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.