Bacterial Leaf Scorch in Landscape Trees

Bacterial leaf scorch is a chronic disease caused by
a bacterium, Xylella fastidiosa, that grows in the xylem of
the tree and physically clogs these water-conducting ves-
sels. As the bacterium multiplies, water transport becomes
more limited. The tree suffers water stress, especially in
mid to late summer, resulting in leaf scorch; a browning
or discoloration of the margins of the leaves with interior
portions of the leaves near the veins remaining green. The
bacterium is spread by leafhoppers, spittlebugs and other
xylem-feeding insects.
Hosts and Symptoms
Bacterial leaf scorch has a wide host range including
many herbaceous and woody species (goldenrod, alfalfa,
clover, blackberries). Tree species most affected are elm,
sweetgum, sycamore, dogwood, mulberry, red maple, sugar
maple and particularly, many species in the red oak fam-
ily – pin, northern red and scarlet oaks. Symptoms usually
appear on one branch and progressively spread throughout
the crown in subsequent years. On large trees, it may take
five to 10 years for the disease to progress through the entire
crown. Infested branches generally releaf for several years
following the onset of scorch disease symptoms. Leaves will
appear normal in the spring, but later show scorch symp-
toms. Growth is reduced and tree crowns become progres-
sively sparse as the tree declines. Eventually, infected trees
will succumb from the disease.
Diagnosis
Symptoms of bacterial leaf scorch are often mis-
taken for those produced by vascular wilt diseases such as
oak wilt and Dutch elm disease. The difference is that the
scorch and decline occurs progressively over several years
rather than occurring over a period of two or three months
with the wilt diseases. Sometimes bacterial leaf scorch is
difficult to diagnose on symptoms alone, since the symp-
toms are similar to other related tree responses to drought,
salt damage or root injury. Early fall coloration of leaves
often coincides with leaf scorch. Therefore, a plant tissue
analysis lab must perform a positive diagnosis to determine
if the bacterium is present.
Integrated Management
Unfortunately, no cure
or treatment for infected trees
or a strategy for preventing
infection is presently avail-
able. Leafhoppers and other
insects that spread the dis-
ease are active for most of
the growing season. This
makes disease prevention by
controlling the insect vector
with insecticide treatments
impractical.
The life of infected
trees can be prolonged with
judicious management. Trunk
injections with antibiotics
Bacterial leaf scorch on northern red oak. leaf scorch on a sycamore leaf. Note the live tissue near the veins of the leaf
and the dead tissue at the leaf margins.
have been shown to suppress the symptoms. Treatments must
be made annually in late May or early June. Antibiotics only
cause a remission of the symptoms, not a cure. Injections
must be applied each year. The continual wounding made by
these injections raises concerns whether the wounds provide
an entrance to secondary disease organisms.
Pruning is another possible treatment if the disease
is detected early. Pruning of infected branches can delay
the spread of the disease within the crown. Mulching and
watering during drought periods may reduce moisture stress
and possibly delay scorch development. The effects of fer-
tilization are unclear with this disease. Fertilization treat-
ments are recommended when a soil or leaf analysis shows
a nutrient deficiency.
Summary
Since there is no effective treatment or cure for bac-
terial leaf scorch, one should expect diseased trees to be
gradually lost over the years. The eventual best remedy for
bacterial leaf scorch is tree replacement once the tree no lon-
ger adds to the landscape. However, the life of the tree can
be prolonged somewhat with judicious management since
tree death is not immediate. Thus, tree replacement can be
done before the infected tree dies, allowing tree establish-
ment before infected trees are removed.

Bacterial Spot of Peach

Bacterial spot, known also as bacteriosis, bacte-rial shothole, or shothole, is caused by the bacterium
Xanthomonas campestris pv. pruni. Although primarily
a peach and nectarine problem, this disease also occurs
on apricots, plums and, to a lesser degree, cherries.
Losses due to bacterial spot occur from affected
fruit and from the devitalization of trees caused by
frequent defoliation. Such weakened trees are more
subject to winter injury. Losses are greatest in light,
low-fertility soils. Trees low in vigor are more suscep-
tible to bacterial spot than vigorous trees.
Symptoms
Numerous small spots form on the leaves. The
spots are angular, purple to purplish-brown or black.
Spots may merge and the centers may fall out, giving
the characteristic “shothole” appearance. Heavily
infected leaves turn yellow and drop. Severe leaf loss
early in the summer reduces fruit size and weakens the
tree. A few lesions can result in severe defoliation on
sensitive varieties; tolerant varieties may require many
more lesions for defoliation.
On fruits, tiny water-soaked, sunken spots form.
The spots enlarge and merge to cover large, irregular
areas. As the fruit grows, cracking or pitting occurs in
the lesions. The brown rot fungus can easily enter these
cracks and become established. Every effort to control
brown rot should be made when deep bacterial spots
form on fruits.
The bacterial spot organism infects only cur-
rent-season growth. On twigs, two types of cankers
form. “Summer cankers” develop on green twigs,
usually after leaf spots are evident. The lesions begin
as water-soaked, purplish spots between the nodes.
The cankers enlarge, become slightly sunken and are
circular to elliptical in shape. Cankers caused by the
peach scab fungus are similar, but scab cankers are
slightly raised.
Steve Bost, Professor
Entomology and Plant Pathology
Infections that occur late in the year on young,
succulent twigs show up as “spring cankers” the fol-
lowing spring. These cankers, which tend to appear at
buds or nodes about the time the fi rst leaves appear,
are water-soaked and slightly darkened. As the season
progresses, the epidermis over the lesion ruptures,
releasing the bacteria.
Disease Cycle
The bacterial spot organism over-winters in peach
twigs infected late in the summer. These infections are
not visible until spring, when they are referred to as
“spring cankers.”
Bacteria are released from cankers in an ooze, and
the bacteria are spread to other tissues by rain and in-
sects. The bacteria infect expanding leaves, green fruit
and current-year twigs. Penetration occurs through
stomata or lenticels when surface moisture is present.
Repeated infections will occur throughout the
growing season if environmental conditions are favor-
able. Severe disease is more apt to occur during warm
weather with frequent rains. The disease makes little
progress during summer weather that is hot and dry.
In late summer and fall, bacteria are carried to
young succulent stems. Cankers resulting from these late
infections do not appear until spring, when they produce
inoculum for early spring disease development.
Control
Bacterial spot is very diffi cult to control during
rainy seasons. No entirely satisfactory control method
is known. Homeowners are particularly limited as to
chemicals available to them for bacterial spot control.
Proper cultural practices are important in minimizing
this disease.
1. Resistance to bacterial spot should be a primary
consideration in peach varietal selection. Some
of the more resistant varieties include Bell of
Georgia, Biscoe, Harken, Loring, Madison, Ranger,
Redhaven, Redskin and Sunhaven. Very susceptible
varieties include Elberta, July Elberta, Halehaven,
Rio-Oso-Gem and Sunhigh.
2. Vigorous trees are less susceptible to the disease
than weak, neglected trees. Fertilize according to
a soil test, to maintain vigorous, but not excessive,
shoot growth.
3. Allow for good air circulation by avoiding low-lying
orchard sites and by adhering to correct pruning
practices. Prune during dry weather in the latter
part of the dormant season.
4. Chemical spray applications may provide some sup-
pression of bacterial spot. Use dodine 65 WP plus
Captan 50 WP (1/2 lb. plus 1 lb. per 100 gal. water).
Observe label precautions, as this mixture can
cause injury under certain conditions. Mycoshield
at 12 oz. per 100 gal. is probably a more effective
treatment. Mycoshield should be tank mixed with
sulphur (6 lbs. per 100 gal.) or Captan (2 lb. per 100
gal.) for broad spectrum disease control. Bacterial
spot sprays should be initiated at shuck-split and re-
peated at seven-day intervals through cover sprays.
Observe harvest restrictions.

Precautionary Statement
To protect people and the environment, pesticides should be used safely. This is everyone's responsibility, especially the user. Read and
follow label directions carefully before you buy, mix, apply, store or dispose of a pesticide. According to laws regulating pesticides, they must
be used only as directed by the label. Persons who do not obey the law will be subject to penalties.
Disclaimer Statement
Pesticides recommended in this publication were registered for the prescribed uses when printed. Pesticides registrations are continuously
reviewed. Should registration of a recommended pesticide be canceled, it would no longer be recommended by the University of Tennessee.
Use of trade or brand names in this publication is for clarity and information; it does not imply approval of the product to the exclusion
of others that may be of similar, suitable composition, nor does it guarantee or warrant the standard of the product.

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.