Red twig dogwood species consisting of Cornus sericea, Cornus alba and Cornus sanguinea are prone to several fungal diseases and their coresponding fungi including powdery mildew (Erysiphe pulchra), dogwood anthracnose (Discula destructiva), leaf spot (Cercospora cornicola and Septoria) and Botryosphaeria canker (Botryosphaeria dothidea). Of these fungal diseases, Botryosphaeria canker and dogwood anthracnose are both capable of producing cankers on the stems of dogwood. To tell these two diseases apart look first to the leaves. In dogwood anthracnose the leaves are spotted, often with baize centers and purplish halos around the spots; Infected twigs form a shepherd’s crook reminiscent of fire blight symptoms and the disease tends to be more severe under cool 18-24°­C (65°­75° F), wet and shady conditions (Smith S. and Carson J. n.d.). With Botryosphaeria canker the leaves tend to suddenly droop and turn brown; twigs do not curl over (shepherd’s crook) and optimal conditions are (24-30C) with high humidity or rainy summer weather or when host plants are heat-stressed (Kenaley S.C. et al. 2010).

What is Botryosphaeria Canker?

Botryosphaeria canker is a fungal disease caused by the fungus Botryosphaeria dothidea. The fungus causes sunken and dying tissues on the stems. The decaying area affects the vascular system of the stem and as the canker grows the transportation of water and nutrients to the plant area beyond the canker is reduced or completely shut off (girdled). Leaves may suddenly wilt and die and whole stems can die back to the ground. The fungus gains entry to the plant through wounds, insect feeding, growth cracks and natural openings like lenticels (raised spores on the stems).

Are Other trees and Shrubs in The Landscape at Risk of This Disease?

There are hundreds of plant genera that are susceptible to Botryosphaeria cankers and dieback. The genera I most commonly find it on is honey locust, rose, redbud, holly, rhododendrons, flowering dogwoods and red twig dogwoods; but they also commonly affect juniper, beeches, arborvitae and many others. For a more complete list of susceptible genera.

Identification of Botryosphaeria Canker

The diseased branches appear darker in colour (often easier to spot when the shrubs are dormant. On closer inspection cankers can be seen, some of them running several feet up the stem. Within these cankers appear tiny black spots, these are the fruiting bodies of the fungi. Entire stems can dieback to the ground. The fungi overwinter on diseased branches and in the spring the spores are splashed about by rain and blown by the wind. Insects and tools can also spread the spores. Affected branches may fail to leaf out in the spring or as the canker grows it may cause the leaves to suddenly wilt and die. Warm temperatures paired with high humidity or abundant rainfall favour it’s development.

Management of Botryosphaeria Canker

Cultural Practices:

• Prune out diseased wood during dry weather to healthy wood that appears creamy in colour; diseased wood appears brownish-red. Make your pruning cuts back a few inches from the canker (diseased wood). Disinfect your pruning tool between cuts with rubbing alcohol or Lysol spray. Black bag the diseased plant material.
• Wounds provide entry points for the fungi so take care in the handling of the plant and keep mowers and trimmers well back from the stems.
• Drought stricken plants are prone to the disease so keep your dogwood well watered especially during periods of drought.
• Sapwood injury caused by freeze/thaw cycles can predispose dogwoods to the disease.
• Manage insect infestations. Some pests to be on the look out for are scale, bores, sawflies, aphids and club-gall Midge.
• Botryosphaeria dothidea are opportunistic fungi that typically affect plants that are weakened by drought, over watering, insect damage, compacted soil or planted in overly sunny or shady areas. When planting dogwood site location is key to avoiding this disease as well as good cultural practices.

Chemical Sprays:

• There is currently no fungicide listed for use against Botryosphaeria Canker. Protective fungicides however may provide some protection.

Summary

Botryosphaeria canker is a serious but common fungal disease of many woody plants. Plants most at risk are those that are weaken by either environmental stressors like drought, insect damage, or freeze/thaw cycles or by cultural practices, like pruning wounds, poor site selection and neglect. The fungus enters the plant through wounds or natural openings. In red twig dogwoods diseased wood appears darker and black fungal fruiting bodies are visible. Cankered areas start off discoloured, with some blackening they then become sunken with raised bark surrounding it. The cankers grow in length up the stem affecting the transport of water and nutrients until the entire stem dies.

Photo Credits: all photos taken by the author.

References

Kenaley S.C., Hudler G.W., O’Brien D.D., Cameron K.D., and Smart L.B. 2010. Cankers caused by Botryosphaeria and related fungi. Cornell University. Retrieved on Mar. 31, 2021 from: willowbotryosphaeria.pdf (cornell.edu)

Smith S. (Plant Pathologist/Instructor) and Carson J. (Associate Professor and Horticulture Specialist) n.d. Anthracnose Diseases of Dogwood. University of Arkansas. Retrieved on Mar 31, 2021 from: Anthracnose Diseases of Dogwood – FSA7564 (uaex.edu)

Other reading resources:

Bush E.A. n.d., Extension Plant Pathologist, Virginia Tech, Botryosphaeria Canker and Dieback of Trees and Shrubs in the Landscape, Publication 450-726. 450-726.pdf (vt.edu)

Neely D. and Nolte D.S., (1989). Septoria Leaf Spot on Dogwoods. Volume 15, Issue 11. Arboriculture & Urban Forestry Online. Articles for ISA Arboriculture Journal (isa-arbor.com)

Nicholas Brazee, rev. Jan. 2, 2021. Botryosphaeria Canker. University of Massachusetts Amherst. https://extension.umd.edu/hgic/topics/botryosphaeria-canker-shrubs

Pataky N., ((2018). Three Dogwood Diseases. Three Dogwood Diseases (illinois.edu)

Reese C., (2009). Three Kinds of Redtwig Dogwoods. Horticulture Magazine. Three Kinds of Redtwig Dogwoods – Horticulture (hortmag.com)

RHS, (n.d.). Cornus anthracnose. Cornus anthracnose / RHS Gardening

University of Illinois Extension 2007. Redosier Dogwood Canker. Home, Yard and Garden Pest News Letter. Redosier Dogwood Canker (illinois.edu)

University of Maryland Extension, ( n. d.). Botryosphaeria Canker – Shrubs. Home and Garden information Center. Botryosphaeria Canker – Shrubs | University of Maryland Extension (umd.edu)

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