Boxwood tends to be very disease resistant. In most instances, proper planting techniques, site selection, maintenance, and smart gardening will lessen disease occurrence and severity. Several diseases typically infect boxwoods.
Phytophthora
Phytophthora nicotianae is also called root rot. It is a disease that is found most commonly in heavy, poorly drained soils. It weakens the plant and will eventually kill it. Usually, the first indication is when it appears the plant has stopped growing. The foliage turns to light green, then to brown, and finally to a straw color. This progression can take several months to a year or longer. Phytophthora will often selectively kill a branch or section of the plant at a time. When a plant with phytophthora is dug up, the ends of the roots will pull off much like pulling the sheath off a knife. Roots will be brown and stringy instead of a healthy white or tan color.
When cultivars struggle with drainage and early stages of phytophthora, their foliage will begin to fade with a yellowish tint or lime-colored tint. If the drainage issues are addressed quickly or the plant is moved or elevated, it can sometimes fight off the fungus and return to a healthy green color.
Phytophthora is difficult to treat once it is visibly present in a plant but you can usually avoid the disease with good gardening practices. Purchase disease-free plants from reputable suppliers. Avoid areas where poor drainage is an issue. Use proper planting techniques to maximize drainage around the plant.
Some cultivars appear to be less sensitive to phytophthora, including many of the micro-phylla cultivars. Many of the sempervirens (American), ‘Suffruticosa’ (English), ‘Elegantissima’, and ‘Justin Brouwers’ are especially susceptible when planted in poorly drained soils.
Volutella
Volutella buxi or stem blight is a fungus characterized by foliage that will turn bronze, then red, then yellow. The damage looks very similar to winter burn but the plant will not recover spring flush of growth. Cankers will form on the branch, and the bark will typically break and fall or peel off the stem. It normally affects one limb at a time. Volutella is often a secondary infection after somethings has weakened or injured a plant. It is most prominent in times of high humidity and poor air movement. Buxus sempervirens ‘Suffruitocos’ (English) is prone to volutella due to its very dense habit.
Avoiding conditions that are conducive to volutella is the best means to control it. Boxwood is more susceptible to volutella when they are under stress, in poor environmental conditions, or suffering from winter injury.
Avoid overhead irrigation, maintain proper drainage, and thin plants to maximize air movement within the plant. Volutella often invades plants that have had winter damage such as heavy snow loads that injured the bark. Sometimes volutella may take a summer or two to invade a damaged plant as it awaits optimum conditions of high humidity and low air movement.
When volutella is present, prune infected limbs several inches below cankers or broken bark. There are some fungicides available on the market but as mentioned above, proper care and culture are the best way to avoid the disease.
Cylindrocladium buxicola
The disease boxwood blight or box blight is caused by the fungal pathogen Cylindro-cladium buxicola (also known as Calonectria pseudonaviculata). Boxwood blight was officially documented in the United States in late 2011. It was found in Europe in the late 1990’s. Understanding boxwood blight is a high priority among research facilities in both Europe and the United States. Boxwood blight fungus that attacks the foliage of boxwood. It first appears as black or chocolate-colored spots on foliage. In a few days, those spots will develop yellow to brown rings around them and cover the leaf. Infected leaves will have streaking black stem lesions or cankers.
Boxwood blight is most prominent in times when foliage is continually wet from constant irrigation, prolonged rain, or high humidity, and the temperatures are in the high 60’s to low 70’s or higher. When optimal conditions exist and the fungus is present, it will move quickly. When conditions are not conducive, it will subside. The fungus will lie dormant in infected foliage and stems for long periods (5-10 years) and reappear when conditions are conducive. The boxwood blight spore is a sticky spore that moves from plant to plant by several means including water splash, pruning equipment, clothing, animals such as dogs or wildlife, or other contacts. It is not known to be moved by wind except in driving rain.
Like many of the diseases, boxwood is subject to, boxwood blight is best avoided with good cultural and care practices. Purchase healthy plants from reputable suppliers. Educate yourself about the disease to minimize its movement on clothing, equipment, and plant debris. Maintain good airflow in and around boxwood, avoid overhead irrigation whenever possible, and properly select cultivars for each application based on the ultimate desired size and shape to avoid over-pruning. If heavy shearing is necessary for the desired application shape avoid over-pruning. If heavy shearing is necessary for the desired application, care should be taken to annually thin the plant to allow air penetration into the plant.
Boxwood blight research is producing a large amount of valuable information. While all boxwood has some susceptibility to boxwood blight, Buxus sempervirens ‘Sufrruiticosa’ is the most susceptible cultivar. We have learned there is some boxwood blight resistance in boxwood cultivars, particularly the microphylla and several other cultivars. Plant architecture, with exceptions, seems to influence susceptibility. More open, taller plants tend to be more resistant while short, compact cultivars that will be resistant to boxwood blight by ascertaining the shapes and sizes that are naturally more susceptible.