Boxwood blight is a destructive fungal disease that poses a significant threat to boxwood (Buxus species) shrubs used extensively in landscaping. The pathogen responsible is the fungus Calonectria pseudonaviculata. This disease is relatively new to North America, first confirmed in the United States in 2011, but it has since spread rapidly across many regions. The infection causes rapid decline in the plant’s aesthetic value, often leading to complete defoliation and, in severe cases, the death of young or highly susceptible shrubs.
Identifying the Signs of Boxwood Blight
The first visible evidence of boxwood blight appears as dark or light brown spots on the leaves, which can be circular or irregular. These spots often enlarge and merge, forming larger brown blotches. Initial symptoms usually begin on the lower branches of the shrub and progress upward through the canopy.
A distinguishing symptom is the development of lesions on the stems, appearing as narrow black streaks or diamond-shaped cankers on the green, non-woody stems. During periods of high humidity, white, fuzzy masses of fungal spores may emerge from these black stem lesions and the undersides of infected leaves.
The rapid and severe loss of leaves, known as defoliation, is a key symptom. Infected leaves turn a tan or straw color and drop prematurely, leaving bare branches. Repeated defoliation and dieback caused by the stem cankers can ultimately kill young plants or severely disfigure mature specimens.
The Cause and Spread of the Disease
The disease is caused by the fungus, which produces asexual spores called conidia. These conidia are the main source of infection and are produced in slimy, sticky masses on the infected plant tissue. The primary mechanism for local spread is the dispersal of these spores by splashing water, such as from rainfall or overhead irrigation.
The sticky conidia easily adhere to anything that brushes against the infected plant, including gardening tools, clothing, footwear, and animals. This allows the disease to be transported over short and long distances, with movement of infected plants through nursery trade being a major factor in distribution. The fungus is most active when temperatures are between 64 and 80 degrees Fahrenheit, requiring extended periods of leaf wetness to initiate infection.
The fungus can also survive in the environment for years, even after the infected plant has been removed. It does this by forming specialized, dark brown, thick-walled survival structures called microsclerotia. These microsclerotia persist in fallen leaf debris and soil, allowing the fungus to re-emerge and infect new plants planted in the same location.
Controlling and Managing the Infection
Managing boxwood blight requires an integrated approach focusing heavily on sanitation and cultural practices, as there is no cure once the plant is infected. If the disease is confirmed, immediately remove all infected plants, including fallen leaves and debris beneath the shrub. All removed material should be double-bagged and disposed of in municipal waste or buried, never composted, to prevent the survival of microsclerotia and spread of the disease.
Cultural practices are employed to create an environment less favorable for the fungus. Homeowners should avoid overhead watering, which minimizes the splash dispersal of spores and reduces the duration of leaf wetness. Increasing air circulation within the plant canopy through careful pruning helps leaves dry more quickly, slowing the progression of the disease. Applying a layer of mulch around the base of the plants helps bury fallen infected debris and prevents spores from splashing back up onto the lower leaves.
Sanitation must extend to tools and equipment, as the fungus is easily moved on contaminated surfaces. Pruning tools used on infected shrubs should be cleaned and disinfected with a solution like bleach or a quaternary ammonia product before being used on healthy plants. Fungicide applications can be used as a protective measure on healthy or at-risk plants, but they are not curative and must be applied preventatively on a consistent schedule during favorable weather conditions. When replanting, choosing less susceptible boxwood varieties, such as certain Buxus microphylla cultivars, is a practical long-term strategy.