Although boxwoods (Buxus sp.) are not native to the U.S., their versatility has made them one of the most popular shrubs for edging, hedges, and topiaries in classic and modern landscapes. We don’t see many infectious diseases on boxwood in Iowa, but Buxus species are often damaged by winter injury.
However, in October 2011 a new pest to the U.S. ornamental industry was reported in North Carolina. Boxwoods were showing yellowing or bronzing of foliage followed by rapid defoliation. This new pathogen is able to kill shrubs as quickly as in two weeks. The fungus responsible for this disease (Cylindrocladium pseudonaviculatum)was first described in 2002, and severe outbreaks up until now had only been reported in New Zealand and Europe. Now the pathogen has been confirmed in at least 5 U.S. states (Virginia, Maryland, Rhode Island, Massachusetts, and Oregon) since October of last year.
What do we know about Box Blight?
Symptoms start with dark or light brown leaf spots, bronze or straw-colored foliage, sudden defoliation, and black streaks on affected branches (click here for pictures. Infections can be severe in young plants and can kill seedlings. Several Buxus species can be affected but English and common boxwood is highly susceptible to this disease. This pathogen can complete its life cycle in about a week under warm and humid weather conditions, and high moisture is required for infection. The fungus can be spread by water, animals, or human activities such as pruning.
Although box blight has not been reported in the Midwest, the potential distribution of this disease goes from USDA Zones 10 to 4, and the pathogen has been reported to survive and grow at low temperatures (50 degrees F).
If you suspect your boxwood is suffering from box blight, don’t panic yet! This disease can be confused with several common boxwood problems. Leaf bronzing and straw-colored branches can be also caused by winter injury and by another fungal pathogen called Volutella. Similar symptoms can also occur from Phytophthora, Verticillium, or nematode infections. To confirm diagnosis it is always best to send samples to a diagnostic laboratory. Look up your local NPDN plant diagnostic clinic and make sure you follow sampling and shipping instructions to ensure a good quality sample. In Iowa, samples can be sent to the Plant and Insect Diagnostic Clinic.
Stay tuned for more information!
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