State extension offices throughout the northeast are alerting arborists and landscapers to be aware of the incursion of a serious new disease of boxwoods known as “boxwood blight.” The disease has been reported since 2011 in North Carolina and Connecticut and more recently in New York, Massachusetts, Maryland, Ohio, Rhode Island and Virginia. There’s almost no doubt that the disease has been transported by shipping infected boxwood stock from state to state.
The disease is caused by a recently discovered fungal pathogen known as Cylindrocladium pseudonaviculatum, also known as C. buxicola or Calonectria pseudonaviculata. The initial symptoms may be hard detect and consist of small dark leaf spots and thin, black streaky cankers on plant shoots. The disease can spread very rapidly under warm, humid conditions and will cause large areas of browned leaves and defoliation If you use a dissecting microscope you can identify fruiting bodies on the tissue. Cornell Cooperative Extension suggests getting samples to them, because it is difficult if not impossible to use field observations to distinguish Boxwood Blight from common boxwood problems like Volutella and Macrophoma blights
Boxwood Blight is going to be tough to control for a number of reasons. Because the disease will usually be brought in on nursery stock, it is vital to buy boxwoods from reliable sources and to carefully inspect (and refuse infected plants) both at the nursery and upon delivery. Treatment with fungicides has not been effective, usually masking the symptoms for a while but not reducing the infection. There has been some success using a combination of chlorothalonil (Daconil) and thiophanate-methyl (3336F.) Because the disease remains active in fallen leaves and debris, the only really effective technique is to immediately rogue out infected plants and then sanitize the area as best you can, removing leaves and litter. The leaves and litter must NOT be used in a compost pile, because the disease can remain active for over 5 years.
Nurseries will be pushing to grow non-susceptible varieties of boxwoods and landscapers need to use these varieties or, even better, find other similar evergreens to supplant boxwoods. It is known at this point that the most susceptible variety is B. sempervirens ‘Suffruticosa’ also known as the Common Boxwood or English Boxwood. This is the mostly commonly planted variety in the United States. The least susceptible variety is believed to be B. microphylla var. japonica or Green Beauty. Below is a pdf file showing the susceptibility or tolerance of different boxwood species published by North Carolina State University