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 but have not been completely successful. The disease has been found in seven Oregon nurseries (Wasington and Marion counties) but also in landscapes in Multnomah and Coos counties. It has also been found in numerous nurseries and landscapes in Eastern North America. It has been a problem for many years in Europe and New Zealand. Calonectria henricotiea is an important species found only in Europe that is resistant to many fungicides. The disease is especially severe where plants are sheared tightly and/or planted in hedges. Due to the history of this disease, its explosive life cycle, and low level in tolerant plants, it is expected to be an ever-increasing problem throughout the PNW.
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 although they still carry the fungus.
The fungus can survive as mycelia 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, released into the soil when the tissues breaks down, and subsequently initiate new infections when splashed onto foliage. Infection can occur very quickly in warm (64°F 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 in-fected 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. Fall and winter are the best times of the year to find the disease especially after significant rainfall.
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 just after warm, wet weather. Limit activity in plantings during rainy and/or 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.
- Do not grow susceptible plants on ground that has been positive for this disease.
- Disinfect pruning equipment, especially between blocks of plants. Labeled rates of bleach, Lysol, ZeroTol, Oxidate, and X3 have been effective.
- Avoid overhead irrigation.
- Space and prune (thin) plants for good air circulation. Thinning is best done in the fall using sharp bypass pruners.
- Grow tolerant species or cultivars but exercise caution to avoid inadvertently bringing in or shipping out infected plants.
- Grow plants in well-drained media or soil with a pH between 6.8 and 7.5. Light shade of 20% can also reduce injury from summer and winter extremes.
- 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. Curative applications will not effectively manage this disease even with the best products. Alternate or tank-mix products from different groups that have different modes of action. Use of tank-mixes or pre-packed mixes are preferred. 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. Good coverage is essential but difficult to achieve with dense foliage. Dilute high-volume applications will work better than low-volume concentrate applications.
Weather data combined with disease modeling has indicated that many infections can occur on young growth in spring during rainy weather. The potential for fall infections of older leaves is also possible when rain returns in the autumn. Focus applications in the spring beginning at bud break and continuing with wet weather. Monitor weather in the fall and apply a fungicide if warm and wet weather is forecasted.
- 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. Not as effective alone. 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 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 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 may legally be used. Good control. Landscape use only. Might leave a slight residue on leaves and/or slight plant growth regulation effect. Group 3 + 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 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 may legally be used. Do not use with a surfactant. Good control. Group M5 fungicide. 12-hr reentry.
- Eagle 20 EW at 6 to 12 fl oz/100 gal water. Boxwood is NOT on the label but if it is not phytotoxic may legally be used. Group 3 fungicide. 24-hr reentry.
- Emblem at 2 to 4 fl 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 cause plant damage. Group 12 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 to fair 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 1.5 lb/A or 1 to 2 lb/100 gal water plus a spreader-sticker.
- Protect DF at 1 to 2 lb/100 gal water plus 2 to 4 oz spreader-sticker.
- Medallion WDG at 1 to 4 oz/100 gal water. Boxwood is NOT on the label but may legally be used. Use with oils or adjuvants may damage plant. Group 12 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 to good control. Group 7 + 11 fungicide. 12-hr reentry.
- Palladium at 2 to 4 oz/100 gal water. Boxwood is NOT on the label but may legally be used. Avoid excessive runoff to small plants, which may result in stunting and/or chlorosis. Group 9 + 12 fungicide. 12-hr reentry.
- ProCon-Z at 8 oz/100 gal 2water. Boxwood is NOT on the label but may legally be used. Group 3 fungicide. 24-hr reentry.
- Spectro 90 WDG at 1 to 2 lb/100 gal water. Boxwood is NOT on the label but may legally be used. Group 1 + M5 fungicide. Good control. 12-hr reentry.
- Strike Plus 50 WDG at 3 to 9 oz/100 gal water. Boxwood is NOT on the label but may legally be used. Good control. Group 3 + 11 fungicide. 12-hr reentry.
- Torque at 4 to 10 fl oz/100 gal water. Boxwood is NOT on the label but may legally be used. Group 3 fungicide. 12-hr reentry.
References LaMondia, J. A. 2015. Management of Calonectria pseudonaviculata in boxwood with fungicides and less susceptible host species and varieties. Plant Disease. 99:363-369.
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.