Summer Tree Diseases

— Written By N.C. Cooperative Extension image

Summer brings us warmer temperatures and with that, sometimes a few
troublesome tree problems not present in colder weather. Let’s look at a
few unique tree problems that you may have seen, but just did not know
what they were.

Bacterial wetwood, also called slime flux, is a major rot of the trunk
and branches of trees. Slime flux has been attributed to bacterial
infection in the inner sapwood and outer heartwood area of the tree. The
bacterial infection is normally associated with wounding or
environmental stress. Numerous bacteria have been associated with this
condition in trees such as willow, maple, hickory, oak, cherry and

A tree with slime flux is water-soaked and "weeps" from visible wounds
and even from healthy looking bark (the weeping liquid is fermented
sap). The "weeping" may be a good thing as it is having a slow, natural
draining effect on a bacterium that needs a dark, damp environment. A
tree with this trunk rot is trying its best to compartmentalize the
damage. There are no control measures to take because nothing can stop
further rot except the tree's ability to isolate the spot by growing
good wood around the diseased portion.

Powdery mildew is a common disease that appears as a white powdery
substance on the leaf surface. The powdery appearance comes from
millions of tiny fungus spores, which are spread in air currents to
cause new infections. It attacks all kinds of trees including oak,
dogwood, crabapple and cherry. Powdery mildew is a product of moist
conditions and is usually seen in wetter seasons. This humidity-loving
fungi can be controlled if moisture can be controlled. Don't plant trees
in heavily-shaded areas and provide plenty of space for air movement and
growing room. Prune the tree for effective air movement. Chemical
control is usually not necessary.

Sooty mold appropriately describes this disease, as it looks just like
chimney soot. Although unsightly, it seldom damages the tree. The
pathogens are dark fungi growing either on the honeydew excreted by
sucking insects or on material coming from leaves of certain trees.
Sooty mold may occur on any tree but is most commonly seen on boxelder,
elm and maple. Sooty molds are associated with high temperatures and
increased stress brought on by limited moisture. During drought, aphid
populations and their honeydew production typically increase on foliage
undergoing moisture stress. Keep plants well watered and free of
insects. A good washing of the infested tree's foliage (if possible) can
dilute the honeydew and wash off mold. That just may be all that is needed.

The term "canker" is used to describe a killed area or blister on the
bark, a branch or the trunk of an infected tree. The canker-causing
fungi commonly invade wounded or injured bark tissues to form a canker
and subsequently produce reproductive structures called fruiting bodies.
Dozens of species of fungi cause canker diseases.

Preventing cankers means growing vigorous trees that can fight off the
entrance of pathogens into the bark. Wounds are essential for most
canker infections so avoid wounds, especially where active cankers are
present. Plant your tree on a good site, use vigorous planting stock,
fertilize trees to promote growth and control weeds for several years
after planting. Landscape trees will benefit by deep watering or trickle
irrigation, especially during dry summer months. To control canker
disease on trees, cut off the affected branch or limb. If a large canker
is on the main trunk, the tree may need to be replaced. No effective
chemicals are available to control the fungi that cause canker disease.
In general, just try to keep your tree healthy.

Jeffrey E. Seiler
County Extension Director – Jackson/Swain Counties
N.C. Cooperative Extension – NC State University
Jackson (828) 586-4009
Swain (828)488-3848