A tree trunk turning black usually points to one of a handful of causes, and the most common by far is sooty mold, a dark fungal coating that grows on the sugary waste left behind by sap-feeding insects. But sooty mold is not the only explanation. Fungal canker diseases, bacterial infections, fire scarring, and even urban air pollution can all darken bark in ways that look similar from a distance. Telling them apart matters because some are purely cosmetic nuisances while others signal a tree in serious decline.
Sooty Mold, the Most Common Culprit
If your tree trunk looks like someone smeared soot or black paint over the bark, sooty mold is the likeliest explanation. Sooty mold is not a single organism but a group of dark-pigmented fungi that colonize surfaces coated in honeydew, the sticky, sugar-rich excrement produced by aphids, scale insects, whiteflies, mealybugs, and other sap-sucking pests. The fungi do not infect living plant tissue. They feed on the honeydew film and form a dark crust over whatever surface it lands on, whether that is bark, leaves, or the sidewalk beneath the tree.
Research on sooty mould growing on lime trees identified Aureobasidium pullulans and Cladosporium herbarum as the most common fungal species in the coating, along with several other saprophytic fungi of lesser importance.1Elsevier. A study of sooty mould on lime trees (Tilia × Vulgaris) These are opportunistic surface-dwellers, not parasites. They thrive wherever there is sugar and moisture, and a heavy aphid infestation can provide both in abundance.
The good news is that sooty mold itself does not harm bark. On leaves, a thick coating can block enough light to reduce photosynthesis and weaken the tree over time. On a trunk, the main damage is cosmetic. The bad news is that the mold’s presence tells you something else is wrong: you have a significant population of sap-feeding insects overhead. Addressing the insect problem removes the honeydew supply, and the sooty mold gradually weathers away on its own.
The Insect Connection
Because sooty mold is always downstream of an insect problem, identifying and managing the pest is the real fix. Aphids are the classic offender on many deciduous trees, but scale insects are equally common and harder to spot because they sit flat against bark and look like small bumps rather than bugs. Whiteflies and mealybugs can also produce heavy honeydew loads.
A newer and increasingly destructive example in the eastern United States is the spotted lanternfly (Lycorma delicatula). This invasive planthopper feeds voraciously on a wide range of trees, and its sheer body size means each individual excretes far more honeydew than a typical aphid. Research has documented that sooty mold colonizes honeydew excreted by spotted lanternflies during feeding, shading leaf surfaces beneath infested trees.2PubMed Central. Sooty mold deposition and leaf-level light attenuation beneath trees infested with Lycorma delicatula The lanternfly reduces plant resources both directly through feeding and indirectly through the sooty mold that grows on its excrement and blocks photosynthesis.3PubMed. Perspective: shedding light on spotted lanternfly impacts in the USA If you live in an area where spotted lanternflies have arrived and your tree’s trunk and lower branches are coated in black, the two are almost certainly connected.
For lighter infestations of aphids or scale, a strong blast of water from a hose can knock populations back. Horticultural oils and insecticidal soaps work well on contact for soft-bodied insects. Systemic insecticides applied as a soil drench can control scale and other persistent pests, though they carry risks for pollinators and should be used carefully on flowering trees. Once the insect population is under control, the honeydew supply dries up and the sooty mold stops expanding. Existing mold fades over weeks to months as rain and weathering break up the fungal mat.
Black Knot Disease
If your tree is a plum, cherry, or another member of the Prunus family and the black growth is hard, rough, and swollen rather than a smooth film, you are likely looking at black knot disease. This is caused by the fungus Apiosporina morbosa, and unlike sooty mold, it is a true infection of living tissue. The fungus invades branches and sometimes trunks, stimulating abnormal growths that start out olive-green and soft, then harden over a season into coal-black, warty knots that can be several inches long.
Black knot affects both Japanese and European plums equally.4PubMed. Comparative Anatomical Responses of Tolerant and Susceptible European Plum Varieties to Black Knot Disease Wild cherry and chokecherry are also highly susceptible and often serve as a reservoir of spores in the landscape. If you have ornamental plums or cherry trees with black, knotty growths on the branches, pruning out infected wood well below the visible knot is the standard management approach. Because spores spread during wet spring weather, removing knots before budbreak reduces the chance of reinfection.
Charcoal Canker on Oaks
Oaks can develop a different kind of blackening caused by Biscogniauxia mediterranea, a fungus associated with charcoal canker disease. This pathogen is considered opportunistic: it typically lives inside oak tissue without causing symptoms, but it turns aggressive when the tree is weakened by drought or heat stress. Infected bark takes on a blackened, charred appearance as the fungus produces dark reproductive structures on the surface.
Research on Biscogniauxia in Iranian oak forests confirmed that the fungus causes charcoal canker in both juvenile and mature trees, with all tested strains capable of causing disease, though aggressiveness varied by region.5Nature. Genetic structure of Biscogniauxia mediterranea fungus populations in Zagros forests of Iran Because the fungus exploits drought-stressed trees, the best prevention is maintaining tree health through proper watering during extended dry spells. Once a tree shows extensive charcoal canker symptoms, treatment options are limited, and an arborist can advise whether the tree can recover or needs to be removed before it becomes a hazard.
Bleeding Cankers and Phytophthora
Sometimes the blackening on a trunk is not a fungal coating on the surface but a dark, wet-looking stain that seems to ooze from the bark. This pattern often points to a bleeding canker, and the pathogen behind it is frequently a species of Phytophthora, a group of water-loving organisms that attack roots and lower trunks. Despite the name (which translates roughly to “plant destroyer”), Phytophthora species are not true fungi but oomycetes, though they behave similarly from the tree’s perspective.
Surveys of European beech trees in the northeastern United States found that roughly 40% of surveyed trees had bleeding cankers, with several Phytophthora species recovered from symptomatic tissue.6Journal of Environmental Horticulture. Prevalence, Distribution and Identification of Phytophthora Species from Bleeding Canker on European Beech The dark staining results from the tree’s internal fluids leaking through bark that has been killed by the pathogen. Over time, the wet stain dries to a dark crust, and the bark beneath may crack and fall away, revealing dead tissue underneath.
Bleeding cankers are a more serious concern than sooty mold because the pathogen is actively killing bark and the cambium layer beneath it. If the canker girdles the trunk entirely, everything above that point dies. Phytophthora thrives in waterlogged soil, so improving drainage around the base of a susceptible tree is one of the most effective preventive steps. Beech, horse chestnut, and some oaks are particularly prone.
Cytospora Canker and Bark Dieback
Fruit trees, especially stone fruits like apricots and cherries, can develop dark, sunken patches on their trunks and branches from Cytospora canker. The pathogen Cytospora cincta attacks both the xylem (water-conducting tissue) and phloem (sugar-conducting tissue), killing bark in expanding patches. When the dead bark completely girdles a branch or trunk, everything above the girdled area dies. When it does not girdle fully, the result is a chronic canker that darkens and may ooze gum.7EPPO Bulletin. Cytospora Canker and Dieback of Apricots
Trees are most vulnerable to Cytospora infection between midsummer and the following spring’s bud opening, a window that coincides with periods of dormancy and environmental stress. Pruning during dry weather, avoiding unnecessary wounding, and keeping trees vigorous through proper watering and nutrition all reduce the risk. If you spot dark, sunken areas on an apricot or plum trunk with amber-colored gum seeping from the margins, Cytospora is a strong suspect.
Fire Scars and Heat Damage
In regions where prescribed burns or wildfires are common, a black trunk can simply be charred bark. Even low-intensity ground fires can scorch bark and produce lasting scars, particularly on the downslope side of the stem where heat and embers accumulate.8Canadian Journal of Forest Research. Fire-scar formation and compartmentalization in oak The blackened patch may look alarming, but many tree species, especially oaks and pines, have thick bark that insulates the living tissue underneath. A fire scar on a healthy tree often heals over years as new bark rolls inward from the wound edges.
You can usually distinguish fire damage from disease by context and pattern. Fire scars tend to appear on one side of the trunk (the side that faced the heat source), have a crisp boundary between charred and uncharred bark, and do not spread or expand over time. A fungal infection, by contrast, typically grows outward in an irregular pattern and may produce visible spore structures, oozing sap, or soft, punky wood beneath the surface.
Urban Pollution and Particulate Buildup
If you live in a city and your tree’s bark is uniformly dark gray to black without any of the signs described above, air pollution may be the cause. Tree bark is porous and rough, making it an efficient trap for airborne particulate matter. Research on urban trees in the Rhine Valley found that bark accumulates atmospheric particles in its outermost structure and becomes enriched in metals like nickel, copper, zinc, cadmium, and lead, especially near busy roads and industrial zones.9PubMed. Atmospheric pollution in an urban environment by tree bark biomonitoring–part I: trace element analysis
This type of darkening tends to be uniform across the entire trunk rather than patchy or concentrated on one side. It builds up slowly over years and does not produce swelling, oozing, or soft spots. The tree itself is not harmed by the surface grime, though the darkened bark serves as a reminder of what you and the tree are both breathing. Rain washes away some particulate matter, but in heavily polluted areas the bark stays persistently dark. There is nothing you need to do for the tree in this case; the discoloration is entirely cosmetic.
Armillaria Root Rot and Secondary Darkening
Sometimes a blackened trunk is a late symptom of a problem that started underground. Armillaria root rot, caused by several species of the fungus Armillaria, decays the root system and lower trunk of many common trees and shrubs.10EDIS. Armillaria Root Rot (Also known as Mushroom Root Rot, Shoestring Root Rot, Honey Mushroom Rot) As the infection advances, bark at the base of the trunk may darken, become waterlogged, or peel away to reveal white fungal mats underneath. During moist conditions, clusters of honey-colored mushrooms may appear around the trunk base, which is a strong diagnostic clue.
Armillaria is a particularly insidious pathogen because it spreads through the soil via dark, shoestring-like fungal strands called rhizomorphs, which can travel from an infected stump or root system to a healthy neighboring tree. By the time the trunk shows visible darkening, the root system may already be extensively compromised. Trees with Armillaria often show thinning canopies, stunted growth, and branch dieback before the trunk symptoms become obvious. There is no reliable chemical cure for established Armillaria infections; removing dead wood, improving soil drainage, and avoiding replanting susceptible species in infected soil are the main management strategies.
How to Tell Them Apart
Standing in front of a blackened trunk, a few observations can narrow down the cause quickly:
- Wipes off on your finger: If you rub the dark surface and it smears or comes off as a fine black powder, you are dealing with sooty mold. Check the canopy for sticky honeydew drips and look for aphids, scale, or other sap-feeding insects on the undersides of leaves and along branches.
- Hard, knotty swellings: Rough, coal-black galls that are physically part of the wood point to black knot disease on Prunus species, or charcoal canker on oaks. These do not wipe off and feel like part of the branch or trunk itself.
- Wet, dark streaks: Dark staining that appears to seep from a crack or wound in the bark suggests a bleeding canker, possibly caused by Phytophthora. Press the bark near the stain; if it feels soft or waterlogged, the tissue beneath is dead or dying.
- Sunken patches with gum: Dark, depressed areas on fruit tree trunks with amber or dark gum oozing from the edges suggest Cytospora or a similar canker pathogen.
- One-sided char: A blackened patch confined to one side of the trunk with a crisp edge is consistent with fire scarring or heat damage.
- Uniform gray-black grime: An even, all-over darkening without soft spots, swelling, or oozing is likely particulate accumulation from road traffic or industrial sources.
When in doubt, peel back a small flap of loose bark at the edge of the dark area (only where bark is already detaching). Healthy cambium underneath is green or cream-colored and moist. Brown, dry, or foul-smelling tissue indicates active disease. If you find white, fan-shaped fungal mats between the bark and the wood, Armillaria is a strong possibility.
When to Call an Arborist
Sooty mold and pollution grime are cosmetic problems. Fire scars on thick-barked species usually heal without intervention. But canker diseases and root rots can threaten the structural integrity of a tree, which becomes a safety concern if the tree is near a house, driveway, or anywhere people spend time. A few signs should prompt a professional evaluation: dark staining that encircles more than half the trunk’s circumference, bark that peels away easily to reveal dead tissue, mushrooms growing from the base or roots, significant lean that has developed recently, or a canopy that has thinned noticeably over one or two seasons. An arborist can probe the trunk for decay, assess how much living tissue remains, and determine whether the tree can be managed or needs removal. Waiting until a weakened tree fails on its own is almost always more expensive and more dangerous than addressing the problem early.