Boxwood blight is caused by the non-native fungus Calonectria pseudonaviculata. American and English boxwood (Buxus sempervirens) are most susceptible to this disease whereas many Asian species of boxwood, and hybrids with Asian parents, are generally more resistant, but can still become infected. The disease causes black spots to form on leaves, along with elongated black lesions on twigs and stems. Rapid defoliation (in a matter of weeks) soon follows the initial symptoms of infection.
Boxwood blight is often introduced into landscapes on infested nursery stock. Spores of pathogen can easily stick to shoes, clothing, and shearing/pruning tools, so the disease can be easily introduced during routine maintenance. New outbreaks commonly occur following shearing plants in summer.
Moisture on leaves and twigs provides ideal conditions for infection by this fungus so years with high rainfall and extended summer temperatures result in greater disease incidence and severity.
There are several ways to prevent boxwood blight.
Planting
- Plant tolerant species such as hybrids like Buxus microphylla var. koreana and B. sempervirens, Monitor for symptom development from planting on.
- Do not install infected plants. If adding boxwood to the landscape, “heal-in” the new plants in an area of the property remote from existing boxwood plantings. Observe them for a growing season to ensure they are disease free before planting near established boxwood.
- Plant boxwood in full sun and prune any over story plants to minimize moisture accumulation on leaves and twigs.
Maintenance
- Do not shear plants, especially during the growing season. Prune (not shear) boxwoods for managing size and shape during the dormant season. If boxwood must be pruned during the growing season, disinfect tools between plants.
- Boxwood blight spores are easily spread by sprinkler irrigation, so drip irrigation or hand watering is recommended.
- Mulch existing plantings, especially in areas that previously contained infected plants
- If boxwood blight develops on a landscape, remove severely diseased plants.
- Avoid working with healthy boxwoods after working with infected ones. The boxwood blight pathogen can spread via pruning tools, clothing, and other equipment that may come in contact with the sticky spores.
Treatments
- Fungicide treatments are available to protect new infections on remaining plants. Once introduced into a landscape, the causal fungus can survive in the soil for years so fungicide treatments must be applied on an ongoing basis to protect plants from infection.
