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FIRE BLIGHT: BIOLOGY AND MANAGEMENT
(TIZÓN DE FUEGO: BIOLOGIA Y MANEJO)
S. KRISHNA MOHAN Plant Pathologist
University of Idaho, Parma, ID 83660 U.S.A.
Fire blight (tizón de fuego, fuego bacteriano) is a serious bacterial disease of apples,
pears, quinces, and many other plants in the rose family (Rosaceae), including several
ornamental plants like Crataegus spp. (hawthorn), Cotoneaster spp., Pyracantha
spp.(firethorn) Sorbus spp. (mountain ash) and Photinia sp. It is particularly destructive
on susceptible varieties of apples and pears, and causes significant economic losses under
favorable weather conditions in several parts of the world. Fire blight has become highly
destructive in high density apple orchards with highly susceptible varieties and rootstocks
and management practices that stimulate rapid and vigorous growth. Severe disease
epidemics can result under warm and humid conditions, especially if bloom periods of
the varieties coincide with such conditions, or following hail damage to young shoots.
The disease incidence is sporadic in most growing areas, being very extensive and severe
in some years and occurring only in traces in others. The name fire blight describes the
Presented at the XII Simposium Internacional 2006 Sobre el Manzano, Cd. Cuauhtémoc,
Chihuahua, México, November 10, 2006.
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scorched appearance of the affected blossoms, leaves and stems.
Cause: Fire blight is caused by the bacterium Erwinia amylovora. The bacterium overwinters primarily in “holdover” cankers on apple trees or on other susceptible host
plants in the vicinity. These cankers become active in the spring, and produce the primary
inoculum that is spread by splashing rain, pruning tools, and insects such as flies, bees,
leafhoppers and aphids. Most of the infections occur through open blossoms. The
bacteria are deposited on the stigma where they multiply and are washed down into the
blossom and invade through nectaries. Temperature of 18oC or higher, and rain or high
(more than 65%) relative humidity are necessary for infections to occur. Young trees and
young and actively growing leaves and shoots are more susceptible. Wounds caused by
high winds and hail can lead to severe disease.
Symptoms: Depending on the part of the tree affected, fire blight symptoms are often referred to as blossom blight, shoot blight, leaf blight, fruit blight, trunk blight or collar
blight. Blossom blight is common during the bloom period in the spring. Infected
blossoms become watersoaked, turn brown, then wilt and dry out, usually remaining
attached to the spur. Under warm and humid weather, droplets of milky bacterial
exudation can be seen at the base of blighted blossom.
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The infection progresses to the fruit spur and into the developing young fruit which turns
brown and shriveled. Adjoining leaves in the infected spur get invaded, showing necrotic
areas along the midrib and major veins causing leaf blight.
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Actively growing, succulent shoots are most susceptible.
Infection
progresses rapidly under favorable weather conditions and the blighted shoots and leaves
turn light to dark brown, and present a “shepherd’s crook” appearance.
Although not common in apple, under certain conditions, immature fruit get infected
either from an infected spur or through wounds caused by insects or, more commonly,
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following hail damage.
Infected areas appear brown to black with a reddish border, and the fruit may produce
bacterial exudates in the form of viscous drops or strands. Blighted fruits turn brown,
shrivel and remain attached to the spur.
Trunk blight results when the disease advances to the trunk from infected spurs, shoots
and branches, and is sometimes accompanied by bacterial ooze along the bark.
Collar blight symptoms are produced at the base of the plant, following infection through
wounds or through infection spreading from roots. Infected bark appears dark and
purplish, usually with a crack developing at the lesion margin. Underneath the infected
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bark, the infected tissue may show brown streaks. Collar blight may progress to
complete trunk girdling, and may kill the tree. Some times, symptoms have been
observed to develop at the graft union, causing tree death, especially in young trees.
Symptoms of collar blight may be confused with those of collar rot caused by the fungus
Phytophthora, and correct diagnosis of the cause is critical for choosing appropriate
control measures.
Management: An effective fire blight management strategy requires integration of several practices aimed at 1) reducing the initial inoculum, 2) protecting the apple trees
from getting infected, and 3) planting less susceptible varieties (on less susceptible
rootstocks).
Reducing the amount of available inoculum in an orchard heavily depends on adoption of
good phytosanitary measures. Removing any perennial sources of inoculum in the
vicinity of apple orchard (abandoned infected apple, crabapple and pear trees, infected
hawthorn plants in the landscape, etc.) will reduce the primary sources of inoculum.
Removing holdover cankers (from the previous growing season) during winter pruning is
critical. It may be helpful to apply delayed dormant sprays of copper compounds to
reduce the surface inoculum. Removing of infected shoots soon after bloom period may
reduce the disease spread under some situations. Clean the cutting tools with solutions of
10% Clorox or in Lysol, between cuts. Do not cut when the trees are wet. In spring and
summer, make cuts at least 12 inches beyond the margin of the canker. Do not use
overhead irrigation.
Since a majority of infections occur through open blossoms under favorable weather
conditions, timely application of bactericide sprays following one of the several models
or local recommendations that predict (forecast) infection periods based on climatic and
phenological data, is a major component of fire blight management. Several products
including copper based products, antibiotics like streptomycin sulfate and terramycin
(oxytetracycline), shoot growth retardants, plant resistance inducers and some biological
control agents are available. Application of streptomycin sprays immediately after
hailstorm can limit the extent of damage by shoot blight. In several growing areas, the
fire blight bacterium has developed resistance to the antibiotic streptomycin and,
therefore, make sure the strain in your orchard is still sensitive to this antibiotic and
discontinue its use if the strain is already resistant to it. Follow a resistance management
strategy when using this antibiotic. Consult your local farm adviser or technical representative, and always follow the pesticide label directions.
Avoid planting highly susceptible varieties and rootstocks. The following are some of
the popular apple varieties known to be highly susceptible to fire blight: Jonathan, Rome
Beauty, Gala, Idared, Fuji, Braeburn, Granny Smith, Pink Lady. The varieties Red
Delicious, Golden Delicious, Enterprise and Liberty are among the less susceptible
varieties. Among the rootstocks, M.9 and M.26 are highly susceptible, whereas M.7 and
some newer rootstocks from Geneva/Cornell program are least susceptible. Always
avoid planting highly susceptible varieties on highly susceptible rootstocks.
Avoid excessive nitrogen fertilization that promotes succulent shoot growth that is
susceptible to fire blight infection.
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