University of Maryland Extension

Boxwood: Preventing and Managing Common Pests and Diseases

boxwood
Boxwood in a foundation planting. Photo:

Key Points

  • Boxwood (Buxus spp.) are broad-leaved evergreen, deer-resistant shrubs that are typically used as foundation plantings and backdrops for planting beds, topiaries, and formal gardens. There are many species and cultivars available.

  • Proper site selection and plant care are essential for maintaining the health of boxwood. In general, boxwood:
    • Need well-drained soil and will not tolerate sites that are constantly moist.

    • Prefer a soil pH of 6.5- 7.2 and a location with some afternoon shade.

    • Benefit from an annual pruning (thinning), a 1” layer of mulch over the shallow root system, and irrigation during severe droughts.

  • The most common pests of boxwood in Maryland are leafminers, psyllids, and boxwood mites. Common diseases include Volutella stem blight and Macrophoma leaf spot.
  • Understand how to identify, prevent, and manage problems of boxwood using an Integrated Pest Management (IPM) approach. Refer to the diagnostic chart below to identify symptoms and possible causes.

Diagnosing Common Problems of Boxwood
Care of Boxwood
Winter Injury of Boxwood
Common Pests, including voles
Common Diseases
Boxwood Decline

Diagnosing Commmon Problems of Boxwood

Symptoms

Details

Possible Causes

Leaf Yellowing

With tiny black spots on leaves

Macrophoma Leaf Spot

Pink spores on leaves during moist conditions in spring

Volutella Blight

Eventual dieback from the top of the plant

Root Rot

Leaves eventually turn brown

Winter Damage

Larger branches die back; bark stripped from base of the plant

Meadow Vole

Leaf Stippling (tiny spots)

Fine stippling (pattern of tiny white/yellow dots) of leaves early in season, followed by general grayish, dingy, unhealthy appearance

Boxwood Mite

Cupped Leaves

Damage appears on new terminal leaves in spring; white wax

Boxwood Psyllid

Blistering of Young Leaves

Blotch mines, underside of leaves appear blistered from late summer through the following spring

Boxwood Leafminer

Leaf Spots

Dark spots coalesce to brown blotches

Boxwood Blight

Branch Dieback

Pink spores on leaves during moist conditions in spring

Volutella Blight

Oystershell shaped scale covers found on bark of affected branches

Oystershell Scale

Larger branches dieback; bark stripped from base of the plant

Meadow Vole

Eventual dieback from the top of the plant

Root Rot

Black Lesions (Cankers) on Stems

Narrow black streaks on young green stems

Boxwood Blight

Defoliation

Starts on lower branches and moves upward in the canopy

Boxwood Blight

Care of Boxwood

Boxwood grows well in full sun to partial shade. Sites exposed to full winter sun can cause foliage to “burn” and turn orange. Boxwood planted with a south or southwest exposure suffer winter burn more than plants with an east or north exposure due to increased sun exposure.

Boxwood requires adequate drainage, ample amounts of organic matter and grow best within a soil pH range of 6.5 to 7.2. Yellowing of older inner foliage or premature leaf drop may indicate a lack of nitrogen. As with all evergreens, some normal leaf drop occurs. The leaves remain functional for three years and then they are dropped.  

Boxwood requires only light applications of mulch. Do not apply more than one inch of mulch over the root zone and keep it clear of the main stem. Excessive mulch may encourage vole activity and production of adventitious roots in the mulch layer which are very prone to desiccation (drying) damage. Do not cultivate deeply near boxwoods or their shallow roots will be damaged. 

The most appropriate pruning method for boxwood is thinning, removing entire stems or branches at their point of attachment. Thinning allows the center of the plant to receive adequate sunlight and air circulation. Properly pruned boxwood will have leaves along the entire branch length. Dense foliage encourages fungal diseases such as Macrophoma leaf spot and Volutella canker. Shearing stresses plants and should only be used in boxwood topiaries. The best time to thin boxwood is December through February.

Common Problems of Boxwood

Winter Injury

Winter injury may be confused with early stages of the fungal diseases Phytophthora root rot or Volutella blight. Leaves turn from bronze to reddish-brown as a result of exposure to cold, dry winter winds. Tissue death is caused by the removal of water in the leaves faster than the plant can replace it through root uptake from frozen water in the soil. Bark splitting can be caused by a rapid temperature drop caused by a mid-winter thaw. Dead twigs and branches in the spring may be the result of ice and snow damage from the winter. 

Management: 

  • Winter damage can be reduced by locating plants in partially shaded areas protected from winter winds. 

  • Physical barriers made from materials such as burlap or plastic, placed about 18 inches from the plants on the windward side, can also lessen winter wind damage by reducing wind velocity. 

  • Maintain adequate soil moisture in the fall to prevent winter desiccation. 

To avoid damage from falling snow and ice do not plant boxwoods under roof eaves. For established boxwoods, tie a string or twine at the base of the plant and spiral the twine up and down the plant to hold it together and gently brush snow off plants as soon as possible. This will help prevent damage from falling ice and snow. Inspect plants for winter damage in the spring and prune out affected areas.

Pests of Boxwood

boxwood-leafminer-symptoms
Symptoms of boxwood leafminer damage. Photo: Bruce Watt, University of Maine, Bugwood.org

Boxwood Leafminer
(Monarthropalpus buxi)

The most destructive insect pest of boxwood is the boxwood leafminer, Monarthropalpus buxi. The larvae of this fly feed on the tissue between the outer surfaces of the leaves. This feeding results in blotch-shaped mines visible on the underside of boxwood leaves. 

The infested leaves appear blistered from late summer through the following spring. New leaves do not show signs of mining until late summer when the larvae are larger. By fall, or in early spring, premature leaf-drop may result from heavy infestation.

leafminer-damage
Boxwood leafminer symptoms. Photo: Jim Baker, North Carolina State University, Bugwood.org

Adult leafminers emerge in late April or early May, depending on the weather. The adults are small (3mm), orange, mosquito-like flies. The adult flies emerge over a period of 10-14 days but each fly only lives about 24 hours.

adult-boxwood-leafminer
Boxwood leafminer (adult). Photo: Jim Baker, North Carolina State University, Bugwood.org


leaf-miner-inside-leaf
Boxwood leafminer (larvae). Photo: Jim Baker, North Carolina State University, Bugwood.org

After mating each female inserts about 30 eggs into the new boxwood leaves. The larvae hatch in about 3 weeks and feed within the leaves from June through early fall. They spend the winter in the leaves and pupate the following April. There is one generation each year.

Many cultivars of American boxwood (Buxus sempervirens) and Japanese boxwood (Buxus microphylla var. japonica), are relatively resistant to this pest (e.g., ‘Winter Gem’, ‘Vardar Valley’, ‘Franklin's Gem’, insularis ‘Nana’, ‘Golden Dream’).

Management:

  • It is difficult to control the adult leafminers because of their short adult life stage. Beginning in late April, shake the branches of boxwoods to detect flying adults. When they are present, thoroughly spray the plants with a registered insecticide (spinosad). 

  • If developing mines are observed in the leaves, larvae can be controlled from late June through the summer by spraying with a registered systemic insecticide. It is best to control larvae in June before serious damage has occurred. Some systemic insecticides may only be applied by certified pesticide applicators, as per Maryland’s Pollinator Protection Act of 2016.

leaf-curling-from-psyllids

Boxwood psyllid damage. Photo: Penn State Department of Plant Pathology & Environmental Microbiology Archives, Penn State University, Bugwood.org

Boxwood Psyllid
(Psylla buxi)

The boxwood psyllid, Psylla buxi, causes cupping of the leaves on the terminal and lateral branches of boxwood. This insect can overwinter as an egg or as a first-instar nymph under the bud scales. As the buds develop in the spring, the eggs hatch and nymphs emerge to infest the leaves. 

wax-on-boxwood-leaves

Boxwood psyllid damage. Photo: Daniel Herms, The Ohio State University, Bugwood.org

The feeding causes the leaves to curl and form a cup that encloses the greenish colored nymphs. The nymphs produce a white, waxy secretion that may cover part of the body or small waxy pellets beside the nymphs. The greenish adults emerge late May into June, mate and lay eggs under the bud scales. Only one generation occurs each year.

adult-psyllid
Boxwood psyllid adult. Photo: John Davidson, University of Maryland

Management:

  • This pest causes aesthetic damage to American and English boxwood. Prune out and dispose of infested branch tips. Sprays are only necessary if infestations are heavy. 

  • Boxwood psyllid nymphs may be controlled with horticultural oil or insecticidal soap sprays in April and May.

  • Adults may be controlled by a registered residual insecticide in late May into June.

leaf-damage-from-mites

Boxwood spider mite damage. Photo: David L. Clement, University of Maryland, Bugwood.org

Boxwood Mite
(Eurytetranychus buxi)

Boxwood mites are yellowish-green or reddish and are 0.5mm long. The yellow eggs overwinter on the leaves and hatch in April. These spider mites breed rapidly and have 5 or 6 generations each summer. They are most active in hot, dry summers. Injury shows as a fine stippling of the leaves early in the season, followed by a general grayish, dingy, unhealthy appearance.

This is a common pest wherever boxwoods are grown. Some Buxus microphylla cultivars appear to be more resistant.

Management:

  • Damage is primarily superficial and aesthetic. For light infestations, use a sprong spray of water from a hose to dislodge the mites.

  • For large infestations, use a horticultural oil or insecticidal soap spray in the summer following label instructions. 

  • A biological control option for heavy mite infestations may be the release of predatory mites that can be purchased from mail-order sources.

  • Some insecticides used to treat boxwood leafminers may exacerbate spider mite problems because they kill natural predators of mites.

 

scale-on-boxwood
Oystershell scale (adult covers and juvenile crawlers). Photo: Whitney Cranshaw, Colorado State University, Bugwood.org

Oystershell Scale
(Lepidosaphes ulmi)

Heavy infestations of this armored scale will cause yellowing and wilting of leaves and eventual dieback of branches. Infested plants have an unhealthy appearance overall. To monitor for this pest look for tiny (3mm), oyster shell-shaped, brown to gray scale covers on the bark of wilting or dead branches. There may be one or two generations each year.

Crawlers, newly hatched scale insects, are about the size of a pinhead and light-colored. Look for crawlers near the old scale covers in May.

Management:

  • Prune out heavily infested branches. A dormant oil (3-4%) spray may be applied in late winter. Be sure to thoroughly cover all of the branches.

  • A summer spray (2%) of horticultural oil may be applied in late May.

damage-from-box-moth
Box tree moth damage on Buxus sempervirens. Photo: Ferenc Lakatos, University of Sopron, Bugwood.org

Box Tree Moth
(Cydalima perspectalis) 

Though not currently present in Maryland (as of March 2020), box tree moth (Cydalima perspectalis) is a potential new threat to boxwoods in the United States. This Asian insect was first detected in North America (in Toronto, Canada) in 2018. Prior to then, it spread from Asia to at least 30 European countries. 

caterpillar-box-tree-moth
Box tree moth larva. Photo: Ferenc Lakatos, University of Sopron, Bugwood.org

Box tree moth larvae (caterpillars) feed primarily on the foliage. Adult moths lay their eggs on the undersides of the leaves. When the eggs hatch, the young larvae feed on the undersides of the leaves; the remaining upper leaf surfaces die and turn brown. Older larvae cause extensive chewing damage and defoliation. Webbing and frass (excrement) also are present around infested plants. Refer to these pages for more information about box tree moth and (PDF) Emerging Threats - Box Tree Moth

Other Animal Damage

vole-damage
Girdling caused by vole feeding at the base of the plant. Photo: Mary Ann Hansen, Virginia Polytechnic Institute and State University, Bugwood.org

Meadow Vole
(Microtus pennsylvanicus)

Older boxwood plantings that are neglected and overgrown with weeds are prime candidates for vole damage. Voles damage boxwoods by girdling the base of the plant, feeding on roots, and tunneling through the root system. They cause plant damage primarily in fall and winter.

Voles or meadow mice are found throughout Maryland. A vole is the same size as a house mouse, with small eyes and ears and a short tail. The color may vary between gray and brown.

Voles are often confused with moles, but they are very different in their feeding habits and are not related to them. Moles live underground and feed on soil insects and earthworms. Voles are plant feeders and usually live on the surface but may travel in mole tunnels.

Management:

  • Voles can be controlled by habitat modification and trapping. Use no more than one inch of mulch around boxwoods. Deep mulch provides a good habitat for voles. Keep boxwood plantings free of weeds which provide protection for the voles.

  • To reduce vole populations, mouse traps baited with apple slices or a peanut butter-oatmeal mixture should be placed across surface runways. Many predators prey on voles, including black rat snakes, owls, cats, etc.

Diseases of Boxwood

boxwood-blight-symptom
Dark leaf spots are a symptom of boxwood blight. Photo: Dave Clement

Boxwood Blight
(Calonectria pseudonaviculata)

The disease is caused by a fungus called Cylindrocladium pseudonaviculatum (synonym: Cylindrocladium buxicola). The first symptoms begin as leaf spots followed by rapid browning and leaf drop starting on the lower branches and moving upward in the canopy.

boxwood-blight-stem-symptoms
Black lesions on the green portion of a boxwood stem indicate boxwood blight. Photo: Dave Clement

A key symptom that differentiates boxwood blight from other boxwood diseases, such as Volutella blight and Macrophoma leaf spot, are numerous narrow black cankers (black streaks) that develop on the green stems. The pathogen does not attack the roots, so larger plants may produce new leaves during the growing season but may lose ornamental value as defoliation becomes severe. Repeated defoliation and dieback from stem cankers will kill entire plants. The fungus can remain alive in fallen leaves which can then serve as the source of infection for subsequent years.

boxwoods-declining-from-boxwood-blight
Boxwood blight causes leaf necrosis and defoliation. Photo: Adria Bordas, Virginia Polytechnic Institute and State University, Bugwood.org

Management:

  • Plant disease-resistant cultivars (e.g. ‘New Gen’, ‘Green Beauty’, ‘Nana’) from reputable nurseries. Ask if they receive plants from producers that participate in the Boxwood Blight Cleanliness Program. 

  • Use landscapers and lawn care professionals who are educated about this disease and best management practices for preventing its spread.

  • Send photos of suspicious boxwood symptoms to the Home & Garden Information Center’s Ask an Expert service.

  • If disease symptoms are diagnosed, immediately bag and remove infected plants along with fallen leaves. Mulch the area to bury the remaining debris. 

  • Do not compost infected boxwood material. Launder all clothing, gloves, and shoes, and sanitize gardening tools. 

  • Removal will not guarantee eradication of the boxwood blight pathogen since it can survive in fungal resting structures in the soil for many years.

  • Fungicide sprays have shown some disease suppression in limited situations. However, these treatments do not eradicate boxwood blight and must be repeated throughout the growing season.

  • Consider replacement of boxwoods with non-susceptible plants such as hollies and conifers.

Root Rots
(Phytophthora spp.)

Several Phytophthora species cause root rots in boxwoods. Symptoms include poor growth, loss of healthy foliage color (leaves eventually turn from green to yellow-green to purplish-brown or straw color), upward turning and inward rolling of leaf margins, dark brown discolored wood at the base of the stem for 2 or 3 inches above the soil line, and loosening and separation of the dead lower bark. As a result of the fungal infection, the root system is reduced and turns dark brown. Root diseases on older established plants can result from changes in water drainage patterns. 

Management: 

  • Although there are no chemical cures for these diseases they can be prevented by proper planting. Avoid planting boxwoods in poorly drained compacted soils or in low areas where water collects. 

  • Avoid placing boxwoods near downspouts. 

  • Construction of raised beds or grade changes may be needed to ensure proper drainage. 

browning-of-boxwood
Symptom of Volutella stem blight. Photo: HGIC, UMD

Volutella Stem Blight or Canker
(Pseudonectria buxi)

Many boxwoods are susceptible to this disease caused by the fungus, P. buxi. Before new growth appears in the spring, leaves on the tips of infected branches lose their green color and then fade to a light straw color. However, the infected branches retain most of their leaves for many months. 

Examination of affected branches reveals loose bark and girdling at varying distances from the tips and discoloration of the wood. In moist weather, the fungus produces salmon pink fruiting bodies on leaves and stems.

Management: 

  • Diseased branches should be pruned out when the foliage is dry.

  • Plants should be thinned to improve air circulation and light penetration. 

  • Old fallen leaves and diseased leaves that have accumulated in the crotches of branches in the interior of the plant should be shaken out and removed. 

  • Improve growing conditions, especially to alleviate drought stress.

spotted-leaf-of-boxwood
Macrophoma leaf spot on boxwood. Photo: Dave Clement

Macrophoma Leaf Spot
(Dothiorella candollei)

Many boxwood are susceptible to infection by the weakly parasitic fungus, Dothiorella candollei. The most obvious symptoms are the many tiny black raised fruiting bodies found on dying or dead straw-colored leaves.

Management: Pruning infected branches is sufficient management for this fungus. Thinning pruning is recommended to increase air circulation helping to reduce moisture. 

By Mary Kay Malinoski, David L. Clement, and Raymond Bosmans, University of Maryland Extension, Home and Garden Information Information Center. 5/1995, 5/2009, revised 3/2020.

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