Boxwood blight

Boxwood blight (also known as box blight or boxwood leaf drop) is a widespread fungal disease affecting boxwoods (box plants), caused by Cylindrocladium buxicola (also called Calonectria pseudonaviculata).

Boxwood blight
Boxwood blight, early leaf symptoms
Causal agentsCylindrocladium buxicola (also called Calonectria pseudonaviculata)
HostsBoxwood plants
VectorsPossibly: Pachysandra(spurge) and Sarcococca (sweetbox)
EPPO CodeCLYDBU
DistributionEast & West Coasts of United States & Canada and Europe

History

The first description of boxwood blight was from the United Kingdom in the mid 1990s. In 2002, when the disease was discovered in New Zealand, the cause was identified as a new species of fungus which was formally named Cylindrocladium pseudonaviculatum. The fungus causing the disease in the UK was later named C. buxicola. These are now known to be the same.[1]

Boxwood blight is found throughout Europe,[2][3] and has spread to North America. In the United States, boxwood blight was first reported in North Carolina in September 2011; the disease was observed in Connecticut several weeks later.[4] To date, boxwood blight has been identified in at least 30 U.S. states and several Canadian provinces.[5]

Hosts

There appears to be some tolerance to the disease within the genus Buxus. In general, the boxwood varieties within the species sempervirens tend to be more susceptible to this disease; this would include the most popular varieties 'English' and 'American' boxwood.[6] Other plants within the family Buxaceae, such as Pachysandra terminalis, as well as a Sarcococca species, have also found to be susceptible to this fungal pathogen.[7]

Symptoms and disease process

The blight initially presents as dark or light brown spots or lesions on leaves. The leaves typically turn brown or straw color, then fall off.[8] The stems develop dark brown or black lesions.[9] The disease is often fatal to young plants.

The spores remain viable for five years in fallen boxwood leaves,[10] and are dispersed by wind and rain over short distances. Since they are sticky the spores may also be spread by birds, animals, and contaminated clothing and footwear. The most common mode of transmitting the blight is by the introduction of asymptomatic plants, or plants treated with fungicide (which can mask the disease) to unaffected areas.[11] Warm and humid conditions facilitate its spread.[12] The fungus does not need a wound to infect a plant, but it does require high humidity or free water.[13]

Treatment

There is no known cure. Fungicides may prevent the spread of the disease. To be effective, they must be applied to the entire plant, leaves and stems, which can be difficult because boxwood leaves are very closely spaced.[14]

Notes
  1. Douglas. p. 1
  2. Douglas pp. 1–2
  3. Milius
  4. Ivors, K. L.; Lacey, L. W.; Milks, D. C.; Douglas, S. M.; Inman, M. K.; Marra, R. E.; LaMondia, J. A. (July 2012). "First Report of Boxwood Blight Caused by Cylindrocladium pseudonaviculatum in the United States". Plant Disease. 96 (7): 1070–1070. doi:10.1094/PDIS-03-12-0247-PDN. ISSN 0191-2917.
  5. Castroagudín, Vanina L.; Yang, Xiao; Daughtrey, Margery L.; Luster, Douglas G.; Pscheidt, Jay W.; Weiland, Jerry E.; Crouch, Jo Anne (2020-01-01). "Boxwood Blight Disease: A Diagnostic Guide". Plant Health Progress. 21 (4): 295. doi:10.1094/PHP-06-20-0053-DG. ISSN 1535-1025.
  6. http://go.ncsu.edu/boxwood_blight_links
  7. Daughtrey, Margery L. (2019-08-25). "Boxwood Blight: Threat to Ornamentals". Annual Review of Phytopathology. 57 (1): 189–209. doi:10.1146/annurev-phyto-082718-100156. ISSN 0066-4286.
  8. Douglas, p. 2
  9. Milius quotes Lynn R. Batdorf, curator of the National Arboretum's boxwood collection (currently not infected): “I’ve never poured diesel fuel on a boxwood, but if I did, that’s what it would look like.”
  10. Douglas, p. 7
  11. Douglas, p. 6, calls this "Trojan horse " or "Typhoid Mary" transmission.
  12. Douglas, p. 5
  13. Douglas, p. 6
  14. Douglas, p. 9 has details on various fungicides

References
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