Boxwoods (Buxus species) are highly valued for their dense, evergreen foliage and versatility in landscape design, from formal hedges to topiaries. Their popularity is matched by their susceptibility to several serious fungal pathogens that can severely diminish their appearance and health. Successfully managing these issues begins with an accurate diagnosis of the specific disease present.
Identifying Common Boxwood Diseases
Understanding the distinct visual symptoms of different boxwood diseases is the first step toward effective management. Two of the most destructive and widespread diseases are Boxwood Blight and Volutella Canker, both caused by fungal pathogens that thrive in moist conditions. Differentiating between these two diseases is crucial because their management strategies vary.
Boxwood Blight
Boxwood Blight is caused by the fungus Calonectria pseudonaviculata. The disease is characterized by circular leaf spots that are tan or light brown with a dark purple or black border. The most distinguishing symptom is the rapid and severe defoliation that follows, often leaving stems bare within a week of initial infection.
Inspection of the stems reveals narrow, elongated black streaks or lesions that run vertically along the green stem tissue. In humid conditions, white spore masses can be observed on the undersides of infected leaves and stems. This disease is highly aggressive and its spores are easily spread by water splash, contaminated tools, or clothing.
Volutella Canker
Volutella Canker is caused by the opportunistic fungus Pseudonectria buxi. This disease often targets plants already stressed from factors like winter injury, poor drainage, or improper pruning. Symptoms typically become apparent in the early spring, affecting branches damaged during the previous season.
Infected leaves turn from green to light green-yellow, then to a bronze or tan color, and often cup upward along the stem. Unlike Boxwood Blight, the dead leaves remain attached to the branch for a prolonged period, creating a straw-colored appearance. The most definitive symptom is the presence of small, salmon-pink or orange-colored fruiting structures (sporodochia), which emerge on the lower surfaces of affected leaves and cankered stems during periods of high humidity.
Cultural Practices for Disease Prevention
Proactive management focusing on environmental control is the most effective way to reduce the likelihood of boxwood infection. Since fungal diseases thrive in extended periods of leaf wetness and high humidity, altering the plant’s microclimate is necessary. These practices aim to create an environment unfavorable for spore germination and spread.
Proper spacing between boxwood plants promotes good air circulation within the canopy, allowing foliage to dry quickly after rain or irrigation. Avoid planting boxwoods in areas subjected to constant shade or poor drainage, as this prolongs moisture retention. Pruning to thin out dense, interior growth also increases air flow and light penetration.
Watering practices require careful attention; overhead irrigation should be avoided, especially in the late afternoon or evening. Water should be directed to the base of the plant using drip irrigation or a soaker hose to keep the foliage dry. Applying a shallow layer of mulch (two to four inches) underneath the plants helps prevent the splash dispersal of fungal spores from the soil onto the lower leaves.
Sanitation is a continuous effort that removes potential sources of fungal inoculum from the landscape. Fallen leaf litter and plant debris should be promptly raked up and removed, as the Boxwood Blight fungus can survive in this material for years. Avoid working with boxwoods when the foliage is wet, since fungal spores can easily be transferred on hands, clothing, and equipment.
Reactive Treatment and Management
Once a boxwood disease is confirmed, immediate action is required to contain the infection and prevent its spread. The initial response involves physical removal of diseased material followed by rigorous sanitation protocols. Chemical intervention is generally used as a preventative measure rather than a cure.
Sanitation and Removal
Any branches showing symptoms of disease must be pruned out immediately, cutting back into healthy wood well below the visible canker or streaking. For Boxwood Blight, severely defoliated plants must be removed entirely, including the top few inches of surrounding soil, as the pathogen can persist. All removed plant material should be securely bagged and disposed of in the trash or buried deeply away from other boxwoods, never added to a compost pile.
Tool sanitation is mandatory after every cut when working with infected plants to prevent spore transfer. Pruning shears and other equipment should be sterilized by wiping them clean and then soaking or spraying them with a solution like 70% isopropyl alcohol or a 10% bleach solution. This practice breaks the chain of infection, which is often the primary route of disease spread.
Chemical Intervention
Fungicides are not a standalone solution but serve as a protective shield for healthy foliage and adjacent plants. They are most effective when applied preventatively, especially in anticipation of environmental conditions favorable for disease development, such as warm, wet weather. Applications need to cover the entire plant thoroughly, which can be difficult due to the boxwood’s dense structure.
For Volutella Canker, protective fungicides containing active ingredients like copper or chlorothalonil are applied in the early spring before new growth emerges. The goal is to protect the new, susceptible tissue from infection. Boxwood Blight requires a more intensive, season-long preventative spray program using broad-spectrum protectants, sometimes in rotation with systemic fungicides to manage resistance. Homeowners should consult local extension services for registered products and adhere strictly to the product label instructions for application rates and safety.