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Boxwoods (Buxus spp.) are adapted to a wide range of light conditions, and prefer fertile, moist, well-drained soil, which is amended with organic matter. Boxwood is susceptible to the following diseases and pest problems.
Diseases

Decline (Twig Blight, Dieback): Boxwood decline is a poorly understood complex involving the fungi Volutella, Paecilomyces, Macrophoma and Phytophthora, as well as cold injury and nematodes (microscopic round worms). This phenomenon is also closely related to cultural problems associated with boxwoods, such as improper pH and nutritional status, drought, poor drainage, and improper mulch management. Volutella can cause a dieback on all types of boxwood. Macrophoma can cause leaf blight, but it usually acts as a weak pathogen. Paecilomyces buxi has been consistently associated with roots of English boxwood exhibiting the syndrome of boxwood decline.

Symptoms consist of weak and spindly plants. Dead or dying branches occur randomly in the bush. The older leaves drop prematurely and the remaining foliage develops a yellow color. Leaves often have pink eruptions of spores on black fruiting bodies. Dead areas or cankers develop along branches or near the crown. Various species of nematodes (microscopic worms that feed on the roots) also appear to be involved.

Prevention & Treatment: A thorough diagnosis of the associated factors is important before corrective action is taken. This should include a nematode analysis, soil analysis, and evaluations of drainage in the area and the degree of rooting in surface duff (litter).

Crowded growth and dead leaves in the branch crotches tend to maintain high levels of humidity in the canopy, making conditions conducive to die-back diseases. Prune dead stems back to healthy tissue. Disinfect pruning shears frequently in household bleach diluted 1:9 with water. Removal of dead branches and leaves from crotches of the plant, as well as yearly renewal of mulch material, will also aid in control.

Proper cultural practices, such as providing water when necessary, avoiding over-watering or excessive fertilizing, and thinning shrubs to allow better air circulation are of utmost importance in maintaining a vigorous condition. To prevent winter injury, make sure sufficient soil moisture is available during the fall. Plants in highly exposed situations may require wind protection. This is especially important in upstate areas where the soil can freeze and remain frozen on sunny days. When this happens, the foliage continues to transpire but the roots cannot replace the lost moisture from the frozen soil.

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.

Root Rot: Root rot is caused by the fungi Phytophthora nicotianae and P. cinnamomi. Leaves turn from normal dark green to light green as the plant declines. Roots are dark and rotted. The bark rots and peels at the crown. Death of the entire plant is characteristic of this disease.

Prevention & Treatment: Root rot is favored by high soil moisture and warm soil temperatures. The disease is more severe in heavy clays or poorly drained soils. Over-watering plants or long periods of heavy rain also favor disease development. Phytophthora root rot must be prevented, as chemicals are often ineffective in controlling this disease after above-ground symptoms become obvious. The following suggestions may aid in the prevention of root rot:

Purchase disease-free plants from a reputable nursery.
In areas where plants susceptible to root rot have died, replant with plants that are not susceptible.
Plant root rot-susceptible plants in well-drained areas or in raised beds. If the soil is heavy clay, mix it with a porous material such as bark.
The soil around infected plants may be treated with the appropriate fungicide according to the directions on the label. This may reduce the spread of the fungus among plants, but these chemicals may not kill the fungus in infected plants. Read and follow all directions on the fungicide label.

Canker: This disease is caused by the fungus Volutella buxi. The first noticeable symptom is that certain branches or certain plants in a group do not start new growth as early in the spring as do others, nor is the new growth as vigorous as that on healthy specimens. The leaves turn from normal to light green to various shades of tan. Infected leaves turn upward and lie close to the stem instead of spreading out like the leaves on healthy stems. The diseased leaves and branches show small, rose-colored, waxy fruiting bodies of the fungus. The bark at the base of an infected branch is loose and peels off readily from the gray to black discolored wood beneath.

Prevention & Treatment: Dead branches should be removed as soon as they are noticeable. The annual removal and destruction of all leaves that have lodged in crotches is recommended. Applications of a copper fungicide or lime sulfur have been shown to be very effective in preventing canker. The first application should be made after the dead leaves and dying branches have been removed and before growth starts in the spring. Read and follow all directions on the label.

Nematodes: Boxwoods are susceptible to several parasitic nematodes (microscopic round worms), including the Southern root-knot nematode (Meloidogyne incognita), ring nematode (Mesocriconema), the lesion nematode (Pratylenchus), and the stunt nematode (Tylenchorhynchus).

Symptoms consist of leaf bronzing, stunted growth and general decline of boxwood. The microscopic worms feed on the roots, which soon die and the plant forms lateral roots above the invaded area. These lateral roots in turn are infested. Repeated infestations and lateral root production result in a stunted root system resembling a witches’ broom.

Prevention & Treatment: The life of infested plants may be prolonged by providing good care (fertilization, mulching) and by watering the plants thoroughly during dry spells. American boxwood (B. sempervirens) is resistant to root-knot nematodes and tolerant to stunt nematodes. Nematodes cannot be totally eliminated from the landscape. The goal is to keep the population low enough to prevent damaging symptoms that weaken the plant. Boxwoods should not be grown in soils heavily infested with nematodes. Growing plants that are not affected by nematodes (grasses, marigolds) will reduce nematode populations in the long term.

Nematode-tolerant shrubs such as yaupon holly and Burford holly can be used to replace boxwood, which are killed by nematodes.
Insects & Related Pests

Boxwood Leafminer (Monarthropalpus buxi): This is the most serious insect pest that attacks boxwood. The leafminer is the larva (immature form) of a small, orangish mosquito-like fly. These flies are less than 1/8-inch long and can often be seen swarming around boxwoods in the spring. The adult female fly inserts eggs with her ovipositor (egg laying structure) into new boxwood leaves through the leaf’s upper surface. When the larvae hatch, they feed inside the leaf, creating a mine. Larvae are orange and about 1/8-inch in length. They overwinter (survive the winter) in the leaves. Adults emerge from the leaves the following spring, just after new growth occurs on boxwoods. There is one generation per year.
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Boxwood leaf miner larva feeding inside leaf.
Clemson University – USDA Cooperative Extension Slide Series, Bugwood.org.

Boxwood leaf miner attacks result in irregularly shaped swellings on the leaf. There may be a slightly blistered appearance on the leaf’s under-surface. Blistering may not be obvious until late summer. Infested leaves typically turn yellow or brown in splotches, are smaller and drop sooner than healthy leaves. A heavy infestation can cause serious loss of leaves and result in death of the boxwood.

Prevention & Control: Use of insecticides against boxwood leaf miner is not recommended unless damage is intolerable. Insecticides are most effective against this pest when adults have emerged and before they can lay eggs. Adults typically emerge over a three-week period but live only a few days.
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Distorted, splotchy leaves afflicted with boxwood leaf miners.
John A. Weidhass, Virginia Polytechnic Institute and State University, Bugwood.org

Boxwood Mite (Eurytetranychus buxi): The boxwood mite or boxwood spider mite, is not an insect but is more closely related to spiders. The adult is green to yellowish brown in color, has eight legs and is tiny, about 1/64-inch long. Since mites are so small and early symptoms are not distinctive, it is easy to overlook the problem until a heavy infestation occurs and greater damage has occurred. This pest overwinters as eggs on the underside of leaves. The eggs hatch in the spring. Boxwood mites develop and breed rapidly, resulting in eight or more generations per year.

All stages of boxwood mite feed on both leaf surfaces. They pierce the leaf to suck out plant sap. During feeding, they inject toxic saliva, which results in stippling (tiny, yellow scratchlike spots) forming on the leaf’s upper surface. Boxwood mites prefer feeding on young leaves, but damage is most obvious on second- and third-year leaves. From a short distance, the infested boxwood appears unhealthy with a dingy silvery color.

Prevention & Control: Naturally occurring enemies of mites include various predator mites, ladybird beetles (ladybugs) and other insects. These predators will usually suppress mite populations. Since insecticide use kills predators as well as mites, insecticides should be avoided unless absolutely necessary.

To determine whether insecticide use is needed, it helps to know how many mites are present. Hold a white sheet of paper under a branch and strike the branch. The mites that are knocked off will be seen crawling around on the paper. If more than 15 mites are seen per whack, serious damage can result.

Mites can be removed with a strong spray of water, if applied on a regular basis. Horticultural oil applied at the summer rate (1 – 2%) will kill eggs and adult mites. Horticultural oil may be sprayed when temperatures are between 45 and 85 degrees. As with all pesticides, read and follow all label instructions and precautions.

Boxwood Psyllid (Psylla buxi): The adult is a small, greenish insect, about 1/8-inch long. It has clear wings and strong legs adapted for jumping. It looks like a tiny cicada that hops or flies away when disturbed. Both the adult and nymph (the immature insect stage which resembles the adult) feed by piercing leaf surfaces and sucking plant sap.

Nymphs hatch from eggs in the spring. They produce a white, waxy material that often covers their bodies. Nymphs feed from buds and young leaves. This feeding results in the typical cupping of leaves and stunted twig growth that are seen with this pest. Plants tend to outgrow the injury by midsummer.
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Boxwood psyllid feeding causes cupped, stunted leaves.
Daniel Herms, The ohio State University, Bugwood.org.

After further development during the spring, adults are formed. Adults also feed on boxwood, but are less damaging than the nymphs. Adult females lay eggs under bud scales. The immature nymphs develop within the eggs, where they remain until spring. They emerge in spring to feed and complete development to adults. Only one generation occurs per year.