Cause Pseudonectria rousseliana (asexual: Volutella buxi) , a fungus that survives in affected branches and leaves. Boxwood is more susceptible to infection when subjected to stress from improper watering, winter injury or wounds. Optimal temperature for infection and disease development is 20 to 25°C when relative humidity is 85% or greater. Infection often occurs at the bases of small dead shoots, or in crotches where leaves have accumulated, or through pruning wounds. Most common cultivars such as 'Green Velvet,' 'Green Gem,' 'Green Mountain' and 'Chicagoland Green' are susceptible. 'Pincushion' has exhibited less disease in inoculation tests and at nurseries in Ontario, Canada, but no cultivars are immune.
Symptoms Branches die back and leaves turn from green to light green-yellow and finally to shades of tan. Affected leaves turn up and lie close to the stem. These symptoms are similar to winter injury, except that numerous, small, salmon-pink fungal spore-producing structures (sporodochia) appear on lower surfaces of affected leaves and branches.
- Prune out and burn dead branches.
- Remove dead leaves from crotches.
- Minimize wounding.
- Maintain good air circulation and drainage.
- Avoid high humidity (>85%) for long periods.
Chemical control Spray fungicides in spring, before growth starts and again in late spring, summer, and fall.
- Banner MAXX at 2 to 4 fl oz/100 gal water. 12-hr reentry.
- Bordeaux. O
- BSP Lime sulfur at 2 gal/100 gal water after leaves are cleaned up. Use at 2 quarts/100 gal water during the season. 48-hr reentry. H O
- CuPRO 2005 T/N/O at 0.75 to 3 lb/A (or 1 to 3 Tbsp/1000 sq ft) dormant or at 0.75 to 2 lb/A when new growth is present. 24-hr reentry.
- Mancozeb-based products. 24-hr reentry.
- Fore 80 WP at 1.5 lb/100 gal water plus a spreader-sticker.
- Pentathlon DF at 1 to 2 lb/A or per 100 gal water.
- Protect DF at 1 to 2 lb/100 gal water plus 2 to 4 oz spreader-sticker.
- Phyton 27 at 1.5 to 2.5 fl oz/10 gal water. 24-hr reentry.
Reference Shi, F. and Hsiang, T. 2009. Management of leaf and stem diseases of Buxus microphylla (boxwood). Canadian Journal of Plant Pathology. 31(4) (Abstract).