Boxwood (Buxus spp.) shrubs are slow-growing, evergreen plants with dense wood and small, rounded leaves. They are frequently cultivated in the home garden due to their attractive appearance and ability to be pruned into formal shapes, including small hedges and topiaries. They are very common here in New England and you have probably seen them in your friends’ and neighbors’ yards if you do not own a few yourself!

Despite their popularity, boxwood have more than their fair share of issues with pests and diseases. Although we published an update on the invasive and highly destructive box tree moth (Cydalima perspectalis) at the end of August, 2021 (please read it if you haven’t already), there are some other common nuisances of boxwood to also be aware of.

Boxwood Psyllid

The boxwood psyllid (Psylla buxi) is the most common insect pest of boxwood. This insect is in the family Hemiptera, like whiteflies and aphids, and all life stages cause feeding damage. The nymphal (juvenile) stage of the insect is wingless and produces a characteristic white, waxy exudate on the leaves. Another identifiable symptom of boxwood psyllid feeding is cupping of terminal leaves at the end of the shoot. Although the feeding damage can be noticeable and managing the pest can be troublesome, the boxwood psyllid does not cause significant damage to the plants they feed on and nearly all rebound well after the pest is managed. Prune away heavily-infested branches and treat with an approved insecticide (according to the label) or a mixture of neem oil (1 tbsp), castile soap (1 tbsp), and warm water (1 cup).

Figure 1. Photo of Boxwood Psyllid damage recently shared with the UConn PDL. Damage is characterized by cupped terminal leaves and waxy nymphal exudate. Photo credit: Suzanne Rinaldi.

Volutella Blight

Volutella Blight is a significant fungal disease of boxwood (and pachysandra)  caused by Volutella buxi (aka Pseudonectria rouselliana). The disease is sometimes called Pseudonectria Canker for this reason. Symptoms include poor vigor and yellowing leaves that turn tan and remain attached to the stem. On the undersides of leaves, orange to salmon-pink colored sporulation will form in moist and humid conditions. Spores are spread by rain splash or contaminated tools. Volutella blight is commonly associated with plants that are stressed.

To manage, prune away all infected tissue during dry weather. Make cuts 3-4 inches below the point where symptomatic tissue ends. Disinfect tools between plants and cuts with 10% bleach solution or 70% alcohol. Discard or burn all pruned tissue and fallen debris as the disease can overwinter in plant material. Do not compost. Remove and destroy fallen leaves whenever possible. Plants that are heavily infected will need to be removed. Prune interior stems of nearby healthy plants to increase airflow. Avoid overhead watering or only water in early morning to minimize periods of leaf wetness. The most effective preventive fungicides include those with the following active ingredients: Chlorothalonil, Chlorothalonil + Thiophanate-methyl, Copper hydroxide, Copper sulfate pentahydrate, Propiconazole, and Mancozeb. Begin spraying in spring, before or at first sign of disease, always following label rates and instructions. Rotate to compounds with a different FRAC code after every third application to minimize resistance.

Figure 2. Salmon-colored sporulation on the underside of a boxwood leaf with Volutella Blight. Photo credit: Nick Goltz, DPM, UConn Plant Diagnostic Lab.

Boxwood Blight

Boxwood blight is another significant fungal disease of boxwood caused by the pathogen Calonectria pseudonaviculata. Disease severity can vary, but is often costly and hard to control, particularly in nursery settings. Symptoms of this disease are dark leaf spots, black stem lesions, sporulation on leaves and twigs, and significant defoliation. The fungus overwinters in infected tissue and fallen debris. It can also survive in the soil for up to 6 years, further complicating management.

To manage, prune away all infected tissue during dry weather. Make cuts 3-4 inches below the point where symptomatic tissue ends. Disinfect tools between plants and cuts with 10% bleach solution or 70% alcohol. Discard or burn all pruned tissue and fallen debris. Do not compost. Remove and destroy fallen leaves whenever possible. Plants that are heavily infected will need to be removed. Prune interior stems of nearby healthy plants to increase airflow. Avoid overhead watering or only water in early morning to minimize periods of leaf wetness. The most effective preventive fungicides include those with the following active ingredients: Chlorothalonil, Chlorothalonil + Thiophanate-methyl, and Mancozeb. Begin spraying in spring, before first sign of disease, always following label rates and instructions. Rotate to compounds with a different FRAC code after every third application to minimize resistance. Fungicides are only preventatively effective. If symptoms are observed, destroy or discard plants immediately to prevent spread to healthy plants nearby.

Figure 3. Symptoms typical of Boxwood Blight. The dark lesions along the stem are a characteristic symptom. Photo credit: Nick Goltz, DPM, UConn Plant Diagnostic Lab.
Figure 4. Unique cylindrical spores of Calonectria pseudonaviculata. Photo credit: Nick Goltz, DPM, UConn Plant Diagnostic Lab.

There are some other pests and diseases of boxwood that one should be on the lookout for in Connecticut, such as the boxwood leaf miner and various root-rot pathogens. If you are ever in doubt about what may ail your boxwood, or if you would like confirmation (essential for proper fungicide or insecticide application), you may send a sample to the UConn Plant Diagnostic Lab for assistance. Submission guidelines and instructions are detailed on our website: plant.lab.uconn.edu

Nick Goltz, DPM

During the spring, many homeowners notice changes or problems arising in their gardens and landscapes. Throughout May, the Plant Diagnostic Lab received many samples and emails of evergreen plants such as junipers, arborvitae, cedars, and boxwoods for diagnosis. Phomopsis Tip Blight on conifers and Volutella Blight on boxwood are two common diseases seen this time of year. Homeowners tend to notice symptoms in the spring and early summer when conditions are wet and damp and conducive for disease development.

Phomopsis blight is caused by the fungal pathogen Phomopsis juniperovora. Plants that are commonly affected include juniper, cedar, cypress, and arborvitae. Most infections occur in April through early June and in the fall but can occur throughout the growing season on young foliage during wet and humid conditions. Symptoms first appear on immature tissues about 3-5 days after infection. Older, mature branches are resistant. After infection, small yellow spots can be seen on the foliage. Eventually, shoot tips will turn a reddish, brown color after the fungus has entered the xylem.

B1 phomopsis juniper Bruce Watt bugwood.org

Infected branch tips on juniper. Bruce Watt UMaine. bugwood.org.

Over time, cankers will form at the base of the blighted shoots, appearing as a gray band. These cankers can girdle stems less than 1 cm in diameter. Black pycnidia, or tiny fruiting structures, form on killed tissue 3-4 weeks after infection. During wet weather, yellowish conidia, or spores, are extruded from the pycnidia and spread by wind and rain splash. The pathogen can remain on the host and continue to spread spores for up to 2 years when environmental conditions are favorable.

Volutella blight on boxwood hosts is caused by the fungal pathogen Pseudonectria buxi. Symptoms begin in spring as poor vigor and leaves turning light green to straw or tan colored. Leaves turn upwards and tend to remain attached to the branch. This key observation can help distinguish Volutella from boxwood blight, which causes rapid defoliation. Additionally, distinctive black stem streaking and cankers are signs of boxwood blight. Volutella can cause sunken lesions on the stems and plants may eventually lose bark.

B2 Volutella blight L. Borbas

Volutella blight symptoms. Photo by Lillian Borbas, UConn 2021

B3 Straw colored leaves stem lesions L. Borbas

Straw-colored leaves attached to branches. Sunken stem lesions. Photo by Lilian Borbas, UConn 2021

B4 Black stem streaking Mary Ann Hansen bugwood.org

Black stem streaking on a boxwood diagnosed with Boxwood Blight. Mary Ann Hansen. VPI & State University. Bugwood.org

On the undersides of leaves, Volutella will form distinctive orange to salmon-colored sporodochia (fruiting bodies) in moist and humid conditions. The pathogen is spread through rain splash or contaminated tools and is commonly associated with plants that are stressed. Maintaining plant vigor and utilizing the cultural controls outlined below can help manage this disease.

B5 Sporodochia boxwood Bruce Watt bugwood.org

Sporodochia on boxwood leaf. Bruce Watt, UMaine. Bugwood.org.

Management recommendations

For both Volutella and Phomopsis blight, control options are similar. When growing plants susceptible to these diseases, preventative methods, proper planting practices, and cultural controls are important for management. Proper site selection can discourage infection, for example, well drained sites with proper light exposure and air circulation. Ensuring that plants are properly spaced will reduce humidity and moisture around the foliage. Make sure the plant’s mature size is considered when planting. Properly irrigate and fertilize based on the needs of the plant and planting site. Do not water with sprinklers or overhead irrigation as this will encourage prolonged leaf wetness. Avoid letting weeds or other vegetation grow close to the plants. Excessive watering or fertilizing can make the plants more susceptible to infection. Avoid shearing, wounding, or pruning plants in wet, humid weather as this will aid in disease spread.

Regularly check your plants for signs of the symptoms described. If these symptoms are observed, cultural practices should be implemented first. Remove all infected tissue during dry weather, cutting 3-4 inches back from the damage. Disinfect tools between plants and cuts with 10% bleach or 70% isopropyl alcohol. Discard or burn the infected branches as they can be a source of disease. If the plant is heavily infected, it may need to be removed. Consider choosing plants that are more resistant or not susceptible to the disease.

Fungicides may need to be applied if infections become severe and these cultural controls are exhausted. However, many fungicides are used preventatively and should only be used after cultural controls have been tried. They are often only effective if followed by this management and applied before new growth begins in the spring. Whenever using fungicides, always read and follow the label for information on proper rates and application times.

Lillian Borbas