Introduction: Fire blight is a destructive bacterial disease of apples and pears that
kills blossoms, shoots, limbs, and, sometimes, entire trees. The disease is generally
common throughout the mid-Atlantic region although outbreaks are typically very erratic,
causing severe losses in some orchards in some years and little or no significant damage
in others. This erratic occurrence is attributed to differences in the availability of
overwintering inoculum, the specific requirements governing infection, variations in
specific local weather conditions, and the stage of development of the cultivars
available. The destructive potential and sporadic nature of fire blight, along with the
fact that epidemics often develop in several different phases, make this disease difficult
and costly to control. Of the apple varieties planted in the mid-Atlantic region, those
that are most susceptible include 'York', 'Rome', 'Jonathan', 'Jonagold', 'Idared',
'Tydeman's Red', 'Gala', 'Fuji', 'Braeburn', 'Lodi', and 'Liberty'. 'Stayman' and 'Golden
Delicious' cultivars are moderately resistant and all strains of 'Delicious' are highly
resistant to fire blight, except when tissues are damaged by frost, hail or high winds.
II. Symptoms: Overwintering cankers
harboring the fire blight pathogen are often clearly visible on trunks and
large limbs as slightly to deeply depressed areas of discolored bark, which
are sometimes cracked about the margins. The largest number of cankers,
however, are much smaller and not so easily distinguished. These occur on
small limbs where blossom or shoot infections occurred the previous year and
often around cuts made to remove blighted limbs (photo 2-17). Since many of
these cankers are established later in the season, they are not often strongly depressed and seldom show bark cracks at their margins.
Also, they are often quite small, extending less than one inch (25
mm), with reddish to purple bark that may be covered with tiny black
fungus fruiting bodies (most notably Botryosphaeria obtusa, the black
rot pathogen of apple).
Blossom blight symptoms
most often appear within one to two weeks after bloom and usually involve the entire
blossom cluster, which wilts and dies, turning brown on apple (photo 2-18) and quite black
on pear (photo 2-19). When weather is favorable for pathogen development, globules of
bacterial ooze can be seen on the blossoms (photo 2-20). The spur bearing the blossom
cluster also dies and the infection may spread into and kill portions of the supporting
limb. The tips of young infected shoots wilt, forming a very typical "shepherd's
crook" symptom (photo 2-21). Older shoots that become infected after they develop
about 20 leaves may not show this curling symptom at the tip. As the infection spreads
down the shoot axis, the leaves first show dark streaks in the midveins,
then wilt and turn brown, remaining tightly attached to the shoot
throughout the season. As with blossom infections, the pathogen often
invades and kills a portion of the limb supporting the infected shoot. The first symptom
on water sprouts and shoots that are invaded systemically from nearby
active cankers is the development of a yellow to orange discoloration
of the shoot tip before wilting occurs (photo 2-22). In addition, the
petioles and midveins of the basal leaves on such sprouts usually become necrotic before
those at the shoot tip.
Depending on the cultivar and its stage of
development at the time infection occurs, a single blossom or shoot infection can result
in the death of an entire limb, and where the central leader or trunk of the tree is
invaded, a major portion of the tree can be killed in just one season. In general,
infections of any type that occur between petal fall and terminal bud set usually lead to
the greatest limb and tree loss. In addition, heavily structured trees tend to suffer less
severe limb loss than those trained to weaker systems for high productivity.
Where highly susceptible apple rootstocks (M.26, M.9, Mark) become infected, much
of the scion trunk and major limbs above the graft union very
typically remain symptomless, while a distinct
dark brown canker develops around the rootstock. As this rootstock canker girdles the
tree, the upper portion shows symptoms of general decline (poor foliage color, weak
growth) by mid to late season. In some instances, the foliage of trees affected by
rootstock blight develop early fall red color in late August to early September, not
unlike that often associated with collar rot disease caused by a soilborne fungus. Some
trees with rootstock infections may not show decline symptoms until the following spring,
at which time cankers can be seen extending upward into the lower trunk (photo 2-23).
III. Disease Cycle: The bacterial pathogen causing fire blight
overwinters almost exclusively in cankers on limbs infected the previous season. The
largest number of cankers and, hence, those most important in contributing inoculum, occur
on limbs smaller than 1.5 inches (38 mm) in diameter, especially around cuts made the
previous year to remove blighted limbs. During the early spring, in response to warmer
temperatures and rapid bud development, the bacteria at canker margins begin multiplying
rapidly and produce a thick yellowish to white ooze that is elaborated onto the bark
surface up to several weeks before the bloom period. Many insect species (predominantly
flies) are attracted to the ooze, and subsequently disperse the bacteria throughout the
orchard. Once the first few open blossoms are colonized by the bacteria, pollinating
insects rapidly move the pathogen to other flowers, initiating more blossom blight. These
colonized flowers are subject to infection within minutes after any wetting event caused
by rain or heavy dew when the average daily temperatures are equal to or greater than 60 F
(16 C) while the flower petals are intact (flower receptacles and young fruits are
resistant after petal fall). Once blossom infections occur, early symptoms can be expected
with the accumulation of at least 103 degree days (DD) greater than 55 F (57 DD greater
than 13 C) which, depending upon daily temperatures, may require 5 to 30 calendar days.
the appearance of blossom blight symptoms, the number and distribution of inoculum sources
in the orchard increase greatly. Inoculum from these sources is further spread by wind,
rain, and many casual insect visitors to young shoot tips, increasing the likelihood for
an outbreak of shoot blight. Recent research conducted in Pennsylvania indicates that
aphid feeding does not contribute to shoot blight. More research is needed to determine
whether or not leafhoppers play a role in the incidence of shoot blight. Most shoot tip
infections occur between the time that the shoots have about nine to ten leaves and
terminal bud set, when sources of inoculum and insect vectors are available, and daily
temperatures average 60 F (16 C) or more.
In years when blossom infections do not occur, the primary sources of
inoculum for the shoot blight phase are the overwintering cankers and,
in particular, young water sprouts near these cankers, which become
infected as the bacteria move into them systemically from the canker
margins. Such systemic shoot infections,
called canker blight, are apparently initiated about 200 DD greater
than 55 F (111 DD greater than 13 C) after green tip, although visible
symptoms may not be apparent until the accumulation of at least 300 DD
greater than 55 F (167 DD greater than 13 C) after green tip. In the
absence of blossom infections, the development of shoot blight
infections is often localized around areas with overwintering cankers.
Although mature shoot and limb
tissues are generally resistant to infection by E. amylovora, injuries caused by hail,
late frosts of 28 F (-2 C) or lower, and high winds that damage the foliage can create a
trauma blight situation in which the normal defense mechanisms in mature tissues are
breached and infections occur. Instances of trauma blight are known to occur even on
normally resistant cultivars like 'Delicious'.
Rootstock blight, yet another phase of fire blight, has
been recognized recently and is associated primarily with the highly susceptible M.26, M.9
and Mark rootstocks. On these trees, just a few blossom or shoot infections on the scion
cultivar can supply bacteria that then move systemically into the rootstock where a canker
often, but not always, develops and eventually girdles the tree. Trees affected by
rootstock blight generally show symptoms of decline and early death by mid to late season,
but may not be apparent until the following spring.
IV. Monitoring: Concentrate monitoring in orchard
blocks where the disease occurred during the previous season. Observe blighted limbs and
shoots for removal during normal pruning operation. There may be a need to remove whole
trees on some occasions.
blight occurred the previous year in orchards grown on susceptible
rootstocks (M.26, M.9, Mark), trees showing poor foliage color or
dieback should be examined for rootstock cankers and, if found,
removed from the orchard immediately and destroyed. A very important
aspect of fire blight management involves monitoring the weather for
the specific conditions that govern the build-up of inoculum in the
orchard, the blossom infection process and the appearance of symptoms.
A weather station (discussed in chapter 10) that records the daily
minimum and maximum temperatures and rainfall amounts is needed. When
50 percent of the buds show green tissue, begin keeping a daily record
of the cumulative degree days (DD) greater than 55 F (12.7 C; see
Appendix B and F). This information can be used to signal when
symptoms are likely to appear in the orchard for blossom
blight [103 DD greater than 55 F (57 DD greater than 12.7 C) after
infection] (photos 2-18, 2-20), canker blight [about 300 DD greater
than 55 F (167 DD greater than 12.7 C) after green tip] (photo 2-22),
and early shoot blight [about 103 DD greater than 55 F (57 DD greater
than 12.7 C) after blossom blight or canker blight symptoms appear]
to monitor and record the daily minimum and maximum temperatures and rainfall amounts, and
continue to accumulate degree days (DD) greater than 55 F (12.7 C; see Appendix B and F).
At the full pink stage (i.e., first flower open in the orchard), a record should also be
kept of the cumulative degree hours (DH) greater than 65 F (18.3 C; see Appendix B and G).
Once a total of 200 or more DH greater than 65 F (111 DH greater than 18.3 C) has
accumulated after the start of bloom, any wetting event caused by rain or heavy dew that
wets the foliage is likely to trigger a blossom infection event if the average daily
temperature is 60 F (15.6 C) or more.
This information can be used to schedule streptomycin
sprays, which are most effective if applied on the day before or the day of an infection
event. Such sprays protect all flowers open at the time of treatment. However, because
other flower buds may open after treatment, reassess the need for additional sprays at
four-day intervals during bloom. Continue to monitor for strikes and remove all blighted
Monitor the orchard to locate blighted limbs (photo 2-22)
for removal. For the greatest effect on the current season's damage severity, infected
limbs should be removed as soon as early symptoms are detected and before extensive
necrosis develops. Where the number and distribution of strikes is too great for removal
within a few days, it may be best to leave most strikes and cut out only those that
threaten the main stem. On young trees, and those on dwarfing rootstocks, early strikes in
the tops of the trees often provide inoculum for later infections of shoots and sprouts on
lower limbs near the trunk, which may result in tree loss. Give these early strikes a high
priority for removal.
Look for symptoms of early tree decline or early fall color
in orchards planted on highly susceptible rootstocks (M.26, M.9, Mark) where the disease
developed this year. These symptoms may appear either on one side or throughout individual
trees. Examine the rootstock area of these trees just below the graft union for evidence
of cankering or bacterial ooze. Remove any tree showing these symptoms during this period.
V. Management: Many practices can
help reduce the incidence of fire blight and may help reduce the severity of the disease
when it occurs. Not all measures suggested below are necessary or even feasible in
every planting, since planting systems play a large role in contributing to the level of
risk of disease development. No single control method is adequate and, in regions
where it is established, a conscious effort must be made to control the disease each year.
Even under the most conscientious efforts, in some years losses from fire blight
can be devastating.
Chemical and biological control: A
copper spray applied at the 1/4-inch green tip stage may reduce the amount of inoculum on
the outer surfaces of infected trees. At bloom, antibiotic sprays are highly
effective against the blossom blight phase of the disease. These sprays are critical
because effective early season disease control often prevents the disease from becoming
established in an orchard. Predictive models, particularly Maryblyt, help to identify potential infection periods and
improve the timing of antibiotic treatments, as well as avoid unnecessary treatments.
Strains of the pathogen that are resistant to streptomycin are present in some
orchards in the eastern U.S., and are widespread in most apple and pear regions of the
western U.S. Biological control agents, although not widely used, have provided
partial control of blossom infections. More effective biological agents are required
if their use is to become widespread.
Removing sources of infection:
Dormant pruning to remove overwintering infections helps reduce inoculum for the next
season. Make cuts about 4 inches below any signs of dead bark. Remove pruned
material from the orchard. Beginning about one week after petal fall, monitor the
orchard to locate blighted limbs for removal. For the greatest effect on the current
season's damage severity, infected limbs should be removed as soon as early symptoms are
detected and before extensive necrosis develops. Where the number and distribution of
strikes is too great for removal within a few days, it may be best to leave most strikes
and cut out only those that threaten the main stem. On young trees, and those on dwarfing
rootstocks, early strikes in the tops of the trees often provide inoculum for later
infections of shoots and sprouts on lower limbs near the trunk, which may result in tree
loss. Give these early strikes in the tops of trees a high priority for removal. Do
not combine the practices of fire blight removal with pruning and training of young,
Insect control: The role of insects
in the transmission of fire blight bacteria is under investigation. It is likely
that insects that cause wounds (leafhoppers, plant bugs, pear psylla) can create places
for bacteria to enter the tree, and some summer infections (shoot blight) are probably
facilitated by insects. Where fire blight is a problem, and until more is known
about their specific role in the spread of the disease, controlling these insects at
levels below their economic injury threshold is advised.
Cultural practices: Use management
systems that promote early cessation of tree growth without adversely affecting tree
vigor. Excessive vigor is an important component of orchard risk for fire blight.
When tree growth continues past mid summer, the likelihood that late season
infections will overwinter increases. Orchards should be established on well-drained
soils, avoiding low, frost-prone or potentially water-logged areas, and nitrogen
fertilizer should be applied based on analyses of foliage N levels.
Resistant cultivars: When
establishing new orchards, consider susceptibilities of the scion and rootstock to fire
blight. Although none are immune, there is considerable variation among apple cultivars (and pear cultivars) in susceptibility to fire
blight. Some cultivar/rootstock combinations
are so susceptible to fire blight that investments in these are extremely high risk.
In the eastern U.S., Gala on M.26 is a good example. Long range plans for
establishing new orchards with fire blight susceptible cultivars should include
contingency plans for controlling the disease without streptomycin.
Chemical control -
Chemical control - home
orchardists (pdf file - Acrobat Reader required)
Text prepared by P.W. Steiner, T. van der
Zwet, and A. R. Biggs
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