literature states the tree should be cut to the ground and burned if more than 15 percent of the crown is affected. No stump area should be left because the fungus can still reproduce even on very small stumps. In my observations, removing a tree before it is dead does little to reduce the amount of fungal spores, since more than likely there is an infected tree or trees next door or across the street.
The best defense against this disease is to maintain trees in a healthy, vigorous growing condition by adequate fertilization and to provide adequate water year round.