Hypoxylon Canker

Hypoxylon Canker in Shade Trees

Hypoxylon Canker - Asexual stage. (Ronald F. Billings, TX Forest Service)

Asexual stage. (Ronald F. Billings, TX Forest Service)

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There are a number of  diseases and maladies that  strictly colonize stressed  shade trees. One of the most  common of these stress  related diseases is called  Hypoxylon Canker. The specific species of this fungus  that appears on Oak trees is  called Hypoxylon atropunctatum. This species of canker  is common in North Texas due to the fact that Live Oak, Red Oak, Blackjack Oak, and Post Oak are very common types of Oak trees making up a significant portion of the urban tree population.  Hypoxylon Canker will appear more often on stressed  Red Oaks and Post Oaks due to the simple fact that they  are particularly intolerant to changes that occur in  their environment.  Thus they become stressed quite  easily as a result of even minor changes around them.

Hypoxylon atropunctatum on Oaks first appears as a  light brown or tan colored area (asexual stage) that is dry and dusty when disturbed.  These are the spores of  the fungus and they can travel far distances in wind  driven currents.  When these infections first develop, the  bark becomes paper thin on the surface of the cankers and flakes off exposing the brown fungal patch.  Within a few weeks, the light brown dusty area will turn a silvery-gray color with scattered black spots (sexual stage) and is no longer dusty but is very hard to the touch.

Hypoxylon atropunctatum is  a weak parasite at best and  causes no problem to  healthy trees.  The tree apparently defends against any significant infection with its normal defense mechanisms.  Hypoxylon often appears rapidly on the exterior of weak or dead limbs.  In this circumstance the organism lives harmlessly in the very outer bark tissue and aids the tree in quickly shedding or discarding limbs for whatever reason.  If Hypoxylon Canker appears on limbs or branches, it is not considered lethal and is often eliminated by simply removing the dead or dying limbs.

Hypoxylon Canker - Sexual stage. (Robert L. Anderson, USDA Forest Service)

Sexual stage. (Robert L. Anderson, USDA Forest Service)

When the disease appears on the main trunk or stem  of a tree, the tree is often dying or is nearly dead.  It is  extremely rare to observe Hypoxylon Canker on the  trunk and the tree recover or survive.  The main trunk of  a tree actively carries moisture and nutrients up the  tree.  The outer layer of the trunk, under the bark, is  merely a few cell layers thick and constitutes a tree’s current growth ring.  The appearance of Hypoxylon  Canker on this portion of a tree indicates that its main transport system has been severely damaged or is dead, and the tree’s basic ability to sustain itself is lost and the tree dies.

The disease does not spread from tree to tree, as many people fear.  The fungus already lives in the outer bark of most healthy Oak trees.  It should be noted that any portion of a tree where Hypoxylon Canker appears, the wood will dry out quickly becoming brittle and dangerous.

Arborists will often use the terminology that “Hypoxylon  Canker simply finished the tree off”, since the organism  is only present due to a tree already being severely  stressed.  It would be inaccurate to say a tree died of  Hypoxylon Canker because the disease is not considered a primary pathogen and is only present  due to the fact the tree has been damaged from some  other type of negative impact.  This is another of many  reasons why urban shade trees sustain significant  benefit from ongoing deep root fertilization programs.  This activity maintains and improves a tree’s ability to  naturally defend itself from Hypoxylon Canker and other common stress related problems.

Hypoxylon Canker - Sexual stage. (Robert L. Anderson, USDA Forest Service)

Sexual stage. (Robert L. Anderson, USDA Forest Service)

There are no effective means of managing a significant  infection of Hypoxylon  Canker.  Removing dead and weak limbs, as well as  maintaining overall health  and vigor, will be the most  effective way to reduce the  incidence of Hypoxylon Canker on your trees.  To avoid Hypoxylon Canker  from becoming a significant inhabitant of your trees:

  1. Maintain and improve the overall health and vigor of  your trees with regular and ongoing deep root  fertilization.  More aggressive programs may be  required to help rehabilitate stressed or damaged trees.
  2. Regularly prune your trees to eliminate any weak,  dying, or dead limbs.
  3. Avoid injury to the trunk and limbs of any tree.  By  reducing injuries, the trees are healthier and the  chances of infection are reduced.
  4. Avoid reductions or additions of soil in a tree’s root  zone, especially up against the main trunk of the tree.  This will keep the bark tissue from breaking down,  reducing entry wounds from developing.
  5. Take extreme caution when disturbing the  environment of any existing shade tree.  A tree  becomes much more likely to develop significant  problems the older and larger the tree.  A mature tree  is much less resilient to even minor changes in its  environment than a younger tree.  This includes any  activities such as landscape additions or  modifications and irrigation system updates or  installations.
  6. Consult with your Certified Arborist when changes  around your trees are going to occur (irrigation installation, landscaping, paving, or construction).

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Just a reminder!  We offer regular and ongoing tree inspections as a  courtesy to our existing clients. This can often avoid or  at least make you aware of potential or developing problems with your trees.

Questions? Contact us.