First described in the 1990s in the United Kingdom, boxwood blight, was not officially identified until 2002 in New Zealand. The fungus that causes this disease, was itself a newly identified species; named Cylindricladium pseduonaviculatum by finders in New Zealand while also being called C. buxicola when found in the UK. The two names are now used synonymously to describe the same disease and fungus. Since the original identification boxwood blight has been reported throughout Europe and although it is not currently regulated, is considered a great concern. The exact, geographic point of origin for the disease cannot be pinpointed and its method of introduction into the US is also unknown, but the presence of boxwood blight has been confirmed in North Carolina, Virginia, New Jersey, Delaware, Massachusetts, Maryland, New York, Ohio, Oregon, Pennsylvania, Rhode Island and Connecticut.
Boxwood blight is known to affect all species in the genus Buxus, although some cultivars seem more susceptible than others. Currently boxwood blight is most frequently occurring on Buxus sempervirens ‘Suffruticosa’; English boxwood and Buxus sempervirens; American or common boxwood. Many other species, cultivars and hybrids have been found to be infected and as of yet none has shown any resistance.
C. pseduonaviculatum /C. buxicola infects all the above ground portions of the plant, but seemingly not the roots. The first visible symptom is usually lesions on the foliage; light or dark brown spots on leaves. Lesions may have dark borders, they eventually coalesce or “zonate”: which is a definitive line formed at the edge of an infected portion of leaf tissue. Infected leaves will turn brown or straw-colored then defoliation will quickly follow.
The disease will also affect the stems, which will appear as distinctive black lesions that display an angular, diamond-shaped pattern. Lesions can generally be found infecting stems from the soil line to the shoot tips.
Fungus readily produces spore structures called, sporodochia, on the underside of leaves. These may only be visible with a hand lens. Sporochodia contain hundreds of sticky, cylindrical spores which will be wind dispersed over short distances. Longer distance travel is possible, and usually through human activity, i.e. contaminated boots, equipment or transport of infected plants. Birds and animals may aid long distance spore dispersal as well.
Plants with severe infection may drop all of their leaves, and although they attempt regrowth the infection and lack of leaves impacts the root system and stresses the plant. Newly planted specimens or young plants may quickly succumb.
Warm, humid weather, especially for extended periods, provides perfect conditions for growth and spread of boxwood blight. This can be especially detrimental in commercial settings where disease conditions are favorable and plants are grown in monoculture in close proximity for ease of care. This fungus has a rapid and aggressive life cycle which can be completed in just 1 week under optimal conditions; 77 degrees fahrenheit is the ideal temperature for reproduction and spread, but the fungus can survive between 44 and 86 degrees and will die after 7 days of temperatures at or above 91 degrees.
If you suspect boxwood blight or are concerned about your landscape plants, contact your arborist immediately to discuss and develop a plan for control and management of disease and treatments to aid recovery.