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).
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.
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.
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
Uconnladybug's Blog 