The appearance of black discoloration on a tree trunk can be an alarming sight for any homeowner. This symptom is not a single disease but a physical manifestation of several possible underlying issues, ranging in severity from a harmless surface condition to a serious internal infection. Understanding the visual clues surrounding the black area—such as texture, location, and whether the area is wet or dry—is the first step in accurately diagnosing the problem.
Black Coating Caused by Sooty Mold
One of the most frequent causes of a black trunk is sooty mold, a harmless fungus that grows on honeydew. Honeydew is a sticky, sugary substance produced as excrement by sap-sucking insects, such as aphids, scale insects, or mealybugs, that feed on the tree’s internal fluids. The mold is superficial, dark, and powdery, resembling soot or ash that can often be rubbed off the bark.
This fungus does not infect the plant tissue directly but uses the sugary honeydew as a food source. While the mold is not parasitic, a heavy coating can block sunlight from reaching the leaves, inhibiting photosynthesis and potentially weakening the tree over time. The presence of sooty mold is a reliable indicator of an active infestation of sap-feeding pests.
Effective treatment requires eliminating the insect population producing the honeydew. Pests can be smothered using horticultural oils or insecticidal soaps, or a systemic insecticide may be used as a soil drench for larger trees. Once the insect infestation is controlled and the honeydew supply stops, the sooty mold will dry up and eventually flake off the tree naturally.
Wet Oozing from Bacterial Slime Flux
Bacterial slime flux, also known as wetwood, is characterized by a dark, wet, foul-smelling liquid oozing from the trunk. This internal issue begins when anaerobic bacteria infect the inner sapwood and heartwood, typically entering through a wound or crack in the bark. Inside the tree, the bacteria ferment the sap, producing gases that build up significant internal pressure.
This pressure forces the resulting liquid—the slime flux—to seep out through the bark, pruning cuts, or branch crotches. The oozing liquid is initially light-colored but turns dark brown or black as it mixes with external contaminants while flowing down the trunk. The dark, watery streaks running vertically down the bark separate slime flux from surface coatings like sooty mold.
The liquid often has a sour odor and can be toxic to the cambium layer, preventing wounds from closing properly. Since there is no chemical cure for this chronic internal infection, management focuses on improving the tree’s overall health and vigor. This is achieved through proper watering, fertilization, and avoiding further trunk wounds.
Fungal Cankers and Other Physical Damage
Blackening can also be caused by localized infections known as fungal cankers, which appear as sunken, discolored, or raised areas of dead bark. Cankers are lesions caused by fungi or bacteria that enter the tree through an injury, such as a broken branch or bark scrape. The dead tissue in the canker area may appear dark brown or black and sometimes weep a dark, gummy sap.
These lesions represent the tree’s attempt to compartmentalize the infection, creating a boundary between diseased and healthy wood. The discoloration is confined to a specific spot, unlike the widespread nature of sooty mold or the vertical streaks of slime flux. If a canker encircles the trunk, it can girdle the tree, blocking the flow of water and nutrients and causing dieback above the lesion.
Physical and Environmental Staining
Other instances of black bark result from physical or environmental stress rather than biological infection. Severe sunburn (sunscald) can kill patches of bark, leaving behind dead, blackened tissue that may eventually crack or peel. Environmental staining can also occur when water runoff from metal structures stains the bark a uniform dark color.
Next Steps for Identifying the Problem and Treatment
Determining the exact cause of the black discoloration requires careful observation of the area.
Sooty Mold Treatment
If the black substance is dry, easily rubs off, and is spread over the surface, the primary action should be pest control. This involves applying a horticultural oil to eliminate the insect infestation generating the honeydew.
Slime Flux Management
If the area is persistently wet, smells strongly, and features dark vertical streaks, the issue is slime flux. Management involves promoting the tree’s natural defenses by ensuring adequate water and avoiding practices that cause further stress or damage. Sanitation around the trunk may be needed to protect surrounding vegetation from the toxic fermented liquid.
Canker and Physical Damage Response
If the black area is a localized, sunken patch of dead bark, it is likely a canker or physical damage. The best approach is to prune the affected branch, cutting back to healthy wood, and then focus on overall tree health to help the tree seal the wound. For any large tree, or when discoloration affects the main trunk and is accompanied by wilting, consulting a certified arborist is the most prudent step.