A Fungal Disease of Shade Trees

Anthracnose is a name for a group of diseases caused
by several closely related fungi that attack many of our
finest shade trees. It occurs most commonly and severely
on sycamore, white oak, elm, dogwood, and maple.
Other host plants that are usually only slightly affected
include linden (basswood), tulip tree, hickory, birch, and
walnut. Each species of anthracnose fungus attacks only
a limited number of tree species. The fungus that causes
sycamore anthracnose, for example, infects only
sycamore and not other tree species. Other
anthracnose-causing fungi have similar life cycles, but
require slightly different moisture and temperature
conditions for infection.
Symptoms
Anthracnose fungi may cause defoliation of most maple,
oak, elm, walnut, birch, sycamore, and hickory species
and, occasionally, of ash and linden trees. Damage of this
type usually occurs after unusually cool, wet weather
during bud break. Single attacks are seldom harmful to
the tree, but yearly infections will cause reduced growth
and may predispose the tree to other stresses. Damage
may be in the form of:
• killing of buds, which stimulates the development of
many short twigs or “witches’ brooms;” these may spoil
the shape of the tree
• girdling and killing of small twigs, leaves, and branches
up to an inch in diameter
• repeated early loss of leaves, which over several
successive years weakens the tree and predisposes it
to borer attack and winter injury
• premature leaf drop, which lessens the shade and
ornamental value of the tree
Specific symptoms of anthracnose vary somewhat
depending on the tree species infected:
• On sycamore, leaves and growing tips of the twigs may
die as they emerge from the bud. This damage is often
confused with late frost injury. Sudden browning and
killing of single leaves or leaf clusters may occur as the
leaves expand. The disease continues to develop later
in the season, resulting in irregular brown to nearly
black, dead areas between or along the main leaf veins
and extending to the margin (Fig. 1). Infected leaves
fall when the
petiole is girdled
or when several
lesions enlarge
and coalesce to
form large, dead
blotches. After
defoliation from
spring infections,
the tree may
appear bare
except for tufts
of leaves at
branch tips.
Regrowth
appears by
midsummer.
Sunken cankers
form on larger
twigs during
cooler weather
in fall, winter,
and spring (Fig.
2). Twigs may die
as a result of
canker
formation.
When terminal twigs are killed, lateral twigs take over
as leaders. Thus, repeated twig dieback results in the
formation of crooked branches.
• On oak, small scattered brown spots or large light
brown blotches form along veins. The leaves look
scorched.
• On maple, purplish brown areas form along the veins
or larger, irregular, light to dark brown spots form
along or between veins (Fig.3), extending to the leaf
Fig. 2. Twig canker caused by the sycamore
anthracnose fungus.
(Photo by E. Dutky-U. Md.).
Fig. 1. Blighting of leaf tissue caused by the
sycamore anthracnose fungus.
(Photo by E. Dutky-U. Md.).
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margin.
• On ash, large,
irregular, light
brown spots
appear, most
often along leaf
margins (Fig. 4).
• On linden, large
brown areas with
black margins
appear, especially
along main leaf
veins. The areas
are small to
large and
circular to
elongate.
• On birch, small,
irregular,
circular, brown
spots with dark
brown margins
are apparent.
• On hickory, large, irregular, reddish brown spots
appear on the upper leaf surface and a dull brown
area is apparent on the lower leaf surface.
• On walnut, irregular, circular, dark brown to black
spots are visible on leaves.
• On dogwood, two different anthracnose diseases may
occur. Symptoms of spot anthracnose include tiny
leaf and bract spots, about the size of a pinhead, with
whitish centers and purplish borders. Symptoms of
Discula anthracnose (dogwood anthracnose)
include irregular, small to large brown blotches with
purplish borders on leaves and bracts, lower branch
dieback, and trunk cankers that culminate in death of
the tree.
Disease Cycle
Anthracnose fungi overwinter in infected leaves on the
ground. Some canker-causing anthracnose fungi, such as
the sycamore anthracnose fungus, also overwinter in
twigs on the ground or in cankered twigs that remain on
the tree. Microscopic spores of most anthracnose fungi
are produced in infected tissues during April and May.
The spores are blown and splashed to the buds and young
leaves and, with favorable moisture conditions, penetrate
and infect the swelling buds and unfolding leaves. Long
rainy periods help the fungus to spread rapidly.
Control
Disease control measures for different trees vary slightly
because the period of infection is different depending on
the fungal species involved. If fungicides are used, sprays
must be applied on a preventative basis, beginning before
infection takes place. Spraying large trees for many
anthracnose diseases may be impractical and
unnecessary, especially in dry springs. Sanitation is
important in reducing the amount of fungal inoculum
available for new infections. For large, high-value
sycamore
trees,
injection
with the
fungicide,
thiabendazole
hypophosphite
(e.g.
Arbotect
20-S), on a
3-year basis
is also an
option
(Fig 5).
For effective anthracnose control of most anthracnose
diseases:
• Rake up and remove infected leaves in the fall. Leaves
may be shredded and composted or burned.
• Prune out and burn or bury dead twigs and small
branches. Prune to thin the crown. Thinning will
improve air movement and promote faster drying of
the leaves.
• If fertilizer is needed, fertilize in the fall about a month
after the average date of the first frost or in early spring
about a month before the date of the last frost to
increase tree vigor.
• If chemical control is desired, spray with a fungicide
containing mancozeb (e.g. Manzate 200, Dithane M-45)
at budswell and twice again during leaf expansion (in
most years, this would be at 10-14 day intervals).

Bacterial Wetwood Disease of Trees

Wetwood is a water-soaked condition of wood in the
trunk and branches of trees. This condition has been attrib-
uted to bacterial infection in the inner sapwood and outer
heartwood area of the tree. Infection is normally associated
with wounding or environmental stress on the tree. The
bacteria, Enterobactor cloacae, has been implicated as the
cause of wetwood in elm, but numerous other bacteria have
been associated with this condition in other trees such as
cottonwood, willow, ash, maple, birch, hickory, beech, oak,
sycamore, cherry and yellow-poplar. Bacteria alter wood
cell walls, causing moisture content of the wood to increase.
Infected wood may also have a high (basic) pH and a high
concentration of microelements.
The most common evidence of wetwood is bleeding
or “fluxing” of sap from the trunk or larger limbs of a tree.
Often this fluxing is associated with a wound, but has also
been observed where no obvious wound existed. Bacteria
associated with wetwood are common in soil and water
Stain associated with bacterial wetwood disease on the trunk
of pin oak.
Closeup of damage to bole (pruning scar) where fl ux of sap
is exuding. Notice the bird peck hole where birds are either
hunting for insects attracted by the fl ow of sap or actually
feeding on the sap. The flux of sap may be the result of insects boring, animal rubbing
or mechanical injuries to the tree, such as frost cracks or
pruning. During extended drought periods, the condition
has been noticed at the base of larger, older trees, espe-
cially oaks.
Bacterial fermentation of the sap during warm
weather produces gases (often methane), causing pressure
in the affected wood. The pressure forces the sap out of the
tree by the path of least resistance. This is why the fluxing
is usually found near wounds and openings in the bark.
The exuding sap will run down the side of the tree, soak-
ing a large area of bark. Once exposed to the air, the sap
will become contaminated with other bacteria, yeasts and
fungi, resulting in a foul-smelling, slimy, foamy substance.
Fluxing of the sap is sometimes referred to as slime flux.
The flux associated with wetwood should not be confused
with the normal bleeding that may occur after pruning.
If slime flux runs down the tree for extended periods,
it may cause the bark to decay and eventually may damage
the cambium. The cambium is the regenerative layer of tis-
sue between the bark and the wood that is responsible for the
tree’s diameter growth. The cambium produces new wood
and bark each year and is directly related to tree vigor.
Fluxing of sap may also cause toxicity in the sap that
is carried to the branches, thus resulting in wilting and
defoliation of the leaves. Plants adjacent to the tree trunk
may also be killed or damaged by toxic sap exuded from
wetwood wounds. Wetwood alone rarely causes tree death,
but may lead to secondary pathogens that combine for con-
tinued tree decline and eventual death.
Wood-infesting and other insects are attracted to the
flux exudates. These insects may lay eggs and reproduce in
the fluxing material. Wood-infesting insects are likely to
invade the tree after being attracted to the slimy exudate.
There is no control for wetwood disease. Preventing
damage and stress to tree roots and stem is probably the best
way to avoid a wetwood problem. Drought conditions tend
to increase wetwood problems, so it is important that the
tree receives adequate water during the growing season.
Treatments for trees already infected with wetwood
are generally only cosmetic and of no remedial value. Trees
affected with wetwood will compartmentalize around the
wetwood-affected area, and limit its spread to other parts
of the tree. This is nature’s way of protecting trees from
infections.
A previously recommended practice of installing
drain tubes in the wetwood-affected area to relieve sap
pressure and remove lateral liquids has been challenged by
researchers in recent years. Research has found that the ben-
efits of tube installation are offset by the injury the instal-
lation causes. In fact, the spread of the infection to other
tree parts may be increased by using drain tubes. Also, the
removal of the internal liquids can create conditions favor-
able for invasion by wood-decay fungi.
For these same reasons, research has shown it is better
not to scrape the wound and clean out the infected wetwood
areas. Wetwood will cause only a small amount of injury
for most healthy trees if they are allowed to compartmen-
talize the diseased area. It is far better for the tree to have
a small section infected by wetwood than to be invaded by
wood-decay fungi that could cause far more damage and
structurally weaken the tree.
Use preventative measures to avoid wetwood disease.
Follow good pruning techniques that result in minimum
injury to the tree. Prune only as required for shaping the
tree and removing the dead wood. Never cut behind the
bark ridge when pruning a branch; i.e., do not make flush
cuts. Protect trees, especially the roots, during construction
projects. Fertilize and water as needed to avoid nutritional or
moisture stress. Trees usually do not require watering unless
prolonged drought occurs. Trees growing in lawns do not
need additional fertilizer if the lawn grass is fertilized.
For trees with wetwood disease, wash the slime flux
from the surface and apply insecticidal spray to protect the
tree from insect infestations. Loose, dead bark or limbs
should be removed. Cutting or scraping the fluxing area is
not recommended. Increased applications of nitrogen fertil-
izer have increased the growth rate and recovery of some
wetwood-affected trees.