Pathogen
Boxwood blight is caused by the fungus Calonectria pseudonaviculata (formerly Cylindrocladium pseudonaviculatum or Cylindrocladium buxicola).
History and Hosts
Boxwood blight was first reported in the United Kingdom in 1994, and has since become established in Europe, Asia, and New Zealand. It was first confirmed in North America in 2011 in North Carolina and Connecticut; by 2019, it had been identified in 28 states and 3 Canadian provinces.
Boxwood blight affects all species of Buxus. Sweet box (Sarcococca hookeriana), Allegheny pachysandra/spurge (Pachysandra procumbens), and Japanese pachysandra/spurge (Pachysandra terminalis) are also susceptible to the disease.
Symptoms, Signs, and Life Cycle
Symptoms tend to begin in the interior of the canopy or on lower leaves. Tan or brown foliar lesions with dark borders appear on leaves. As the lesions expand, they may coalesce and entire leaves may turn brown. Defoliation is typically dramatic and rapid. Black lesions or streaking may be observed on stems, and twig dieback may occur. Under humid conditions, white sporulation may be observed on the undersides of infected leaves and in stem lesions. Roots are unaffected by boxwood blight.
The pathogen may be introduced into the landscape on asymptomatic plants. Fungal spores are spread by wind, splashing water, and human or animal activity. High humidity and temperatures of 64°F–80°F (18°C–27°C) favor disease development. The fungus survives the winter in infected leaves and twigs.
Management
Start with healthy plants. Ideally, new boxwoods should be isolated without fungicide treatment for one month prior to planting and examined regularly for symptom development. Plant boxwoods in well-drained soil in areas where they will receive some shade in the afternoon. Ensure adequate space around each plant to facilitate good air circulation. Provide supplemental irrigation during periods of drought. Avoid wetting foliage when watering. Do not work with plants when they are wet.
If boxwood blight is confirmed, remove infected plants. Rake up and remove fallen leaves and twigs as thoroughly as possible. Plant debris should be burned, buried at least 2 feet underground, or disposed of in the trash. Never compost infected plants or plant debris. Decontaminate hand tools with 70% alcohol and wash hands thoroughly. In early spring, spread 2 inches of mulch underneath any remaining boxwoods.
Holiday wreaths and other decorations may contain infected boxwood sprigs. Dispose of these decorations in the trash or avoid them altogether.
Consider planting blight-tolerant cultivars such as those in the NewGen or Better Boxwood collections.
Consider replacing boxwoods with similar evergreen shrubs. Alternatives to boxwood include:
- Small varieties of inkberry holly (Ilex glabra), such as ‘Gembox’ or ‘Strongbox’
- Small varieties of Japanese holly (Ilex crenata) such as ‘Helleri’ or ‘Compacta’
- Small varieties of arborvitae (Thuja occidentalis) such as ‘Anna’s Magic Ball’
- Yew (Taxus x media) ‘Stonehenge Dark Druid’
- Junipers (Juniperus sp.), various cultivars
Fungicide applications will not cure boxwood blight, but they can help protect healthy plants from becoming infected. Apply fungicides when the temperature is above 60°F (16°C) and rain is expected. The best option is to tank mix a systemic fungicide with a protectant fungicide or use a product that combines a systemic and a protectant. Fungicides labeled for boxwood blight management include chlorothalonil, copper, cyprodinil, fludioxonil, metconazole, propiconazole, pydiflumetofen, and thiophanate-methyl. Homeowners should consult a professional landscaper about this option.