David J. Moorhead, University of Georgia, Bugwood.org
Symptoms and Diagnosis & Disease Cycle
Hypoxylon canker is caused by an
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
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,
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
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,
overcrowding, and other factors. The hypoxylon fungus is
considered a weak pathogen in that it is not aggressive enough
invade healthy trees. In addition
to the hypoxylon fungus, weakened and stressed trees may
become susceptible to a host of other insect and
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
laboratory diagnosis is made. The disease is usually
associates with stresses caused by drought, heat, wound or
Healthy trees are more resistant to the
Hypoxylon canker activity usually increases when prolonged
drought occurs and
trees are now predisposed to disease development.
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
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.
hypoxylon actively infects a tree, the tree will likely
Leaves of trees infected with Hypoxylon turn yellow and wilt,
and the entire branches die, but these symptoms are merely
that the tree is under stress not
necessarily caused by the fungi. This dieback continues from
branch to branch through the stem until eventually
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
with numerous black fruiting structures. Mature fruiting
structures (perithecia) can forcibly discharge sexual spores (ascospores)
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
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
to black and become hard. They have
the appearance of solidified tar.
After several months, the areas will become a silver-gray
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
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
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.
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 chemical control for hypoxylon canker other
than maintaining tree vigor.
Apparently the spores of this fungus are so common in
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
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.
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
Sometimes several different color phases of hypoxylon canker
can be found at the same time on a single tree. Tyler County
Photos: Texas Forest Service
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,
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).
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,