Scientific name: Calonectria pseudonaviculata and Calonectria henriocotiae
Native range: First discovered in the UK in the 1990s
Boxwood (Buxus) is the primary host for boxwood blight, but also infects Pachysandra and sweet box (Sarcococca). Boxwood is a broad-leaved evergreen (leaves do not drop in winter) shrub and is sometimes used in decorative wreaths, which can be infected with the disease.
Boxwood blight was first found in the U.S. in 2011 (Connecticut) and is now known to occur in 22 states, in both landscape and nursery settings. Boxwood blight is not known to occur in Minnesota.
Boxwood blight is caused by two closely related fungal pathogens that can infect all above-ground plant parts, resulting in leaf lesions, leaf drop, stem lesions and severe dieback. Spread to new areas is mainly through movement of infected plants, but not all infected plants show symptoms, which can make long-distance spread difficult to manage. Once the disease is on a site, infections can develop and spread rapidly, especially when humidity is high. The pathogen can remain viable in infected debris for several years.
Confirming the presence of boxwood blight will require a laboratory test available from the University of Minnesota Plant Disease Clinic. Symptomatic plants will show leaf spots with a dark-brown border that grow until the entire leaf is brown and drops from the plant. White spores may be seen on the undersides of leaves. Stems may have dark lesions that encircle them and which eventually produce small, whitish, tuft-like fruiting bodies.
Volutella leaf blight can also affect boxwood and be mistaken for boxwood blight. The University of Minnesota Extension website has information on diagnosing problems with boxwood.
Regulatory Status: Potentially Regulated
Boxwood blight is considered reportable by the United States Department of Agriculture Animal and Plant Health Inspection Service (USDA APHIS). This means that the first find of boxwood blight in each state should be sent to the National Mycology Laboratory. Discovery of boxwood blight in Minnesota could result in regulations to prevent spread and in measures to control or eradicate the disease.
What Can I Do?
The University of Minnesota Plant Diagnostic Clinic is available to test samples of unknown plant problems. Contact the Minnesota Department of Agriculture via email at Arrest.the.Pest@state.mn.us if you suspect you have found boxwood blight.