English boxwood, a popular shrub in traditional gardens in Virginia, and American boxwood, used in both traditional and contemporary plantings, are susceptible to several diseases that can decrease their effectiveness in established plantings. 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 also affect boxwoods. The major diseases of boxwood are discussed below.
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, bronze, or straw-colored. Leaves turn upward and lateral leaf margins roll inward. 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.
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.
External and internal stem discoloration usually accompanies the root rot phase of the disease. Plants dying from decline have vascular discoloration well up the main stem. The discoloration may be continuous or discontinuous in the stem. Sections of the foliage of infected plants turn a light green color. Later, foliage of infected plants turns yellow and then straw-colored. By the time foliar symptoms are observed, the root system has been severely impaired by root rot.
Damage to roots of both American and English boxwoods can occur from the feeding of several types of plant parasitic nematodes. The most common nematodes that feed on boxwood roots in Virginia are ring, lesion, and spiral nematodes. These nematodes obtain nutrients by inserting their syringe-like mouthparts into root cells and removing the contents. When populations of any of these nematodes are high in soil, their feeding can cause severe damage to roots and may also predispose roots to infection by fungi. Symptoms on roots include stunting and browning. Aboveground symptoms resemble those caused by root rot fungi: the plant undergoes a gradual decline characterized by yellowing and bronzing of the leaves and dieback of large sections of the plant.
For several years, the role of the fungus Volutella buxi in the decline of boxwood has been open to question. This fungus is asso-ciated with wilt and canker, but its role as a primary pathogen has not been clearly established. Both English and American boxwood have been found to develop symptoms of Volutella stem blight.
In the spring, before the new growth appears, leaves on the tips of twigs turn orange or bronze, then straw-colored. Infected twigs die back for some distance. A dark brown to black canker is easily discernible after cutting bark away with a sharp knife. The Volutella fungus produces numerous clusters of conidia, which appear pink en masse. Winter injury causes foliar symptoms similar to those caused by Volutella buxi, and Volutella infection often follows winter or frost injury. If winter injury alone is the problem, new, healthy leaves will appear in spring and eventually hide the bronze-colored leaves.
Boxwood leaves that die as a result of various root diseases or environmental stresses are frequently colonized by the fungus Macrophoma candollei. This fungus produces numerous black fruiting bodies, which can be seen as dark specks on dead leaves. It is a secondary colonizer of dead leaves and its presence indicates that the plant is stressed by some other factor. No controls for Macrophoma are recommmended; however, predisposing factors should be addressed.
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Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Alan L. Grant, Dean, College of Agriculture and Life Sciences; Edwin J. Jones, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; Jewel E. Hairston, Administrator, 1890 Extension Program, Virginia State, Petersburg.
May 1, 2009