Friday, 30 August 2019

Bacterial Diseases of Fruit Crops

Bacterial Diseases of Fruit Crops


APPLE AND PEAR

Fire Blight of Apple and Pear

Pathogen: Erwinia amylovora

Fire blight, also written fireblight, is a contagious disease affecting apples, pears, and some other members of the family Rosaceae. Fire blight is a destructive bacterial disease of apples and pears that kills blossoms, shoots, limbs, and sometimes the entire trees. It poses a serious concern to producers of apples and pears. Under optimal conditions, it can destroy an entire orchard in a single growing season.

Pears are the most susceptible, but apples, loquat, crabapples, quinces, hawthorn, Cotoneaster, Pyracantha, raspberry, and some other rosaceous plants are also equally vulnerable to fire blight. The disease is believed to be indigenous to North America, from where it spread to the rest of the world.

Pathogen

E. amylovora is a native pathogen of wild, rosaceous hosts in eastern North America. These hosts include hawthorn, serviceberry, and mountain ash. Early European settlers introduced apple and pear to North America. The first report of fire blight as a disease of apple and pear occurred in 1780, in the Hudson Valley of New York. In California, the disease was first reported in 1887. Early nineteenth- and twentieth-century horticultural texts and bulletins recognized fire blight as a serious disease of pear, provided descriptions of symptoms, and outlined pruning practices for control. Nonetheless, in the eastern United States, fire blight proved to be destructively epidemic on pear, limiting the cultivation of this host. Even today, the threat of fire blight restricts commercial production of pear to semi-arid, desert areas west of the Rocky Mountains.

E. amylovora has the distinction of being the first bacterium shown to be a pathogen of plants. Koch’s postulates for E. amylovora were fulfilled by J.C. Arthur in 1885, but the genesis of the concept that bacteria can be plant pathogens required the contributions of many scientists (notably T.J. Burrill) and growers over a period extending from 1846 to 1901. E. amylovora is also one of the first plant pathogens to be associated with an insect vector. In the late 1890s, M.B. Waite linked blossom infection to the movement of the pathogen from flower to flower by pollinating insects. The disease was introduced to Europe in the 1950s. It has since spread to most countries in Western Europe. Introductions of infested plant material served to establish E. amylovora in Europe, the Middle East, and New Zealand. In 1995, fire blight was first observed in the Po River Valley of northern Italy, which is the largest pear production area in the world. Since 1995, the Italian government has destroyed 500,000 pear trees in an attempt to eradicate E. amylovora.

Fire blight was first recorded in Ireland in 1986, but to date, the disease has not become established there. In Ireland, Cotoneaster appears to be the most susceptible host, especially the larger eaf varieties like Cotoneaster bullatus, Cotoneaster cornubia, and Cotoneaster lacteus. Fire blight is not believed to be present in Australia, though it might possibly exist there. It has been a major reason for a long-standing embargo on the importation of New Zealand apples to Australia. Japan was likewise believed to be without the disease, but it was discovered in pears grown in northern Japan. Japanese authorities are, however, still denying its existence and the Japanese scientist who discovered it is believed to have committed suicide after his name was leaked to affected farmers.


Pears appear to be more susceptible than apples to fire blight pathogen. This is because pears tend to have more flowers per spur than apples, and these flowers tend to remain open and susceptible for a longer period than those on apple. For example, individual apple flowers stay open for about 80 degree days (DD) above 4°C (44 DD above 4°C), while pear flowers stay open for an average of 120 DD above 4°C (67 DD above 4°C). Because of this longer flower life, nearly 90% of the total flower buds are open at full bloom on pears compared with only 65%–70% of those on apples. 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 expensive 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.

Host
Amelanchier (June berry), Malus (apple), Chaenomeles (Quince) Mespilus (Medlar), Pyracantha (firethorn) Cotoneaster Crataegus (hawthorn/whitethorn), Pyrus (pear) Cydonia (quince), Sorbus (mountain ash, whitebeam) Eriobotrya (loquat), Photinia davidiana (formerly Stranvaesia davidiana)
Symptoms Fire blight is a systemic disease. The term “fire blight” describes the appearance of the disease, which can make affected areas appear blackened, shrunken, and cracked, as though scorched
by fire.

Primary infections are established in open blossoms and tender new shoots and leaves in the spring when blossoms are open. Symptoms of fire blight can be observed on all above-ground tissues, including blossoms, fruits, shoots, branches and limbs, and in the rootstock near the graft union on the lower trunk. Generally, symptoms of fire blight are easy to recognize and distinguishable from other diseases.

On Blossom Clusters
Blossom symptoms are first observed 1–2 weeks after petal fall. The floral receptacle, ovary, and peduncles become water-soaked and dull, grayish-green in appearance. Later these tissues shrivel and turn brown to black (Photo 7.1). Similar symptoms often develop in the base of the blossom cluster and young fruitless as the infection spreads internally. During periods of high humidity, small droplets of bacterial ooze form on water-soaked and discolored tissues. Ooze droplets are initially creamy white, becoming amber-tinted as they age.

E. amylovora—fire blight on Malus.

PHOTO 7.1 E. amylovora—fire blight on Malus. (Courtesy of Charlie, flickr.com.)

On blossoms, E. amylovora acts as an excellent colonizer of the surfaces of stigmas and, to a lesser extent, the surface of the nectary. This reproduction on blossom surfaces is called epiphytic growth and occurs without the bacterium causing disease. Epiphytic growth of E. amylovora on stigmas and movement of the pathogen from blossom to blossom by pollinating insects are two important processes that regulate the incidence of blossom infection. E. amylovora also can survive on other healthy plant surfaces, such as leaves and branches, for limited periods (weeks), but colony establishment and epiphytic growth on these surfaces do not occur. Cells of E. amylovora excrete large amounts of an extracellular polysaccharide (a major component of bacterial ooze), which creates a matrix that protects the pathogen on plant surfaces. In propagation nurseries, cells of E. amylovora surviving on woody surfaces can initiate disease when scions and rootstocks are wounded during grafting. E. amylovora can also reside as an endophyte within apparently healthy plant tissue, such as branches, limbs, and budwood. Migration of the pathogen through xylem is one mechanism by which blossom infections of pear/apple can lead to rootstock infections near the graft union.

On Young Shoot
Shoot symptoms are similar to those in blossoms but develop more rapidly. Tips of shoots may wilt rapidly to form a “shepherd’s crook.” Leaves on diseased shoots often show blackening along the
midrib and veins before becoming fully necrotic (Photo 7.2). Numerous diseased shoots give a tree a burnt, blighted appearance, hence the disease name.

Advanced Foliar Symptoms
Infections initiated in blossoms and shoots can continue to expand both up and down larger branches and limbs. Bark on younger branches becomes darkened and water-soaked (Photo 7.3). At advanced stages, cracks will develop in the bark, and the surface will be sunken slightly. Amber-colored bacterial ooze mixed with plant sap may be present on the bark. Wood under the bark will show streaked discolorations.


E. amylovora—fire blight on Pyrus calleryana on young shoot, Bradford pear.

PHOTO 7.2 E. amylovora—fire blight on Pyrus calleryana on young shoot, Bradford pear. (Courtesy of Charlie, flickr.com.)

Advanced foliar symptoms of fire blight on Pyrus calleryana, Bradford pear

PHOTO 7.3 Advanced foliar symptoms of fire blight on Pyrus calleryana, Bradford pear. (Courtesy of Charlie, flickr.com.)

On Pear and Apple Fruits
Indeterminate, water-soaked lesions form on fruit surface and later turn brown to black. Droplets of bacterial ooze may form on lesions (Photo 7.4), usually in association with lenticels. Severely diseased fruits blacken completely and shrivel.

Fire blight of apple caused by E. amylovora. (Courtesy of Scoth, N.)

PHOTO 7.4  Fire blight of apple caused by E. amylovora. (Courtesy of Scoth, N.)

Apple Rootstocks
Rootstock infections usually develop near the graft union as a result of internal movement of the pathogen through the tree or from infections through water sprouts or burr knots. The bark of infected rootstocks may show water-soaking, a purplish-to-black discoloration, cracking, and signs
of bacterial ooze. Red-brown to black streaking may be apparent in the wood just under the bark. Symptoms of rootstock blight can be confused with Phytophthora collar rot. Malling 26 and 9 rootstocks are highly susceptible to fire blight.

Pathogen Biology
E. amylovora is a member of the family Enterobacteriaceae. Cells of E. amylovora are Gramnegative, rod-shaped, and measure 0.5–1.0 × 3.0 mm, and are flagellated on all sides (peritrichous). Physiologically, E. amylovora is classified as a facultative anaerobe. It grows on most standard microbiological media and on several differential media. The optimum temperature for growth is 27°C, with cell division occurring at temperatures ranging from 5°C to 31°C. Identification of E. amylovora isolates is based on biochemical and serological tests, inoculation of immature pear fruits and apple seedlings, and DNA hybridization assays.

Disease Cycle
Honeybees and other insects, birds, rain, and wind can transmit the bacterium to susceptible tissue. Injured tissue is also highly susceptible to infection, including punctures and tears caused by plant-sucking or biting insects. Hailstorms aggravate the infection of an entire orchard in a few minutes, and growers do not wait until symptoms appear, normally beginning control measures within a few hours.

Once deposited, the bacterium enters the plant through open stomata and causes blackened, necrotic lesions, which may also produce a viscous exudate. This bacteria-laden exudate can be distributed to other parts of the same plant or to susceptible areas of different plants by rain, birds, or insects, causing secondary infections. The disease spreads most quickly during hot and wet weather and is dormant in the winter when temperatures drop. Infected plant tissue contains viable bacteria, however, and will resume production of exudate upon the return of warm weather in the following spring. This exudate is then the source for new rounds of primary infections. The disease cycle of fire blight of apple and pear is illustrated in Figure 7.1. The pathogen spreads through the tree from the point of infection via the plant’s vascular system, eventually reaching the roots and/or graft junction of the plant. Once the plant’s roots are affected, the death of the plant often results. Overpruning and overfertilization (especially with nitrogen) can lead to watersprout and other midsummer growth that leave the tree more susceptible.

Overwintering
E. amylovora overwinters in a small percentage of the annual cankers that were formed on diseased branches in the previous season. These overwintering sites are called “holdover cankers.” As temperatures warm in spring, the pathogen becomes active in the margins of holdover cankers. Free bacterial cells are released onto the bark surface, sometimes as visible ooze. Insects attracted to the ooze (e.g., flies) or rain disseminates the bacteria from the canker to blossoms.

Floral Epiphytic Phase
Stigmas, which are borne on the ends of the style, are the principal site of epiphytic colonization and growth by E. amylovora. During the floral epiphytic phase, the ultimate population size that the pathogen attains is influenced by temperature, which regulates the generation time of the pathogen, and by the number of blossoms in which the pathogen becomes established, which is facilitated by pollinating insects, honey bees in particular. Under ideal conditions, stigmas of each flower can support 10 6 cells of the pathogen.

Primary Infection in Flowers
Blossom blight is initiated when cells of E. amylovora are washed externally from the stigma to the hypanthium (floral cup). On the hypanthium, E. amylovora gains entry into the plant through secretory cells (nectarthodes) located on the surface. In pear, the importance of blossom blight is expanded further by the tendency of this species to produce nuisance, secondary or “rattail” blossoms during late spring and early summer, long after the period of primary bloom.

Disease cycle of fire blight of pear and apple caused by E. amylovora.
FIGURE 7.1 Disease cycle of fire blight of pear and apple caused by E. amylovora.

Secondary Phases
Secondary phases include shoot, fruit, and rootstock blight. These phases are usually initiated by inoculum produced on diseased tissues as a result of blossom infection. Wounds are generally required by E. amylovora to initiate shoot and fruit blight. Insects such as plant bugs and psylla create wounds on succulent shoots during feeding. Strong winds, rain, and hail can create numerous, large wounds in host tissues. Infection events induced by severe weather are sometimes called “trauma blight.” Rootstock blight of apple can result from shoot blight on watersprouts or from
internal translocation of E. amylovora from infections higher on the tree.

Canker Expansion

Both primary and secondary infections can expand throughout the summer, with the ultimate severity of an infection being dependent on the host species, cultivar, environment, and age and nutritional status of the host tissues. Young, vigorous tissues and trees are more susceptible to fire blight than older, slower growing tissues or trees. Similarly, trees that have received an excess of nitrogen fertilizer, and therefore are growing rapidly, are more susceptible than trees growing under a balanced nutrient regime. Rates of canker expansion also can be enhanced by a high water status in a tree caused by excessive or frequent irrigation or poorly drained soils. Canker expansion slows in late summer as temperatures cool and growth rates of trees and shoots decline.

Epidemiological Models

Blossom blight is sporadic from season to season owing to the requirement for warm temperatures to drive the development of large epiphytic populations. Several epidemiological models (e.g., Cougarblight and Maryblyt) predict the likelihood of blossom blight epidemics based on observed climatic conditions. The models work by identifying the periods conducive for epiphytic growth of E. amylovora on blossoms before infection occurs, and thus are used widely to aid decisions on the need for and timing of chemical applications. Blossom blight risk models accumulate degree units above a threshold temperature of 15.5°C or 18°C. Data on rain or blossom wetness during periods of warm weather are also used in the models to indicate more precisely the timing and likelihood of blossom infection. Other temperature-based models predict the time of symptom expression after an infection event (i.e., the length of the incubation period) based on heat unit sums. These models are used to time orchard inspections and/or pruning activities.

How Does the Disease Spread?

Fireblight can be spread in the following ways:
• Movement of infected host plants, which may or may not display symptoms, can transmit fireblight over long distances.
• Rain splash: Bacteria emerge from infected material in the form of sticky ooze and droplets that harden on drying but which readily dissolve in water and can be spread by rain splash from an infected host to adjacent host material.
• Wind: Fine strands of ooze may be extruded especially from young tissue and these become brittle on drying and are dispersed by wind.
• Insects can spread the pathogen from overwintering cankers to early blossoms and between blossoms.

Disease Management

Effective management of fire blight is multifaceted and largely preventative. The grower must utilize a combination of sanitation, cultural practices, and sprays of chemical or biological agents to keep the disease in check.

Action Following Discovery of Fire Blight

All infected plants must be destroyed under the supervision of the Department of Agriculture and Food. It may also be necessary to destroy other adjacent host plants in order to control spread of the disease. The source of the infected material is then traced so that other infected plants may also be
destroyed. The plant passport system facilitates the tracing of fireblight host plants traded within the European Union.

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. Orchards grown on susceptible rootstocks (M.26, M.9, Mark), where fire blight occurred the previous year in 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 buildup of inoculum in the orchard, the blossom infection process, and the appearance of symptoms. A weather station that records the daily minimum and maximum temperatures and rainfall amounts is needed. When 50% of the buds show green tissue, begin keeping a daily record of the cumulative DD greater than 12°C. This information can be used to signal when symptoms are likely to appear in the orchard for blossom blight (103 DD greater than 12°C [57 DD greater than 12.7°C] after infection), canker blight (about 300 DD greater than 12°C [167 DD greater than 12.7°C] after green tip), and early shoot blight (about 103 DD greater than 12°C [57 DD greater than 12.7°C] after blossom blight or canker blight symptoms appear).

Continue to monitor and record the daily minimum and maximum temperatures and rainfall amounts and continue to accumulate DD greater than 12°C (12.7°C). 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 18.3°C. Once a total of 200 or more DH greater than 18.3°C (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 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 on 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 nee 
for additional sprays at 4-day intervals during bloom. Continue to monitor for strikes and remove all blighted limbs.

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 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. 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.

Elimination of Overwintering Inoculums
Vigilant sanitation through the removal of expanding and overwintering cankers is essential for the control of fire blight in susceptible cultivars. Removal of overwintering (“holdover”) cankers is accomplished by inspecting and pruning trees during the winter.

Removing Sources of Infection

Dormant pruning to remove overwintering infections helps reduce inoculum for the next season. Make cuts about 4 in. below any signs of dead bark. Remove pruned material from the orchard. Beginning about 1 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.

Midseason Suppression of Established Infections

In summer, established infections are controlled principally by pruning. Effective control through pruning requires that cuts are made 20–25 cm (8–10 in.) below the visible end of the expanding canker and that between cuts the pruning tools are disinfested with a bleach or alcohol solution to prevent cut-to-cut transmission. Repeated trips through an orchard are necessary, as some of the infections are invariably missed and others become visible at later times. Prunings harboring the pathogen are usually destroyed by burning.

In severely affected orchards, cultural practices that slow the growth rate of the tree will also slow the rate of canker development. They include withholding irrigation water, nitrogen fertilizer, and cultivation. Similarly, practices that reduce tree wounding and bacterial movement can reduce secondary infection. They include controlling insects such as plant bugs and psylla, limiting the use of limb spreaders in young orchards, and avoiding the use of overhead sprinklers. Chemicals such as streptomycin or copper can suppress trauma blight if applied immediately after a hailstorm.

Prevention of Blossom Blight
Prevention of blossom infection is important in fire blight management because infections initiated in flowers are destructive and because the pathogen cells originating from blossom infections provide much of the inoculum for secondary phases of the disease, including the infection of shoots, fruits, and rootstocks. Management actions to suppress blossom blight target the floral epiphytic phase. Sprays of antibiotics, streptomycin, or oxytetracycline have effectively suppressed blossom infection in commercial orchards.

Treatment
Sprays of the antibiotics streptomycin or terramycin can prevent new infections. The use of such sprays has led to streptomycin-resistant bacteria in some areas, such as California and Washington. Certain biological controls consisting of beneficial bacteria or yeast can also prevent fire blight from infecting new trees. The only effective treatment for plants already infected is to prune off the affected branches and remove them from the area. Plants or trees should be inspected routinely for the appearance of new infections. The rest of the plant can be saved if the blighted wood is removed before the infection spreads to the roots.

Chemical and Biological Control
Copper compounds also are effective but not used widely because copper can be phytotoxic to the skin of young fruits. E. amylovora has become resistant to streptomycin in some production areas, limiting the effectiveness of this chemical. Nonpathogenic, bacterial epiphytes sprayed onto blossoms can preemptively suppress fire blight by colonizing the niche (stigmatic surface) used by E. amylovora to increase its epiphytic population size. The bacterium Pseudomonas fluorescens strain A506 is registered and sold commercially for this purpose (BlightBan A506).

A copper spray applied at the 1/4 in. 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 United States and are widespread in most apple and pear regions of the western United States. Fire blight bacteria in Utah County have been tested and are documented to be resistant to streptomycin. Use of streptomycin, to control fire blight, is not recommended in Utah County. Instead, use of compounds containing oxytetracycline is recommended.

Dormant Spray
In orchards with a history of severe fire blight, it is advisable to carefully prune out overwintering cankers and spray with a copper-plus-oil mixture at the delayed dormant stage (silver tip to green tip). Copper compounds can be phytotoxic if they are sprayed much past the bud burst stage (1/2 in. green). This spray is thought to reduce the levels of inoculum in the orchard, but they may also be effective in reducing insect vectors. Bordeaux mixture (see the following formulation) plus 1 gal of 60–70 s spray oil. Preparation of Bordeaux mixture: Dissolve 8 lb of crystalline copper sulfate in 100 gals of water in the spray tank. After the copper sulfate is dissolved, add 8 lb of hydrated spray lime (350 mesh), either mixed in water or as powder, to the tank. Constant agitation is needed to thoroughly mix the contents of the tank. Finally, add 1 gal of spray oil. Copper hydroxide plus oil. Copper oxychloride sulfate + basic copper sulfate.

Blossom Sprays
Streptomycin and oxytetracycline, and fixed copper sprays, have proven very effective in reducing fire blight provided they are properly applied at correct times. They are preventive sprays only and must be repeated every 4–5 days as long as new flowers are opening. A biological control compound (a bioantagonistic bacterial competitor) has recently been licensed for the control of fire blight as well (Bloomtime FD [Pantoea agglomerans]). Agricultural antibiotics available for the treatment of fire blight in pears and apples in Utah include Agri-Mycin 17 (streptomycin sulfate [not recommended for use in Utah County]) and/or Mycoshield (oxytetracycline calcium complex).

Fixed Coppers
Copper oxychloride sulfate (C-O-C-S WDG) and Kocide branded products (20/20, 101, 2000, and 3000); read labels as some are phytotoxic and may not be labeled for pear, only for apple. Timing bactericide applications is critical. A delay of even several hours can reduce the level of control. It is not necessary to spray until mean daily temperature during bloom (average of maximum and minimum temperatures from midnight to midnight) first exceeds 16°C in the spring. 

Use forecasting models such as Maryblyt or Cougarblight to determine when to spray. Sprays should be repeated at 4- to 5-day intervals throughout the bloom period as long as temperatures are above the mean threshold. Applications are most important on young pears and susceptible varieties of apples. Fixed coppers may have phytotoxic effects, causing russeting in fruits. Do not use fixed coppers on d’Anjou pears. 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.

Insect Control
The role of insects in the transmission of fire blight bacteria is well known. 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, 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 midsummer, it is likely that late-season infections will increase. 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 the analyses of foliage N levels. Any practice that promotes excessive succulent growth should be avoided. Trees should be fertilized to promote good health, but overfertilization with nitrogen or applications late in the season often cause excessive new growth that is susceptible to fire blight infection. Remove blighted blossoms and twigs as soon as they are evident. The infected blossoms and twigs should be pruned a minimum of 8–12 in. below the obvious infection. Avoid heavy pruning in the early summer because it stimulates succulent growth, which is very susceptible to blight. 

During the dormant season, remove any cankers or blighted tissue. Also, remove any suckers growing up from the roots or on the trunk. It is advisable to avoid using pruners on small twigs; use hands to break out the blighted twigs. Pruners should be soaked in a 10% solution (one part bleach and nine parts water) of household bleach, or good surface disinfectant, between cuts when pruning out active fire blight in the summer. Bleach solution is effective but is corrosive to pruning tools. Rinse, dry, and oil the tools several times during the pruning. Pruners need not be sterilized between cuts when pruning is done during the dormant season.

Hosts of fire blight such as Pyracantha, hawthorn, Cotoneaster, and crabapple growing near the orchard should be eradicated. This will reduce fire blight inoculum in the orchard. Avoid excessive irrigation to reduce humidity in the orchard. If using sprinkler irrigation, do not allow water
to wet the foliage since it will act in the same manner as rain in spreading bacteria. No cultivars are completely resistant to fire blight, but some are less susceptible than others. If fire blight is a common problem in your area, plant less susceptible varieties.

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 United States, 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. Fire blight–resistant cultivars of apple include Red Delicious, Liberty, Prima, Pricilla, Redfree, Spartan, and Sir Prize. There are many moderately resistant cultivars including Honeycrisp, Duchess, Empire, Golden Delicious, Granny Smith, Jonagold, McIntosh, Mutsu, and Winesap. This is not an exhaustive list as there are many cultivars of apples. Some less-susceptible cultivars of pear include Harrow Delight, Harvest Queen, Kieffer, Moonglow, Seckel, LaConte, and Magness. Of the pear varieties most commonly grown in the mid-Atlantic region, Bartlett, Bosc, D’Anjou, and Clapp’s Favorite are the most susceptible, while Magness, Moonglow, Maxine, and Seckel are highly resistant. All varieties of Asian pears, except Seuri, Shinko, and Singo are moderate to highly susceptible to fire blight.

No comments:

Post a comment