The common hackberry (Celtis occidentalis) is a durable shade tree known for thriving in challenging urban environments where many other species fail. It tolerates poor soils, variable pH levels, heat, and drought, making it a popular choice for street plantings and landscapes. Despite this inherent toughness, the hackberry is susceptible to specific, recurring problems caused by insects, pathogens, and environmental stressors. Addressing these issues involves understanding the cause and applying targeted management strategies to maintain the tree’s health.
Addressing Common Insect Infestations
The most frequently noticed insect issue is the Hackberry Nipple Gall, caused by the tiny psyllid, Pachypsylla celtidismamma. These insects stimulate the leaf tissue to form distinct, raised, nipple-like structures that protrude from the underside of the leaf. The gall shields the developing psyllid nymph inside throughout the summer, making topical insecticide treatments ineffective during this period.
The damage is primarily cosmetic and rarely threatens the overall health or survival of a mature tree, though heavy infestations may cause premature leaf drop. The greatest annoyance occurs in the fall when adult psyllids emerge from the galls and seek sheltered places to overwinter, often migrating indoors. To control severe infestations, a systemic insecticide like imidacloprid can be applied as a soil drench in the fall or very early spring before budbreak. This allows the product to reach the new foliage where the pests lay eggs.
Hackberry trees also host several sap-feeding insects, including various types of scale and the Asian woolly hackberry aphid. These pests extract plant fluids, excreting a sugary substance called honeydew that coats leaves, branches, and objects beneath the tree. Sooty mold, a black, crusty layer, then grows on this sticky residue, which can block sunlight and reduce photosynthesis.
Management for these sucking insects often begins with cultural control, such as blasting populations off leaves with a strong jet of water. Dormant horticultural oil sprays, applied in late winter before new growth begins, can suffocate overwintering eggs and nymphs on the bark. Systemic insecticides are reserved for dense, persistent infestations where the tree’s vigor is compromised.
Management of Fungal Diseases
The most distinctive pathological problem affecting the species is the formation of Witches’ Brooms, which are dense, distorted clusters of short, twiggy shoots. This symptom is a disease complex resulting from the combined irritation of a powdery mildew fungus and an eriophyid mite. The fungus and mite disrupt the normal hormonal balance in the developing buds, causing the proliferation of abnormal growth.
While the brooms are highly visible, especially during the winter months, they seldom cause severe damage or tree decline. The primary treatment for Witches’ Broom is pruning out the affected branches during the dormant season to improve the tree’s appearance. Cuts should be made several inches below the point of the broom’s attachment to ensure all affected tissue is removed.
Other common fungal issues include leaf spot diseases and powdery mildew, which appear as dark blotches or a white, powdery coating on the foliage. These are seasonal problems tied to periods of high humidity and are rarely fatal to the tree. Chemical treatment is seldom practical or necessary for a large shade tree.
Control focuses on cultural improvements rather than relying on fungicides, which are often difficult to apply effectively to tall trees. Improving air circulation within the canopy through selective thinning of branches reduces the moist conditions where fungi thrive. Raking and destroying fallen, infected leaves in the autumn removes a source of fungal spores that could reinfect the tree the following spring.
Solving Environmental and Cultural Issues
Many hackberry problems that appear to be caused by pests or disease are actually rooted in environmental stress from the urban setting. The tree’s tolerance for tough sites does not exempt it from requiring proper cultural care for long-term health. Water stress, resulting from severe drought or chronic overwatering, is a common issue that manifests as leaf scorch, yellowing, or premature leaf drop in mid-summer.
To mitigate drought stress, deep and infrequent watering is necessary, applying water slowly to saturate the root zone rather than just the surface layer. Soil compaction, prevalent in high-traffic areas and construction sites, severely limits the tree’s ability to absorb water and nutrients. Aeration techniques or the installation of a root collar can help alleviate this stress.
Nutrient deficiencies, particularly iron or manganese, can cause leaves to display interveinal chlorosis, which is a yellowing of the tissue while the veins remain green. This often occurs when hackberries are planted in highly alkaline soils with a high pH, which binds these micronutrients and makes them unavailable. Applying chelated forms of iron or manganese can sometimes correct this issue, though addressing the soil pH is the long-term solution.
Improper pruning techniques, such as topping or leaving ragged cuts, create large wounds that invite opportunistic fungal pathogens causing decay and cankers. Pruning should follow correct arboricultural standards, making clean cuts just outside the branch collar and avoiding removal of too much canopy at once. The use of broadleaf herbicides near the tree can also cause significant leaf deformation or sudden defoliation due to chemical drift from lawn treatments.