Arborvitae (Thuja genus) are popular evergreens used widely in landscapes. When the vibrant green foliage begins to darken or turn black, it signals a problem requiring attention. Determining the exact cause of this discoloration is the first step toward restoring the plant’s health, as the black appearance results from various biological and environmental issues. Diagnosis involves distinguishing between surface-level coatings and actual tissue necrosis.
The Relationship Between Pests and Black Foliage
A common cause of black foliage is a superficial layer known as sooty mold. This fungus does not infect the plant tissue but grows on “honeydew,” a sticky excretion. Honeydew is left behind by sap-sucking insects, such as aphids or various types of scale insects. The mold covers the surface of the leaves, giving the plant a dark or sooty look.
A simple diagnostic test involves gently rubbing the affected foliage. If the black material wipes away easily, leaving the leaf tissue beneath intact, sooty mold is the likely culprit. The presence of this mold indicates an active infestation by honeydew-producing pests, which are drawing out the plant’s fluids and causing stress. Treating the sooty mold directly with fungicides is ineffective because it does not address the underlying food source. Effective management focuses instead on eradicating the population of scale or aphids using targeted horticultural oils or insecticidal soaps.
Direct Fungal Diseases Causing Blackening
Beyond surface coatings, true fungal pathogens can directly infect arborvitae tissue, resulting in black or dark brown necrosis. A prevalent disease is Pestalotiopsis Tip Blight, caused by the fungus Pestalotiopsis funerea. This blight often attacks foliage already weakened by environmental stress. It causes the foliage to progress from green to yellowish, then to a dark brown or nearly black color as the tissue dies. The discoloration typically begins at the tips of the branches and moves inward.
Unlike sooty mold, this blackening is a sign of tissue death and cannot be wiped off. Diagnosis is confirmed by observing small, black, pimple-like fruiting structures, called conidia, dotting the surface of the dead twig tips. These structures release spores that can spread the infection, making sanitation a high priority. Management requires precise pruning to remove all dead and infected material, which limits the spread of the fungal spores. While cultural improvements are the primary defense, fungicides containing copper can be used as a preventative measure in cases of severe or recurring infection.
Non-Biological Stressors and Damage
When neither pests nor primary fungal diseases appear to be the cause, the blackening is often a sign of significant environmental or physiological stress. One major abiotic factor is poor soil drainage or overwatering, which can lead to root rot. When roots are saturated for extended periods, they drown from a lack of oxygen and become susceptible to decay-producing fungi like Phytophthora. This root damage prevents the plant from absorbing necessary water and nutrients, causing the canopy to show symptoms like yellowing, browning, and eventual dark dieback.
Desiccation, or winter burn, is another common non-biological cause that presents as dried, dark foliage, particularly on the side of the plant exposed to winter sun and wind. This occurs when the evergreen foliage loses moisture through transpiration but the frozen ground prevents the roots from taking up replacement water. Chemical injury can also result in blackening, such as localized damage from excessive de-icing road salt spray or concentrated animal urine. These contain salts and nitrogen compounds that scorch the sensitive foliage.
Steps for Diagnosis, Treatment, and Future Health
A systematic inspection provides the best chance for accurate diagnosis and successful treatment. Begin by performing the rub test on the black material to distinguish between the superficial sooty mold and necrotic tissue. If sooty mold is present, focus treatment on the underlying pest using a targeted horticultural oil application to smother the insects. For necrotic tissue, examine the pattern of dieback: tip blackening suggests a blight, while interior browning or a general decline points toward a root or environmental issue.
Address environmental factors by ensuring the soil drains well and adjusting watering practices to avoid saturation, especially during wet seasons. If root rot is suspected, improving the drainage around the root zone is the most effective long-term solution. For blight issues, immediately prune out all dark or dead branches, cutting back into healthy wood to remove the source of fungal spores. Proper pruning, combined with deep, infrequent watering during dry periods and an application of mulch to regulate soil temperature, helps the tree recover and strengthens its defense against future problems.