Insect and Disease Fact Sheet Compliments of
New Century 
1-877-79TREES
Fire Blight of Apples,
Crabapples and Pears
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Fire blight is a common and very destructive
bacterial disease of apples and pears. The disease is so named because
infected leaves on very susceptible trees will suddenly turn brown, appearing
as though they had been scorched by fire. As a result of this disease, blight
susceptible pear cultivars are no longer grown in many parts in the Midwest.
Damage and losses from fire blight on apple result from: death or severe
damage to trees in the nursery; death of young trees in the orchard; delay of
bearing in young trees due to frequent blighting of shoots and limbs; loss of
limbs or entire trees in older plantings as the result of girdling by fire
blight cankers; and direct loss of fruit due to blighting of blossoms and
young fruit. Fire blight may cause severe damage to many other members of the
Rosaceae family. Quince, crabapple, mountain ash, spirea, hawthorn, pyracantha,
and cotoneaster are all susceptible. Cultivars within some of these species
are resistant. Symptoms Blossom and twig blight symptoms appear in the
spring. Diseased blossoms become water-soaked and turn brown. The bacteria
may then grow down into the blossom bearing twigs (spurs). Leaves on the spur
become blighted, turning brown on apple and black on pear. Droplets of milky
tan-colored bacterial ooze may be visible on the surface of diseased tissue.
These droplets contain millions of bacteria which can cause new infections. |
Fire blight on apple
twigs. Note the curved “Sheperd’s Crook” at the tip
of the infected twigs. |
Twig blight starts at the growing tips of shoots
and moves down into older portions of the twig. Blighted twigs first appear
water-soaked, and then turn dark brown or black. Blighted leaves remain
attached to the dead branches through the summer. The end of the branch may
bend over, resembling a shepherd's crook or an upside down "J". As the
fire blight bacteria move through blighted twigs into main branches, the bark
sometimes cracks along the margin of the infected area on the main branch
causing a distinct canker.
Both apple and pear fruit may be blighted.
Rotted areas turn brown to black and become covered with droplets of ooze. The fruit remains firm but later dries out
and shrivels into mummies.
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Causal Organism Fire blight is caused by the bacterium, Erwinia amylovora. The
fire blight bacteria over winter in living tissue at the margins of cankers
on the trunk and main branches. The bacteria become active in the spring when
temperatures get above 65 degrees F. Their growth is favored by rain, heavy
dews, and high humidity. By the time trees are blossoming, droplets of ooze
containing the bacteria are present on the surface of cankers. Relatively few
over wintering cankers become active and produce bacteria in the spring, but
a single active canker may produce millions of bacteria, enough to infect an
entire orchard. The bacteria in droplets of ooze are spread by splashing rain
or insects (mostly bees, flies, and ants) to open blossoms. The bacteria
multiply rapidly in the blossom nectar, and invade the blossom tissue through
natural openings called nectaries. The optimum
temperature range for blossom blight infection is 65 to 86 degrees F. The
bacteria are spread from blossom to blossom by rain or pollinating insects. |
Fire blight canker on
apple trunk. |
Actively growing shoot tips are infected by
bacteria that have been spread by rain or insects from both cankers and infected
blossoms. Invasion can occur directly through natural openings, such as
lenticels and stomata, under conditions of prolonged rain and high humidity.
However, shoot infection more commonly occurs through wounds created by sucking
insects, such as aphids, leafhoppers, and tarnished plant bugs; by wind
whipping; or by hail. Fire blight bacteria multiply rapidly within an infected
shoot. Droplets of ooze can form on the shoots within 3 days. Shoots remain
highly susceptible to infection until vegetative growth ceases and the terminal
bud is formed.
Control
Fireblight is
one of the most difficult diseases of apple to control, and there is no one
procedure that will give complete control. Though control is not an easy task,
the use of several practices in an integrated manner should result in minimal
damage from fire blight.
1. Plant apple, crabapple, and pear varieties
that are less susceptible to fire blight. Fireblight
is not as severe a disease problem on
most crabapple varieties. A few crabapple varieties which can develop severe fireblight include: Silver Moon, Snowdrift, Red Jade, and Van Esseltine.
2. Prune out fire blight cankers and blighted
twigs. To decrease the inoculum level for the following season,
prune out blighted twigs and cankers during the dormant season. During
the dormant season (winter) there is much less chance of spreading bacteria.
Branches that are more than half-girdled by cankers should be removed. Cut off
blighted twigs by making cuts at least 4 inches below the visible dead wood.
Cankers can be cut out of trunks or large branches by removing dead tissue down
to wood that appears healthy. If blighted twigs are pruned out during summer,
cuts should be made 12 to 15 inches below diseased wood and pruning tools
should be disinfested by dipping in a 2: 10 solution of household bleach in
water after each cut.
3. Follow proper pruning and fertilization
practices. Excessive nitrogen fertilizer and heavy pruning will promote
vigorous growth of succulent tissue which is more susceptible to fire blight.
Adjust management practices on susceptible varieties to promote moderate
growth. Make fertilizer applications in early spring or late fall after growth
has ceased.
4. Sucking insects create wounds through which
fire blight bacteria can enter. These pests should be controlled throughout the
growing season. To protect bees, 90 not apply insecticides during bloom.
5. Commercial growers should consider following
a recommended spray program for fire blight. Sprays for fire blight control are
generally not recommended for backyard growers. Instead, backyard growers are
encouraged to plant less susceptible varieties and use other non-chemical
control measures.
Information obtained
through the Ohio State Extension Fact Sheet HYG-3002-94
Insect and Disease Fact Sheet Compliments of
New Century
1-877-79TREES