By Chris Stroupe
Piedmont Master Gardener
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. My colleague Yvette Graham, 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. Here are some of the common diseases, pests and environmental issues facing boxwoods in Virginia, as well as tips for how to diagnose, treat and prevent them. You will find more details and many helpful photos on the website for Virginia Cooperative Extension’s Boxwood Blight Task Force at https://ext.vt.edu/agriculture/commercial-horticulture/boxwood-blight.html.
Diagnosis The first sign of this devastating fungal disease is brown spots, ringed with darker brown areas, on the leaves. The spots are followed by sudden defoliation and black streaks on stems. Fluffy white clusters of spores may appear, but not always.
Treatment There is no cure for boxwood blight. To prevent it from spreading, remove infected plants 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 two inches of mulch under remaining plants to help prevent spores from splashing up onto foliage. Wash dirt off tools, then disinfect them with bleach, 70 percent ethanol, or hydrogen peroxide. Wash clothes, gloves and footwear with soap and hot water. Plants in the same family as boxwoods can be hosts for boxwood blight, including Japanese spurge (Pachysandra terminalis), Allegheny spurge (P. procumbens), fragrant spurge (P. axillaris) and sweetbox (Sarcococca species). Avoid growing them near your boxwoods.
Prevention Healthy boxwoods are less susceptible to blight. To keep them healthy, have your soil tested every two to three years and follow the amendment recommendations that come from the lab. Improve drainage by amending heavy clay soil with compost. Prune boxwoods to promote airflow, keep leaves dry, and minimize fungal growth. Do not work on plants when their leaves are wet, which can spread spores. Avoid overhead irrigation. Mulch under the plants to avoid splashing spores onto leaves and to reduce the need for watering. Rake up any fallen leaves and branches. Finally, sanitize tools, clothes and footwear after working on boxwoods.
Diagnosis Dieback of branches without leaf drop, randomly distributed around the plant, is the most notable symptom of this fungus. 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.
Treatment Like blight, there is no treatment for Colletotrichum. The only surefire way to stop it from spreading is to remove and dispose of infected plants as described above. Again, sanitize clothes and tools.
Prevention At present, the only recommended preventative measures are general cultural practices like those described above.
Diagnosis Volutella also causes dieback without leaf drop and can be distinguished from Colletotrichum by looking at the fruiting bodies: Volutella’s are orange or pink, 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 Keep plants healthy and dry. If there’s an infected plant nearby, consider a preventative fungicide spray as described on page 4-9 in Virginia Cooperative Extension’s 2023 Pest Management Guide. (Search online for VCE Publication 456-018.)
Root Rot (Phytophthora and Pythium)
Diagnosis Root damage first manifests as subtle yellowing or bronzing of a section of leaves. It then progresses to stunting and eventually sectional dieback. Leaves 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.
Treatment VCE recommends potassium salts of phosphorous, applied as a soil drench or foliar spray. (Refer to Table 4.2 in the VCE Pest Management Guide.)
Prevention Prevent root rots by not over-watering. When planting boxwoods, avoid low-lying areas and poorly drained soil, such as heavy clay. Amend clay with compost to improve drainage; instead of adding compost to the hole where the boxwood will be planted, work it into the soil around the planting hole.
Diagnosis Psyllids cause leaf cupping near the ends of branches. 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. Unless there is severe infestation, preventative insecticide use is not recommended. Consult VCE’s Pest Management Guide (page 4-52) for more information.
Prevention Halt the pest’s life cycle by removing affected branches.
Diagnosis Look for small yellowish blisters in leaves, where leaf-miner larvae live between the upper and lower tissue layers.
Treatment Remove branches with affected leaves and dispose of the waste as described previously. For severe infestations, consult VCE’s Pest Management Guide (page 4-49) for more information on insecticide treatments.
Prevention Prevent future infestations by removing affected branches.
Mites (Boxwood Mites and Spider Mites)
Diagnosis Tiny pale dots, termed “stipples,” appear on leaves. Despite the name, spider mite webbing is apparent only during heavy infestations. Heavy feeding can cause leaf yellowing and death.
Treatment Knock mites off plants with a strong stream of water. Keep plants well-pruned. Conventional miticides are not recommended because they can kill beneficial mites that prey on harmful mites. Some garden stores and catalogs sell these beneficial mites.
Prevention Horticultural oils, applied early in the spring, can kill mite eggs—but can also harm predatory mites and insects. Use them only if there was a severe infestation the previous year. Consult page 4-51 in VCE’s Pest Management Guide for more information.
Diagnosis These microscopic worms attack roots, so nematode damage looks like root disease: leaf yellowing or bronzing, stunting and dieback. Root-knot nematodes may cause nodules to form on roots, but in general, nematode damage is hard to distinguish from root disease without a microscope. Virginia Tech’s Plant Disease Clinic (see below) can check boxwoods for harmful nematodes.
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 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–but extremely susceptible to blight.
Non-living causes can damage boxwoods as badly as diseases and pests. These include:
Water—Too Much or Too Little. Excessive watering can promote root rot, while drought stress can cause leaves to turn yellow and die, especially on newly planted boxwoods. One inch of rain or irrigation per week is recommended for new boxwoods. This works out to five gallons in a three-foot diameter circle. Established boxwoods do not need irrigation, except in times of extreme heat and/or drought.
Salt. Salt used for melting ice on roads and sidewalks can desiccate plants and cause leaf yellowing and/or death, both through direct contact with leaves and by leaching into the soil.
Winter Cold. If leaves turn dark red or bronze, particularly in the spring, winter damage is the likely cause. Simply cut off the affected parts; the rest of the plant will grow and fill in the missing area. Do not fertilize plants in late summer or fall. This may spur new growth that is not winter-hardy.
Virginia Tech’s Plant Disease Clinic Can Help
Virginia Tech’s Plant Disease Clinic is a fantastic resource for diagnosing boxwood problems. However, it is critical to collect and submit a proper sample. Follow this step-by-step guide at spes.vt.edu/affiliated/plant-disease-clinic/submitting-samples.html. The cost is $35 per sample.
In the Albemarle-Charlottesville area, bring samples to the Virginia Cooperative Extension office at 460 Stagecoach Rd. (the back entrance of the Albemarle County Office Building off 5th Street Extended). It’s best to bring in samples on a Monday to ensure they reach the clinic before the weekend. For assistance, call the VCE office at (434) 872-4580 or the Horticultural Help Desk at 434-872-4583.