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Nancy F. Gregory
Boxwood Blight Caused by the Fungus Cylindrocladium pseudonaviculatum
The fungus Cylindrocladium pseudonaviculatum causes severe defoliation and death of young and container grown boxwood plants, and dieback in older plantings. Most boxwood species are susceptible, including American and English boxwood (Buxus sempervirens), little leaf boxwood (B. microphylla) and hybrids such as B. sinica. Pachysandra is also a host, and can be a source of spores capable of causing new infections. There are a number of other diseases that affect boxwood, and environmental stress plays a role in susceptibility and predisposition to disease. Timely and accurate identification is important in order to control this aggressive new disease.
Boxwood blight was first identified in the United Kingdom in 1994 and then in the U.S. in October of 2011. The disease has been identified in nurseries and landscapes on boxwood and pachysandra. It has now been confirmed in ten states, including NC, CT, VA, RI, MD, MA, OR, NY, PA, OH, and Delaware. It was identified in the UD Plant Diagnostic Clinic in 2012, but came from plants in Maryland. In June of 2013, boxwood blight was identified in a retail nursery in Delaware.
Cylindrocladium is favored by temperatures between 18 and 25 C and by rainfall or high humidity. Early symptoms include brown leaf spots or tip blight, which can be rather inconspicuous. Twigs and stems later develop black streaks and leaves drop. After moist weather conditions or incubation of samples in a moist chamber, signs of the fungal pathogen may be seen with a hand lens or a microscope. Signs include white clusters of spores of the fungus. It is very important to have this disease accurately identified due to similarity with other diseases, especially Volutella blight, which has been common in Delaware for many years. Volutella blight is most often associated with boxwood plants under environmental stress, whereas Cylindrocladium can affect healthy plants.
Spore masses of Volutella are pink to orange in color. There is no good control for Volutella except to trim out and reduce stress on plants. Boxwood blight, however, can be severe and lead to death of infected plants, and requires remediation and control.
Management and Control of Boxwood Blight
Control is dependent on rapid and accurate diagnosis. Diseased plants should be removed and destroyed if in a retail or nursery situation. Boxwood within 10 feet of affected plants should also be removed. Make sure to clean up all debris and leaves, as sanitation is very important to reduce plant material producing spores. The pathogen can survive for several years in soil and debris, so infected material should not be composted. Following removal of affected material and trimming and sanitation of remaining area, fungicides can be used to protect new growth and unaffected plants nearby. Pruners and tools should be cleaned with bleach or alcohol. Avoid overhead irrigation. Do not bring new boxwood into established landscape plantings.
Chemical Control Options
Due to the fact that this is a relatively new disease to the U.S., boxwood blight may not appear on a fungicide label on existing product. Preventive applications are helpful on asymptomatic plants or plants near to confirmed cases. Fungicide spray coverage needs to be very thorough due to the dense canopy of boxwood plants. It is recommended that plants be trimmed first and then sprayed with the high rate of chemical according to label. There are no effective curative treatments. Rotation of chemistries is important to avoid development of resistance. Some options include:
- Pageant at 8-12 oz/100gal + chlorothalonil (or mancozeb) (Pageant is Boscalid and Pyraclostrobin in a combination). Use no more than 2 consecutive sprays.
- Medallion (fludioxonil) 1-4 oz/100gal + mancozeb (or chlorothalonil)
- Chlorothalonil 1.4 pints/100gal or mancozeb
For more information please see the following websites of the American Nursery and Landscape Association (ANLA) and the Connecticut Agricultural Experiment Station:
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