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Figure 1.  Fireblight of pear.
Note shepherd’s crook symptom
on the blighted twig.

Figure 2.  Fireblight on trunk of
English hawthorn.

Fireblight affects a wide range of rosaceous plants including apple, flowering crab, pear, firethorn, hawthorn, cotoneaster, quince, and mountain ash. Although fireblight can be a devastating disease, it is not as common in Kansas as scab and cedar-apple rust.

Symptoms of fireblight are most noticeable in the spring on blossoms and succulent new growth. Infected blossoms become water-soaked and turn dark green or brown. Young, infected shoots rapidly wilt as if scorched by fire. The terminal end of the diseased shoot becomes hooked and is commonly referred to as a shepherd’s crook. During wet weather, small droplets of amber-colored ooze containing millions of bacteria can be seen leaking from infected tissue.

After initial infection of the shoots, the bacterium may move long distances within the living tissue and kill large portions of the tree. Infected areas of the bark on branches and trunks become slightly depressed and darker in color than surrounding healthy bark. When the outer bark is peeled away, the inner tissues appear red and water-soaked.

Cause

The bacteria (Erwinia amylovora) overwinters in dead portions of the trunk or scaffold branches called holdover cankers. During wet weather in the spring, bacteria ooze from canker margins in gelatinous strands. The bacteria in the strands are splashed to flowers by rainfall or carried to the flowers by insects. The bacteria enter through natural openings in the floral parts. Wet weather and temperatures between 65 and 86 F favor infection. Honeybees visiting diseased flowers become contaminated with the bacteria, and spread them to adjacent healthy flowers. Shoot infection may also occur at wounds caused by pruning cuts or hail injury.

Control

Several cultural practices will reduce or prevent the occurrence of fireblight on both apple and flowering crab. During the winter, remove all dead shoots, infected spurs, and larger holdover cankers that harbor the fireblight bacterium. Pruning cuts should be made at least 6 inches below the margin of dead tissue. Pruning tools should be disinfected in a 10% bleach solution (1 part household bleach to 9 parts water) or in a 70% ethanol solution. Household bleach is corrosive, so pruning tools should be cleaned thoroughly and oiled after use. During the summer, periodically inspect the trees for symptoms of fireblight. Diseased areas may be pruned out during the summer, provided a few precautions are taken. Never prune during wet weather. Pruning cuts in the summer should be made 8 to 10 inches below diseased tissue, and pruning tools should be disinfected between each cut.

Fireblight is most severe on trees and shrubs that have an abundance of succulent shoot growth. Avoid overfertilization, fertilizing late in the growing season, and overpruning which leads to the formation of numerous adventitious sprouts. Do not plant susceptible cultivars (Tables 1 and 2) of flowering crab or apples in areas where fireblight has been a problem.

The antibiotic streptomycin sulfate (Agristrep) may be used to protect susceptible apple cultivars during the bloom period. Applications should begin at the first sign of blossoms and continue at four-day intervals through petal fall. Sprays should be made at night or under slow-drying conditions. The use of streptomycin after bloom to control the twig blight phase of the disease is of limited value.

Reference to products in this publication is not intended to be an endorsement to the exclusion of others which may be similar. Persons using such products assume responsibility for their use in accordance with current label directions of the manufacturer.

Web updated 9/01/06
Web comments to jpierz@ksu.edu