Boxwood Diseases and Insect Pests in South Carolina

Quick Guide (At-a-Glance)

Affected Plants:

• Boxwoods (Buxus sempervirens, B. microphylla, B. sinica var. insularis)

Primary Disease Types:

• Decline complex
• Root Rot (Phytophthora nicotianae, P. cinnamomi)
• Canker/Stem Blight (Volutella buxi)
• Boxwood Blight (Calonectria pseudonaviculata)

Primary Insect Pests:

• Boxwood Leafminer (Monarthropalpus flavus)
• Boxwood Mite (Eurytetranychus buxi)
• Boxwood Psyllid (Psylla buxi)

Primary Causes:

• Fungal pathogens
• Poor drainage
• Excess soil moisture
• Drought stress
• Winter injury
• Nematodes (Meloidogyne, Mesocriconema, Pratylenchus, Tylenchorhynchus)

Most Important Prevention Strategy:

• Maintain proper cultural practices
  • Well-drained soil
  • Correct irrigation
  • Adequate air circulation
  • Sanitation
  • Site selection

Chemical Control:

• Fungicides: chlorothalonil, thiophanate methyl, and copper-based fungicides
• Insecticides: acephate, spinosad, malathion, dinotefuran, imidacloprid
• Less toxic alternatives: horticultural oil, insecticidal soap

Boxwoods (Buxus spp.) are adapted to a wide range of light conditions and prefer fertile, moist, well-drained soil, amended with organic matter. For information on species, varieties, and culture, see HGIC 1061, Boxwood.

In South Carolina, boxwoods are vulnerable to several diseases and insect pests under stressful growing conditions, such as improper drainage or environmental stress.

Boxwood Diseases: Boxwood Decline on Boxwood

Boxwood decline is a complex disorder affecting boxwood that involves multiple fungi, environmental stress factors, and cultural problems.

Cause

Boxwood decline is a poorly understood complex involving the fungi Paecilomyces, Volutella, Macrophoma, and Phytophthora, as well as cold injury, drought stress, and nematodes (microscopic round worms). This phenomenon is also closely related to cultural problems associated with boxwoods, including improper pH and nutrient levels, inadequate irrigation, poor drainage, and improper mulch management.

Paecilomyces buxi has been consistently associated with roots of English boxwood exhibiting the syndrome of boxwood decline, and this decline often follows periods of drought stress.

Root rots by Phytophthora are usually more of a problem in wet soils (see Root Rot section below). Macrophoma candollei can cause leaf blight, but it usually acts as a weak pathogen following root diseases or environmental stresses. Volutella buxi may cause a dieback or stem blight and often follows winter injury (see the Canker section below).

More than one problem may be present in stressed boxwoods.

Symptoms

Symptoms consist of:

• Weak, spindly plants
• Random dead or dying branches
• Premature leaf drop of older foliage
• Yellow foliage
• Pink spore eruptions on black fruiting bodies
• Dead areas or cankers develop along branches or near the crown

Nematode (microscopic worms that feed on the roots) feeding may also contribute to root damage and decline (see Nematodes section below).

Prevention & Treatment

A thorough diagnosis of the associated factors is essential before corrective action is taken.

Cultural Controls

• Submit samples for nematode and root disease analysis to the Clemson University Plant & Pest Diagnostic Clinic.
• Evaluate soil drainage and surface rooting conditions
• Prune dead stems back to healthy tissue
• Disinfect pruning shears in household bleach (1:9 dilution with water) or rubbing (isopropyl) alcohol for 10 seconds
• Removal of dead branches and leaves from branch crotches
• Renew mulch yearly
• Provide water when necessary
• Avoid over-watering or excessive fertilizing
• Thin shrubs to improve air circulation
• Ensure sufficient soil moisture in the fall to prevent winter injury
• Provide wind protection in exposed Upstate locations

If decline is confirmed:

• Do not plant English boxwood (Buxus sempervirens ‘Suffruiticosa’) at the same site
• American boxwood (Buxus sempervirens) and littleleaf boxwood (B. microphylla) can be used as replacements if decline (not Phytophthora root rot) is the primary issue, as they are resistant to decline.

If Phytophthora root rot is confirmed:

• Avoid replanting boxwood at the site

Resistant Varieties

• American boxwood (Buxus sempervirens)
• Littleleaf boxwood (Buxus microphylla)

Biological Control

No biological controls available

Chemical Control

No effective chemical controls for boxwood decline. Management focuses primarily on proper diagnosis and cultural correction.

Boxwood Diseases: Root Rot on Boxwood

Root rot on boxwood is a destructive fungal disease caused by Phytophthora species that attack roots in poorly drained soils.

Cause

Root rot is caused by the fungi Phytophthora nicotianae and P. cinnamomi.

High soil moisture, due to overwatering plants or prolonged periods of heavy rain, and warm soil temperatures favor the development of root rot. The disease is more severe in heavy clays or poorly drained soils.

Symptoms

Symptoms include:

• Leaves turn from normal dark green to light green as the plant declines
• Dark and rotted roots
• Bark rotting and peeling at the crown
• Death of the entire plant

Prevention & Treatment

Phytophthora root rot must be prevented, as chemicals are often ineffective after above-ground symptoms become apparent.

Cultural Controls

• Purchase disease-free plants from a reputable nursery.
• Replant with non-susceptible plants in areas where susceptible plants have died.
• Plant root-rot-susceptible plants in well-drained areas or in raised beds.
• Amend heavy clay soils with porous materials such as composted pine bark.
• Avoid over-watering.

Resistant Varieties

No fully resistant boxwood varieties are available.

Biological Control

No biological controls are available.

Chemical Control

• Soil treatments with the appropriate fungicide may reduce the spread of the fungus among plants.
• Fungicides may not kill the pathogens in infected plants.
• Read and follow all directions on the fungicide label.

Boxwood Diseases: Canker (Stem Blight) on Boxwood

Canker, also called stem blight, is a fungal disease of boxwood caused by Volutella buxi, resulting in dieback and branch death.

Cause

This disease is caused by the fungus Volutella buxi.

It often follows winter injury and may occur on stressed English and American boxwoods.

Symptoms

Symptoms include:

• Delayed or weak spring growth on certain branches or plants
• Leaves turn from normal green to light green to tan
• Infected leaves turn upward and lie close to the stem instead of spreading out
• Small, rose-colored, waxy fruiting bodies on diseased leaves and branches
• Loose bark at the base of infected branches
• Gray to black discolored wood beneath peeling bark

Prevention & Treatment

Early sanitation and preventive fungicide applications are essential for management.

Cultural Controls

Remove dead branches as soon as they are noticed.

Annually remove and destroy leaves lodged in branch crotches.

Resistant Varieties

No resistant varieties are available.

Biological Control

No biological controls are available.

Chemical Control

• Copper-based fungicides are very effective in preventing canker.
• Make the first application after dead leaves and dying branches have been removed and before growth starts in the spring.
• Read and follow all label instructions and precautions.

See Table 1 for examples of products.

Boxwood Diseases: Boxwood Blight on Boxwood

Boxwood blight is a serious fungal disease of boxwood caused by Calonectria pseudonaviculata that leads to leaf spots, stem cankers, and defoliation.

Cause

Boxwood blight is caused by the fungal pathogen Calonectria pseudonaviculata (synonym Cylindrocladium pseudonaviculatum).

Other related plants, such as pachysandra and sweet box (Sarcococca species), may also be hosts.

The pathogen can survive for at least five years on blighted and fallen foliage, as well as on infected stems. Rainfall and overhead irrigation may splash the spores onto adjacent plants.

Symptoms

Symptoms include:

• Initial leaf spots
• Leaf spots that enlarge and coalesce
• Blighted leaves that drop from the plant
• Dark brown to black stem lesions or cankers
• Stem dieback when cankers encircle stems
• Severe defoliation

 

Prevention & Treatment

Accurate diagnosis is essential, as boxwood blight may be confused with other boxwood diseases.

Plant samples can be sent to the Clemson Plant & Pest Diagnostic Clinic for identification.

Cultural Controls

• Prune out the diseased stems.
• Rake up fallen foliage.
• Do not compost diseased plant material.
• Bury, burn, or bag infected material for landfill disposal.
• Apply fresh mulch beneath the plants to reduce splash dispersal of spores.

Non-susceptible alternative landscape replacements include:

• Dwarf yaupon holly
• Japanese holly

Resistant Varieties

There are no known resistant boxwoods.

More tolerant selections include:

• Buxus sinica var. insularis ‘Nana’
• Buxus microphylla var. japonica ‘Green Beauty’

Biological Control

No known biological controls are available.

Chemical Control

• Chlorothalonil
• Chlorothalonil mixed with thiophanate methyl

Read and follow all label instructions and precautions.

See Table 1 for examples of brands and specific products.

Boxwood Diseases: Nematodes on Boxwood

Nematodes on boxwood are microscopic parasitic roundworms that damage roots and contribute to overall plant decline.

Cause

Boxwoods are susceptible to several parasitic nematodes (microscopic round worms), including:

• Southern root-knot nematode (Meloidogyne incognita)
• Ring nematode (Mesocriconema)
• Lesion nematode (Pratylenchus)
• Stunt nematode (Tylenchorhynchus)

These microscopic worms feed on the roots. As roots die, the plant forms lateral roots above the infested area that are subsequently infested.

Symptoms

Symptoms include:

• Leaf bronzing
• Stunted growth
• General plant decline
• Root system resembling a witches’ broom due to repeated infestations and lateral root production

Prevention & Treatment

Management focuses on reducing nematode populations and maintaining plant vigor.

Cultural Controls

• Provide good fertilization and mulching practices.
• Water plants thoroughly during dry spells.
• Avoid planting boxwoods in soils heavily infested with nematodes.
• Grow non-host plants such as grasses and marigolds to reduce nematode populations over time.

Resistant Varieties

• American boxwood (Buxus sempervirens) is resistant to root-knot nematodes.
• American boxwood is tolerant of stunt nematodes.

Biological Control

No biological controls are available.

Chemical Control

No chemical controls are available for eliminating home landscape nematodes.

Nematodes cannot be eliminated from the landscape. The goal is to keep the population low enough to prevent damaging symptoms that weaken the plant.

Use nematode-tolerant shrubs, such as yaupon holly and Burford holly, to replace nematode-killed boxwoods.

Several common boxwood insect pests can cause cosmetic injury or significant plant stress when populations are high.

Boxwood Insect Pests: Boxwood Leafminer on Boxwood

Boxwood leafminer is the most serious insect pest of boxwood and feeds inside the leaves during its larval stage.

Cause

Boxwood leafminer (Monarthropalpus flavus) is the larva (immature form) of a small, orangish, mosquito-like fly.

Adult flies are less than ⅛-inch long and emerge, often seen swarming around boxwoods, in the spring.

Females insert eggs with their ovipositor (egg-laying structure) into new boxwood leaves through the leaf’s upper surface.

Larvae hatch and feed inside the leaf, creating a mine. There is one generation per year, and larvae overwinter inside the leaves.

American boxwood is the preferred host plant, but English and Japanese boxwoods (B. microphylla var. japonica) are also susceptible.

Symptoms

Symptoms include:

• Irregularly shaped leaf swelling
• Slight blistering on the leaf undersurface (more evident in late summer)
• Yellow or brown splotches
• Smaller leaves
• Premature leaf drop
• Heavy infestation can lead to serious leaf loss and possible plant death

Prevention & Control

Insecticide use is not recommended unless damage is intolerable.

Insecticides are most effective when adults have emerged and before they lay eggs. However, as adults typically emerge over three weeks but live only a few days, insecticide application is challenging.

Cultural Controls

• Monitor adult emergence in mid-April to early May.
• Observe for hovering adult flies around plants.

Resistant Varieties

No resistant varieties are available.

Biological Control

No biological controls are available.

Chemical Control

Contact insecticides (targeting adults):

• Malathion

Foliar systemic insecticides (targeting larvae in mines):

• Acephate
• Spinosad

Soil-applied systemic insecticides:

• Dinotefuran
• Imidacloprid

Treatments may take two weeks or more to begin providing season-long control. Dinotefuran may move into shrubs more quickly than imidacloprid.

See Table 1 for examples of products.

Read and follow all label instructions and precautions.

Boxwood Insect Pests: Boxwood Mite on Boxwood

Boxwood mite is a spider-mite pest of boxwood that feeds on foliage and causes stippling and discoloration.

Cause

Boxwood mite is not an insect but is more closely related to spiders.

Adults are greenish to yellowish-brown, have eight legs, and are tiny, about ¹/₆₄-inch long.

This pest overwinters as eggs on the underside of leaves, and the eggs hatch in the spring.

Boxwood mites develop and breed rapidly, resulting in eight or more generations per year.

All stages feed on both leaf surfaces. Mites pierce the leaf to suck out plant sap. Prefer young leaves.

Symptoms

Symptoms include:

• Fine stippling on the upper leaf surface
• Tiny scratch-like yellow spots (caused by toxic saliva)
• Feeding damage is most evident on second-and third-year leaves
• Overall dingy silvery plant appearance
• Heavy infestations resulting in significant foliage damage

Tiny mite size and non-descript early symptoms lead to the problem being overlooked, and greater damage has occurred.

Prevention & Control

Naturally occurring predators often suppress mite populations.

Insecticides should be avoided unless necessary, as they may kill beneficial predators.

Cultural Controls

• Monitor mite populations using a white sheet of paper, hold the paper under a branch and strike the branch to dislodge mites.
• Observe for mites crawling around on the paper.
• If more than 15 mites are seen per strike, serious damage can result.
• Remove mites with a strong spray of water applied regularly.

Resistant Varieties

No resistant varieties are known.

Biological Control

Naturally occurring predators include:

• Predatory mites
• Ladybird beetles (ladybugs)
• Other beneficial insects

Chemical Control

Horticultural oil:

• Apply at 1% to 2% summer rate (2½ to 5 tablespoons oil per gallon of water) kills eggs and adult mites
• Spray when temperatures are between 45°F and 85°F.

Insecticidal soaps:

• Apply before populations get high.

Miticides labeled for homeowner use:

• Tau-fluvalinate

Reapply miticides in seven to 10 days as needed.

See Table 1 for examples of products.

Read and follow all label instructions and precautions.

Boxwood Insect Pests: Boxwood Psyllid on Boxwood

Boxwood psyllid is a sap-feeding insect on boxwood that causes cupped leaves and stunted twig growth.

Cause

The adult is a small, greenish insect, about ⅛-inch long, with long, transparent wings and strong jumping legs.

Adults resemble tiny cicadas and hop or fly 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.

There is one generation per year. Eggs are laid under bud scales, and immature nymphs emerge in spring to feed on buds and young leaves.

Symptoms

Symptoms include:

• Cupped or curled leaves
• Stunted twig growth
• White, waxy material covering nymphs
• Distorted new growth

Plants tend to outgrow the injury by midsummer.

Prevention & Control

Insecticides should only be used if infestations are heavy.

Cultural Controls

• Monitor plants in early spring for cupping of new growth.
• Recognize that injury is usually cosmetic and temporary.

Resistant Varieties

No resistant varieties are available.

Biological Control

No biological controls are listed in the original document.

Chemical Control

Foliar systemic insecticide:

• Acephate

Soil-applied systemic insecticides:

• Dinotefuran
• Imidacloprid

Treatments may take two weeks or more to begin providing season-long control. Dinotefuran may move into shrubs more quickly than imidacloprid.

See Table 1 for examples of products.

Read and follow all label instructions and precautions.

Fungicides for Boxwood Diseases (Table 1)

Active Ingredient	Examples of Brand Names & Products
Chlorothalonil	Bonide Fung-onil Multi-purpose Fungicide Concentrate Bonide Fung-onil Multi-purpose Fungicide Ready to Use Ferti-lome Broad Spectrum Landscape & Garden Fungicide GardenTech Daconil Fungicide Concentrate GardenTech Daconil Fungicide Ready-to-Use Hi-Yield Vegetable, Flower, Fruit And Ornamental Fungicide Ortho MAX Garden Disease Control Concentrate Southern Ag Liquid Ornamental and Vegetable Flowable Fungicide Tiger Brand Daconil
Copper-based Fungicides	Bonide Captain Jack’s Liquid Copper Fungicide Bonide Captain Jack’s Liquid Copper Fungicide Bonide Copper Fungicide Spray or Dust Ready to Use Camelot O Fungicide/Bactericide Espoma Organic Copper Soap Ready-to-Use Fungicide Monterey Copper Fungicide Garden Spray Liqui-Cop Natural Guard by ferti-lome Copper Soap Fungicide Concentrate Natural Guard by ferti-lome Copper Soap Fungicide Ready to Use Southern Ag Liquid Copper Fungicide
Horticultural Oil	Bonide All Seasons Horticulture Spray Oil Ready-to-Use Bonide All Seasons Horticulture & Dormant Spray Oil Ready-to-Spray Ferti-lome Horticultural Oil Spray Monterey Horticultural Oil Concentrate Monterey Horticultural Oil RTU2 Safer Brand Horticultural & Dormant Spray Oil Concentrate Southern Ag Parafine Horticultural Oil Summit Year-Round Spray Oil
Thiophanate methyl	Southern Ag Thiomyl Systemic Fungicide
Note: Read and follow all label instructions and precautions.

Insecticides & Miticides for Boxwood Insect Pests (Table 1)

Active Ingredient	Examples of Brand Names & Products
Acephate	BioAdvanced Science-Based Solutions 2-in-1 Systemic Rose & Flower Care Granules with 6-9-6 Fertilizer Bonide Systemic Insect Control II
Dinotefuran	Gordon’s Zylam Liquid Systemic Insecticide Valent Safari 20 SG Insecticide
Horticultural Oil	Bonide All Seasons Horticulture Spray Oil Ready-to-Use Bonide All Seasons Horticulture & Dormant Spray Oil Ready-to-Spray Ferti-lome Horticultural Oil Spray Monterey Horticultural Oil Concentrate Monterey Horticultural Oil RTU2 Safer Brand Horticultural & Dormant Spray Oil Concentrate Southern Ag Parafine Horticultural Oil Summit Year-Round Spray Oil
Imidacloprid	BioAdvanced Science-Based Solutions 12 Month Tree & Shrub Insect Control Concentrate Landscape Formula BioAdvanced Science-Based Solutions 12 Month Tree & Shrub Protect & Feed Concentrate 2-1-1 BioAdvanced Science-Based Solutions 12 Month Tree & Shrub Protect & Feed II Concentrate 2-1-1 BioAdvanced Science-Based Solutions 12 Month Tree & Shrub Protect & Feed II Ready to Use Granules 2-1-1 BioAdvanced Science-Based Solutions 12 Month Tree & Shrub Protect & Feed Ready to Use Granules 2-1-1 BioAdvanced Science-Based Solutions 2 in 1 Systemic Rose & Flower Care Ready-to-Use Granules II Bonide Annual Tree & Shrub Insect Control w/ Systemaxx Bonide Systemic Granules Insect Control Bonide Systemic Insect Spray w/ Systemaxx Ready to Spray Ferti-lome Tree & Shrub Drench Hi-Yield Systemic Insect Granules Hi-Yield Systemic Insect Spray Ready-to-Spray Martin’s Dominion Tree & Shrub Insecticide Concentrate Monterey Once A Year Insect Control II
Insecticidal Soap	Bonide Insecticidal Soap Multi-purpose Insect Control Ready to Use Espoma Organic Insect Soap Multi-Purpose Insect Killer Garden Safe Brand Insecticidal Soap Insect Killer Miracle-Gro Nature’s Care Insecticidal Soap Natria Insecticidal Soap Ready-to-Use Natural Guard by ferti-lome Insecticidal Soap Concentrate Safer Brand Insect Killing Soap Concentrate II Whitney Farms Insecticidal Soap 1
Malathion	Bonide Malathion 50% Insect Control Gordon’s Malathion 50% Spray Hi-Yield 55% Malathion Insect Spray Martin’s Malathion 57% Ortho Max Malathion Insect Spray Concentrate Southern Ag Malathion 50% EC Spectracide Malathion Insect Spray Concentrate Tiger Brand 50% Malathion
Spinosad	Bonide Captain Jack’s Deadbug Brew Concentrate Bonide Captain Jack’s Deadbug Brew Ready to Spray Bonide Captain Jack’s Deadbug Brew Ready to Use Bonide Colorado Potato Beetle Beater Concentrate Conserve SC Turf & Ornamental Monterey Garden Insect Spray Monterey Garden Insect Spray RTU2 Natural Guard by ferti-lome Spinosad Bagworm, Tent Caterpillar & Chewing Insect Control Natural Guard by ferti-lome Spinosad Soap Ortho Insect Killer Tree & Shrub Concentrate Southern Ag Conserve Naturalyte Insect Control
Tau-Fluvalinate	BioAdvanced Science-Based Solutions 3-in-1 Insect, Disease & Mite Control BioAdvanced Science-Based Solutions 3-in-1 Insect, Disease & Mite Control Ready-To-Use BioAdvanced Science-Based Solutions 3-in-1 Insect, Disease & Mite Control I
Abbreviations Used in Table 1 RTS = Ready to Spray (hose-end applicator) RTU = Ready to Use (pre-mixed spray bottle)

Pesticide Safety

• Do not apply oil sprays when the temperature is above 85 °F or to drought-stressed plants.
• Do not apply soap sprays when the temperature is above 90 °F or to drought-stressed plants.
• Spray late in the day when no rainfall is forecast.
• Read and follow all label instructions and precautions.

Pesticides are updated annually. The last updates were made on 3/26 by N. Jordan Franklin.

Originally published 3/00