Calonectria pseudonaviculata (syn. Cylindrocladium buxicola and Cylindrocladium pseudonaviculatum)
Click on images to see larger view
Boxwood blight is a serious disease of boxwood. It was first confirmed in the United States in 2011 in North Carolina and Connecticut and has since spread to many eastern states and others in the Midwest and Pacific Northwest. The disease is considered widespread in Europe. Rapid defoliation under favorable environmental conditions destroys the aesthetic value of the plants. If you believe your plants may be infected with boxwood blight, an accurate laboratory diagnosis is recommended to prevent spread through recommended management practices
Boxwood (Buxus species), Pachysandra and Sarcococca (sweet box) are currently known to be hosts of this disease. All are in the Buxaceae family. Susceptibility varies among boxwood species and cultivars but none are completely resistant. Buxus sempervirens ‘Suffruticosa’ (English boxwood) and B. sempervirens ‘American’ are especially susceptible.
Symptoms occur on leaves and stems. Brown spots appear on leaves following infection and after the spots enlarge and coalesce, leaves drop. As the spots enlarge, concentric lines of darker and lighter shades of brown may develop. Infected leaves range in color from tan to bronze. Black streaks or lesions occur on infected stems from the top to the soil line. Root infection does not occur. The most striking symptom is extensive defoliation. Favorable conditions are temperatures of 64-80°F and high humidity. Symptoms can be similar to those of other boxwood problems including Volutella blight, root rot, boxwood leafminer and winter injury. Volutella may occur on the same plant with the boxwood blight pathogen.
Leaf Spots Image by Mary Hansen, Virginia Polytechnic Institute and State University, Bugwood.org
Stem Lesions Image by Mary Hansen, Virginia Polytechnic Institute and State University, Bugwood.org
Under ideal conditions (warm and humid), the fungus will produce spores on the undersides of infected leaves and in stem lesions in as short a time as one week after infection. Spore masses are white and under magnification have a crystalline or quartz-like appearance. The spores are quite sticky and easily adhere to people, animals, insects or tools that come into contact with them, allowing them to be spread easily to new plants and infection sites. Additional spread occurs via splashing water or wind-driven rain. Long distance spread is via human movement of infected plant material. The fungus can survive in plant debris or in the soil during the winter and in the absence of a host plant for at least 5-6 years.
Sporulation Cylindrocladium pseudonaviculatum
The primary control measures for boxwood blight fall under the category of sanitation and include:
Protective fungicides can be used to prevent infection in remaining plants, especially when the conditions favorable for infection are expected. Active ingredients registered for use by home gardeners include chlorothalonil and mancozeb. Thorough coverage is essential and this is a challenge on boxwood due to its compact growth habit so spray until runoff.
- Plant only disease free new boxwood and other host plants.
- Purchase plants free of any symptoms including leaf spots or stem lesions.
- When introducing new boxwood into a landscape with established host plants, quarantine the new plant(s) away from the others for at least one month to observe for symptom development prior to planting if possible.
- If symptoms are noticed, have the plant(s) checked at the UConn Plant Diagnostic Laboratory
- If boxwood blight is confirmed, removal of the plant(s) with symptoms and those within 10 feet of those is recommended. The safest approach is to remove all boxwood/hosts from the property. Plants should be double bagged and placed in the trash or buried two feet deep. Do not compost.
- Replant with non-host plants.
Despite good cultural practices, pests and diseases at times may appear. Chemical control should be used only after all other methods have failed.
For pesticide information or other questions please call toll free: 877-486-6271.
UConn Home and Garden Education Center, 2016
The information in this material is for educational purposes. The recommendations contained are based on the best available knowledge at the time of printing. Any reference to commercial products, trade or brand names is for information only, and no endorsement or approval is intended. The Cooperative Extension system does not guarantee or warrant the standard of any product referenced or imply approval of the product to the exclusion of others which also may be available. All agrochemicals/pesticides listed are registered for suggested uses in accordance with federal and Connecticut state laws and regulations as of the date of printing. If the information does not agree with current labeling, follow the label instructions. The label is the law. Warning! Agrochemicals/pesticides are dangerous. Read and follow all instructions and safety precautions on labels. Carefully handle and store agrochemicals/pesticides in originally labeled containers immediately in a safe manner and place. Contact the Connecticut Department of Environmental Protection for current regulations. The user of this information assumes all risks for personal injury or property damage. Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Gregory J. Weidemann, Director, Cooperative Extension System, The University of Connecticut, Storrs. The Connecticut Cooperative Extension System offers its programs to persons regardless of race, color, national origin, sex, age or disability and is an equal opportunity employer.