Boxwood Blight, Mites, and other Frights

I was a novice volunteer at the Piedmont Master Gardeners’ Horticultural Help Desk when a “client” came in, clutching a boxwood branch with brown, desiccated leaves. “Is this boxwood blight?” she asked. Yvette, a Help Desk veteran, glanced at the sorry-looking twig. “Do you have dogs?” “Yes, we just adopted two Lhasa apsos,” the client replied.

Unfortunately, not all boxwood problems are so easy to solve. Many fungal diseases have similar signs and symptoms. Some diseases are merely opportunistic, taking advantage of weaknesses caused by hidden problems. Root damage usually manifests as foliage issues. Non-living causes, like road salt and cold weather, can damage foliage as badly as fungal infections.

This article discusses common diseases, pests, and environmental problems facing boxwoods in Virginia. It starts with diagnosis and possible treatment. More importantly, it then covers how to prevent these issues from arising in the first place. Finally, this article covers how to get professional diagnoses and treatment recommendations from the plant disease, nematode, and insect identification clinics at Virginia Tech.

The main source for this article is the Boxwood Blight Task Force at the Virginia Cooperative Extension. Never let it be said that we don’t take boxwoods seriously in Virginia! References cited below offer in-depth descriptions and recommendations.

Boxwood Blight

Diagnosis The first sign of boxwood blight is brown spots ringed with darker brown areas (see picture). The spots are followed by (1) sudden defoliation and (2) black streaks on stems (see pictures below). Fluffy white clusters of spores may appear, but not always, and they can be difficult to see even if present.

Treatment There is no cure for boxwood blight. You might, however, prevent it from spreading to other plants. Remove the infected plant entirely. Thoroughly rake up all fallen leaves and twigs. Throw this debris away, burn it if legal, or bury it as far from other boxwoods as possible. Spread 2 inches of mulch under remaining plants to help prevent spores from splashing up onto foliage. Wash dirt off tools, then disinfect with 70% ethanol, bleach, or hydrogen peroxide (consult the “Sanitizers” section of the Boxwood Blight Task Force website). Wash clothes, gloves, and footwear with soap and hot water. Finally, consider a preventative fungicide spray on neighboring plants, as described in the “Fungicides” section of the Boxwood Blight Task Force website (PDF version).[symple_column size=”one-half” position=”first” fade_in=”false”]

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[/symple_column]Prevention

• Avoid introducing blight to your area via infected plants. Only buy plants from growers that participate (PDF link) in Virginia’s Boxwood Blight Cleanliness Program.

• Several boxwood cultivars are tolerant of boxwood blight, according to the Virginia Cooperative Extension (PDF link). Note that none are resistant.

Table 1. Boxwood Blight-Tolerant Cultivars

Species	Cultivar
Buxus microphylla var. japonica	Green Beauty
B. microphylla	Northern Emerald
B. microphylla	Wedding Ring
B. microphylla	Wintergreen
B. microphylla	Golden Dream
B. microphylla	Winter Gem
B. sinica var. insularis	Nana
B. sinica var. insularis	Franklin’s Gem
B. sinica var. insularis	Wee Willie
B. harlandii	Richard

Avoid the following cultivars, which are particularly susceptible to blight:

Table 2. Cultivars Quite Susceptible to Boxwood Blight

Species	Cultivar
Buxus sempervirens	Suffruticosa
B. mycrophylla var. japonica	Morris Midget
B. sempervirens	Justin Brouwers
B. sempervirens	American
B. sempervirens	Halifax American
B. sempervirens	Fineline
B. sempervirens	Black American
B. sempervirens	Arborescens
B. sempervirens	Aurea Pendula
B. sempervirens	Latifolia Maculata

Other plants in the same family as boxwoods can be hosts for boxwood blight. The most common of these are: Japanese spurge (Pachysandra terminalis), Allegheny spurge (P. procumbens), fragrant spurge (P. axillaris), and sweetbox (Sarcococca species). Try not to grow these plants near your boxwoods.

• Use proper cultural practices to promote plant health. Healthy boxwoods are less susceptible to blight. Have your soil tested every 2-3 years and follow the amendment recommendations that come from the lab. The PMG Horticultural Help Desks can help interpret these recommendations. Keep roots healthy by amending heavy clay soil with compost to improve drainage.

Prune boxwood plants to promote airflow, keep leaves dry, and minimize fungal growth. Try not to work on plants when leaves are wet, which can increase the spread of spores. Avoid overhead irrigation. Mulch under the plants to avoid splashing spores onto leaves. This also conserves water and reduces the need for irrigation in the first place. Rake up any fallen leaves and branches. Finally, sanitize tools, clothes, and footwear after working on boxwoods.

Other Boxwood Fungal Diseases

• Colletotrichum

Diagnosis Dieback of branches without leaf drop, randomly distributed around the plant, is the most notable symptom of Colletotrichum (see picture). Black discoloration can be found under the bark of the dead wood. Sometimes black “fruiting bodies” (the structures that produce spores) will appear on leaves. Roots will be healthy, that is, they will be brown, and the outer layer will be firmly attached to the core.

Treatment Like blight, there is no treatment for Colletotrichum. The only surefire way to stop it from spreading is to remove and dispose of the infected plant as described above. Thoroughly clean up plant debris and sanitize clothes and tools. There is no preventative spray recommended by the Virginia Cooperative Extension.

Prevention At present, the only recommended preventative measures are general cultural practices like those described above.

• Volutella:

Diagnosis Volutella also causes dieback without leaf drop. The best way to distinguish Volutella from Colletotrichum is by looking at the fruiting bodies: Volutella’s are orange or pink (see picture), whereas Colletotrichum’s are black. For diagnostic purposes, encourage fruiting bodies to grow by placing an infected branch in a sealed plastic bag with a few drops of water.

Treatment There is no treatment for Volutella, but in general it is not as serious as Colletotrichum. Usually it’s an opportunistic infection that indicates an underlying issue, such as root problems (see below). Prune infected branches a few inches below the lowest affected leaves and dispose as described above. Clean up plant debris and sanitize tools, footwear, and clothes. If a plant is seriously infected, remove it completely.

Prevention As for the above fungal diseases, prevent Volutella by keeping plants healthy and dry. If there’s an infected plant nearby, consider a preventative fungicide spray as described in the Virginia Cooperative Extension Pest Management Guide (PDF link)  (page 4-9 in the 2023 guide).

• Root rot, e.g. Phytophthora and Pythium (though technically these two pathogens are oomycetes, not fungi):

Diagnosis Root damage first manifests as subtle yellowing or bronzing of a section of leaves. It then progresses to stunting, and eventually sectional dieback (see pictures). Leaves will remain attached. Unlike Colletotrichum, there will be no discoloration under the bark of dying wood. The fibrous roots will be blackened and weak, and the outer layer will detach easily from the core.[symple_column size=”one-half” position=”first” fade_in=”false”]

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[/symple_column]Treatment The Virginia Cooperative Extension Pest Management Guide (PDF link) (page 4-8 in the 2023 guide) has one recommendation for treating Phytophthora and Pythium root rots: potassium salts of phosphorous, applied as a soil drench or foliar spray. (Note: these products don’t act as fertilizers, because they contain phosphorous in a different form than that used by plants.) Remember, it’s always better to prevent diseases than to treat them (see below).

Prevention Prevent root rots by not over-watering plants. When planting boxwoods, avoid low-lying areas and poorly drained soil, such as heavy clay. Amend clay with compost to improve drainage. However, don’t add compost to the hole where the boxwood will be planted; instead add it to the soil around the planting hole.

Insects, Mites, and Nematodes

• Boxwood psyllids:

Diagnosis Leaf cupping near the ends of branches is the signature of boxwood psyllids (see picture). This is noticeable in the spring, when nymphs hatch and begin feeding on the plants’ sap. Adults appear in late spring or early summer, but they cause minimal damage apart from laying eggs in buds. Fortunately, boxwood psyllids have only one generation per year.

Treatment Trim and dispose of affected branches. Insecticides are needed only for severe infestations. Consult the Virginia Cooperative Extension Pest Management Guide (PDF link) (page 4-52 of 2023 guide) for more information.

Prevention Halt the life cycle by removing affected branches. Unless there is severe infestation, preventative insecticide use is not recommended.

• Boxwood leaf-miners:

Diagnosis Look for small yellowish blisters in leaves (see picture) where leaf-miner larvae live between the upper and lower tissue layers.

Treatment Most infestations can be halted by removing branches with affected leaves. Dispose of the waste as described previously. For severe infestations, consult the Virginia Cooperative Extension Pest Management Guide (PDF link) (page 4-49 of 2023 guide) for more information on insecticide treatments.

Prevention As for psyllids, prevent future infestations by removing affected branches.

• Mites, primarily boxwood mites and spider mites:

Diagnosis Mite damage appears as tiny pale dots, termed “stipples,” on leaves. Despite the name, spider mite webbing is apparent only during heavy infestations. Heavy feeding can also cause leaf yellowing and death.

Treatment Knock mites off plants with a strong stream of water. (This conflicts with the advice to avoid wetting leaves. Do this only in dry weather, and keep plants well-pruned.) Conventional miticides are not recommended because they can harm beneficial mites that feed on harmful mites. Some garden stores and catalogs sell beneficial predatory mites, e.g. phytoseiid mites.

Prevention Horticultural oils, applied early in the spring, can kill mite eggs. Consult the Virginia Cooperative Extension Pest Management Guide (PDF link) (page 4-51 of 2023 guide) for more information. Note that horticultural oils can harm predatory mites and insects, so only use them if there was a severe infestation the previous year.

• Nematodes

Diagnosis  Nematodes – microscopic worms – attack roots, so nematode damage looks like root disease (see pictures above): leaf yellowing or bronzing, stunting, and dieback. As detailed below, the Virginia Tech Nematode Clinic can check boxwoods for harmful nematodes. Root-knot nematodes may cause nodules to form on roots, but in general, nematode damage is hard to distinguish from root disease without using a microscope.

Treatment The best treatment is to promote the health of the infected plant with fertilization (guided by a soil test), watering, and pruning. Nematicides are not recommended because they kill beneficial nematodes that feed on harmful nematodes.

Prevention Nematodes cannot be eliminated from the environment. Help your boxwoods to fight off nematode damage by promoting general health as described above. If you are planting new boxwoods, American boxwoods are somewhat resistant to nematode damage.

Non-biological Issues

• Water (too much or too little):

Excessive watering can promote root rot, as described above. On the other hand, drought stress can also cause leaves to turn yellow and die. Drought stress is a particular problem for newly planted boxwoods, so monitor new plantings carefully, especially during hot weather. One inch of rain or irrigation per week is recommended for new boxwoods. (This works out to 2.5 quarts per square foot, or 5 gallons in a 3-foot diameter circle.) Established boxwoods do not need irrigation, except in times of extreme heat and/or drought.

• Salt

Salt (e.g. for melting ice) can desiccate plants and cause leaf yellowing and/or death. If boxwoods near a sidewalk or driveway show this kind of damage, particularly in the spring, consider salt as a cause. Salt can harm boxwoods in two ways: through direct contact with leaves, and by leaching into the soil.

• Winter/cold damage

Winter injury is very common for boxwoods. It begins as leaf browning or bronzing in the spring, possibly followed by death of the affected area (see picture). If the leaves of a boxwood turn dark red or bronze, particularly in the spring, winter damage is the likely cause. A good diagnostic for cold injury is that the damage only appears on the upper part of a plant, where it is not protected by snow. Simply cut off the affected parts; the rest of the plant will grow and fill in the missing area.

Avoid winter damage by planting boxwoods in areas protected from wind. Northern and eastern exposure are better than southern or western because cold injury takes place when cold plants suddenly warm up. Do not fertilize plants in late summer or fall; this may spur new growth that is not winter-hardy.

Plant Disease Clinic at Virginia Tech

Needless to say, not all boxwood problems look like the descriptions and pictures in this article. The Plant Disease Clinic at Virginia Tech is a fantastic resource for professional diagnosis of plant diseases. Moreover, if the disease clinic suspects that a problem is actually caused by insects or nematodes, they will send a sample directly to Virginia Tech clinics that specialize in those organisms.

Collecting a proper sample is critical to achieving an accurate diagnosis. The clinic will reject insufficient samples. The optimal sample is a whole plant, including roots; be sure to leave a pint of soil around the roots to keep them moist. If a whole plant isn’t feasible, the sample should include branches or twigs (more than one) that contain both living and dead or dying tissue. This helps ensure that the disease-causing organism, if there is one, will be actively growing somewhere on the sample. The sample must also include fibrous roots in a pint of soil. Read the clinic’s sampling instructions carefully before collecting a sample.

“Digital samples,” i.e. pictures, are a useful addition to physical samples because they help the diagnostician examine the overall health of the plant and the distribution of any diseased portions. Read the disease clinic’s guide to good digital samples (PDF link) for tips on how to take the most useful pictures.

The cost to send a sample to the Plant Disease Clinic is $35. Contact your local Virginia Cooperative Extension office for instructions on where to deliver samples. In the Albemarle-Charlottesville area, bring samples to the Horticultural Help Desk in the Virginia Cooperative Extension office at 460 Stagecoach Rd., Charlottesville. Go to the back entrance of the Albemarle County Office Building off 5^th Street Extended. The help desk is open 9 AM – noon, Monday – Friday from April to October, and Monday/Wednesday/Friday the rest of the year. Try to bring samples on Monday to allow them to reach the clinic before the weekend. Email pictures to albemarlevcehelpdesk@gmail.com. The Help Desk phone number is 434-872-4583.

Closing Thoughts

Boxwoods are popular for many good reasons. Personally, I am very fond of the smell. But boxwoods can be difficult to grow, particularly in hot, humid places like Virginia. The cultural practices described here will help prevent diseases, pests (and pets), and the environment from damaging your boxwoods. If something does go wrong, hopefully this article – and the Plant Disease Clinic – will help diagnose and halt any problems to get you back on track with beautiful, fragrant boxwoods.

References and further reading

featured image: Johannes Bergsma, CC BY-SA 3.0

Agromyzid Leafminers (Virginia Cooperative Extension)

Best Management Practices for Boxwood Blight in the Virginia Home Landscape (Virginia Cooperative Extension)

Boxwood Blight Cleanliness Program (Virginia Department of Agriculture and Consumer Services)

Boxwood Blight Task Force (Virginia Cooperative Extension)

Boxwood Diseases and Insect Pests (Clemson Cooperative Extension)

Boxwoods: Identify and Manage Common Problems (University of Maryland Extension)

Boxwood Psyllid (University of Kentucky)

Boxwood Psyllid Insects on Shrubs (University of Maryland Extension)

Browning of Boxwood: Is it Boxwood Blight? (Michigan State University Extension)

Home Grounds and Animals 2023 Pest Management Guide (PDF link) (Virginia Cooperative Extension)

Plant Health Alert: Improving Clay Soils for Better Gardens (Henderson County Center, North Carolina Cooperative Extension)

Reddish-brown Boxwood Leaves Usually Caused by Winter (University of Georgia Extension)

Spider Mites on Trees and Shrubs (University of Maryland Extension)

Types of Boxwood (New York Botanic Garden)

Virginia Tech Plant Disease Clinic

Virginia Tech Soil Testing Laboratory

Volutella Stem and Leaf Blight on Boxwoods (University of Missouri)