Buxus plants, commonly known as boxwood, are found in gardens all over the world. The Buxus genus includes approximately 90 species of shrubs and hundreds of cultivars that range in size from foot-tall dwarfs to specimens that are dozens of feet tall. The use of the evergreen in gardens dates back hundreds of years as topiaries, privacy screens, mazes and borders. Overall, boxwood plants are low-maintenance, but they may suffer damage when infected by some pests and diseases.
Boxwoods can survive in a variety of soil conditions in U.S. Department of Agriculture hardiness zones 5 to 9. Nevertheless, the plants are vulnerable to several pests, such as boxwood psyllids (box suckers), scales, mealybugs, whiteflies, spider mites, boxwood leafminers and nematodes. Psyllids, scales, mealybugs, whiteflies are sap-sucking pests. Spider mites also feed on plant fluids. Boxwood leafminers are tiny flies; their larvae feed in groups on leaves. Nematodes, microscopic roundworms, attack buxus plants at the root.
Pest Damage and Control
Psyllids, scales, mealybugs and whiteflies cause stunted growth and distorted foliage. They also secrete honeydew, a sticky substance that is a host for black, sooty mold. Spider mites leave behind unsightly light dots and webbing on leaves. Damaged plants have yellowed leaves. Leaves with boxwood leafminer damage have yellow blotches and drop prematurely.
Low-toxicity sprays such as neem oil and insecticidal soap provide temporary control of these pests. There is nothing you can do protect plants from nematodes in infected soil. The parasites cause stunted growth and leaf discoloration. The best way to prolong the life of boxwood is to take good care of the plant. This includes providing mulch during the fall, pruning and thinning during the winter, conducting regular inspections for signs of infestations, making sure soil is well-drained and keeping debris from dead foliage away from plants.
Boxwoods are vulnerable to collar, root and crown rot as well as leaf spot diseases, canker and wood decay. The two types of leaf spot diseases that attack Buxus are Macrophoma candollei and Phyllosticta spp. The fungus Volutella buxi causes canker in boxwood. Wood decay generally infects older boxwood plants. It comes from fungi that damage the center of branches, destroying the boxwood from the inside.
Plants infected with rot, leaf spot and canker will show signs of decline as well as discolored leaves and premature leaf dropping. Rot begins to develop in plants located in waterlogged soil, especially after heavy rains. Rot may also cause bark to ooze sap. Boxwood with leaf spot will also have circular yellow and brown spots on leaves. Leaves of plants with canker begin to look faded as they turn from light green to tan. Leaves also have waxy, rose-colored fungal growth and bark begins to peel away, uncovering dark or gray wood. Branches with wood decay will eventually fall and are vulnerable to breaking off during wind or rain.
Disease Prevention and Management
Gardeners use fungicides to help control rot. The best solution for avoiding rot is to keep plants in well-drained soil and use raised beds to improve drainage. When you notice signs of leaf spot, the best solution is to remove the infected leaves and clean up any debris around the plant. You should also remove leaves and branches with canker. Fungicides that contain lime sulfur and copper are effective for managing canker. There is not much you can do to manage wood decay in boxwood plants outside of keeping the plant as healthy as possible.