Boxwood blight was first identified in the UK in 1994. It was found in Connecticut and North Carolina in October 2011 and has since then spread to other states. This disease is caused by the fungus Cylindrocladium pseudonaviculatum, or C. buxicola. Infected plants have round leaf spots in various shades of brown and black. Eventually the leaves turn entirely brown or light tan and may drop off, leaving bare stems. Warm and humid conditions provide the perfect environment for the disease to ravage boxwoods. The fungus’s ideal temperature range is 64 to 77˚F. It stops growing at 86˚F and is killed at 95˚F. The disease cycle from initial infection to the release of spores can occur in as little as seven days. The disease is spread from one plant to another by wind, water and even tools, your hands, animals or birds passing by and so on.
If you find your boxwood affected by boxwood blight, you can cut back diseased twigs and remove any fallen plant matter from the surrounding soil. If your plant is severely sick, it should be removed and disposed of. Seal diseased trimmings, etc., in a garbage bag and throw them away; do not compost them. Keeping foliage dry may discourage fungal growth. There is no known cure. Fungicides have proven somewhat effective, but not a great fix.
As this disease seems likely only to spread further, you might consider other planting options. Some suitable evergreen hedging alternatives to boxwoods include inkberry holly (Ilex glabra), ‘Globe’ arborvitae, Japanese holly (Ilex crenata) and sweet box (Sarcococca hookeriana).