Winter Tree Care: Identifying Common Tree Diseases in Winter, Part 11

This is the eleventh part of a series on winter tree diseases and disorders. This article examines sooty mold and thousand cankers disease.


During the winter months, many of the fungal pathogens that affect trees enter dormancy. The pathogens overwinter on their hosts or in the soil, awaiting spring’s arrival. Despite the frigid temperatures, trees suffering from fungal diseases may still exhibit infection symptoms in winter, especially if the disease has advanced into its later stages. The following describes some of the most common diseases to overwinter on trees, and how they may be detected.

Sooty Mold

Sooty mold is a collective term applied to several species of dark fungi. The fungi grow on honeydew excreted by insects, or exudates from leaves of certain plants. Sooty mold growths are composed of fungal complexes consisting of ascomycetes, and fungi imperfecti. Some of the common genera include Aureobasidium, Antennamariella, Limacinula, and Capnodium.


Sooty mold can appear on a multitude of ornamental trees and shrubs.

Symptoms of Sooty Mold Growth

Sooty molds grow on surfaces where honeydew deposits accumulate. Coatings of sooty mold reduce or block sunlight penetration, making photosynthesis less efficient. Without adequate sunlight, plant growth is stunted, causing leaves to wither and drop prematurely. Sooty mold growths can also develop on outdoor structures and furniture. Large masses of sooty mold create an unseemly appearance, and are often difficult to remove. Sooty molds have high allergenic potential, particularly the Cladosporium and Aureobasidium components found in sooty molds of the Eastern United States.


  • Sooty mold can be managed by reducing populations of insects that excrete honeydew. Pressure washing can help dislodge insects from trees.
  • An important biological consideration is ant management. Ants are attracted to, and use honeydew as a source of food. As such, they protect insects that produce honeydew from predators and parasites. If ants are eliminated, predators and parasites will become more prevalent. As their populations increase, they begin feeding on scale insects, aphids, psyllids, whiteflies, and mealybugs. Ant stakes and other baits can be placed under trees and shrubs to prevent ants from foraging. Sticky compounds are another effective deterrent; they may be placed around the base of trees.
  • If insect populations fail to decline, horticultural oils, insecticide, fungicide, miticide, or insecticide soap can be applied to suppress insects; one or more applications may be required.
  • Neem oil is an organic broad spectrum pesticide that can used to quash insect populations on house plants, flowers, vegetables, trees, shrubs, and fruit. It is a biodegradable substance, and has not been shown to be toxic to mammals, birds, bees, earthworms, or beneficial insects.
  • Judicious pruning cuts should be applied to remove infested plant parts.
  • Branches close to buildings or other access points should be trimmed back to prevent insects from invading the tree.
  • Trees should be fertilized in late spring or early summer to maintain tree vigor.
  • Ensure trees are sufficiently watered, especially during periods of extreme heat.
  • Apply a layer of organic mulch around the base of trees to improve soil quality, moderate soil temperature, and maintain soil moisture.
  • Outdoor furniture can be cleansed with water during periods of honeydew excretion, particularly during drought.
  • Sooty mold fungi growth can be inhibited by preservatives used in treated wood.
  • The following cleaning solution can be used to remove sooty mold from plastic or painted surfaces:

– 1/3 cup of powdered house detergent

– 2/3 cup of trisodium phosphate

– 1 quart of household liquid bleach

– 3 quarts of water

  • Always wear rubber gloves when cleaning with this solution.
  • A mixture of lukewarm water and mild soap can be used to remove sooty mold from fruits and vegetables. Fruits and vegetables covered with sooty mold remain edible

Thousand Cankers Disease

Thousand cankers disease (Geosmithia morbida) is a disease complex that affects walnut trees. The complex begins with the infestation of vulnerable walnut trees by the walnut twig beetle, Pityophthorus juglandis. During its adult stage, Pityophthorus juglandis makes contact with the fungal pathogen, Geosmithia morbida. The adults subsequently bore into the bark of infested trees to overwinter or lay eggs. As the beetles tunnel into the host, they unwittingly introduce the fungus, which germinates in the woody tissue. Once the fungus has become established, it induces the formation of cankers on the diseased branches. Large beetle populations can overwhelm infested trees, causing them to decline rapidly. Infected trees may experience tree mortality within 2 to 3 years of the initial infestation.


Thousand cankers disease infects walnut trees. Black walnut is the most common host.

Symptoms of Infection

Unlike other canker diseases, when a tree is infected by thousand cankers disease, the bark does not slough off. Instead, it remains firmly attached to the trunk, preventing the necrotic areas from being easily discerned. If the outer bark is stripped away, the beetle tunnels and branch cankers will often be apparent. Infected parts of the tree may become shallow or sunken. Severely infected trees may exude sap from the beetle tunnels. The sap will often trickle down the trunk, causing a dark amber stain to form on the bark surface.

External symptoms become more visible as the host declines. Once the disease has advanced into the trunk, more branches are girdled. This results in a discoloration of the tree’s crown, which can be widespread, or relegated to individual branches. The leaves of infected trees will often abruptly wilt. The upper portion of the crown will gradually die back. Host trees typically succumb to infection within 2 to 3 years.


  • When planting, select varieties of walnut that exhibit an increased resistance to thousand cankers disease. Arizona, California, and little walnut are some of the most resistant varieties.
  • The wood from infected trees may be used for commercial purposes, but the bark, phloem, and cambium should be removed first, to reduce the spread of the disease.
  • After they have been harvested, walnut trees intended for shipment to garden centers, households, or nurseries should be inspected for the beetle galleries, cankers, or dieback.
  • There is currently no chemical treatment available for control of thousand cankers disease.
  • Infected trees can be culled to prevent the disease from spreading to healthy trees.
  • Areas where infections have been detected can be placed under quarantine to manage the disease.