Boxwood plants are a popular choice for many landscapes, but they can sometimes develop brown spots on their foliage. These spots are often a symptom of underlying issues that can be addressed with proper care. Understanding the reasons behind these brown patches is the first step toward restoring your boxwood’s health.
Understanding the Causes
Brown spots on boxwoods result from either fungal diseases or environmental stressors. Fungal pathogens are common culprits, with Boxwood Blight (Calonectria pseudonaviculata) being an aggressive disease that leads to rapid browning and defoliation. Volutella Blight (Volutella buxi) often affects stressed plants, and Macrophoma Leaf Spot (Macrophoma candollei) can also cause discoloration.
Environmental factors also play a substantial role. Winter burn occurs when cold winds and sun exposure cause desiccation, leading to foliage browning, particularly on exposed sides. Drought stress manifests as browning foliage due to insufficient water, especially in newly planted boxwoods or during prolonged dry periods. Poor drainage can lead to root rot (Phytophthora spp.), restricting the plant’s ability to absorb water and nutrients, causing leaves to wilt and turn brown. Nutrient deficiencies, such as a lack of nitrogen, can result in yellowing or browning of lower leaves.
Identifying the Specific Problem
Distinguishing between causes requires careful observation of visual cues. Boxwood Blight presents as dark leaf spots merging into brown blotches, often with narrow black streaks (cankers) on green stems. Infected leaves drop rapidly, sometimes within a week. Volutella Blight causes leaves to turn light green-yellow, then bronze or tan, with affected leaves often turning upward and remaining attached. Under humid conditions, small, salmon-pink fungal spore-producing structures may be visible on the undersides of leaves and stems.
Macrophoma Leaf Spot is identifiable by tiny black, raised fruiting bodies on dead or dying straw-colored leaves. When environmental stress is the culprit, the browning pattern helps diagnosis. Winter burn appears uniformly on the side most exposed to wind and sun. Drought stress results in wilting and a crispy texture; leaves affected by drought remain firmly attached. Root rot, caused by soggy soil conditions, leads to wilting, discoloration, and eventual dieback, with roots appearing brown and water-soaked.
Effective Treatment Strategies
Once the problem is identified, targeted treatment can begin. For fungal diseases, prune out affected branches and dispose of infected material to prevent further spread. Sterilize pruning tools between cuts to avoid contaminating healthy plant tissue. For Boxwood Blight, fungicides are preventative or suppressive and do not cure existing infections; severely infected plants may need removal.
For Volutella Blight, copper-based fungicides can be applied preventatively in spring before new growth and after pruning. Macrophoma Leaf Spot does not require fungicide treatment; proper pruning and thinning to improve air circulation are sufficient for management.
Addressing environmental stressors involves adjusting cultural practices. For drought stress, deep and infrequent watering encourages robust root development. Improving soil drainage prevents root rot. Mulching around the base helps retain soil moisture and moderates soil temperature. Providing winter protection like burlap wraps shields boxwoods from harsh conditions.
Preventing Future Brown Spots
Proactive measures maintain healthy boxwoods and prevent recurrence. Proper planting ensures well-drained soil and avoids planting too deeply. Adequate spacing promotes good air circulation, which reduces humidity around foliage and deters fungal growth. Regular, balanced fertilization supports vigorous growth; a soil test helps determine nutrient needs and soil pH (6.5-7.0 for boxwoods).
Pruning should thin dense canopies to enhance airflow and remove dead or weak branches that could serve as entry points for pathogens. Routine inspection allows early detection of potential issues, enabling prompt intervention. Practicing good sanitation, such as cleaning up fallen leaves and plant debris, removes fungal spores that can overwinter and cause new infections.