Fire Blight caused by Erwinia amylovora, is a serious bacterial disease that affects many members of the rose family (Rosaceae), such as purple sand cherry, apple, crabapple, quince, pear, plum, mountain ash and more. The trees and shrubs that have been infected have the appearance of having been scorched by fire. The disease is systemic and spreads rapidly leading to extensive damage, even death. There is no cure only management.

Symptoms

Blossoms: at first appear water soaked then wilt and turn brown/black and remain on the tree. This is called blossom blight.

Leaves: wilt and turn brown but remain on the tree, giving it the appearance of having been burnt by fire. Thus, the name fire blight.

Stems or shoots: Infected stems darken, and stems may bend over and appear hooked. This is called ‘shepherd’s crook’. This stage of the disease is called shoot blight.

Larger branches and trunk: Cankers form on the older wood and bacterial ooze (as pictured below), may be seen oozing from them. The cankers girdle the branch and the area from the canker to the tip appears darker in colour and dies. The cankers first appear brown to purple in colour. Eventually they become sunken with cracked margins (OMAFRA (Nov. 24, 2020). This stage of the disease is called limb and trunk blight.

Life Cycle

Fire blight bacteria overwinter in blighted branches and at the edges of cankers (areas of bark killed by bacteria). In the spring, as temperatures increase above 18ºC (65ºF) the cankers become active and begin to ooze (OMAFRA (Nov. 24, 2020)). This ooze is laden with bacteria which can be spread to open flower petals by insects, wind, splashing rain, humidity and dew. Pollinating insects further contribute to the spread of the pathogens by carrying it from infected to non-infected blossoms. “Once in the blossom, bacteria multiply rapidly in the nectar and eventually enter the flower tissue. From the flower, the bacteria move into the branch. When the bacteria invade and kill the cambial tissue of the branch, all flowers, leaves and fruit above the girdled area die” (Koski R.D. & Jacobi W.R. rev. 2009). Note: the open blossoms are the most susceptible tissues.

Secondary infections occur throughout the growing season through natural openings in leaves (stomata), branches (lenticels), pruning wounds, insect feeding, ovipositing (insect egg laying in branches), and hail.

Warm temperatures with high humidity combine to form ideal conditions for fire blight. Under these conditions the bacteria can move 15-30 cm (6-12 inches) or more in the shoot within a few days after infection (OMAFRA (Nov. 24, 2020).

Treatment

There is no cure for fire blight but there are several things the gardener can do to manage the disease.

• Cultural Practices:
  • Monitoring: Regularly monitor your susceptible trees and shrubs for any sign of infection. Examine the branches in late winter for any signs of cankers and prune off any found. Next carefully monitor the tree at blossom time (the most vulnerable period), for any water soaked, wilted or browning flowers. Once flowering is complete continue to monitor stems watching for signs of ‘shepherd’s crook’ and bacterial ooze. Droplets of bacterial ooze can form on twigs within three days after infection (OMAFRA (Nov. 24, 2020)). Continue to monitor the remainder of the season constantly being on the lookout for any signs of the infection.
  • Pruning: Prune off infected branches about 1 foot (30-40cm) below the canker during dormancy. Continue to prune off infected areas throughout the garden season. Burn them if you can others wise black bag them. Disinfect your pruning tool between cuts with rubbing alcohol, bleach or Lysol (spray or multi surface cleaner). When pruning for overall size and shape prune lightly, as heavy pruning will spur on a lot of new growth that is very vulnerable to infection.
  • Fertilizing: Ease up on the fertilizers, especially nitrogen, to prevent excess young succulent growth, which is most susceptible to infection.
• Sprays:
  • Copper or Bordo sprays are effective against bacterial diseases. They serve to provide an inhibitory barrier. The spray is best applied at dormancy (before bud break).
  • Streptomycin: is an antibiotic that works by inhibiting the growth of the bacterial pathogen on the surface of the plant (UAP Canada, Feb. 2006). It is used at blossom time to protect the blossoms against blossom blight.
  • Serenade Garden: an organic, broad spectrum, preventative, bio-fungicide. It inhibits the growth of the bacteria on the surface of the plant. Begin spraying at bud break and repeat spray according to manufactures recommendations.
  • Organocide Plant Doctor: is an natural, earth friendly, broad spectrum, systemic fungicide. It works its way through the entire plant to prevent disease and attack existing disease. Even though it is labeled a fungicide the manufacturer claims it is effective for fire blight.

Note: Before purchasing any spray make sure it is legal to use in your province, state or country. Also be sure to follow the manufacturer’s instructions.

Susceptible Trees and Shrubs

They are particularly destructive to apples (Malus spp.), pears (Pyrus spp.), and crabapples (Malus spp.). The disease also occurs on purple leaf sand cherry (Prunus x cistena), (serviceberries (Amelanchier spp.), flowering quinces (Chaenolmeles spp.), cotoneasters (Cotoneaster spp.), hawthorns (Crataegus spp.), quinces (Cydonia spp.), pyracanthas (Pyracantha spp.), blackberries (Rubus spp.), raspberries (Rubus spp.), and mountain ashes (Sorbus spp.) (Koski R.D. & Jacobi W.R., re, 2009).

Summary

Fire blight is a serious bacterial disease that can cause extensive damage, even death to your purple Sand Cherries and other susceptible members of the rose family. The disease spreads very quickly and must be caught early to prevent extensive damage. There is no cure so careful monitoring and cultural practices are of prime importance to both prevention and the management of the disease.

Photo Credits: all photos taken by the author.

Updated: April 3, 2022

References:

Koski R.D. & Jacobi W.R., re, 2009, Extension Specialist-Turf College of Agriculture Sciences Department of Horticulture and Landscape Architecture, ‘Fire Blight – 2.907’, Colorado State University Extension, Retrieved from: https://extension.colostate.edu/staff-directory/pg/2/?cn-s=R.D.+Koski+and+W.R.+Jacobi&cn-cat=

Solymar B. (2018) and Tim MacDonald, “Integrated Management of Fire Blight on Apple and Pear in Canada”, Retrieved on Jan.8th, 2021, from: https://www.agr.gc.ca/eng/agriculture-and-the-environment/agricultural-practices/agricultural-pest-management/agricultural-pest-management-resources/integrated-management-of-fire-blight-on-apple-and-pear-in-canada/?id=1544193381450

OMAFRA (Nov. 24, 2020); ‘Fire Blight’ Excerpt from Publication 310, Integrated Pest Management for Apples; http://www.omafra.gov.on.ca/english/crops/facts/fireblight.htm

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