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A continuación se exponen diversos
artículos que esperamos ilustren e instruyan a nuestros lectores.
1. Fuego bacteriano, (Erwinia amylovora)
 Es
una enfermedad producidad por la bacteria Erwinia amylovora.
Pueden padecerla los frutales peral y manzano
En peral es muy espectacular el efecto del ataque, puesto que aparece
como quemado por fuego, ennegrecido. En manzano y membrillero es igual,
pero con menos incidencia.
El Fuego bacteriano ataca a las ramas jóvenes y a los brotes, los cuales
aparecen como quemados y destacan durante el verano.
Después se forman chancros en las ramas, que produce una viscosidad
bacteriana amarilla o blanca según el huesped. Los chancros en tronco y
cuello de la raíz provocan la muerte de la planta completa.
Control
Buen cultivo para tener las plantas fuertes.
Que no se produzcan heridas. Por ahí entran las bacterias (poda, rotura
de ramas, grietas por el frío, insectos, etc.). El Oxicloruro de cobre
sirve para proteger las heridas de las infecciones.
Poda y destrucción de las partes enfermas.
Retira las plantas afectadas o poda hasta 60 centímetros más allá del
área afectada.
Sumerge el serrucho en desinfectante antes de usarlo para otro árbol.
En Holanda y Estados Unidos se emplean bactericidas.
www.infojardin.com
Fire Blight, Erwinia amylovora
 I.
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.
With 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.
Where fire 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] (photo 2-21).
Continue 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 limbs.
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,
high-density trees.
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.
Text prepared by P.W. Steiner, T. van der Zwet, and A. R. Biggs
www.caf.wvu.edu
Apple
Fire Blight
Pathogen: Erwinia amylovora
SYMPTOMS
Fire blight causes blossom clusters to wilt and collapse in late spring.
A brownish, sticky exudate is produced from diseased tissue. The tips of
infected young succulent shoots curve into a characteristic shepherd's
hook.
COMMENTS ON THE DISEASE
 Erwinia
amylovora overwinters in twig and branch cankers, is spread by rain
splash and insects, and favored by warm, humid weather during bloom.
Overhead sprinklers may aggravate the problem, especially in fire blight
susceptible cultivars. Damage ranges from individual flower or shoot
infections to death of limbs.
Apple cultivars vary in susceptibility and extent of damage. For example,
in Granny Smith, infections are usually limited and do not cause severe
structural damage to the tree whereas Gala and Fuji trees may be
devastated. The rootstocks M26 and M9 are very susceptible to this
disease, and infections of these rootstocks can cause the trees to die.
MANAGEMENT
Fire blight development is influenced primarily by seasonal weather.
Warm spring weather, accompanied by intermittent rain and hail, is ideal
for disease development. Other influences on disease development are the
varieties and rootstocks used in the orchard, location of the orchard,
application of too much nitrogen fertilizer, heavy pruning, or over-irrigation.
Management relies on maintaining trees in the proper range of vigor,
applying blossom sprays of antibiotics or copper, and most importantly,
promptly finding, removing, and destroying blight strikes. Remove
holdover cankers by cutting back to healthy wood. This disease can be
difficult to control.
Organically Acceptable Methods
Cutting out diseased wood and treatment
with Bordeaux or fixed copper materials are organically acceptable
methods.
Chemical Control
Blossom applications of copper materials or the antibiotic streptomycin
are necessary in some apple-growing areas to reduce the spread of fire
blight bacteria. The timing of the first application is critical. In
California, average daily temperatures of degree-hours are used to
schedule fire blight sprays. For detailed information on these methods,
see Integrated Pest Management for Apples and Pears, 2nd ed. UC ANR
Publication 3340. Rain or hail requires immediate re-spray of the
orchard.
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