Documents: Latest From: Liesbeth Leatherbarrow:

Fire Blight: Tips On Control And Prevention
by Liesbeth Leatherbarrow
by Liesbeth Leatherbarrow

Liesbeth has written for several western gardening publications, including Gardens West and The Gardener for the Prairies. She has also co-authored three gardening books: The Calgary Gardener (with the Calgary Horticultural Society), The Calgary Gardener, Volume Two: Beyond the Basics (with Lesley Reynolds), and 101 Best Plants for the Prairies (with Lesley Reynolds).

December 10, 2000

1pt.gif (86 bytes)Fire blight, a bacterial disease that ravages familiar trees and shrubs of the rose family (apple, crabapple, pear, mountain ash, spirea, cotoneaster, hawthorn, saskatoon) has reared its ugly head in our community. I know this because our 'Royalty' crabapple and all the others in the neighbourhood have succumbed; 'Royalties', especially, are poorly equipped to ward off this disease and are usually the first to take a hit. 1pt.gif (86 bytes)1pt.gif (86 bytes)Faced with making a difficult decision (do I replace the tree or try to save it?) I consulted several arborists and did some research of my own. Here is what I learned about this insidious disease that can sneak up on you if you are not vigilant and for which there is no known cure.

Cause and Transmission

  • Fire blight is caused by the bacterium Erwinia amylovora.

  • It overwinters at the margins of cankers and becomes active when the ambient air temperature reaches 18° C (65° F).

  • Few overwintering cankers become active in the spring, but it only takes one to spread the disease throughout an entire community.

  • Bacterial growth is favoured by rain, heavy dews, and high humidity, conditions that often prevail during Calgary springs.

  • Fire blight bacteria are spread by splashing rain or insects (bees, flies, ants); blossoms are a common entry point for the bacteria, so trees and shrubs are most vulnerable when they are in bloom; bacteria multiply rapidly in blossom nectar.

  • Actively growing shoot tips are also highly susceptible to infection; bacteria may enter through natural openings on twigs or through wounds created by sucking insects, wind-whipping, or hail.


  • Diseased blossoms become water-soaked and turn brown; droplets of bacterial ooze may be visible; infected trees often experience premature blossom drop.

  • Infected leaves suddenly turn brown, as though scorched by fire; they remain attached to dead branches throughout the summer.

  • Twig blight starts in the soft growth at the tips of new shoots and works its way down into the older portions of the twig; twigs turn a wrinkled brown or black; the ends of infected branches often bend over to resemble a shepherd's crook.

  • Clear amber liquid may be found oozing from diseased twigs.

  • On badly infected trees, bark cankers may also be present; these are indented, discoloured areas, often characterized by the presence of bacterial ooze.

Control and Prevention

  • Be vigilant and prune blighted twigs and cankers as soon as they appear by making cuts at least 10 cm (4 in.) below or around the infected area.

  • Prune during the dormant season (winter) when there is less chance of spreading the bacteria.

  • If you must prune during active growth, cuts must be made at least 40 cm (15 in.) below the infected area.

  • Remove an entire branch if it is more than half girdled by cankers.

  • To prevent spreading the infection, use a 10% bleach solution to disinfect pruning shears BETWEEN EVERY CUT.

  • Dispose of all diseased leaves, branches, twigs in the garbage or burn them; do not compost or use as mulch; bacteria can remain active for a long time.

  • Refrain from using a high nitrogen fertilizer around susceptible trees and shrubs; this promotes vigorous new growth which is more susceptible to fire blight

  • Suckers and water sprouts are also vulnerable to infection, so should be pruned as soon as they appear.

  • Hose down susceptible shrubs and trees on a regular basis to remove any sucking insects that create wounds through which fire blight bacteria enter.

  • There is no known chemical cure for fire blight although agricultural streptomycin, an antibiotic, is recommended by some as a preventive measure; this can be an expensive and time-consuming proposition; to be effective, trees must be sprayed frequently during their entire bloom period and on an annual basis as long as there are other infected trees (e.g. the neighbours') in the vicinity.

  • If fire blight infection continues to spread in your tree despite your best efforts to control it, then you may be better off cutting the tree down and replacing it with a disease resistant genus, species, or cultivar.

  • New Eden
  • Kids Garden
  • Plant a Row Grow a Row