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A Guide for Detecting Boxwood Blight and Managing Its Impact on Nursery Stock and in Landscape Plantings

By Dr. Fulya Baysal-Gurel, Associate Professor of Ornamental Plant Pathology and Bhawana Ghimire, PhD Graduate Research Assistant, Tennessee State University, Otis L. Floyd Nursery Research Center

How and when were boxwood blight found and what are its plant hosts?

Recently, a 150-year-old boxwood plantation, the focal point of the Carl Sandburg Home National Historic Site in North Carolina, was cut down because of a plant disease spreading like wildfire among boxwood plantings. Boxwood blight disease is caused by two fungal species, Calonectria pseudonaviculata and C. henricotiae. To date, only C. pseudonaviculata is present in the United States. This fungal pathogen was first reported in North Carolina and Connecticut in 2011, affecting boxwood shrubs in nurseries and landscape plantings. Since 2011, boxwood blight has been confirmed in 28 additional states. Boxwood, sweet box (Sarcococca spp.) and a few species of Pachysandra have been reported as host plants. Due to the potential for economic harm, the Tennessee Department of Agriculture established boxwood blight quarantine in February 2018. Proper precautionary actions should be taken to prevent the introduction of this threatening pathogen into healthy boxwood production areas and landscapes.

How do we monitor symptoms and signs of boxwood blight?

Symptoms often first appear on the lower part of the plants, or within the plant interior. Initially, reddish-brown to blackish-brown concentric circular spots with tan to light brown centers are visible on infected leaves (Fig. 1). Angular dark brown to black lesions also may appear from the base of stems to the shoot tip. With disease progression, whole leaves will turn brown; defoliation usually begins from the lower branches and spreads to the upper part of the plant (Fig. 2). When temperature is warm (64–77ºF) and humidity is high, continuous leaf wetness may facilitate fungal sporulation. White spore masses that can be seen without magnification may be produced on leaf undersides (Fig. 3) and on the stem. Sudden and severe defoliation is one of the characteristic symptoms of boxwood blight disease.

FIGURE 1 — Black to brown necrotic lesions may be apparent on the stems and leaves

FIGURE 2 — Defoliation on infected boxwood plants may start from the canopy interior or be initiated from the bottom of the shrub

FIGURE 3 — Spores of Calonectria pseudonaviculata can be seen on the under-sides of infected boxwood leaves

What can be done to prevent, limit spread and severity boxwood blight disease?

1.1 Precautionary measures

One of the most important precautionary measures that can be used to prevent the disease is excluding pathogen/infected plant materials from nurseries and landscapes. The fungus produces sticky conidia which can stick to the cutting tools, workers’ boots, and clothes. The accidental introduction of the disease from one site to another can be prevented by sanitizing tools, equipment, hard surfaces, vehicle tires and footwear using disinfectants such as Simple Green D Pro 5, KleenGrow (0.12%) and Greenshield. Boots and clothes can also be changed or cleaned before entering a new production area or client landscape.

Purchase plants and liners from nurseries that are licensed and/or certified according to applicable phytosanitary laws and regulations. Proper inspection should be performed for all incoming host plants, including cuttings, at the time of delivery. Isolate newly received host plants or rooted cuttings away from existing plant hosts in the nursery for at least 30 days. Isolation should involve a minimum of three meters distance between blocks. Provide a physical barrier and practice other sanitary best practices. For example, within isolation keep newly received host plants that were obtained from different vendors separate; maintain a clean isolation area; and remove plant debris on at least a weekly basis. Fungicides should not be applied during isolation. Do not allow run off from the isolated retention ponds, other surface water, into or production areas.

There are several other steps that can be taken to protect your nursery stock and client landscapes. Vehicles that are used to transport boxwood plants should be cleaned and free of host plants and plant tissue debris before entering production areas. At delivery destinations, plants should be unloaded within an isolation area. Proper cleaning and disposal of host materials from the vehicle, as well as loading area, should be conducted after each unloading. Every boxwood plant returned by customers or clients also should be received and held within the isolation area and monitored for boxwood blight symptoms. Within managed landscapes, if there are boxwood plantings or historic gardens near homes, then decoration of those homes using boxwood greenery in holidays should be avoided to prevent the introduction of boxwood blight disease in those plantings.

1.2 Best management practices

Host Plant Resistance: Production and landscape use of Buxus species and cultivars which are moderately tolerant or tolerant is a practical way to reduce incidence and severity of boxwood blight. Readily-available boxwood blight-tolerant cultivars include Buxus harlandii ‘Richard’, B. microphylla ‘John Baldwin’, B. microphylla var. japonica ‘Green Beauty’, ‘Golden Dream’, ‘NewGen Independence’, ‘Jim Stauffer’, ‘NewGen Freedom’, ‘Cranberry Creek’, ‘Calgary’, ‘Little Missy’ and ‘Sprinter’, B. sempervirens ‘Dee Runk’, and B. sinica var. insularis ‘Nana’ and ‘Franklin’s Gem’.

Scouting and Early Diagnosis: Scouting and making early diagnosis of infected plants are critical in limiting the spread and severity of boxwood blight disease. If infection is suspected, plant samples should be submitted for proper diagnosis in Department of Agriculture or diagnostic labs. If infection is confirmed, the Tennessee Department of Agriculture should be informed immediately. Infected plants and their plant parts/debris should be immediately eradicated; sanitation should follow using various methods including flaming. Other host plants within the nursery or landscape should also immediately be scouted and any suspect symptoms should be screened for boxwood blight infection. If confirmed, then other infected host plants should also be eradicated.

Cultural Practices: Recognize that pathogen inoculum can survive in plant debris and fallen leaves for a long time. Removal of older and senescent leaves should be performed on a regular basis, with plant parts destroyed or disposed of off-site.

Irrigation Practices: Drip irrigation instead of overhead irrigation is preferred in nursery production areas and if there is a need to use recycled irrigation water, scheduled monitoring and water treatments inoculum may be necessary.

Plant Spacing and Interplanting: Maintaining proper plant to plant distance and planting of non-host plants in between the plantings can be done to prevent the spread of disease within a production system. Mulching of boxwood could also be done to prevent accidental plantto-plant spread of the pathogen in the landscape.

Preventative Treatment with Fungicides: Many fungicides work preventatively and growers can initiate protective sprays, particularly if the disease has been confirmed locally, to prevent the further spread of this disease. Fungicide efficacy is best when used in combination with the other management strategies described above. When there is a risk of boxwood blight occurrence, repeated applications of fungicides may be necessary at 7 or 14-day intervals. A spray program that includes fungicides with different modes of action, such as chlorothalonil, benzovindiflupyr + azoxystrobin, boscalid + pyraclostrobin, etc. should be used to reduce the risk of fungicide resistance development.

For more information about boxwood blight in Spanish and English, please visit Tennessee State University Extension Publications (https://www.tnstate.edu/extension/publication_index.aspx) and Horticultural Research Institute Boxwood Blight Insight Group (https://www.boxwoodhealth.org). This work was supported, in part, by USDA-NIFA SCRI grant #2020-51181-32135. If you encounter boxwood plants in nurseries or managed landscapes that present disease problems, contact Dr. Fulya Baysal-Gurel, fbaysalg@tnstate.edu.