Japanese Maple Problems…Coral Spot Nectria Canker

Coral Spot Nectria Canker on a weeping, green, split leaf Japanese Maple (Acer palmatum ‘Viridis’). The photo was taken in early spring when the coral coloured fungal fruiting bodies were clearly visible.

What is Coral Spot Nectria Canker?

Coral Spot Nectria Canker is a fungal disease caused by the fungus Nectria cinnabarina. These opportunistic fungi typically attack trees and shrubs that have been weakened by stress or injuries like mechanical injury, storm damage, insect feeding, animal damage, pruning, disease, frost cracks, cold injury or drought damage. Once the fungi gain entry to the plant they cause the formation of cankers. These cankers first damage the bark then the phloem (which carries the sugars down to the branches, trunks and roots), then the cambium (the area where diameter growth occurs. It produces both phloem and xylem cells), and finally the xylem aka. sapwood (which carries water and nutrients from the roots up to the leaves). These cankers thus reduce the flow of water and nutrients throughout the tree weakening it and making it vulnerable to other diseases or insect infestations. Coral Spot Nectria Canker is very difficult to eradicate once infection takes place thus a good offence is your best defense.

What Woody Plants Are at Risk of Coral Spot Nectria Canker?

Nectria cinnabarina attacks over 90 different genera of woody plants (Brazee N., n.d.). Some of those genera include: (with the bolded genera being the most susceptible).

  • Apple (Malus pumila)
  • Ash (Fraxinus)
  • Barberry (Berberis Vulgaris)
  • Beech (Fagus)
  • Birch (Betula)
  • Boxwood (Buxus)
  • Crabapple (Malus sylvestris)
  • Elm (Ulmus)
  • Hickory (Carya)
  • Honey locust  (Gledistia)
  • Hornbeam (Carpinus)
  • Horse chestnut (Aesculus)
  • Japanese maple (Acer palmatum or Acer japonicum)
  • Linden aka. Lime (Tilia)
  • Maples (Acer spp.)
  • Pear (Pyrus)
  • Rose (Rosa)
  • Sycamore (Platanus occidentalis)
  • Walnut (Juglans)

Symptoms of Coral Spot Nectria Canker

The first symptom of infection is a sunken discolored area or a water soaked area of bark near wounds or at the base of dead twigs or branches. In the spring coral, reddish/orange, or sometimes white fungal fruiting bodies may become apparent. Leaves may fail to come out in the spring or be smaller than usual and branch die back may be observed. As the outer bark dies the area of infection appears more sunken. In response to this fungal invasion the tree attempts to stop the spread of the disease by building a calloused ridge around the area. If the infection continues to spread into healthy wood the tree will again build another calloused ridge. This back and forth attack and defense can occur for several years. Mature cankers appear round or elongated with pronounced calloused ridges around them and a target like appearance. As the infection spreads via fungal spores splashing or by being wind blown to other parts of the tree it grows increasing more stressed. Branch dieback commonly occurs and in some cases the entire tree may die.

Coral Spot Nectria Canker on a weeping, green, split leaf Japanese Maple (Acer palmatum ‘Viridis’). The photo was taken in early spring when the coral coloured fungal fruiting bodies were clearly visible. Notice the water soaked bark, a common early symptom.
Coral Spot Nectria Canker on a weeping, green, split leaf Japanese Maple (Acer palmatum ‘Viridis’). The photo was taken in early spring when the coral coloured fungal fruiting bodies were clearly visible.
Coral Spot Nectria Canker on a weeping, green, split leaf Japanese Maple (Acer palmatum ‘Viridis’). The photo was taken in early spring when the coral coloured fungal fruiting bodies were clearly visible.

Life Cycle of Coral Spot Nectria Canker

This fungal infection is most likely to take place in spring and fall (although it can take place at any time). Usually the fungus attacks woody plants that are already stressed. The fungus gains entry to the plant through a wound. Once inside the plant it causes twigs and branches to die back. In the spring dense clusters of soft, coral coloured fruiting fungal bodies break through the thin bark on these dead branches. The large number of fungal spores contained in these fruiting bodies are then splashed and wind blown onto healthy wood and the infection spreads. As the season progresses the coral coloured fruiting bodies harden and darken in colour. Newly infected tissues may at first appear sunken or water soaked. As the disease progresses sunken lesions with a calloused ridge form (cankers).

Treatment of Coral Spot Nectria Canker

Proper site location: Make sure to select plants well suited to your soil type, soil pH, available sunlight, water requirements and drainage.

Cultural practices:

  • Prune out dead and diseased wood a few inches back of the infected area during dry periods. Disinfect your tools (and ideally your gloves) between cuts with either rubbing alcohol or Lysol spray. DO NOT cut into the canker itself, doing so could cause greater fungal spread and damage. When in doubt hire a qualified professional.
  • Keep lawn mowers and trimmers well back to avoid injury.
  • Keep the trees properly watered and fertilized.
  • General pruning is usually best done in late winter, well before the wet season. Make sure your pruning tools are clean and sharp. Make a clean cut just above the branch collar (this will help the wound to heal as quickly as possible and eliminate the risk of damage caused by stub dieback).
  • Remove heavily infected trees to reduce the risk of spreading the disease to other healthy trees and shrubs near by.

Chemical control:

  • “Apply fungicide after pruning but before a rain or irrigation to protect wounds against infection. No specific registrations are for this purpose. Many fungicides recommended for anthracnose control also help protect trees from nectria canker.” (Pacific Northwest Extension n.d.)
  • “If plants are heavily infected or susceptible species are newly planted, the frequency of new infections may be reduced by thoroughly spraying a fixed copper or a freshly prepared Bordeaux mixture during early leaf drop, just before the rainy weather. If leaf fall is prolonged by warm fall weather, a second application may be warranted when three-fourths of leaves have dropped.” (Regents of the University of California, © 2017).

Summary

Coral Spot Nectria Canker is a canker causing fungal disease of many woody plants. Although it is not as serious as some of the other fungal diseases it none the less can cause extensive damage even the death of the tree. The specific fungus that causes this disease is Nectria cinnabarina. These fungi produce the brightly coloured coral fruiting bodies associated with the disease. This fungus tends to attack woody plants already weakened by other factors be it disease, insect feeding or injury. The fungi damage the vascular system of the tree reducing the flow of water and nutrients to the plant. This further weakens the tree making it even more vulnerable to other diseases, insect problems, and other environmental stressors.

Photo credits: all photos taken by the author.

References:

Brazee N., n.d. Coral-spot Nectria Canker. University of Massachusetts Amherst. Retrieved on Apr. 9, 2021 from: https://ag.umass.edu/landscape/fact-sheets/coral-spot-nectria-canker

Pacific Northwest Extension n.d. Maple (Acer spp.)-Nectria Canker. PNW Plant Disease Management Handbook. Retrieved on Apr. 9, 2021 from: https://pnwhandbooks.org/plantdisease/host-disease/maple-acer-spp-nectria-canker

Regents of the University of California, © 2017. Nectria canker—Nectria spp.. Retrieved on Apr. 9, 2021 from: http://ipm.ucanr.edu/PMG/GARDEN/PLANTS/DISEASES/nectriacanker.html

Other Reading resources:

https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/diseases/cankers/nectria-canker-and-dieback.aspx

https://www.rhs.org.uk/advice/profile?pid=135

https://homeguides.sfgate.com/stem-canker-japanese-maple-27174.html

Nectria Canker

Nectria cinnabarina Coral Spot fungus (first-nature.com)

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