Boxwoods (Buxus species) are valued for their dense, evergreen foliage and ability to be shaped into formal hedges and topiaries. Despite their hardiness, these shrubs face threats that cause decline, dieback, and death. These problems often present with similar symptoms, making accurate identification difficult. Understanding whether the decline is caused by a pathogen, an insect, or a cultural issue is the first step toward saving the plant.
Fungal Diseases Causing Decline
Fungal pathogens pose the most urgent threat to boxwoods, spreading rapidly and often proving fatal. The most devastating is Boxwood Blight, caused by the fungus Calonectria pseudonaviculata. Symptoms begin with dark leaf spots that quickly cause leaves to turn brown and drop off the plant. A distinguishing feature is the development of narrow, elongated black streaks, known as cankers, on the green stems. This rapid defoliation and the presence of black stem cankers signal an immediate crisis.
Volutella Canker, caused by the fungus Pseudonectria buxi, presents symptoms often confused with winter injury. Leaves on infected branches change from green to a light straw or tan color, often cupping upward along the stem. Unlike Blight, the dead leaves infected with Volutella remain attached to the branches for months. The definitive sign is the appearance of numerous small, salmon-pink fungal spore masses on the undersides of affected leaves and branches during high humidity. This fungus is opportunistic, primarily infecting boxwoods weakened by stress factors like pruning wounds.
Insect Pests That Stress Boxwoods
Several insect pests target boxwoods, causing aesthetic damage and weakening the plant. The Boxwood Leafminer (Monarthropalpus flavus) is a significant pest, with damage occurring inside the leaf tissue. Adult females lay eggs inside new leaves in the spring, and the hatched larvae feed internally throughout the summer. This internal feeding causes the leaves to develop characteristic blisters or mines, visible as crinkled, yellow, or brown swellings on the undersides of the leaves.
The Boxwood Psyllid attacks the shrub’s new growth. Feeding by these small insects causes the terminal leaves to become cupped or spoon-shaped. While the distortion is unsightly, Psyllid infestations rarely cause plant death. Spider Mites (Eurytetranychus buxi) are active during hot, dry weather and cause injury that appears as fine stippling on the leaves, eventually leading to bronzing or a grayish appearance.
Environmental Factors and Cultural Issues
Non-biological factors frequently cause boxwood decline, often mimicking disease or increasing vulnerability to pathogens. Poor soil drainage, resulting in “wet feet,” is a common problem since boxwoods require well-drained soil. Symptoms include general decline, yellowing foliage, and eventual collapse, often mistaken for root rot.
Winter injury, or desiccation, occurs when foliage loses moisture to cold winds faster than the roots can replace it while the soil is frozen. This damage typically affects the outer, exposed parts of the canopy, causing foliage to turn bronzed or straw-colored.
Improper cultural practices, such as deep planting and poor pruning, also contribute to decline. Planting a boxwood too deeply, where the root flare is buried, predisposes the shrub to stress and root issues. Tightly shearing boxwoods creates a dense, humid canopy that restricts air circulation, making inner foliage susceptible to fungal diseases like Volutella Canker. Maintaining good air circulation and avoiding the accumulation of dead leaves are essential cultural controls to reduce disease risk.
Identifying the Problem and Taking Action
Diagnosing the precise cause of boxwood decline requires careful observation of the specific symptoms present on the plant. If you observe rapid leaf drop accompanied by distinct black streaks on the green stems, Boxwood Blight is the likely culprit. Conversely, if the leaves remain attached and turn tan, with salmon-pink spore masses visible, Volutella Canker is indicated.
When dealing with fungal issues, immediately prune out all infected material, cutting well below any visible cankers. Sanitize all pruning tools between cuts using a bleach solution or alcohol to prevent spreading spores. For severe fungal infections, a systemic fungicide may be necessary; consult a local agricultural extension office for guidance. If pests are suspected, targeted systemic insecticides or horticultural oils can be used, but treatment timing must align with the insect’s specific life cycle. If symptoms indicate cultural issues, correct poor drainage, ensure adequate watering, and confirm the plant is not buried too deeply to improve overall health.