Q. I have been hearing about a new disease that affects boxwood, and it concerns me because I have a lot of boxwood in my landscape. Can you tell me more about it, and what I can do to protect my plants?
A. I imagine you are referring to boxwood blight or box blight, which was identified in the United States for the first time in 2011. This disease was first discovered in the United Kingdom in the mid-1990s and is now widespread in Europe. To date it has been found in Connecticut, Maryland, Massachusetts, New York, North Carolina, Ohio, Oregon, Pennsylvania, Rhode Island and three Canadian provinces.
Boxwood blight is caused by a fungus, Cylindrocladium pseudonaviculatum or Cylindrocladium buxicola. Both names refer to the same pathogen. This fungus is extremely pathogenic and difficult to control; avoiding infection is the best way to protect your plants. Other members of the Buxaceae family include Japanese pachysandra (Pachysandra terminalis), Allegheny pachysandra (P. procumbens) and sweet box (Sarcococca spp.). Boxwood blight has been identified in plantings of pachysandra, which complicates the management of this disease when both hosts are present in a landscape. Sweet box has been infected in the laboratory, but no naturally occurring infections have been identified to date.
Older, established plantings are only at risk if new boxwood plants have been installed since 2010. The spores of this fungus are very sticky and are not easily spread by wind; long-distance transmission is mainly due to the unintentional movement of infected nursery stock and cuttings. However, those sticky spores are easily moved short distances on tools and equipment, hands and clothing, and by animals that have been in contact with infected plants.
Symptoms of boxwood blight include light to dark brown circular leaf spots that often have darker margins. Those spots merge together and cause infected leaves to turn straw brown and drop prematurely. The disease also causes dark brown to black elongate or diamond-shaped stem lesions. Symptoms are generally most severe low on the plant and where the plant is shaded. Shade and warm (68-80 degrees), humid conditions favor disease development, and defoliation can occur rapidly under those conditions. Under high humidity, you may observe fuzzy white spores on the underside of infected leaves or on stems. Because boxwood has other problems, such as volutella blight or nematodes, laboratory testing is necessary for a definitive diagnosis.
There are steps you can take to minimize the chance of blight impacting your landscape. If you do bring any new boxwood plants into your landscape, quarantine them for at least a month (two is better) in an area well away from other boxwoods. You may want to cover the area with black plastic and place the pots on it; that will make cleaning up any fallen leaves easier in case any of them are symptomatic. The fungus can remain dormant in fallen leaves for years, and boxwood leaves do not break down readily.
Much of the research on boxwood blight has been conducted by Kelly Ivors at North Carolina State University and Sharon Douglas from the Connecticut Agricultural Experiment Station in New Haven. North Carolina has done susceptibility trials of commercially available boxwood varieties that rate boxwood from most susceptible to least susceptible. The entire list is available at http://plantpath.cals.ncsu.edu/ornamentals. If you are adding new boxwood to your landscape, it is wise to stick with those rated as less susceptible or tolerant, such as Buxus 'Green Gem'; Buxus sempervirens 'Dee Runk' or 'Fastigiata'; Buxus microphylla 'Winter Gem' or 'Golden Dream'; and Buxus microphylla var. japonica 'Green Beauty.'
Remember that there are other common diseases and environmental conditions that can cause dieback and defoliation of boxwood shrubs, including poor drainage and winter injury. After the subzero, windy weather we have experienced this winter, winter injury on broadleaf evergreens such as boxwood is likely to be quite common.
Send questions to Sandy Feather by e-mail at email@example.com or by regular mail c/o Penn State Extension, 400 N. Lexington Ave., Pittsburgh 15208.