Boxwood diseases can be hard to diagnose. Several unrelated problems all show up the same way at first, as patches of yellow, tan or bronze leaves on a shrub that looked fine a few weeks earlier. Some of these problems are minor and grow out with better care. Others, such as boxwood blight, can strip a mature hedge of its leaves in a few weeks and stay in the soil for years after the plants are gone. Correct identification matters because the treatment for one disease can do nothing against another, or even make it worse.
Boxwood is a major plant in the American nursery trade. According to USDA Census of Horticultural Specialties data analyzed in the Journal of Environmental Horticulture, U.S. boxwood sales rose from $102.9 million in 2009 to $126.4 million in 2014. That was a 23% increase, about twice the growth rate of nursery stock overall. In 2014, boxwood production was worth more nationally than azalea, holly, hydrangea or arborvitae. With that many plants in yards and nurseries, a serious disease affects colonial gardens in Virginia, suburban foundation plantings in Ohio and commercial growers in Oregon alike.
Boxwood disease problems have changed a lot in the last 15 years. For most of the 1900s, gardeners mainly dealt with root rot in wet soils, stem canker after hard winters and a slow decline in old English boxwood. Boxwood blight reached the United States in 2011 and spread quickly. By 2019 it had been confirmed in 30 states and the District of Columbia, and those states grow about 95% of the country’s boxwood. Because of that, any sudden leaf drop on a boxwood now deserves a closer look.
This guide covers the six diseases American gardeners see most often. For each one it explains how to recognize the symptoms, what causes it and what treatment works. It also explains how to tell real disease from winter injury and insect damage, which cause more boxwood browning than any pathogen. The last sections cover prevention and when to send a sample to a diagnostic lab.
Boxwood Blight
Identifying Boxwood Blight
Boxwood blight is the most destructive boxwood disease in North America, and three symptoms separate it from everything else. The first is round leaf spots, light to dark brown, often with a darker border and sometimes a zonate, target-like pattern. The second is narrow black streaks or diamond-shaped lesions on the green stems. No other common boxwood disease causes these streaks, so they are the most reliable field sign. The third is fast leaf drop: infected plants can shed most of their leaves within 1 to 2 weeks of the first spots, starting at the bottom and moving up.
In humid weather, fuzzy white spore masses can appear on the undersides of infected leaves and on stem lesions. A hand lens makes them easier to see. Infected shrubs often look bare at the base while the top still has green leaves, and the fallen leaves pile up under the plant. Plants that aren’t killed outright usually put out new leaves, but they get infected again each season until they are weak and thin. Pachysandra and sweet box are related to boxwood, so check any nearby plantings of them too.
What Causes Boxwood Blight
The disease is caused by a fungus first reported in the United Kingdom in the mid-1990s, which then spread across Europe and into New Zealand. On October 26, 2011, federal inspectors confirmed the first U.S. detection at a nursery in Surry County, North Carolina, and Connecticut confirmed cases at about the same time. Since then it has spread mainly on infected nursery stock, holiday greenery and cut boxwood. Once at a site, its sticky spores move by splashing water, pruning tools, gloves, clothing, shoes, pets and wildlife.
Boxwood blight needs warm, wet conditions. It infects fastest at roughly 64 to 77 degrees Fahrenheit with leaves that stay wet for several hours. Long rainy spells in late spring, summer and early fall are the highest-risk periods, especially for dense hedges and plants watered with overhead sprinklers. The fungus forms tough resting structures that survive in fallen leaves and soil for at least several years. That is why replanting boxwood where blight was confirmed is so risky.
Treating Boxwood Blight
Once a plant has boxwood blight, it cannot be cured, only contained. If you suspect blight, have it confirmed first. University of Connecticut’s Home and Garden Education Center and other state extension services recommend lab confirmation because Volutella blight and winter injury can look similar. After confirmation, dig out infected plants with their root balls and as much fallen leaf debris as possible. Bag everything and send it to a landfill or bury it deeply. Never compost it, because home compost piles don’t get hot enough to kill the fungus.
Fungicides only protect healthy plants. They do not cure infected ones. Commercial growers and landscapers in high-pressure areas apply protective sprays every 7 to 14 days during wet weather, but this is expensive and must be kept up all season. Homeowners usually do better by switching to tolerant plants. Littleleaf and Japanese boxwood types are generally less susceptible than English boxwood, and blight-tolerant lines such as the NewGen series from Virginia are widely available. Clean pruning tools between plants, and avoid working in boxwood plantings while the leaves are wet.
Volutella Blight (Boxwood Canker)
Identifying Volutella Blight
Volutella blight, also called boxwood canker or stem blight, is the most common fungal disease of landscape boxwood in the eastern United States. It usually first appears in spring as single branches or sections that turn light green, then yellow, then a straw or tan color. Unlike boxwood blight, the dead leaves stay attached for weeks or months instead of dropping. The bark at the base of affected branches often comes loose and peels away easily, showing a canker that has girdled the stem. Green stems never show the black streaks of boxwood blight.
The clearest sign is small salmon-pink to orange spore masses on the undersides of leaves and on infected stems during cool, moist weather. They often show up in early spring after rain and are visible with or without a hand lens. Affected branches usually pull out of the plant easily because the bark and wood at the base have rotted. Damage is often scattered through the shrub instead of moving from the bottom up. Volutella blight can occur together with boxwood blight on the same plant, which makes field diagnosis harder.
What Causes Volutella Blight
The fungus behind Volutella blight is mainly an opportunist. It enters through damaged or stressed tissue instead of attacking vigorous growth. According to the University of Kentucky Cooperative Extension Service, winter injury, poor vigor and stem wounds increase the risk of infection, and all species and cultivars of boxwood are susceptible. Bark split by a hard freeze, branches cracked under heavy snow, mower and string-trimmer wounds, and cuts from shearing are all common entry points. Plants already weakened by root rot, nematodes or drought are especially likely to be infected.
Thick, heavily sheared boxwood is especially prone to the disease. Shearing creates a dense outer shell of leaves that traps moisture and fallen leaves inside the shrub. That humid interior gives the fungus good conditions to grow and spread. Outbreaks are usually worst after harsh winters in the Mid-Atlantic, Ohio Valley and Northeast. In the Deep South the disease is generally milder but still common after ice storms.
Treating Volutella Blight
Pruning and sanitation are the main treatment, and in most home landscapes they work well. Cut out every dead and dying branch back to healthy, green-wooded tissue, ideally in dry weather so the cuts don’t spread spores. Then reach into the shrub and remove the dead leaves and debris collected in the branch crotches and around the base. Disinfect your pruners between cuts with a 10% bleach solution or 70% alcohol. Bag and remove the debris instead of leaving it under the plant.
Thinning the shrub keeps the disease from coming back. Instead of shearing the outside, reach into the canopy and remove 6- to 8-inch sections of interior branches so light and air can get into the middle. Many extension services recommend doing this during the dormant season. After a severe outbreak, a protective copper-based or other labeled fungicide applied before new growth begins in spring and repeated as the label directs can reduce new infections. Fungicides won’t save branches that are already girdled, so pruning is still the most important step.
Phytophthora Root and Crown Rot
Identifying Phytophthora Root Rot
Phytophthora root rot starts underground, so the first symptoms above ground are vague. Leaves on part or all of the plant lose their deep green color and turn light green, then yellow, tan or bronze. Leaves often curl upward along the edges and point upward against the stem. Plants grow slowly, wilt in warm afternoons even when the soil is moist, and eventually die back, either a section at a time or all at once. Like Volutella blight, the dead leaves usually stay on the plant.
To confirm root rot, look below the soil line. Scrape the bark at the base of the main stem with a knife. Healthy wood underneath is creamy white to pale green, while infected crown tissue is discolored gray to dark brown. Dig up a few feeder roots. Healthy roots are white to tan and firm, while rotted roots are brown, water-soaked and slip apart when pinched. The outer root layer often peels away from the inner core like a sleeve.
What Causes Phytophthora Root Rot
Phytophthora root rot is caused by several species of water molds. These organisms act like fungi but are actually more closely related to algae. They produce swimming spores that move through saturated soil, which is why the disease is closely tied to wet ground. According to Tennessee State University Cooperative Extension, the disease is promoted by rising soil temperature and moisture, poor drainage, over-irrigation, long periods of heavy rain, and planting too deeply. The pathogens persist in soil and potting mix as long-lived resting spores.
Boxwood roots are shallow and need oxygen, so heavy clay soils in the Southeast and Mid-Atlantic are especially risky. Planting in low spots, at the bottom of downspouts or next to lawns that get frequent irrigation keeps roots wet for long periods. New plantings are often infected through contaminated nursery stock, or because they were set deeper than they grew in the container. Clemson Extension notes that the disease is caused mainly by two water mold species in South Carolina and that these thrive in poorly drained soils.
Treating Phytophthora Root Rot
Phytophthora root rot has to be prevented, because chemical treatments rarely save a plant once symptoms appear above ground. By the time leaves change color, a large share of the root system is usually already gone. Plants with severe crown rot or more than half their canopy dead should be removed along with their roots and the soil right around them. Don’t replant boxwood in the same spot without first improving drainage.
For plants in early decline, fix the conditions that let the disease develop. Redirect downspouts, reduce irrigation, and pull back mulch so no more than 1 inch sits against the base of the stems. On heavy soils, set new boxwoods in raised beds or berms so the top of the root ball sits slightly above the surrounding grade. Fungicides containing potassium phosphite or similar active ingredients, used as a soil drench or as directed on the label, can help protect healthy plants in high-risk sites. Treat them as a preventive step, not a cure.
English Boxwood Decline
Identifying English Boxwood Decline
English boxwood decline is a slow disease that mostly affects older English boxwood, often plants 20 years old or more. It starts as one or more sections of the shrub where the leaves lose their gloss and turn light green, then orange-tan to bronze. Over several seasons, more sections fade until the plant has an uneven, patchy look with dead areas mixed among living ones. The dead leaves usually stay on the branches. In historic gardens across Virginia and Maryland, decline has slowly destroyed boxwood allées that were more than a century old.
At the base of affected branches, peeling back the bark shows a dark, sunken canker and discolored wood beneath. These cankers usually sit near the soil line or just above it and gradually girdle the stems. The roots may also be damaged, but usually not to the extent seen with Phytophthora root rot. Because the symptoms overlap with root rot, lab testing is the only reliable way to tell the two apart. The pattern over time also helps, since decline develops much more slowly.
What Causes English Boxwood Decline
Decline is a disease complex, not a single infection. Clemson Cooperative Extension describes it as a poorly understood problem involving several fungi along with cold injury, drought stress and nematodes. One canker-forming fungus is considered the main pathogen, but it generally gets a hold only on plants already stressed by poor growing conditions. Old English boxwood planted in compacted soil, shaded by trees that grew up around it, or exposed to repeated drought is the most vulnerable.
Root damage from construction, foot traffic, or lawn mowers and edgers adds to the stress. So does deep mulch, which keeps the bark at the base wet and encourages cankers. The disease is much more common in English boxwood than in other types. According to Clemson, American boxwood and littleleaf boxwood resist decline, which makes them good replacements when decline, not root rot, is the main problem.
Treating English Boxwood Decline
There are no effective chemical controls for English boxwood decline, so management depends on proper diagnosis and fixing growing conditions. Prune out dead and dying branches back to healthy wood during dry weather, and remove the dead leaves collected inside the shrub. Pull mulch back from the base and keep it to 1 inch or less. Water deeply during droughts, but don’t keep the soil wet all the time. Protect the root zone from foot traffic, construction and string trimmers.
In historic gardens, careful pruning, thinning and soil improvement can keep declining boxwood alive for many more years. In home landscapes, badly damaged plants are often best replaced. Get Clemson Extension’s boxwood disease guidance or a local diagnostic lab report first, so you know whether the problem is decline or root rot before choosing replacement plants. If decline is the cause, American or littleleaf boxwood will usually do well in the same spot.
Macrophoma Leaf Spot
Identifying Macrophoma Leaf Spot
Macrophoma leaf spot is one of the most common boxwood problems, but it is also one of the least serious. The symptom is many tiny black dots scattered over leaves that have already turned yellow or straw-colored. The dots are the fruiting bodies of the fungus, each about the size of a pinpoint. They usually appear on older leaves inside the plant or on leaves damaged by winter cold. Healthy green leaves rarely show any spotting.
Gardeners often see these black dots first and assume they have found the cause of the browning. In most cases the fungus is a secondary invader that moved into leaves already weakened by something else. The black specks look striking, but the shrub’s health depends on what caused the leaves to yellow in the first place. Look for winter injury, root rot, nematodes or Volutella blight before blaming leaf spot.
What Causes and Treats Macrophoma Leaf Spot
The fungus behind this disease is a weak pathogen that colonizes leaves after root diseases, cold injury, drought or other environmental stress. It overwinters in fallen and attached dead leaves and spreads by splashing water in spring. Dense, heavily sheared plants with poor air movement inside carry more of it. In most American landscapes it causes cosmetic damage only.
Treatment is mostly sanitation and better care. Shake or rake out the dead leaves inside the shrub, thin the canopy for better airflow, and remove spotted leaves where they’re easy to reach. Fungicides are rarely justified for this disease alone. Spend the effort finding and fixing the underlying stress. Once the plant grows strongly again, new growth usually covers the old spotted leaves within a season or two.
Nematode Damage
Identifying Nematode Problems
Plant-parasitic nematodes are microscopic roundworms that feed on boxwood roots. Their damage looks a lot like root rot and drought stress. Infected plants grow poorly, look stunted and gradually turn a dull bronze or yellow-green. They wilt quickly during hot, dry weather and recover slowly when rain returns. Over several years, branches die back and the shrub thins.
Below ground, the root system is usually small and has few fine feeder roots. Root-knot nematodes cause small swellings, or galls, on the roots, while other types cause stubby, darkened or pruned-looking roots. Nematodes are a bigger problem in the light, sandy soils of the Southeast coastal plain than in heavier northern soils. They often weaken plants enough to invite Volutella blight, decline or Macrophoma leaf spot, so a plant can have several problems at once.
What Causes and Treats Nematode Damage
Nematodes live in soil and move short distances on their own. They travel much farther on infested nursery stock, in contaminated soil on tools and equipment, and in moving water. Warm soil temperatures and sandy texture favor their reproduction, which is why South Carolina, Georgia and Florida see more damage than New England. Littleleaf boxwood types are generally considered more sensitive to root-knot nematodes than American and English boxwood, though susceptibility varies by nematode species.
No chemical treatment for nematodes is available or practical for homeowners. Management means reducing stress and replacing badly affected plants. Water deeply during drought, add organic matter to improve the soil, and avoid heavy fertilizing, which pushes top growth the weakened roots can’t support. A soil nematode test from your state’s diagnostic lab will identify which species are present and how many. When counts are high, replace boxwood with plants that aren’t hosts for those nematodes.
How to Tell Boxwood Diseases from Winter Injury and Pests
A large share of browning boxwoods brought to extension offices have no infectious disease at all. Winter injury is the most common look-alike. It appears in late winter and early spring as orange, reddish-bronze or tan leaves, usually on the south and west sides of the plant or on the side facing prevailing winds. It happens when frozen soil stops the roots from replacing water lost through the leaves on sunny, windy days. The damage is usually uniform across the exposed side, and buds underneath often leaf out normally by late spring.
Several insects and mites cause damage that is easy to mistake for disease. Boxwood leafminer, the most damaging boxwood insect in the United States, makes blister-like swellings on the undersides of leaves that turn yellowish-brown, and tiny orange larvae are visible inside if you peel a blister open. Boxwood spider mites cause fine pale stippling across the upper leaf surface, while boxwood psyllids make new leaves at the shoot tips cup inward. In parts of New York, Ohio, Michigan and the Northeast, the invasive box tree moth now causes heavy defoliation with webbing, which gardeners can confuse with blight. The webbing, chewed leaves and green-and-black caterpillars tell it apart.
Preventing Boxwood Diseases
Prevention starts when you buy and plant the boxwood. Buy plants from reputable nurseries that take part in their state’s boxwood blight cleanliness program, and inspect every plant for leaf spots, stem streaks and bare interiors before buying. Keep new plants separate from existing boxwood for about 4 weeks if you can, watching for symptoms before planting them near established beds. Choose cultivars suited to your region and site, and use blight-tolerant types where the disease is already known in your county. Avoid using cut boxwood in holiday wreaths and decorations unless you know where it came from, since that greenery has introduced blight to many home landscapes.
Good planting and care do a lot to prevent the stress-related diseases. Plant boxwood in well-drained soil at the same depth it grew in the container, or slightly higher on heavy clay. Use drip irrigation or soaker hoses instead of overhead sprinklers so the leaves stay dry. Keep mulch thin and away from the stems, and get a soil test before fertilizing, since boxwood grows best at a soil pH of about 6.5 to 7.2. Thin plants every year or two instead of shearing, to keep air and light reaching the interior.
When to Get a Laboratory Diagnosis
Because so many boxwood problems look alike, a lab diagnosis is the most reliable way to choose a treatment. Nearly every land-grant university runs a plant disease diagnostic clinic, usually through the state cooperative extension service, and fees for homeowner samples are generally modest. A good sample includes several branches showing a range of symptoms from healthy to dying, still attached to a section of stem. If root rot or nematodes are suspected, include roots and soil as well. Seal the sample in a plastic bag, keep it cool and ship it early in the week so it doesn’t sit in a warehouse over the weekend.
Get a lab diagnosis whenever you see black stem streaks or sudden leaf drop, because confirming boxwood blight early protects your other plants and your neighbors’ gardens. A diagnosis is also worth the cost before removing an expensive or historic hedge, and before replanting a site where boxwoods have died. Your county extension agent can tell you how to submit samples and which fungicides are approved and effective in your state. A few dollars spent on a correct diagnosis usually costs far less than replacing the wrong plants or spraying the wrong product.
Sources
University of Connecticut Home and Garden Education Center, “Boxwood Blight”
University of Kentucky Cooperative Extension Service, “Volutella Blight of Boxwood”
Tennessee State University Cooperative Extension, “Boxwood Phytophthora Root and Crown Rot”
Clemson Cooperative Extension Home and Garden Information Center, “Boxwood Diseases & Insect Pests”