Boxwood blight is a significant disease that poses a serious threat to boxwood plants in landscapes across the United States. This devastating fungus, caused by the pathogen Calonectria pseudonaviculata, spreads rapidly through infected plant material and water splash. The disease can quickly turn healthy, mature boxwoods into defoliated shrubs. Taking immediate, aggressive action is necessary to manage an active infection and protect surrounding plants. This article details the steps for accurate identification and provides strategies for managing and treating boxwood blight.
Recognizing the Symptoms of Boxwood Blight
Accurate identification is the initial step toward effective treatment, as symptoms can be confused with issues like winter injury or Volutella blight. The first visual signs appear on the leaves as light or dark brown spots, often surrounded by a darker border. These spots quickly coalesce, causing the entire leaf to brown or yellow before dropping off prematurely. This rapid defoliation is a damaging characteristic of the disease.
The distinguishing symptom is the presence of black streaking or cankers that form on the stems. These dark, diamond-shaped lesions develop along the green, woody tissue. During periods of high humidity, the fungus may also produce masses of white, fuzzy spores on the undersides of infected leaves or on the stem cankers. The presence of these stem lesions warrants immediate and aggressive treatment.
Immediate Physical Removal and Sanitation
Once boxwood blight is confirmed, the immediate removal of infected material is the most effective way to stop the spread of the disease. Any plant or portion of a plant showing leaf spots, defoliation, or black stem cankers should be pruned out or completely removed. When pruning, cut well below any visible signs of the stem lesions to ensure the entire infected section is removed.
After removal, all fallen leaf debris, clipped branches, and symptomatic material must be collected and bagged immediately. The spores of Calonectria pseudonaviculata are sticky and heavy, spread primarily by physical contact, water splash, and contaminated tools. Disposal requires double-bagging the material and sending it to a municipal landfill or incineration facility. Never place infected material in a compost pile, as the fungus can persist for years.
Sanitation of all tools and equipment is required to prevent mechanical transfer of spores to healthy plants. Pruning shears, shovels, and wheelbarrows should be sterilized between individual plants using a disinfectant solution. A 5% solution of sodium hypochlorite (1:10 dilution of household bleach) or 70% rubbing alcohol are effective options. Allowing the disinfectant to remain in contact with the tool surfaces for at least ten minutes ensures that any adhering fungal spores are destroyed.
Utilizing Fungicides for Chemical Control
Fungicide application is necessary for managing boxwood blight, particularly in landscapes where the disease is established or risk of infection is high. These chemical treatments are strictly protectant, coating healthy foliage and stems to prevent new infections. Fungicides cannot cure an infected plant, so they must be used in conjunction with aggressive sanitation and removal of diseased tissue.
For homeowners, products containing chlorothalonil are recommended as a broad-spectrum preventative measure. Professional applicators may rotate this with other active ingredients, such as tebuconazole or fludioxonil, to manage potential fungicide resistance. Application timing depends on environmental conditions, as the fungus thrives when temperatures are above 60°F and moisture is present.
A regular application schedule, typically every 7 to 14 days, is required throughout the growing season. Thorough coverage of all foliage, including the inner canopy and lower leaves, is necessary because the fungicide must physically be present to block spore germination. A consistent, multi-year fungicide program may be required to protect susceptible boxwoods from re-infection, as the fungus can persist in the soil.
Long-Term Strategies for Management and Prevention
Implementing long-term cultural strategies is essential for reducing the risk of boxwood blight recurrence. Poor air circulation keeps the foliage wet for extended periods, creating an ideal environment for spore germination. Pruning boxwoods to create a more open canopy and spacing new plantings adequately helps the leaves dry more quickly after rain or irrigation.
Altering watering practices is another preventative measure, as the sticky spores are easily spread by splashing water. Switching from overhead sprinklers to drip irrigation or soaker hoses keeps the foliage dry and minimizes the dispersal of fungal spores from the soil surface. Applying a layer of mulch beneath the boxwoods covers any fallen, infected leaf debris and reduces the chance of splashing spores back onto the healthy plant tissue.
When replacing plants that have died from boxwood blight, selecting cultivars that exhibit higher resistance is a forward-thinking management decision. While no boxwood variety is completely immune, certain cultivars of Buxus microphylla, such as ‘Green Beauty’, and newly introduced hybrids like ‘NewGen Independence’ show significantly improved tolerance compared to highly susceptible varieties like English or American boxwood. Choosing these resistant plants, combined with proper sanitation and cultural practices, is the best strategy for maintaining boxwoods in the landscape over time.