caused by the bacterium Erwinia amylovora, is a common and very serious
bacterial disease. The disease is also referred to as blossom blight,
spur blight, fruit blight, twig blight, or rootstock blight depending
on the plant part that is attacked. Erwinia amylovora infects approximately
75 different species of plants, all in the family Rosaceae. The hosts
for this bacterium include apple, blackberry, cotoneaster, crabapple,
firethorn (Pyracantha), hawthorn, Japanese or flowering quince, mountain-ash,
pear, quince, raspberry, serviceberry, and spiraea. The cultivated apple,
pear, and quince are the most seriously affected species, but many ornamentals
serve as overwintering hosts for the bacterium and are important sources
of new infections each year.
fire blight in apples and pears include: (1) death or severe damage to
trees in the nursery; (2) death of young trees in the orchard; (3) delay
of bearing in young trees due to frequent blighting of shoots and limbs;
(4) loss of limbs or entire trees in older plantings as the result of
girdling by fire blight cankers; and (5) partial loss of the crop by the
blighting of the blossoms and young fruits.
of fire blight is demonstrated by its effect on the commercial pear industry.
At one time, the pear was a popular and widely grown fruit in the Midwest,
comparable in importance to the apple. Fire blight has eliminated the
possibility of commercial pear production in most areas of the Midwest.
The fire blight bacterium can infect any portion of a susceptible plant.
The common types of infection are blossom blight, shoot blight, and branch
and trunk canker. Blossom blight is most common on pear, apple, hawthorn,
mountain-ash, and Pyracantha. Infected blossoms become water-soaked and
darker green as bacteria invade new tissues. Within 4 or 5 days fruiting
spurs may begin to collapse, turning dark brown to black (spur blight)
on image for larger version
Figure 1. Infected Blossoms Wilt, Turn Light to Dark Brown
on Apple, Black on Pear.
on image for larger version
Figure 2. Rough Corky Tissue Forms at Margin of Canker.
wilt, die, and turn dark brown to black, usually remaining attached to
the tree throughout the summer. As the bacteria move through the pedicel,
the tissue becomes water-soaked and dark green. Infected tissues may exude
either small droplets of a milky-white ooze or fine, hairlike strands
containing millions of fire blight bacteria that can initiate new infections.
The ooze, which later turns an amber color, contains countless bacteria
that also are capable of causing new infections.
is recognized by the rapid dieback of shoots. Infections begin in the
shoot tips and move rapidly down from one to twelve inches a day. Newly
infected tissue becomes water-soaked and dark green or reddish brown in
color. As in spur blight, infected leaves die and turn either dark brown
(apple, crabapple) or black (pear) and remain attached throughout the
growing season. Frequently, the tip of the blighted shoot bends over and
resembles a shepherd's crook (Figure 3). Hawthorn leaves turn yellow,
then brown, shrivel, and fall prematurely.
on image for larger version
Figure 3. Apple Shoots Affected by Fire Blight. Note "shepherd's
crooks" at the tips. (Photo courtesy of A.L. Jones, Michigan
shoots of Jonathan apple trees are often blighted back 12 to 36 inches
(30 to 90 centimeters). The infection may continue down a shoot or flower
spur into a larger branch or trunk, forming a canker. These cankers continue
to enlarge during the growing season and may girdle the affected part,
resulting in the death of the entire branch or tree. The surface of a
canker is somewhat sunken, relative to the surrounding healthy tissue,
and the bark is usually darker in color. A distinct zone of rough, corky
tissue may form at the margin of the canker (Figure 2 and 6). In some
cultivars, it is difficult to determine the margin of the canker without
cutting into the wood to expose the discolored and infected tissue. The
diseased inner bark of older branches becomes reddish brown and marbled,
in contrast with the whitish color of normal wood. The surface of smooth-barked
branches darkens; also, cracks usually develop at the margins of the diseased
on image for larger version
Figure 4. Droplets of Bacterial Ooze on Immature Apple. (Photo
courtesy of A.L. Jones, Michigan State University.)
susceptible to infection until just before maturity. The incidence of
fruit infection is usually low; however, infections can follow mechanical
injury such as hail or insect feeding. Diseased fruit is first water-soaked,
turns brown, shrivels, and turns black. Droplets of milky and sticky bacterial
ooze are commonly observed on the fruit surface during wet, humid weather
called "rootstock blight" usually occurs in high density orchards
planted with susceptible rootstock such as M.9 and M.26. Rootstock blight
is caused by formation of cankers on susceptible rootstocks which can
completely girdle and kill the tree in one to a few months (Figures 5
and 6). The bacteria in the infected blossoms or shoots pass through healthy
limbs and trunks and reach the rootstock and produce the cankers. Root
suckers and sprouts from susceptible rootstocks also may become infected,
much as do the shoots. These infections can also lead to the invasion
of the entire root system and the rapid death of the tree. Fire blight
is often followed by Black Rot and Wood Rot.
on image for larger version
Figure 5. A Three-Year Old Fugi Apple Tree Killed from Infection
of M.9 Root Stock by Fire Blight.
The fire blight bacteria overwinter in living tissue at the margins of
trunk and branch cankers that were formed by infections initiated in previous
years, and possibly in buds. The bacteria resume growth in the spring
when temperatures are above 65°F (18°C) with survival favored
by rain, heavy dews, and high humidity. By the time trees are blossoming,
ooze containing bacteria are present on the surface of cankers. Relatively
few cankers survive winter, become active, and produce bacteria in the
spring. However, a single active canker will produce millions of bacteria,
enough to infect an entire orchard. The cankers most likely to produce
bacteria in the spring are those with smooth margins between healthy and
infected tissue, and those formed in older wood. Cankers produce bacteria
in droplets of ooze that are spread by splashing rain or insects (mostly
bees, flies, and ants) to open blossoms. The bacteria multiply rapidly
on the blossom and invade the tissue through the nectaries (non-cutinized
or flower parts). The bacteria then spread from blossom to blossom by
rain or pollinating insects. The optimum temperature range for blossom
blight infection is 65° to 86°F (18° to 30°C).
on image for larger version
Figure 6. Fire Blight on the M.9 Root Stock of an Apple Tree.
shoot tips are frequently infected by bacteria that have been spread from
cankers and infected blossoms. The invasion of shoot tips can occur through
natural openings, such as lenticels and stomata, but more commonly through
wounds created by sucking insects such as aphids, leafhoppers, and tarnished
plant bugs, by wind whipping, or by hail. The fire blight bacteria reproduce
rapidly within an infected shoot. Droplets of ooze form on the shoots
within three days. This ooze serves as a source of inoculum for the further
spread of the disease. Shoots remain highly susceptible to infection until
vegetative growth ceases and the terminal bud forms.
temperature 76° F or 24° C) and moist weather is favorable for
infection, and rapid growth encourages disease development. Nitrogen fertilization,
late fertilizer application, poor soil drainage, and other factors that
promote succulent growth or delay the hardening of the tissues from midsummer
into autumn tend to increase the severity of this disease.
ooze remains infectious for more than a year if it is not subjected to
alternate wetting and drying. Contaminated boxes or other containers that
are taken into orchards for fruit picking may serve as a potential source
single method is adequate to effectively control fire blight. A combination
of practices is needed to reduce the severity of the disease.
Choose the proper cultivars. Apple cultivars differ widely in their
susceptibility to fire blight (Table 1 and Table
2). During warm and rainy weather, cultivars rated moderately susceptible
or moderately resistant will develop shoot infections; however, the extent
to which shoot infections progress will be less in resistant cultivars than
in susceptible cultivars. Commercial growers should select rootstocks
that are less susceptible to fire blight. Bartlett pears are extremely susceptible
to fire blight and are not recommended for planting in Illinois.
Select planting sites with good soil drainage. Trees are more susceptible
to fire blight in poorly drained sites than in well-drained ones. Tree productivity
will also be lower on such sites. Drainage can often be improved by tiling.
Follow proper pruning and fertilization practices. Using nitrogen
containing fertilizer and/or doing heavy pruning promotes vigorous growth
and increases susceptibility. Fertilization and pruning practices on susceptible
cultivars should be adjusted to limit excessive growth. For bearing
trees, moderate shoot growth is 6 to 12 inches (15 to 30 centimeters) per
year. If the growth is more than 12 inches, do not apply fertilizer until
shoot growth is reduced to less than 6 inches.
Apply fertilizer in the early spring (6 weeks before bloom) or apply in
late fall after growth has ceased. Applications in midseason prolong the
time during which shoots are susceptible to infection and increase the likelihood
of winter injury to tender wood.
Prune out fire blight cankers during the dormant season. Delay
the removal of infected shoots until the dormant season in order to avoid
spreading infection to healthy shoots. Make pruning cuts at least 6 inches
(15 centimeters) below the last point of visible infection. After each pruning
cut, sterilize the pruning shears by dipping them in a freshly made solution
of 1 part liquid bleach (Clorox, Purex, Saniclor, Sunny Sol) added to 4
parts of water. Examine the larger branches and trunks carefully for cankers,
since these are likely to overwinter and produce new infections in the spring.
Root suckers and watersprouts should also be removed because infection of
these parts can lead to infection and death of entire trees. Certain dwarfing
rootstocks used for apples are prone to suckering. Commercial growers should
select rootstocks that are resistant to fire blight or that show little
tendency to produce root suckers.
Examine wild, neglected, ornamental hosts of the fire blight bacterium growing
in the vicinity of home or commercial orchards for cankers. In addition
to seedling apples, crabapples, pears, and quince, check hawthorns (Crataegus
spp), firethorns (Pyracantha spp), cotoneasters, mountain-ashes (Sorbus
spp), and spiraea. Remove the cankers when found or destroy the entire plant
Follow a bactericide spray program. Like most bacterial diseases
of plants, fire blight is very difficult to control; however, it can be
reduced by spraying. Commercial orchardists should follow the spray schedule
outlined in the annual Midwest Tree Fruit Pest Management Handbook. The
antibiotic streptomycin is the most effective material for controlling fire
blight; timely sprays will reduce the incidence of fire blight but must
be applied before the appearance of symptoms.
Temperatures at the prebloom and bloom stages are important in determining
whether fire blight will occur in any given year. The bacteria reproduce
only when the temperature is warmer than 65°F (18°C).
The following concept was developed for predicting outbreaks of blossom
blight in Illinois. The first idea to understand is that of a "heating
degree-day." A "degree-day" occurs when the maximum
daily temperature reaches 66°F (19°C). Start counting degree
days after each spring frost. A freeze greatly reduces the number
of fire blight bacteria in holdover cankers and on tree surfaces.
Bacteria reach dangerous population levels ONLY after 30 degree-days
have elapsed since the last frost. Degree-days may be accumulated
in a variety of ways; for example, 2 days with a maximum daily temperature
of 80°F (27°C), 3 days of 75°F (24°C), or 6 days of
70°F (21°C) following a freeze will provide enough accumulated
warmth to allow bacterial populations to increase greatly in number
and present a serious fire blight threat to blossoms. When 30 degree-days
have occurred and when blossoms are still present (including secondary
bloom), apply the first streptomycin spray. Repeat the spray at 4-day
intervals through the bloom period. At temperatures above 86°F
(30°C), bacteria will not multiply. Therefore, it is not necessary
to apply streptomycin when the temperatures average below 65°F
or above 86°F.
Dr. Paul Steiner at the University of Maryland has developed a computer
software package which very accurately predicts the potential for fire blight
occurrence. The prediction is based upon temperature and moisture during
bloom. This computer software is called MARYBLYT and is available from the
University of Maryland, Department of Botany, College Park, MD 20742.
Streptomycin can effectively protect the susceptible apple and pear
flowers, but for maximum effect it must be applied the day of, or
the day before infection event occurs. Missing the critical window
of effectiveness by even 24 hours can result in plant infection and
buildup of a significant amount of bacteria for later infections.
If the blossom blight is well controlled, the subsequent increase
of fire blight in summer is often prevented. To prevent development
of streptomycin-resistant strains of the pathogen, no more than 4
applications of streptomycin per season is recommended. Streptomycin
is more effective in preventing blossom infection and the management
of the shoot blight phase of fire blight should not be attempted with
streptomycin. However, application of streptomycin immediately following
hail storms is highly recommended. Streptomycin is most effective
when applied alone, as a dilute spray, under slow drying conditions
(generally between 10 p.m. and 3 a.m.), and when daytime temperatures
reach 65°F or above. Apply 100 parts per million (ppm) of streptomycin
if the temperature is below 65°F, and 50 ppm if the temperature
is above 65°F.
Bordeaux mixture (6-6-100), made by mixing 6 pounds of crystalline copper
sulfate (bluestone or blue vitriol) and 6 pounds of fresh hydrated spray
lime in 100 gallons of water, will help control fire blight but may cause
russeting of the fruit. Bordeaux mixture is recommended for use by growers
who had a severe epidemic the previous year. Bordeaux mixture should be
applied at the green tip to quarter inch green stage of flower bud growth.
Do not mix bordeaux with other chemicals, and use it as soon as it is prepared.
Do not follow bordeaux mixture with streptomycin, and do not concentrate
bordeaux mixture greater than 2 times. Bordeaux is not as effective as streptomycin
in controlling fire blight.
Copper sulfate (4 lb/100 gal or 2kg/400 liters), applied when trees are
dormant in early spring also helps reduce the number of bacteria present
in ooze on cankers and thus slows the buildup of bacteria in the orchard
prior to bloom.
- Control sucking insects. Good control of aphids, leafhoppers, plant
bugs, and psylla on pears helps prevent shoot infection. Commercial orchardists
should follow a spray program outlined in Midwest Tree Fruit Pest Management
Handbook. [Copies of the circular mentioned are available at your nearest
Extension Office or from Information Technology and Communication Services
(ITCS), 1401 S. Maryland Dr., Urbana, IL 61801 (217/333-2007).]
1. Relative Susceptibility of Common Apple Cultivars and Rootstocks
to Fire Blight.
of these pear-apples are relatively unknown
bSusceptible to trunk infections
cOrnamental tree - no edible fruit
Modern Crabapples not suggested for Illinois but highly resistant
or immune to Rusts, Scab, Fire Blight, Powdery Mildew and Frogeye
cv. Case Seedling
cv. Golden Gem
cv. Golden Gem
Golden Gem (PLT 788-58)
cv. Henry Kohankie
cv. Honeywood #14 hybrid
(scab immune clone GR 700-58)
cv. Minn. 1492
cv. Morden 19-27
c. Professor Sprenger
c. R.M.F. 102
cv. Simpsin 4-28
cv. Simpson 11-57
cv. Simpson 11-58
information concerning diseases of crucifers and other vegetables, contact
Mohammad Babadoost, Extension Specialist in Fruit and Vegetable Pathology,
Department of Crop Sciences, University of Illinois at Urbana-Champaign.