Boxwood blight is a fungal disease that causes leaf spots, twig cankers, rapid defoliation, and eventual death of boxwood if left unmanaged. English and American boxwood are most severely affected, although there is some tolerance to boxwood blight in a growing number of species and hybrids—especially Asian species. Pachysandra is among the many varieties of boxwood that are considered susceptible.
Since discovery of this disease in the United States in 2011, boxwood blight has spread rapidly across the country and can now be found from coast to coast. The causal fungus, Calonectria pseudonaviculata, thrives in moderately warm (60°F–80°F) and wet weather. Abnormally warm, humid, and exceptionally rainy seasons (such as occurred in 2018) can result in severe boxwood blight pressure in landscapes. In the seasons following 2018, there was a large amount of infectious spores in our landscapes which persisted for several years. The 2021 and 2022 seasons saw lower incidence, although by autumn of 2023 some regions were battling it again.
Successful management of boxwood blight involves a multifaceted program that includes sanitation (e.g., removal of infected tissues, cleaning of tools), avoidance of overhead irrigation, and application of products that protect new growth and reduce fungal sporulation. Severely affected plants, which have been infected for multiple years, should be removed when appropriate. Removal of the defoliated leaf litter can help reduce inoculum. This can be followed with a thin layer of mulch to minimize the remaining infectious spores splashing up onto healthy branches. When possible, boxwood should not be pruned during the summer months. Lastly, applications of preventive products can protect new flushes of growth. Multiple applications throughout the season are needed for good success. Although boxwood blight can result in plant death, good responses are seen with our curated management programs.
