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FIRE BLIGHT, (Erwinia amylovora)
BACKGROUND
Fire blight is a bacterial disease native to North America. It is one of the most serious diseases of
fruit trees and shrubs. This disease causes severe blighting on many parts of its host plants
including blossoms, leaves, shoots, limbs and fruit. It has a significant impact on the commercial
orchard industry and on ornamental plantings. It can occasionally reach epidemic levels.
DISTRIBUTION
Fire blight is distributed throughout most of North America, most of Europe and in some Middle
Eastern countries.
HOST SPECIES
Fire blight attacks a number of species in the rose family. Preferred hosts include mountain ash,
apple, choke cherry, pear, crab apple, hawthorn, Saskatoon and a number of common rose
varieties. In Saskatchewan, the most common hosts are crabapple, mountain ash, and
cotoneaster.
LIFE CYCLE
The fire blight bacteria overwinter in live tissue at the margin of cankers on main stems and larger
branches. Bacteria become active in the spring when temperatures exceed 18 C. Growth of the
bacteria is favored by rain, heavy dew, and high humidity. When trees are blossoming, droplets
of bacterial ooze are present on the surface of cankers. This bacterial ooze is spread to open
blossoms by rain splash or insects, mostly pollinators such as bees, wasps, ants and flies. The
bacteria multiply rapidly in blossom nectar, invade the blossom tissue through nectary glands and
cause blossom blight. Further spread of the bacteria from blossom to blossom continues by rain
and pollinating insects. The optimum temperature for blossom blight infection is 18 C to 30 C.
Later, bacteria moving from infected blossoms within the same blossom-bearing twig can infect
developing fruits. New succulent vegetative shoots also become infected early in the growing
season from bacteria spread by rain or insects from cankers and infected blossoms. Infection of
shoots occurs through mechanical wounds, wounds created by sucking insects or through
lenticels and stomata. New shoot infection can be extensive during periods of prolonged rain and
high humidity. Within the infected shoot, the bacteria grow rapidly often killing much of the shoot.
Bacteria will grow from the infected shoots into woody tissue, where its advancement will slow
and cankers will form. The bacteria will then overwinter in these cankers to renew the disease
cycle the following spring. In addition to disease spread occurring from the bacterial ooze in the
spring, bacteria can grow internally from canker margins into nearby shoots, where they cause
systemic infections called canker blights. The bacteria can also grow into the root area and
cause a blight.
SIGNS, SYMPTOMS AND DAMAGE
Fire blight affects many parts of the host plants. Consequently, there are a wide variety of
signs/symptoms, which include:
- Diseased blossoms become water-soaked, turn black or brown and become wilted.
- Infected young fruits appear water soaked, quickly turn brown to black and eventually
shrivel up.
- Leaves on infected new shoots suddenly turn brown or black, appearing as though they
had been scorched by fire.
- Infected shoots wilt and often form a shepherd's crook at their tips.
- Shoots with canker blight are wilted and yellow-orange in color.
- During warm and humid weather, droplets of yellowish-white bacterial ooze may be
visible on the surface of cankers and on the blighted shoots and fruit.
- Dried up blighted leaves may remain on the tree throughout the winter.
- Cankers in the woody tissue appear as dark, water soaked areas that often have cracks
in the bark around the margins.
- Oozing near the base of the tree can be a sign of infection in the root.
Fire blight causes various forms of damage to infected trees including blossom blight, fruit blight,
shoot blight, branch and main stem cankers and root blight. This damage can cause severe
losses in fruit orchards, nurseries and ornamental plantings. There is direct loss of fruit due to
blighting of blossoms and young fruit. Fruit bearing in young trees can be delayed by repeated
blighting of shoots. Extensive blighting and dieback of new shoots causes growth loss, is
unsightly and can cause tree deformity. Fire blight cankers girdle and kill limbs or entire trees.
Root infections can kill trees.
Blossom blight
Fruit blight with ooze on crabapple
Image: University of Georgia Plant Pathology
Archive, University of Georgia, Bugwood.org
Image: P.G. Psallidas, Benaki Institute, Athens,
Bugwood.org
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Fire blight shepherd’s crook
Extensive shoot blight
Image: Manitoba Conservation
Fire blight canker
Image: William Jacobi, Colorado
State University, Bugwood.org
Image: Claude Moffet, Natural Resources
Canada, Canadian Forest Service
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MANAGEMENT PRESCRIPTIONS
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During the dormant period, cankers should be removed by pruning to reduce sources of
infection for the following season. The cankered branch should be pruned about 20 cm
below the margin of the canker.
In the spring, current season infections should be pruned when first noticed. The
punning cut should be about 30 cm below the visible infection. Pruning tools should be
disinfected between each cut during the growing season. Bleach or alcohol solutions can
be used to disinfest tools.
There are chemical products, such as streptomycin and copper compounds, which are
registered to protect trees from fire blight infection. These products are for disease
prevention and must be applied prior to infection at the time of blossoming. Repeated
applications every three days are required throughout the bloom period.
REFERENCES FOR ADDITIONAL INFORMATION
Fire Blight of Apple and Pear
British Columbia Ministry of Agriculture and Lands
http://www.agf.gov.bc.ca/cropprot/tfipm/fireblyt.htm#symptoms
Fire Blight
Cornell University
New York State Integrated Pest management Program
http://www.nysipm.cornell.edu/factsheets/treefruit/diseases/fb/fb.asp
Fire Blight
Kearneysville Plant Disease Fact Sheet
http://www.caf.wvu.edu/Kearneysville/disease_descriptions/omblight.html
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