Hypoxylon Stroma     Image: David J. Moorhead, University of Georgia, Bugwood.org
 

Hypoxylon Canker
 

Symptoms and Diagnosis & Disease Cycle  

Hypoxylon canker is caused by an opportunistic fungi, (hypoxylon antropunctatum) attacking primarily drought stressed trees and it will finish the kill.  Hypoxylon is unable to cause disease in healthy trees but is quick to colonize weakened or dying bark and wood. Hypoxylon canker is a fungus
that causes cankers and death of oak and other hardwood trees. The disease is common in here in Texas Hill Country, and all across the southern
United States.  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.  Hypoxylon canker is a tree disease appearing as a necrotic (dead) lesion on limbs, branches and trunks of affected  trees.                                                                                                                                                                                                                      
                                                                                                                                                                                                                                                                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.

In some areas, red oaks have developed oak wilt symptoms, but also quickly developed additional symptoms of Hypoxylon Canker.
Since Hypoxylon Canker infects and kills weakened or stressed trees, it is thought that oak wilt weakens the tree and then Hypoxylon
fungus moves in and kills the tree. The oak wilt fungus is not a strong competitor with other fungi, the Hypoxylon fungus is the only one
found when laboratory diagnosis is made. The disease is usually associates with stresses caused by drought, heat, wound or chemical injury.
Healthy trees are more resistant to the disease.

Hypoxylon canker activity usually increases when prolonged drought occurs and trees are now predisposed to disease development.  
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 120% - 160%.  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.  Once hypoxylon actively infects a tree, the tree will likely die.

Symptoms

Leaves of trees infected with Hypoxylon turn yellow and wilt, and the entire branches die, but these symptoms are merely general indicators
that the tree is under stress not necessarily caused by the fungi. This dieback continues from branch to branch through the stem until eventually
the tree dies. Hypoxylon canker attacks more trees during extended drought periods.  The fungal spores reproduce rapidly as the tree's water
level drops and can build up enough pressure to blow bark off the tree, exposing the stroma, which may appear dusty brown, one-celled spores
(conidia),  silver, or white depending on its age as it progresses from it’s asexual to sexual stage.  Sunken or depressed areas appear in infected
areas where the fungus has killed the cambium and sapwood.  These spores are gone within a few weeks and a grayish surface is visible. This is
covered with numerous black fruiting structures. Mature fruiting structures (perithecia) can forcibly discharge sexual spores (ascospores) for
distances of 60 mm. Spores produced at a rapid rate are then wind blown to surrounding trees where infection occurs again. Entry appears to
be through injured surfaces on limbs or trunk. The fungus grows best at 86 degrees F but can grow at 50 and 100 degrees F.

An early indication that hypoxylon canker may be invading a tree is a noticeable thinning of the crown.  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. 
Where the bark has sloughed off, tan, olive green, or reddish-brown, powdery spores can be seen.  Different tree species that are infected with
hypoxylon canker will produce the different colors of spores.  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. Hypoxylon canker causes a dark brown discoloration of the sapwood. With age the infected wood is lighter in
color and has black zones or patterns in the wood when observed in cross section.

Once the fungus invades the tree, the sapwood begins to rapidly decay. As the fungus grows into the cambium and often girdling the
tree very quickly killing the cambium and sapwood.  Dark decay lines can be seen running through the wood.  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.  Instead, use bucket trucks or other mechanical lift devices.

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.

Control  

There is no known chemical 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 innoculation.  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. Control is achieved by maintaining the trees in a healthy condition.
Avoid injury to the trunk and limbs and never apply fill soil around the trees. Chemical treatments would not be effective because the fungus
is located within the tree. Water is the main issue, a soaker hose is probably the most effective way to get (water) right down into the ground. 
Water trees at the drip line, the ground area at the edge of the leaf canopy. The primary absorbing roots stretch to the outer edge of the leaf
cover. Mulching will also help combat stress because it helps keep moisture in the soil. As long as some supplemental water is provided,
2 to 3 inches of mulch can make a huge difference in keeping moisture in the ground, Just make sure the mulch isn't piled around the tree trunk.               

The bark sloughs off, exposing the stroma, which may appear dusty brown silver, or white depending on its age as it progresses from it’s asexual to sexual stage.

The above 4 Photos : Ron Billings TFS   These "bark chips" can be found on the ground at the base of the tree
Sometimes several different color phases of hypoxylon canker can be found at the same time on a single tree. Tyler County (TX)

           
Photos: Texas Forest Service

(1)  Evidence of hypoxylon canker on oak trees can appear black and crusty, similar to dried tar.  This stage of the fungus usually appears after the tree has been dead for several months. 
These black eruptions were found on a dead water oak (Lufkin, TX).

 (2) The hypoxylon canker fungus will exhibit several different color phases after an infected tree has died.  Here the fungus has taken on a silver-gray appearance on the bole (trunk) of
a young water oak (Lufkin, TX).

 (3) Certain species of oak trees are very susceptible to hypoxylon canker.Tan, powdery spores are shown on a water oak soon after the tree died in a Lufkin TX. Typical tan hypoxylon
spores on a water oak in a Tyler County, TX.