Volutella blight is the most common disease of both American and English boxwood in the landscape. Volutella blight is caused by an opportunistic fungal pathogen that attacks leaves and stems of damaged or stressed plants. Winter injury, poor vigor, and wounds increase risk for Volutella blight.
The pathogen survives winter on affected branches and leaves from the previous season. Disease symptoms begin to be visible when there is high humidity and temperatures between 65 and 75F.
Leaves will turn red or bronze, eventually becoming straw-yellow. Branches then die back from tips and girdling may occur lower on stems. Bark may also be loose on infected branches. In moist, humid weather, salmon-to-pink fruiting structures may be visible.
Maintaining a sanitation program and promoting plant vigor are critical for disease management. Prune out diseased branches and be careful not to wound the stems or make improper cuts. A boxwood getting adequate nutrition and irrigation will recover better. Avoid excess water. Gather and destroy infected plant debris. When planting new boxwood, give sufficient space between boxwood to allow for air circulation which helps reduce Volutella blight.
If disease continues to be a problem after following other management practices, fungicides may be used preventatively beginning in spring. Homeowners may use fungicides that contain mancozeb or copper. Always follow label directions when utilizing fungicides.
— Kelly Jackson