Boxwood (Buxus sp.)-Box Blight

Cause The fungus Calonectria pseudonaviculata (synonyms Cylindrocladium pseudonaviculatum or C. buxicola) was found for the first time late in 2011 in only three Oregon nurseries and four in British Columbia. Eradication efforts were initiated in Oregon and, at this writing, were successful. The disease has been found in numerous nurseries and landscapes in Eastern North America. It has been a problem for many years in Europe and New Zealand. Two genetic groups of the fungus have been documented (G1 and G2), but it is unknown what groups are present in North America. G2 is important, as it is resistant to many fungicides. The disease is especially severe where plants are sheared tightly and/or planted in hedges.

Hosts include many Buxus spp. and cultivars, Pachysandra, and Sarcococca. The three main boxwood species-Buxus sempervirens, B. microphylla (littleleaf boxwood), and B. sinica var. insularis (Korean boxwood)-are known hosts. Buxus colchica and B. microphylla var. japonica (Japanese boxwood) are also hosts. There are no resistant cultivars; however, tests in North Carolina have found B. sinica var. insularis 'Nana' and B. microphylla var. japonica 'Green Beauty' to be tolerant.

The fungus can survive as mycelium on fallen leaves for up to 5 years and can produce spores in a sticky, slimy matrix when environmental conditions become suitable. Microsclerotia are also formed within infected tissue and may help survival in the soil. Infection can occur very quickly in warm (64 to 77°F), wet conditions. High humidity levels or free water are needed in order for infection to occur. No infections occur below 43°F. Only young leaves became infected at 54°F but need to be wet for 48 hours. The fungus can penetrate the leaf through the cuticle or enter through leaf stomata. New lesions and spores can be produced within a single week under favorable conditions. Short-distance dispersal occurs through water or rain splash (especially wind-driven rain) and human activities such as pruning. Long-distance dispersal can occur through the transport of infected nursery stock or infected plant debris. Spores are unlikely to travel long distances by wind alone.

Volutella buxi, Volutella blight, often colonizes diseased stems associated with this disease.

Symptoms Dark or light brown spots on leaves, often in a circular or zonate pattern, black streaks or cankers on stems, straw- to bronze-colored blighted foliage, and defoliation. Blighting and defoliation can occur suddenly, with complete leaf loss in severe cases. Leaf spots can grow together to cover the entire leaf surface. Downy white fungal growth may be seen on the underside of leaves. Tufts of white spores (sporodochia) may be seen along lesions. Heavily infected plants may lose leaves from the bottom of the canopy first leaving stems with a "lion tail" look. Propagation flats may have more blight toward the center than the margin of the flat.

Infected stems can have multiple diagnostic dark brown or black lesions, either linear or diamond-shaped. The black streaks found on stems progress from the bottom of the plant up. New growth continues to develop on healthy stems, and often the root systems remain healthy and intact. Young seedlings, however, may die from this disease.

Spores of the pathogen can sometimes be seen on the underside of the infected leaves. Under high humidity, white fuzzy masses comprised of large numbers of spores are sometimes visible to the naked eye or with a hand lens on infected stem and leaf tissue.

Scouting Inspect the lower and interior canopy for symptoms of the disease. It may not be found unless the canopy is parted and interior leaves and stems are examined for spots and streaks.

Cultural control To limit spread and movement of the pathogen, all confirmed infected plants should be destroyed.

• Isolate and contain new shipments of boxwood for a 30-day observation period. Locate the isolation area well away from existing blocks or outgoing shipments, and scout weekly.
• Break up large blocks of Buxus with non-host species to limit exposure and potential losses due to this disease.
• Do not reuse or recycle or compost infected plants, media, or containers.
• Scout all plants on a weekly basis, especially during warm, wet weather.
• Remove and destroy dead leaves and plant debris from within and underneath plants. Flaming was only partially successful in reducing inoculum from under contaminated plants in Virginia.
• Disinfect pruning equipment, especially between blocks of plants.
• Avoid overhead irrigation.
• Space and prune plants for good air circulation.
• Grow tolerant species or cultivars.
• Under severe landscape situations in Europe, Japanese holly (Ilex crenata) has been recommended as an alternative plant.

Chemical control Combine fungicide use with intensive scouting and cultural management. Alternate or tank-mix products from different groups that have different modes of action. Test for phytotoxicity on a small group of plants or cultivars before widespread use, especially if surfactants are included in the mix. High rates of Group 3 materials may cause some marginal burning of leaves.

• Affirm WDG at 0.25 to 0.5 lb/100 gal water. Boxwood is NOT specifically mentioned so must be tank-mixed with another fungicide. Group 19 fungicide. 4-hr reentry.
• Cleary's 3336 WP at 12 to 16 oz/100 gal water plus another fungicide from another fungicide group. Boxwood is NOT specifically mentioned on the label but if it is not phytotoxic may legally be used. Inconsistent control. Group 1 fungicide. 12-hr reentry.
• Compass O 50 WDG at 1 to 4 oz/100 gal water. Do not use organosilicate additives. Boxwood is NOT specifically mentioned on the label but if it is not phytotoxic may legally be used. Group 11 fungicide. 12-hr reentry.
• Concert at 26 to 35 fl oz/100 gal water. Boxwood is NOT on the label but if it is not phytotoxic may legally be used. Landscape use only. A Group 3 and M5 fungicide. 12-hr reentry.
• Cygnus 50 WG at 1.6 to 3.2 oz/100 gal water plus a non-organosilicone spreader-sticker. Boxwood is NOT on the label but if it is not phytotoxic may legally be used. Group 11 fungicide. 12-hr reentry.
• Daconil Weather Stik at 1.4 pints/100 gal water. Boxwood is NOT on the label but if it is not phytotoxic may legally be used. Do not use with a surfactant. Group M5 fungicide. 12-hr reentry.
• Heritage at 1 to 4 oz/100 gal water plus a non-silicone-based wetter sticker. Group 11 fungicide. 4-hr reentry.
• Insignia at 4 to 16 oz/100 gal water. Do not use with organosilicate-based adjuvants. Mediocre control. Group 11 fungicide. 12-hr reentry.
• Mancozeb-based products. Group M3 fungicides. 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.
• Medallion at 1 to 4 oz/100 gal water. Boxwood is NOT on the label but if it is not phytotoxic may legally be used. Use with oils or adjuvants may damage plant. Group 9 fungicide. 12-hr reentry.
• Pageant at 8 to 12 oz/100 gal water. Do not use more than two (2) consecutive applications before switching to a different fungicide group. Mediocre control. Group 7 and 11 fungicide. 12-hr reentry.
• Palladium at 2 to 4 oz/100 gal water. Boxwood is NOT on the label but if it is not phytotoxic may legally be used. Avoid excessive runoff to small plants, which may result in stunting and/or chlorosis. A Group 9 and 12 fungicide premix. 12-hr reentry.
• Spectro 90 WDG at 1 to 2 lb/100 gal water. Boxwood is NOT on the label but if it is not phytotoxic may legally be used. Group 1 and M5 fungicide. 12-hr reentry.
• Torque at 4 to 10 fl oz/100 gal water. Boxwood is NOT on the label but if it is not phytotoxic may legally be used. Group 3 fungicide. 12-hr reentry.

References Henricot, B., C. Gorton, G. Denton, and J. Denton. 2008. Studies on the control of Cylindrocladium buxicola using fungicides and host resistance. Plant Disease 92:1273-1279.

Ivors, K. L., Lacey, L. W., Milks, D. C., Douglas, S. M., Inman, M. K., Marra, R. E., and LaMondia, J. A. 2012. First Report of Boxwood Blight Caused by Cylindrocladium pseudonaviculatum in the United States. Plant Disease, 96:1070.

LaMondia, J. A., Li, D. W., Marra, R. E., and Douglas, S. M. 2012. First Report of Cylindrocladium pseudonaviculatum Causing Leaf Spot of Pachysandra terminalis. Plant Disease, 96:1069.