There are many common but less severe diseases of boxwood shrubs, and all are occasionally mistaken for boxwood blight. Learning the following combination of three key symptoms will help you be confident with field diagnoses and differentiate boxwood blight from most other boxwood problems. Just remember that there can be multiple diseases present in any situation.
The most noticeable symptom associated with boxwood blight is defoliation. Unlike other boxwood fungal diseases, boxwood blight causes rapid leaf drop on infected shrubs, leaving completely bare twigs. You will see piles of yellow to green leaves on the ground below defoliated, cankered twigs. Other diseases and disorders often leave dry, straw-colored leaves attached to dead branches.
The second differentiating symptom is the presence of twig cankers. Infected twigs develop distinct blackened lesions extending along the twigs between nodes and eventually produce spores.
3. Leaf spots
The third symptom is leaf spotting. Rapidly dying leaves may appear greasy and water-soaked prior to falling, or they may develop circular spots, often with a dark ring and orange halo. These leaves are more devious than they appear. The fungus can produce many thousands of tiny structures called microsclerotia in each infected leaf. These microsclerotia persist in the leaf litter and soil beneath affected plants, serving as the overwintering stage. They can persist for several years, germinating to cause new infections in later seasons even if the plants were removed.
Matthew Borden, DPM, is a plant health consultant specializing in diagnostics and integrated management of landscape plant pests and diseases.