Contact: Diane Relf, Extension Specialist, Environmental Horticulture
Posted April 1997
One of the most common diseases ascribed to boxwood is decline. In general terms, decline refers to the general debilitation of a woody plant. It is manifested by slower than normal growth, small leaves, loss of green color, and dead twigs and branches. Decline usually occurs when the root system of a plant is impaired by parasitic organisms or unfavorable soil environment. Nutrient deficiencies or toxicities can also cause decline. In some plant species, viruses and/or mycoplasma-like organisms cause decline.
Decline of boxwood usually refers to a disease of English boxwood incited by root and crown colonizing fungi. Because the symptoms are similar, decline is often confused with symptoms incited by parasites or environmental factors acting directly on twigs and foliage. And a single plant may have both twig blight and root rotting fungus. The only way to determine if the roots are healthy is to wash them free of soil and inspect them for discoloration and decay, or submit roots and soil samples to a diagnostic lab.
English boxwoods exhibiting decline symptoms have roots and/or crowns that are colonized by one of several fungi. Root lesion and spiral nematodes can also debilitate roots and cause decline.
The fungi and nematodes are not mutually exclusive; that is, all of them may occur at the same time in various proportions.
Symptoms of decline appear on individual stems or whole plants as a yellow to bronze coloration of foliage. Death of sections of the plant is common. Very often, the basal area of the stem is discolored. The bark must be removed to see this symptom.
Currently, no fungicides or nematicides are recommended for controlling boxwood diseases in the home landscape. To help prevent diseases, use recommended cultural practices such as providing water when necessary; avoiding over-watering or excessive fertilizing; and thinning shrubs to allow better air circulation. To prevent winter injury, make sure sufficient soil moisture is available during the fall. To conserve moisture and moderate soil temperature, coarse organic mulch such as wood chips is also beneficial.
For plants that are declining or have blighted stems and foliage, pruning is in order. Prune dead stems back to healthy tissue. Disinfect pruning shears frequently in household bleach diluted 1:9 with water. When the job is finished, bleach should be washed thoroughly from tools to prevent rusting. Have soil tested for nutrients and fertilize as recommended.
Many factors can cause poor health in boxwoods and typically more than one factor will be involved at any given time. Weather conditions, especially severe winter temperatures and drying winds, can cause leaves to brown and twigs to die back. Extremely cold temperatures when there is no snow cover can even kill plants.
If boxwoods have died and boxwood decline disease is confirmed, do not plant English boxwood in the same site. American boxwood can be used as a replacement as it is resistant to decline. But if Phytophthora root rot is confirmed, the site should be avoided for future boxwood plantings or modified to improve drainage. Fungicides are available to protect certain plants from Phytophthora root rot; however, they will not cure the disease.
(Originally published as "Diseases of Boxwood," by Robert Wick, Assistant Professor, Plant Pathology, Virginia Tech, in The Virginia Gardener Newsletter, Volume 3, Number 7.)
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