An unsightly nuisance
Boxwood blight is a disease caused by the non-native fungus Calonectria pseudonaviculata. All species and varieties of boxwood are considered susceptible with English and American boxwood being most severely affected. While Asian species of boxwoods as well as hybrids with Asian parents can become infected, they typically demonstrate a greater level of tolerance. Boxwood blight can cause rapid defoliation and death of boxwood plants, but it is a manageable disease.
The disease thrives in warm temperatures and high humidity so instances of boxwood blight will increase in years that have long, wet summers. It is often introduced when new shrubs infected with the disease are purchased and planted in a landscape. With this in mind, tolerant species should be selected for any new plantings. When a new boxwood is added to a landscape, it should be kept separate from existing shrubs and monitored for signs of the disease before planting.
The disease is also easily spread via pruning tools, clothing or other equipment that comes in contact with the pathogen’s spores so it is important to sanitize tools and avoid working with healthy boxwoods after working with infected ones.
Since discovery of the disease in the U.S. in 2011, boxwood blight has spread rapidly and can now be found in most states.
The first signs of boxwood blight are circular spots on leaves and dark lesions on twigs, also called cankers. It is easy to miss early signs because shortly after infection, the shrub will turn brown or straw-colored and begin to lose its leaves. In severe cases, complete loss of leaves can occur in as little as one week after infection.
Management of the disease is best achieved by combining a number of different tactics. The first step is selection of tolerant species that are carefully monitored when added to a landscape. Proper sanitation of any tools or equipment used on or near boxwoods is also critical.
Planting new boxwoods in full sun will help create an environment that is unfavorable to disease development. New and existing plantings should also be mulched to make it more difficult for the disease to spread.
Severely diseased plants should be removed, but note that the fungus persists in the soil for many years so if there are other boxwoods on the site or plans to add them, appropriate preventative treatments will be required. Applications should be made monthly or biweekly depending on a number of factors including the season and weather conditions. The summer and early fall months are the most critical times, especially when there are high levels of rainfall.