Keeping plant disease out of a nursery is always a management goal. In the case of boxwood blight, a fatal disease of a popular, high-dollar woody plant, maintaining clean stock through a variety of practices is a challenge.

Boxwood blight was first confirmed in North America in 2011, and it’s now present in more than half of U.S. states, primarily in the Midwest, South and Northeast. Through the ongoing work of AmericanHort and the Horticultural Research Institute, Dr. Fulya Baysal-Gurel, associate professor, Tennessee State University and member of the Boxwood Blight Insight Group, presented information on sanitation practices to prevent boxwood blight.

Boxwood blight is caused by two fungal species, Calonectria pseudonaviculata and C. henricotiae, said Baysal-Gurel. “It’s a devastating disease in the landscape or nursery production. Boxwood blight affects all growth stages of the plant, and the disease will result in defoliation and decline of susceptible boxwood plants. If introduced to a nursery, landscape or garden, boxwood blight is very difficult and costly to manage.”

Despite the difficulty of managing such a devasting disease, nurseries and garden centers can take steps to prevent introduction. To help prevent and manage the disease, Baysal-Gurel explained some of the characteristics of the causative organisms.

“Boxwood blight persists as mycelium in infected leaves left on soil surface or buried in the soil for up to five years,” said Baysal-Gurel. “Microsclerotia have been shown to survive for at least 40 weeks in optimal conditions in soil.”

Under ideal environmental conditions, microsclerotia produce new mycelium and new lesions can be observed within one week. The fungus enters through stomal openings and not by penetration of the cuticle. Some of the symptoms of boxwood blight are observed on the leaves, including circular spots and zonate lesions on the leaves, depending on cultivar. There are also distinctive black streaks on the stems. In severe cases, dieback and extreme defoliation may occur.

“During favorable conditions,” said Baysal-Gurel, “the fungus sporulates and enters on the undersides of leaves and on the stem. This is visible to the naked eye.”

Boxwood blight can spread over short or long distances. “Short distance movement can happen through water splash, rain, irrigation water and sometimes wind,” said Baysal-Gurel. “It may happen through plant debris under trees. Contaminated tools and animals are also sources of short distance introduction. Long distance movement may happen through contaminated plants and boxwood cuttings used for holiday decorations.”

Critical control points for nurseries include sanitation of cutting tools, potting media and hard surfaces. Employee training is also critical for a successful prevention program.

Boxwood blight (Calonectria pseudonaviculata). Photo courtesy of David L. Clement, University of Maryland,

“There are potential gaps between awareness and adoption of management practices, which include sanitation practices,” said Baysal-Gurel. “Sanitation is the formation and application of measures designed to protect plant health. It’s an important cultural strategy for reducing the impact of disease on crops. Generally, sanitation involves the removal of both infected and potential sources of infection followed by disinfection of surfaces and sterilization of soil propagation substrate.”

Sanitation practices for disease management fall into four categories, which may overlap in some areas. “The first is exclusion,” said Baysal-Gurel. “That starts with using clean cut materials, examining plant material, using clean potting media, keeping tools, floors and working surfaces clean and using clean production equipment to exclude the introduction of the disease. The other practice is eradication when disease is occurring.”

Eradication may include hot water treatment, monitoring and inspecting plant material, removal and/or burning of diseased material, removal of potential alternative hosts and soil substrate treatment. Other critical eradication steps include proper cleaning of tools and clothing.

“We would like to reduce the risk of reproduction and spread of the disease,” said Baysal-Gurel. “It’s more effective for disease suppression, important for the management of fungicide resistance and will help improve the impact of biocontrol programs.”

Baysal-Gurel recommended a risk management program that begins with purchasing boxwood plants from suppliers or nurseries that have been inspected by their state’s department of agriculture. Plants should be apparently free of boxwood blight and have a boxwood cleanliness program compliance agreement. Baysal-Gurel also emphasized the importance of careful inspection upon receiving plant material along with routine scouting and early diagnosis of infected plants for effective implementation of disease control strategies.

Incoming boxwood should be isolated from existing boxwood as well as alternative hosts such as sweet box and spurge. Hold new plants in an isolation area for at least one month. During isolation, fungicide applications are not recommended because such treatments can suppress the disease and hamper diagnosis. Moderately tolerant and tolerant cultivars should be inspected carefully during this period since they may be infected without obvious symptoms. Separate boxwood flats with non-host plants, ensure good air circulation and provide appropriate spacing between plants in the nursery.

“Good drainage in the holding and production areas will be critical for boxwood blight risk management,” said Baysal-Gurel. “Prevent runoff from the holding or production block to another. Proper irrigation can reduce disease spread. A drip irrigation system is better than overhead irrigation because it supplies water to the roots without the potential of spreading disease through splashing.” Also power wash tools, disinfect work surfaces and structures and disinfect irrigation lines and drips.

If infected plants are detected, they should be destroyed immediately, including the entire root system. “Leaf and stem debris should be removed from the landscape,” said Baysal-Gurel. “Flaming soil surfaces with a propane push flamer can significantly reduce levels of inoculum in the upper layer of soil or gravel paths.”

Baysal-Gurel advised growers to avoid handling any suspect plants when they are wet. She also recommended anyone who handles infected plant material wear clean, disposable coveralls and booties while disposing of infected material, then disposing of such clothing prior to entering other boxwood areas. Ideally, those working with infected boxwood should wait until the end of the day to enter infected areas.

“Our recommendation is using disinfectants in footbaths at the entrance of production areas,” said Baysal-Gurel. (Footbaths can be purchased online.) “Install a disinfection mat for everything that goes to the production site to help reduce the risk of introduction.”

A variety of disinfection products are effective against the causative organisms of boxwood blight. Consult with your state’s recommendations and follow labeled instructions for all products. For any fungicides used, be aware of the FRAC group to help prevent resistance.

by Sally Colby