CHARLESTON, W.Va. -- Traveling through Virginia recently, I noticed many historic, stately homes with immaculate boxwood hedges. While this sort of landscaping doesn't match the gardening style at my house, I love the look. I've got boxwoods in my garden beds, but they are left unclipped and natural.
While boxwoods are not native to the United States, they are widely cultivated as ornamentals in two major production areas: the Pacific Coast from Washington to north-central California, and in the East from Maryland to South Carolina and west to Tennessee. The three main species grown as ornamentals in the United States are Buxus sempervirens (common or American boxwood), Buxus microphylla and Buxus sinica var. insularis.
A friend said she's had trouble with her boxwoods. After some research, I found there are several major diseases that affect American and English boxwoods.
Mary Ann Hansen, extension plant pathologist at Virginia Tech, said the most common and most important diseases observed in boxwoods are root diseases that cause a gradual and irreversible decline of the plant. A few minor stem and foliar diseases occasionally affect boxwoods. Hansen explains the major diseases of boxwood below.
Phytophthora root rot
Both English (Buxus sempervirens cv.'Suffruticosa') and American boxwood (B. sempervirens cv.'Arborescens') are susceptible to this disease, which is caused by the fungus Phytophthora parasitica. The disease has also been observed in littleleaf boxwood (Buxus microphylla) in Virginia. Aboveground symptoms include poor growth and off-color foliage. Leaves are at first light green and may turn yellow or bronze. Leaves turn upward and the edges curl.
Leaf symptoms may appear on just a few branches or on the entire plant, depending on the extent of infection of the roots. Usually, the bark at the base of the infected plant dies and can be easily separated from the wood. By the time foliar symptoms are observed, roots are few in number and many are brown in color. The lack of functioning roots precedes the yellowing and death of the top of the plant.
Plants growing in soils that have become waterlogged following over watering or heavy rains in summer are predisposed to infection by Phytophthora parasitica. Abundant moisture allows spores of the fungus to move in the soil, infecting new roots on the same or adjacent plants. New plantings should always be made with healthy-appearing plants in well-drained soil. Avoid planting a susceptible plant in infested soil unless drainage can be improved before planting. Planting on raised beds may help improve drainage around plants.
English boxwood decline
The disease called English boxwood decline can best be described as a slow but progressive decline occurring commonly in large plants 20 years or more in age. Decline symptoms resemble those of root rot caused by Phytophthora parasitica. However, Phytophthora root rot is primarily a problem in wet soils, whereas English boxwood decline often follows drought stress.
A complex of fungi has been associated with English boxwood decline, but the fungus Paecilomyces buxi is believed to be the primary pathogen. Plant parasitic nematodes have also been recovered from the roots of dying plants, but not consistently enough to explain the disease.