Learn to recognize this plant disease and strategies for keeping your garden healthy.
Boxwood blight is a deadly plant disease affecting boxwood shrubs and closely related plants. The pathogen was first identified in the United States in 2011 and has since spread to 26 states, ranging from coast to coast. European gardeners have been fighting boxwood blight for decades, giving rise to the breeding program that developed blight-resistant Better Boxwoods. Preventing boxwood blight using resistant varieties is our best tool for protecting our gardens. Learn additional management strategies and how to recognize the signs and symptoms of boxwood blight so you can help slow the spread of this devastating disease.
What is Boxwood Blight
Boxwoods are resilient plants, but like any organism, they are susceptible to certain pests and diseases. Among the more devastating of these is boxwood blight, a fungal disease caused by two fungal species, Calonectria pseudonaviculata and C. henricotiae. Of these, only C. pseudonaviculata is known to be present in the United States. The pathogen infects all above ground portions of the plant and ultimately causes plant death.
Early symptoms of boxwood blight include dark reddish-brown spots on the leaves and narrow black streaks or cankers on the stems. As the disease spreads, the spots expand and merge into brown patches that eventually cover the entire leaf surface. When conditions favor reproduction, white spores are visible on the undersides of leaves and along the stem lesions. Blighted leaves will drop from the plant, beginning on the lower branches. As the stem cankers expand, they cut off the movement of water and nutrients, causing whole stems to die.
Boxwood blight can spread quickly through the landscape. The disease cycle is supported by warm, humid conditions. The close spacing of boxwood plants in many landscapes favors these humid conditions and facilitates the spread of blight through the plant canopy.
How Do Plants Become Infected with Boxwood Blight
Although boxwood blight can move quickly once it enters a landscape, it has a harder time reaching gardens. The spores are heavy and do not travel very far on the wind. The primary way boxwood blight enters the landscape is on new plant material, including boxwoods, pachysandra, sweet box, and boxwood cuttings and swags used for holiday decorating. It can also come in from neighboring properties on infected wind-blown leaves or be carried by animals. You can also move the boxwood blight pathogen on your shoes and clothing.
Within a landscape, boxwood blight spores can splash onto plants from the soil or neighboring plants via rainfall or overhead watering. The disease can also be spread from one plant to another on infected tools. Once boxwood blight is in the landscape, it can persist on infected and fallen leaves, stems, and other plant residues for many years.
What Plants are Susceptible to Boxwood Blight
The boxwood blight fungus infects boxwoods (Buxus species and hybrids) and related species in the boxwood plant family (Buxaceae). In the ornamental landscape, boxwood relatives include sweet box (Sarcococca species), Japanese spurge (Pachysandra terminalis), Allegheny spurge (P. procumbens), and Chinese pachysandra (P. axillaris).
Boxwood species and hybrids vary in their susceptibility to boxwood blight. Varieties of common boxwood (Buxus sempervirens) and English boxwood (B. sempervirens ‘Suffruticosa’) are among the most susceptible, along with hybrids that include B. sempervirens genetics. Although littleleaf boxood (B. microphylla), Korean boxwood (B. insularis), and Harland boxwood (B. harlandii) are typically more tolerant of boxwood blight, many cultivars within these species are highly susceptible.
Varieties in the Better Boxwood collection are resistant to boxwood blight. These selections were developed in Europe, where the disease has persisted since the early 1990’s. Plants in the Better Boxwoods collection have been specifically bred for resistance to boxwood blight while maintaining ornamental characteristics.
How to Prevent Boxwood Blight
The number one way to prevent boxwood blight is to plant resistant varieties like those in the Better Boxwood collection. Over time, the widespread adoption of resistant varieties will help to reduce the spread and prevalence of boxwood blight across the country.
For gardens with existing boxwood plants, purchasing only blight resistant boxwoods will help reduce the likeliness of introducing blight into the landscape.
Remember, boxwood blight can move on sweet box and pachysandra as well as boxwood. When adding these plants to the landscape, purchase them from a reputable nursery that participates in your state’s boxwood blight compliance program. Isolate new pachysandra, sweet box, and boxwood plants for one month before planting in the garden and monitor them regularly for disease symptoms.
You can also take steps to make your garden less favorable to disease. Reduce humidity in the plant canopy by using drip irrigation instead of overhead watering. Increasing airflow around plants can also help. This may require selective pruning or thinning of boxwoods, as well as nearby trees and shrubs. Apply a fresh layer of mulch beneath boxwood plants to reduce the chances of spores splashing from the soil onto the foliage.
When pruning boxwoods, avoid shearing, which encourages dense growth. Selective pruning and thinning are better choices for maintaining plant health. Prune when plants are dry and do not compost infected plant material. Dispose of all infected plant debris by burning or bag it and add it to the trash can. Keep tools clean and sharp. Soak tools in disinfectant for five minutes to kill pathogens before and after pruning.
Fungicides can be used to protect existing boxwoods from infection but do not cure plants that are already infected. Fungicides must be applied regularly during the growing season, as often as every seven to fourteen days, which becomes expensive over time. Unless you have a valuable boxwood collection or rare varieties, replacing plants with resistant varieties provides a more effective long-term management strategy.
What to Do If You Suspect You Have Boxwood Blight
Keep an eye out for symptoms of boxwood blight on existing plantings. Look for leaf spots, stem lesions, and browning foliage. Remember to include pachysandra and sweet box plants when scouting for symptoms. If you suspect a plant is infected, submit a sample to your local university’s plant diagnostic lab or visit your local county extension office for assistance. The symptoms of boxwood blight can be easily confused with other diseases, so it is best to have the disease properly diagnosed.
There is no cure for infected plants, but you can limit the spread of boxwood blight to other plant material by removing and disposing of infected plants. Also collect and dispose of fallen debris from the soil surface (as well as mulch). Where practical, flaming the soil surface with a propane weed torch can reduce remaining pathogen levels in the soil. Also treat hard surfaces that may have been in contact with the pathogen, such as sidewalks and patio pavers. If infected plants were grown in containers, dispose of the soil and soak pots in disinfectant for ten minutes to kill the pathogen.
Replace infected plants with disease-resistant species and varieties, such as Better Boxwoods. Dwarf yaupon hollies and Japanese hollies are also good substitutes, as they have a similar appearance. Do not replant with susceptible varieties, as the pathogen can remain active in the soil for up to five years.
While it is disheartening to see this disease take hold across the country, Better Boxwoods provide a promising solution for gardeners. Disease-resistant plants are our best tool for fighting boxwood blight and maintaining beautiful, resilient plantings.