Boxwoods are valued in landscapes for their dense, evergreen foliage and versatility in garden designs. When these shrubs develop brown spots, it signals a potential decline in health. This browning is not a single ailment but a symptom resulting from a range of issues, including fungal infections, insect infestations, and environmental stressors. Understanding the specific cause is necessary to implement an effective treatment plan.
Differentiating the Causes of Brown Spots
Identifying the source of brown spots requires observing visual clues related to fungal disease, pests, or environmental damage. Fungal infections generally present as localized lesions that spread rapidly, often starting low on the plant in high-humidity areas. Boxwood Blight (Calonectria pseudonaviculata) forms dark brown or black spots on the leaves, leading to rapid defoliation and black streaks or cankers on the stems. Volutella Blight (Pseudonectria buxi) often results in salmon or pink spore masses on the undersides of leaves and branches that die back, with dead leaves remaining attached.
Pest damage creates characteristic feeding marks rather than fungal lesions. Boxwood Mites (Eurytetranychus buxi) cause fine, yellowish-white stippling across the foliage by feeding on the leaf surface. Boxwood Leafminer (Monarthropalpus buxi) larvae feed internally, creating blister-like swellings or mines between the leaf surfaces, which eventually turn yellow and then brown.
Environmental stresses usually result in uniform damage across exposed areas. Winter injury, or sunscald, manifests as a reddish-brown or bronze discoloration, particularly on foliage facing winter sun and wind. Improper watering or poor drainage can also cause widespread browning and decline, sometimes indicating root rot.
Management of Severe Fungal Disease
Boxwood Blight, caused by the fungus Calonectria pseudonaviculata, is a serious concern. This disease spreads quickly through water splash and contaminated tools, leading to rapid defoliation and death of susceptible boxwood cultivars. Management for confirmed or suspected Boxwood Blight requires immediate sanitation to contain the pathogen and prevent its spread.
Infected plant material, including fallen leaves and symptomatic shrubs, should be removed entirely and sealed in bags for disposal. This material must never be composted or chipped, as the fungus can persist for years via survival spores (microsclerotia). Pruning tools used on infected plants must be disinfected thoroughly between cuts and between plants using diluted bleach or a quaternary ammonium product to prevent mechanical transmission.
A preventative fungicide program is necessary for surrounding plants and tolerant cultivars where Boxwood Blight has been confirmed. Fungicides containing active ingredients such as propiconazole, tebuconazole, or chlorothalonil are used in rotation to protect new growth during periods of high moisture. This chemical approach is strictly preventative, as no fungicide can cure a plant already heavily infected with the disease.
Addressing Pests and Environmental Stress
Treatment for brown spots caused by pests must be targeted to the specific organism. Boxwood Leafminer larvae tunnel inside the blistered leaves throughout the year. Control is achieved by applying systemic insecticides, such as products containing imidacloprid or dinotefuran, as a soil drench in the spring. The plant absorbs the chemical, poisoning the larvae as they feed internally.
Boxwood Mites cause stippling damage and are arachnids, meaning they are not effectively controlled by many insecticides. These mites prefer cooler weather and build up populations quickly, sometimes having up to eight generations per year. Targeted treatments include horticultural oils or insecticidal soaps applied to the undersides of the leaves in the late winter or early spring to smother the overwintering eggs and active mites.
The uniform reddish-brown bronzing or browning on the exposed side of a shrub is environmental damage known as winter burn or sunscald. This occurs when leaves lose moisture through transpiration on sunny winter days, but frozen ground prevents the roots from replacing the lost water. Management involves pruning out the damaged foliage in the spring and protecting the plant from harsh winds with burlap screens.
Cultural Practices for Long-Term Health
Long-term health and resistance to brown spots depend on optimizing the boxwood’s growing environment. Proper site selection ensures planting in well-drained soil, as poor drainage increases the risk of root rot and fungal diseases. Afternoon shade is beneficial, and adequate air circulation helps keep the foliage dry, discouraging fungal spores.
Pruning should focus on thinning the dense canopy rather than shearing the outer surface. Thinning allows light and air to penetrate the interior of the shrub, which reduces humidity. When watering, apply deep, infrequent irrigation directly to the root zone, avoiding overhead sprinkling that wets the foliage.
Selecting species and cultivars with known resistance to Calonectria pseudonaviculata is an effective preventative measure. Cultivars derived from Asian species, such as Buxus microphylla and Buxus sinica, typically show greater resistance than European species like Buxus sempervirens.
Resistant Cultivars
Specific varieties have demonstrated improved tolerance to Boxwood Blight, offering a sustainable choice for new plantings:
- ‘Dee Runk’
- ‘Green Beauty’
- NewGen series (‘Independence’ and ‘Freedom’)