Severe leaf necrosis and defoliation caused by boxwood blight. M.A. Hansen, VT, Bugwood.org
Leaf symptoms caused by boxwood blight. Y. Li, CAES, Bugwood.org
White sporulation of boxwood blight fungus on leaves.
D.L. Clement, Univ. of Maryland, Bugwood.org
Black streaking on stems and leaf necrosis of boxwood blight. M.A. Hansen, VT, Bugwood.org
Boxwood blight is a fungal disease that affects plants in the boxwood family (Buxaceae). It is caused by the fungal pathogen Calonectria pseudonaviculata (syn. Cylindrocladium pseudonaviculatum and C. buxicola), which infects the aboveground parts of susceptible plants. The pathogen causes rapid defoliation of leaves and dieback of stems. Repeated defoliation can kill young plants. Defoliation of larger specimens weakens plants, making them susceptible to infection by other pathogens which can lead to plant death. Hosts include boxwood (Buxus), pachysandra (Pachysandra), and sweet box (Sarcococca). The first confirmed occurrence of boxwood blight in the United States was in 2011. Since then, it has been found in many Eastern and Midwestern states, including Missouri and Illinois.
Symptoms and Diagnosis
Symptoms first appear on the leaves as tan or brown spots with a dark edge and often with a yellow halo. As the spots enlarge, entire sections of a leaf will turn brown until the entire leaf is affected and drops. These lesions are usually visible on both sides of the leaf. If conditions are right, masses of fuzzy, white-colored spores develop on the underside of infected leaves. Because the progression from leaf spots to defoliation happens so rapidly, the leaf spot stage may go unnoticed. The stem cankers that develop appear as distinct dark brown or black streaks on the green stems. Unlike other problems of boxwood, defoliation progresses from the base and moves upward. The absence of lower leaves is the easiest way of distinguishing boxwood blight from other common problems of boxwood. Defoliated stems may produce new shoots later in the season, but the plant will not recover health and vigor.
Boxwood blight was probably introduced into this country and spread via infected nursery stock. Spores of Calonectria pseudonaviculata are spread over short distances (e.g., the distance between two plants) by wind-driven rain or splashing water. Transmission occurs when air temperatures are greater than 60 degrees F, the humidity is high, and the foliage is wet. The spores are sticky and can infest soil, plant debris, equipment, clothing, and animals. Fallen leaves may serve as a ready source of the disease, spreading it as the leaves are moved about blown by the wind or leaf blowers, or dumped in the compost pile. The pathogen is extremely resilient, capable of surviving for years on buried leaf debris.
Integrated Pest Management Strategies
1. Avoid introducing the disease into your garden. Inspect plants for symptoms of boxwood blight including resistant cultivars. Gently open the inner canopy and check for leaf spots or black streaks on the stems. Even if free of symptoms, consider quarantining new plants for at least one month and monitoring for symptoms before transplanting. Also, be sure to check holiday greenery that includes boxwood cuttings.
2. Plant Resistant species and cultivars: Note - Resistant does not mean immune. Resistant plants can still carry the pathogen and should be inspected carefully before purchasing and planting. North Carolina State University has identified several cultivars that have low susceptibility to boxwood blight, but a fully disease-resistant species or cultivar has not yet been identified.
Boxwood Cultivars with Low Susceptibility to Boxwood Blight
Buxus microphylla 'Golden Dream'
B. microphylla 'Wedding Ring'
B. microphylla var. japonica 'Green Beauty'
B. sinica var. insularis 'Nana'
Results of the study can be found here: 2012 study, 2013 study
3. Practice careful sanitation. Avoid working in plants when they are wet. After possible exposure to boxwood blight, sanitize clothing, shoes, and tools. Pruning tools should be dipped in a sanitizer for a minimum of 5 minutes between sites or between blocks of plants. Sanitizers available for home use that are effective for the boxwood blight pathogen include a solution of household bleach (1 part bleach: 14 parts water) or Lysol Concentrate Disinfectant (at label rates). Oil tools after use to prevent corrosion. Zerotol 2.0 (hydrogen dioxide + peroxyacetic acid) is a sanitizer that is available for use by commercial growers.
4. Remove and destroy infected plants. Also remove all leaf debris, which can harbor the pathogen. Do not compost. Dispose in the landfill. Monitor remaining plants regularly for symptoms of disease so that new infections can be managed promptly.
5. Chemical controls. Fungicides cannot cure this disease, so do not use fungicides on plants that have already been diagnosed; instead, remove and destroy them. However, some fungicides have been effective as a preventive measure: chlorothalonil at 10 to 21-day intervals (refer to label) as long as conditions are conducive for disease transmission (air temperatures above 60 degrees F, high humidity, wet foliage).
6. Suspected samples of boxwood blight should be sent to a lab for verification. The sample should show all of the symptoms that are present. Place samples on a dry paper towel in a zip-type plastic bag. This bag should be sealed, and then placed inside a second zip-type plastic bag which should also sealed.
University of Missouri Plant Diagnostic Clinic
University of Illinois Plant Clinic
Strategies 1, 2, 3 and 4 are strictly organic approaches.