DISEASE FOCUS: Boxwood blight — a new threat to California

by Deborah M. Mathews

A new disease of boxwoods (Buxus spp.) was first detected in Connecticut last fall and has since been found in nine eastern states and Oregon, as well as British Columbia and Ontario in Canada.  It has been common in Europe and New Zealand for two decades.  It has not yet been detected in California, but our industries should  become informed on what to look for to ensure early detection and management. 

The disease is caused by the fungus Cylindrocladium pseudonaviculatum (= C. buxicola, Calonectria pseudonaviculata).  Symptoms include brown leaf spots with dark edges, black cankers on stems, and severe defoliation and dieback (figs. 1-3).  Fungal fruiting bodies called sporodochia, which are composed of many sticky spores, are found on the undersides of leaves and on black stem lesions and can be seen with a hand lens.  The disease can develop rapidly, within a week, especially in warm, humid environments with a temperature optimum of 77°F, although it is capable of survival in a wide temperature range of 41 to 86°F.  Some plants are able to remain symptomless for a few weeks when the amount of inoculum is low and suboptimum weather conditions exist.  The disease can occur on all boxwood species, however varieties of B. sempervirens seem to be the most susceptible.  C. buxicola has been shown to also infect Sarcococca spp. (fragrant sweet box) and Pachysandra terminalis (Japanese spurge), both members of the Buxacea, in the laboratory; other species are being tested for susceptibility.  The fungus does not require a wound for entry and can penetrate plants directly or through stomates, but does require high humidity or free water to be present for infection.  Overhead watering and tightly spaced plants encourage spread of the disease. The sticky spores can be carried by water and on anything coming into contact with them like tools, equipment, shipping containers, shoes/clothing, etc. Hardened masses of mycelium can survive for years on leaf debris in the soil. 

Fig. 1.  Boxwood blight getting started; note defoliated leaves in bottom of pot. 
Photo by Kelly Ivors, North Carolina State University.

Fig. 2.  Multiple boxwood plants showing severe blight and defoliation. 
Photo by Kelly Ivors, North Carolina State University.

Fig. 3.  Close up of dark stem lesions caused by Cylindrocladium buxicola, causal agent of boxwood blight. 
Photo by Mike Munster, North Carolina State University.

Care must be taken in diagnosing this disease because other pathogens can cause symptoms similar to boxwood blight including Volutella, Phytophthora nicotianae, Pratylenchus root lesion nematodes, and cold injury.  Multiple infections by one or more of these with the boxwood blight pathogen further complicates accurate diagnosis.  Fungicides cannot control the disease once it starts, but may be useful for preventing or reducing sporulation and spread; research is in progress to assess the most effective chemicals.

The basic management tools include: Know the signs and symptoms of boxwood blight and scout your plants regularly; Inspect plants before unloading and reject problem plants immediately; Limit entry of new plant material and keep isolated from existing plants for 30 to 60 days. If this disease is suspected at your location you should immediately notify your County Agricultural Commissioner, UC Cooperative Extension office or the California Department of Food and Agriculture (CDFA) so that an accurate diagnosis can be made and mitigation steps started. 

More information can be found at: http://boxwoodblight.org, hosted by the American Nursery and Landscape Association (ANLA). A 45-minute video seminar is available for viewing at this website and multiple articles with information and current best management practices can be found in the “Additional Resources” section.

Deborah Mathews is UC Cooperative Extension Specialist/Plant Pathologist for Ornamental Crops, Department of Plant Pathology and Microbiology, UC Riverside.