Identification and Treatment
Throughout the US primarily on cultivars of Buxus sempervirens, B. mycrophylla and B. sinica. The disease can also infect Sarcococca and Pachysandra spp. Leaf spots leading to leaf necrosis and falling are one sign. Lesions (cankers) of stem are also visible especially on new stems.
The disease was first documented in the United Kingdom in 1994. It now has a worldwide distribution. In the US it is primarily found throughout the Eastern US and the Pacific Northwest. Diseases in nursery plants are not uncommon. There are two species of the disease with one primarily found in parts of Europe. Calonectria henricotiae is the pathogen of concern in the US.
Humidity exacerbates boxwood decline. Thus, hedging boxwoods causes two problems. First, it increases the humidity of the inner parts of the shrub (lower air movement and less UV exposure) and second, if tools are not cleaned between shrubs it will spread the disease. Disease spread is also via wind and water splash. Mulching and pruning of lower branches can reduce reinfection from ground up. Water splashed (rainfall or irrigation) on dead, infected leaves at soil is a source of further plant infection. Mulch must be replaced on a regular schedule or treated with a fungicide to reduce the number of fungal spores.
There are no effective fungicide treatments, to date, for Boxwood Blight. Carefully selecting species and cultivars with resistance to the disease is the most effective remedy.
photo from ask.extension.org