Contents

Fire Blight

A shepherd’s crook at the end of an apple branch caused by fire blight.

Ann Joy and Brian Hudelson, UW-Madison Plant Pathology
Revised: 4/24/2004
Item number: XHT1090

What is fire blight? Fire blight is the most destructive bacterial disease affecting plants in the rose family, including apple, pear, crabapple, hawthorn, cotoneaster, mountain ash, quince, rose, pyracantha, and spirea. It can kill or disfigure a tree or shrub, depending on the susceptibility of the host and weather conditions.

What does fire blight look like? Blossoms, leaves, twigs, and branches of plants affected by fire blight can turn dark brown to black, giving the appearance of having been scorched in a fire. The blighted blossoms and leaves tend to stay on the tree instead of falling. Current year’s twigs often wilt and bend approximately 180°, forming a “shepherd’s crook.” Cankers develop on branches and stems, and emit a sticky bacterial ooze. Sapwood around cankers may discolor to a reddish brown.

Where does fire blight come from? Fire blight is caused by the bacterium Erwinia amylovora, which overwinters on the margins of cankers and starts to multiply when temperatures rise in the spring. The bacteria-laden ooze from the cankers is dispersed by splashing rain, and insects. Bacteria multiply in blossoms and are carried to other plant parts where they penetrate through wounds and natural openings. They can also be spread through the plant’s water-conducting (vascular) system.

How do I save a plant with fire blight? There is no cure for fire blight, but its spread can be limited. Prune diseased branches preferentially during the dormant season at a time when branches are dry. When removing diseased branches, prune six to eight inches below tissue showing visible symptoms. If pruning is required during the growing season, prune at least 12 inches below the diseased area. Always, disinfect pruning tools by dipping them for at least 30 seconds in 10% bleach or alcohol after each cut (spray disinfectants that contain at least 70% alcohol can also be used). Burn or bury diseased branches.

How do I avoid problems with fire blight in the future? By far the most effective strategy is to choose plants with resistance to fire blight. Select a well-drained site with a soil pH of 5.5 to 6.5. Avoid applying high nitrogen fertilizer, which may stimulate succulent new growth susceptible to the disease. Treatment with Bordeaux mixture (copper sulfate) before buds open can be effective in reducing the amount of bacteria present on branches. Make one or two applications, with four days between applications.

For more information on fire blight: See UW-Extension Bulletins A1616, A3565, A2072 or contact your county Extension agent.

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Tags: blight, disease Categories: Fruit Problems, Tree & Shrub Problems

Fire blight

Fire blight, plant disease, caused by the bacterium Erwinia amylovora, that can give infected plants a scorched appearance. Fire blight largely affects members of the rose family (Rosaceae). It has destroyed pear and apple orchards in much of North America, in parts of Europe, and in New Zealand and Japan. Many other economically important agricultural and ornamental plants can also be affected, including almond, apricot, cherry, cotoneaster, crabapple, flowering quince, hawthorn, loquat, medlar, mountain ash, plum, quince, raspberry, rose, serviceberry, and spirea.

fire blightFire blight on the branch of an apple tree.Peggy Greb, Agriculture Research Service/U. S. Department of Agriculture (Image Number: K10805-2)

Symptoms of fire blight include a sudden brown to black withering and dying of blossoms, fruit spurs, leaves, twigs, and branches. Very susceptible plants appear as if scorched by fire and may die. Cankers—slightly sunken, encircling, dark brown to purplish black lesions with a sharp, often cracked margin—form on twigs, branches, and trunk, causing terminal dieback. Fruits are water-soaked, later turning brown or black and shrivelled. In warm moist spring weather, droplets of bacterial ooze appear on the surface of “holdover” cankers. The oozing bacteria are carried by insects, wind, and rain to infect new plants and tissues. The bacteria spread intercellularly and up to 1.2 metres (4 feet) through vascular tissue in the wood, during late spring and early summer, darkening and killing the tissue. A small percentage of the bacteria overwinter at the margins of branch and trunk cankers, ready to repeat the disease cycle starting the following spring about blossoming time.

Fire blight is difficult to control, especially in warm moist weather conditions. Infected wood should be removed in late summer, fall, or winter, when the bacteria are not actively spreading. Copper blossom sprays can be applied when plants first begin to flower but are of limited effectiveness and can damage fruits. Streptomycin sprays have been used to prevent new infections but have also contributed to antibiotic-resistant outbreaks in some areas. Resistant varieties of several susceptible plants have been developed.

How to Manage Pests

Pests in Gardens and Landscapes

Fire Blight

Revised 7/11

In this Guideline:

  • Identification and damage
  • Life cycle
  • Management
  • About Pest Notes
  • Publication
  • Glossary

Bacterial ooze on a twig with fire blight infection.

Blackened fruit is typical of fire blight infection.

Flower clusters infected with fire blight bacteria.

Branches infected with fire blight.

Red-streaked wood underneath the bark in a fire blight canker.

Fire blight, caused by the bacterium Erwinia amylovora, is a common and frequently destructive disease of pome fruit trees and related plants. Pear (Pyrus species) and quince (Cydonia) are extremely susceptible. Apple, crabapple (Malus species), and firethorns (Pyracantha species) also are frequently damaged. Fire blight is less common on hawthorn (Crataegus species), Spiraea, Cotoneaster, toyon (Photinia species), juneberry or serviceberry (Amelanchier species), loquat (Eriobotria), mountain ash (Sorbus species), and other related plants. The disease can destroy limbs and even entire shrubs or trees.

IDENTIFICATION AND DAMAGE

In spring, branch and trunk canker symptoms can appear as soon as trees begin active growth. The first sign is a watery, light tan bacterial ooze that exudes from cankers (small to large areas of dead bark that the pathogen killed during previous seasons) on branches, twigs, or trunks. The ooze turns dark after exposure to air, leaving streaks on branches or trunks. However, most cankers are small and inconspicuous; thus infections might not be noticed until later in spring when flowers, shoots, and/or young fruit shrivel and blacken. The amount of fruit loss depends upon the extent and severity of the disease.

Open flowers are the most common infection sites (Figure 3) and remain susceptible until petal fall. Infected flowers and flower stems wilt and turn black on pear trees and brown on apple trees. Fire blight infections might be localized, affecting only the flowers or flower clusters, or they might extend into the twigs and branches, causing small shoots to wilt (Figure 4) and form a crook at the end of each infected shoot. Succulent tissues of shoots and water sprouts (root suckers) also are subject to infection. Dead, blackened leaves and fruit cling to branches throughout the season, giving the tree a scorched appearance, hence the name “fire blight.” Infections can extend into scaffold limbs, trunks, or root systems and can kill highly susceptible hosts. Less susceptible varieties might be severely disfigured. Once infected, the plant will harbor the pathogen indefinitely.

When the pathogen spreads from blossoms into wood, the newly infected wood underneath the bark has pink to orange-red streaks (Figure 5). The bacteria also spread into the wood surrounding overwintered cankers that have become active in spring. If the bark is cut away from the edge of an active canker, reddish flecking can be seen in the wood adjacent to the canker margin. This flecking represents new infections the bacteria cause as they invade healthy wood. As the canker expands, the infected wood dies, turns brown, and dries out; areas of dead tissue become sunken, and cracks often develop in the bark at the edges of the canker. The pathogen tends to move in trees from the infection site toward the roots. In fall, leaves on infected pear shoots often turn red and then black.

LIFE CYCLE

Fire blight bacteria overwinter in cankers on twigs, branches, or trunks of host trees. In spring when the weather is sufficiently warm and moist and trees resume growth, a small percentage of the cankers become active as bacteria multiply and ooze from branch or twig surfaces in a light tan liquid. Splashing rain or insects transmit the bacteria to nearby blossoms or succulent growing shoots. Once blossoms are contaminated with the bacteria, honey bees become efficient carriers of the pathogen.

Injuries on tender young leaves and shoots, caused by wind, hail, or insect punctures, are easily invaded by the fire blight bacteria. Such infections lead to shoot blight. Ideal conditions for infection, disease development, and spread of the pathogen are rainy or humid weather with daytime temperatures from 75° to 85°F, especially when night temperatures stay above 55°F.

Fire blight bacteria generally don’t move uniformly through the bark but invade healthy wood by moving in narrow paths up to 1 1⁄2 inches wide in the outer bark ahead of the main infection. These long, narrow infections can extend 2 to 3 feet beyond the edge of the main infection or canker. If you expose bark from an infected woody area, you will see that the diseased tissue closest to the main canker is brown. Farther out, the infection turns red and then appears as flecking. Just beyond the visible infection the tissue will look healthy.

Tree vigor has a major influence on the extent of fire blight damage. Once established, the distance the pathogen moves relates directly to the susceptibility of the tree and rate of tree growth. Vigorously growing shoots are the most severely affected; therefore, conditions such as high soil fertility and abundant soil moisture, which favor rapid shoot growth, increase the severity of damage to trees. In general, trees are more susceptible when young and suffer less damage as they age.

MANAGEMENT

Fire blight development is influenced primarily by seasonal weather. When temperatures of 75° to 85°F are accompanied by intermittent rain or hail, conditions are ideal for disease development. The succulent tissue of rapidly growing trees is especially vulnerable; thus excess nitrogen fertilization and heavy pruning, which promote such growth, should be avoided. Trees shouldn’t be irrigated during bloom. Monitor trees regularly, and remove and destroy fire blight infections. (See Removing Diseased Wood.) If fire blight has been a problem in the past, apply blossom sprays. Sprays prevent new infections but won’t eliminate wood infections; these must be pruned out. In years when weather conditions are very conducive to fire blight development, it can be difficult if not impossible to control the disease.

Choosing Tolerant Varieties

Most pear tree varieties, including Asian pears (with the exception of Shinko) and red pear varieties, are very susceptible to fire blight. Varieties of ornamental pear trees that are less susceptible to fire blight are Bradford, Capitol, and Red Spire; Aristocrat is highly susceptible. Among the more susceptible apple varieties are Fuji, Gala, Golden Delicious, Granny Smith, Gravenstein, Jonathan, Mutsu, Pink Lady, and Yellow Newtown. Wherever possible, plant varieties less prone to fire blight damage. Because most infections originate in the flowers, trees that bloom late or throughout the season (i.e., rat-tail bloom) often have severe fire blight damage.

Removing Diseased Wood

Successful removal of fire blight infections is done in summer or winter when the bacteria no longer are spreading through the tree. At these times infections have ceased enlarging, canker margins are clearly visible, and cleaning pruning shears is unnecessary. Rapidly advancing infections on very susceptible trees (pear, Asian pear, and some apple varieties) should be removed as soon as they appear in spring. In these cases, dipping shears in 10% bleach between cuts might be wise. However, the location of the cut is far more important than the cleansing of tools. New infections that originate at pruning cuts haven’t been observed on orchard trees; instead, the greater risk is “short cutting,” wherein the cut isn’t made far enough below the canker to remove all the infected tissue. To locate the correct , find the lower edge of the visible infection in the branch, trace that infected branch back to its point of attachment, and cut at the next branch juncture down without harming the branch collar. This will remove the infected branch and the branch to which it is attached.

If a fire blight infection occurs on a trunk or major limb, the wood often can be saved by scraping off the bark down to the cambium layer in infected areas (i.e., removing both the outer and inner bark). When scraping, look for long, narrow infections that can extend beyond the margin of the canker or infection site. If any are detected, remove all discolored tissue plus 6 to 8 inches more beyond the infection. This procedure is best done in winter when trees are dormant and bacteria aren’t active in the tree. Don’t apply any dressing to the wound. If the limb has been girdled, scraping won’t work, and the whole limb must be removed.

Chemical Control

Copper products are the only materials available to homeowners for fire blight control, and they often don’t provide adequate control even with multiple applications. A very weak (about 0.5%) Bordeaux mixture or other copper product applied several times as blossoms open might reduce new infections but won’t eliminate all new infections or those already existing in wood. The spray must be applied to open blossoms; thus the number of applications needed depends on the length of the bloom period. Once blossoms begin to open, make the first application when the average temperature (average of the maximum and minimum temperatures for a 24-hour period) exceeds 60°F. Apply at four- to five-day intervals during periods of high humidity and until late bloom is over. For pear trees, this might mean five to 12 applications per season. Copper products also might cause russeting or scarring of the fruit surface. The risk of this damage begins during bloom and increases as fruits enlarge.

WARNING ON THE USE OF PESTICIDES

Broome, J. C., and D. R. Donaldson. June 2010. Pest Notes: Bordeaux Mixture. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 7481.

Ohlendorf, B. 1999. Integrated Pest Management for Apples and Pears. 2nd ed. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 3340.

PUBLICATION INFORMATION

Pest Notes: Fire Blight
UC ANR Publication 7414

Author: B. L. Teviotdale, UC Cooperative Extension (emeritus), Kearney Agricultural Center, Parlier.

Produced by UC Statewide IPM Program, University of California, Davis, CA 95616

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Fire Blight of Apple and Pear

  • Symptoms
  • Life Cycle
  • Fire Blight Management
    • Cultural Control
    • Chemical and Biological Control
    • Forecasting Fire blight
    • Varietal Resistance
    • Prevention
  • Summary
  • Further Information

Symptoms:

Fire blight symptoms may appear on the blossoms, shoots, branches, trunk and rootstock. Blighted blossoms appear wilted, shriveled and brown. Young fruitlets are also very susceptible and appear water soaked and slightly off-colour soon after infection. Fruitlets quickly turn brown to black and eventually shrivel up.

Blighted pear shoots are black in colour, while infected apple shoots are usually a lighter shade of brown. Infected shoots (or “strikes”) wilt rapidly, and often form a shepherd’s crook at their tips. During warm and humid or rainy weather drops of milky to amber coloured bacterial ooze frequently appear on the blighted shoots and fruit. Blighted leaves may remain attached to the tree throughout the winter. When shoots attached to scaffold limbs or trunks are attacked, the pathogen may spread into the structural wood causing cankers. In susceptible hosts or young trees the disease may travel rapidly down branches causing girdling and death of the branches or sometimes the main trunk.

Blossom blight on pear. Note blackening of pedicels (flower stems) Shoot blight on apple. Note browning of leaves and shepherd’s crook and at end of shoot.
Droplets of amber coloured bacterial ooze on fire blight-affected pear shoot Fire blight-infected apple fruitlet, with bacterial ooze
Shoot blight on pear. Note blackened leaves and fruitlets Pear orchard heavily damaged by fire blight

Cankers appear as slightly darker, water soaked areas in the wood, which may produce amber coloured bacterial ooze that runs down the bark. Reddish brown streaks may be seen in the cambium under the bark of diseased branches. Later in the season the bark often cracks around the margins of the canker.

Shoot blight caused by reactivating cankers in the spring is known as canker blight. Bacteria that overwintered in holdover cankers begin to multiply and invade nearby shoots or water sprouts. Invaded shoots wilt and die, and may be mistaken for early shoot blight. Cankers may also expand to girdle limbs.

Fire blight may also spread into the root area, leading to tree death. Rootstock blight may not exhibit typical fire blight symptoms. It is sometimes mistaken for crown rot due to brown discoloured tissue under the bark at the base of the tree. Trees may weaken and die beginning one to several months after planting. Oozing bark may be observed near the base of the tree.

Fire blight rootstock blight. Note red-brown discolouration under bark Leaf discolouration in late summer caused by fire blight rootstock blight

Life Cycle:

Fire blight bacteria overwinter primarily in cankers on infected trees. During spring and early summer, cankers that were not removed the previous season may reactivate and produce bacterial ooze, which may or may not be visible. This ooze, consisting of millions of bacterial cells, is easily transported to blossoms by insects such as flies, ants, and beetles. If weather conditions are warm and humid, the bacteria are able to multiply rapidly in the blossom nectar. Bacteria can then be spread very efficiently from blossom to blossom by honey bees. This gives fire blight explosive potential if the conditions for blossom infection are met.

Once blossoms are infected, the bacteria can quickly spread into shoots and branches. Blossom infections become visible as “strikes”, or dying shoots in anywhere from 1 to 4 weeks, depending on the temperature.

Infected shoots provide additional sources of fire blight bacteria, which can be spread by rain, (especially wind-blown rain), insects, and contaminated pruning tools. Secondary infections may continue to occur throughout the growing season. The worst epidemics always follow blossom infection. However, it is possible for twig or shoot infection to occur in orchards where little or no blossom infection was found. In these situations the disease pressure is usually low and the damage is more easily controlled. Bacteria can enter the host through both wounds and natural openings such as lenticels. Hail storms often result in severe fire blight outbreaks if inoculum is present in an orchard. Wind damaged leaves are also susceptible to infection.

Fire Blight Management – Cultural Control

Dormant Season:

Overwintering cankers should be cut out during the dormant season to reduce sources of bacteria for the next season. Fire blight cankers have either smooth or cracked margins. Both types of cankers should be removed. The smooth margined cankers are harder to see, but they are also more likely to be active than rough margined cankers. Active cankers may enlarge in the spring causing further structural damage. They also provide inoculum for new infections.

One or more separate operations to prune out cankers are recommended. Since cankers may be hard to locate, it is always best to go over the orchard several times. Cankers are most visible on bright, sunny days. Make cuts 15-30 cm below the canker margins. It is not considered necessary to sterilize pruning tools during the dormant season. Do, however, disinfect your tools if spring pruning is extended into late spring when temperatures have warmed up and/or the budburst stage has arrived. These conditions may also reactivate the infectious bacteria in cankers. In fact, an additional inspection for cankers around the budburst stage may reveal cankers that were missed earlier.

Fire blight canker on apple Fire blight canker with bark removed to show discoloured cambium.

Growing Season:

Remove current season infections as soon as they are noticed. Prune out infected branches at least 30-40 cm below the visibly diseased part. This is necessary as bacteria are usually present beyond the discoloured area. Dip tools in a disinfectant between each cut. Flag trees that have been pruned, and watch for further symptoms or the development of cankers. Prunings should be removed and burned immediately.

Scout for new fire blight strikes every 3 or 4 days. Frequent scouting will aid removal of new infections before they have a chance to invade the structural wood.

Where infections occur on shoots attached to scaffold limbs or the trunk, it is not always possible to cut back 30 cm without sacrificing the limb or even the tree. An option on large trees is to scrape out discoloured inner bark using a hatchet or knife, down to clean wood, and disinfect the cut surface.

Cankers often form at the sites of pruning wounds, where blight was cut out during the summer. Such cankers may be hard to detect. To overcome this problem, it is sometimes recommended that a short (10 cm) stub be left beyond the next healthy spur or branch union when pruning out strikes. Remove the stub later, during dormant pruning. Marking the stubs with bright paint will make them more visible.

During severe epidemics, give priority to young trees and high density plantings. Concentrate on salvaging as much of the tree structure and bearing surface as possible. Excessive pruning during the summer will encourage a late flush of growth, which will be susceptible to continued infections.

Summer pruning (other than removal of strikes) should be avoided during a serious outbreak, due to the danger of spreading the disease. If there is any fire blight in the area, disinfect your tools while summer pruning. Avoid pruning during wet weather or when storms are expected within the next 24 hours.

Root suckers and rootstock sprouts may put the entire tree at risk if they become infected. Common dwarfing rootstocks such as M9 and M26 are highly susceptible to blight. Do not cut rootsuckers or rootstock sprouts during a blight outbreak, because the wounds may become infected. They may be safely removed during the dormant season.

Disinfectants:

Good disinfectants for tools include household bleach (eg. Javex, Chlorox), Lysol Concentrated Disinfectant, and PineSol, as well as commercial disinfectants such as Chemprocide. Mix according to label instructions. Bleach can be diluted up to 1:5 with water, and needs to be mixed fresh every day. Tools can either be dipped into, or sprayed with the disinfectant solution. If you use bleach, be aware that it will corrode metal tools and damage your clothing. Ideally, tools should be disinfected after every cut. Dilute disinfectant can also be sprayed on the bark after cutting out an infected branch.

Management Practices:

There are several management practices that can reduce the severity of fire blight. Practices that reduce the vigour of the trees tend to reduce fire blight, because young, fast growing tissue is the most susceptible to infection.

Don’t over fertilize the trees. Excess nitrogen causes vigorous shoot growth, which is more susceptible. Nutrient application should be balanced, preferably based on soil and leaf analysis. Application of fertilizer should also be timed to avoid a late flush of growth, because late season infections are more likely to produce cankers that allow the bacteria to overwinter.

Do not run overhead sprinklers while blossom is present on the tree and weather is favorable for fire blight infection. Overhead irrigation may increase fire blight by splashing bacteria around from tree to tree, and by increasing moisture and humidity levels in the canopy. Cutting back on irrigation may also help to slow down over-vigorous trees.

Control insects with sucking mouthparts such as aphids, leafhoppers and pear psylla. These insects can spread fire blight.

Chemical and Biological Control:

Products recommended for fire blight control in Canada include antibiotics, several biopesticides, a growth regulator, and copper sprays. These products are all used preventatively. There are no treatments that can cure fire blight after infection has occurred.

Timing of fire blight sprays is critical. Read the sections on blossom protection and forecasting fire blight for more information.

Also see the Tree Fruit Production Guide for application rates.

Table 1. Products Registered for fire blight control or suppression in Canada

Always follow label directions and precautions when using pesticides.

Product Resistance Group Crops PHI (days)* Timing Notes
Kasumin (kasugamycin) Group 24 bactericide, antibiotic all pome fruit 90 open blossom only Preventitive. Provides 2-3 days of protection. Maximum 4 applications/ season.
Streptomycin 17 WP Group 18 antibiotic apple, pear 50 (apple); 30 (pear) open blossom only Preventitive. Provides 2-3 days of protection. Maximum 3 applications/ season. Do not tank mix witih other pesticides. Refrigerate Product.
Blossom Protect (Aureobasidium pullulans) biopesticide all pome fruit n/a open blossom only Preventative. Apply up to 4 times at 10%, 40%, 70% and 90% open blossoms, or 5 times when model indicates risk of infection. Do not tank mix with fungicides.
Bloomtime Biological FD (Pantoea agglomerans) biopesticide apple, pear n/a open blossom only Preventative. Suppression only. Apply at 15-20% bloom, followed by a second application at full bloom to petal fall. Maximum two applications per season. Not compatible with copper. Keep frozen prior to use.
Serenade Max (Bacillus subtilis) biopesticide all pome fruit n/a open blossom only Preventative. Suppression only. Apply at early (1-5%) bloom. Repeat at 4-7 day intervals during high risk periods during bloom.
Double Nickel (Bacillus amyloliquefaciens) biopesticide all pome fruit 0 blossom and post-bloom Preventative. Suppression only. Compatible with copper. Can be applied post-bloom for shoot blight management.
Apogee (prohexadione ca) growth regulator apple 45 2.5 – 7.5 cm of new shoot growth Suppresses fire blight by decreasing host susceptibility. Apply at 2.7-7.5 cm shoot growth; repeat at 14-21 day intervals up to 4 times/season. Will also reduce shoot growth. Do not use on pear.
Cueva Commercial (copper octanoate) Group M fungicide/ bactericide all pome fruit 1 Dormant, bloom and in-season Preventative. Less likely to cause russet injury than other coppers. Use highest rate for dormant application.

Guardsman Copper Oxychloride (copper oxychloride)

Group M fungicide/ bactericide pear n/a blossom Use with caution due to risk of fruit russet. Do not use on apple or anjou pear.
Copper Spray Fungicide (copper oxychloride) Group M fungicide/ bactericide apple, pear n/a silvertip, blossom, post-harvest Do not use during blossom on apple or Anjou pear. May cause fruit russet. Dormant copper applications have been shown to reduce or delay the production of inoculum in overwintering cankers.
Copper 53W (copper sulphate) Group M fungicide/ bactericide apple, pear n/a blossom Use with caution due to risk of fruit russet. Mix with hydrated lime to prepare Bordeaux mixture as per label. Do not tank mix with other pesticides.

* Pre-harvest interval.

Resistance Management: Both Kasumin and Streptomycin are at high risk of resistance development by fire blight bacteria. Streptomycin resistance is known to occur in the Okanagan Valley and throughout most of the Pacific Northwest. To help preserve effectiveness of these products:

  • Use only when necessary, when weather conditions create a risk for fire blight
  • Never use Streptomycin or Kasumin after the blossom stage
  • Do not use Streptomycin more than 3 times per season or Kasumin more than 4 times per season.
  • Alternate with other fire blight products.

Blossom Protection and Spray Timing:

Warm temperatures, along with rain, dew or high humidity, favour blossom infections. In British Columbia, favourable fire blight weather during the main blossom period was rare in the past, but has become increasingly more common in recent years. Favourable conditions are even more likely to occur in late spring and early summer with secondary bloom. Spraying for blight is not necessary until daytime temperatures reach the low to mid 20’s C. Several warm days in a row during bloom will greatly increase the risk. See section on forecasting fire blight for more information.

Blossom infection may be reduced by spraying with Kasumin, Streptomycin, Blossom Protect, Serenade, Bloomtime, Blossom Protect, Double Nickel, Cueva or fixed copper. Apply preventatively in early bloom when weather is favourable for blight. Repeated sprays at 48-72 hour intervals may be necessary if weather conditions remain warm and moist while blossoms remain on the tree.

Biopesticides should be used as part of an integrated fire blight management program, which includes using a risk assessment model, as well as cultural controls and antibiotics when necessary. For best results, apply a biopesticide during early bloom at the beginning of a warming trend, and follow with a streptomycin or Kasumin spray 2 or 3 days later if warm temperatures continue to favour blossom infection.

Forecasting Fire blight:

There are several models available which can be used to forecast fire blight blossom infection based on the weather. They can be used to determine whether or not sprays are needed to protect bloom.

The Cougarblight model, developed by Tim Smith at Washington State University, is very easy to use. It is necessary to note the daily maximum temperatures while open blossom is present, and determine whether blossoms have been wetted by rain, irrigation or heavy dew. Daily risk values based on temperature are added up over a 4 or 5 day period. Once a certain threshold is reached, there is risk of blossom infection.

Cougarblight is now integrated into the BC Decision Aid System (DAS). To use DAS, set up a free account, select a weather station near your orchard, and add the fire blight model. DAS Spray recommendations are linked to the BC Tree Fruit Production Guide.

Post-Bloom:

Most fire blight products registered for control or suppression of blossom blight are not effective after petal fall. Cueva and Double Nickel are the only registered fire blight products that can be used after bloom. Apply as soon as possible following hail or wind storm damage in blocks affected by or exposed to fire blight.

Physical removal of late or secondary blossoms would greatly reduce the risk of fire blight infection. Although labour intensive, this could be feasible on young trees that are at high risk of infection.

Post-harvest:

Post-harvest application of fixed copper or Bordeaux mixture in the fall could be considered in orchards with a late flush of growth and an active fire blight problem.

Dormant season:

Dormant application of fixed copper or Bordeaux mixture in the spring will help to delay production of inoculum on overwintering cankers. Apply to runoff around the silver tip to green tip stage.

Varietal Resistance

Varieties rated as susceptible are more likely to have fire blight problems. The disease is more severe on pear, but susceptible apple varieties can also be hard hit. See below for susceptibility ratings of apple and pear varieties.

Factors Affecting Susceptibility:

Susceptibility to fire blight can be influenced by many factors, including tree nutrition, irrigation management, soil factors, and cultural practices in the orchard. Young trees are more likely to be severely damaged or killed than older trees. Varieties which have a late or prolonged blossom period are more likely to have bloom on the tree when the weather warms up, and are therefore more likely to suffer from blossom blight. Even relatively resistant trees, such as Red Delicious, may get fire blight after a hail storm or if they are in a mixed planting with susceptible varieties or pears.

Relative Fire Blight Resistance of Apple and Pear Varieties and Rootstocks:

Apple:

  • Most Resistant: Red Delicious, Liberty, Enterprise, Freedom
  • Moderately Resistant: Golden Delicious, Empire, Granny Smith, McIntosh, Mutsu, Spartan, Summerred, GoldRush, Nova Easygro
  • Susceptible: Braeburn, Fuji, Gala, Ginger Gold, Idared, Jonagold, Rome, Winter Banana

Crabapple:

  • Moderately Resistant: Dolgo
  • Susceptible: Manchurian, Snowdrift

Pear:

  • Susceptible: Anjou, Bartlett, Bosc, Cascade, Flemish Beauty, Starkrimson

Asian Pear:

  • Most Resistant: Seuri, Shinko, Singo
  • Moderately Resistant: Kosui, Chojoro, Shinsui
  • Susceptible: Hosui, Shinseiki, 20th Century

Apple Rootstocks:

  • Most Resistant: M.7
  • Moderately Resistant: MM.106, MM.111, M.4
  • Susceptible: M.9, M.26, M.27, Mark, Ottawa 3, B.9

Pear Rootstocks:

  • Most Resistant: Old Home (OH), Old Home x Farmingdale (except OHF 51)
  • Susceptible: Bartlett seedling, quince

(Note: Resistance ratings are not exact, and can be influenced by growing conditions)

Fire Blight Prevention

If you are growing susceptible varieties, be aware that fire blight may strike. Keep your eyes open for symptoms while working in the orchard, and periodically inspect your crop for blight. Take immediate action if you find anything suspicious. Have the disease confirmed by a field advisor or plant pathologist. If fire blight was in your orchard or a nearby orchard the previous year, be sure to monitor for fire blight favourable weather during bloom and apply preventative sprays if necessary.

You can minimize the chances of introducing fire blight into your orchard by selecting nursery stock from a reliable source. Ask your supplier if they have had fire blight problems in the nursery. The varieties and rootstocks you grow will also influence the potential for fire blight.

Summary

Fire blight poses a serious threat to new and established orchards of apple and pear. Fire blight has a tendency to appear in unexpected places. It could be well worth your time to regularly scout for symptoms in your orchard, even if you’ve never had fire blight before. Successful fire blight control requires good management. The following summary lists the major management practices used to control fire blight:

Dormant season

  • remove cankers and dead branches
  • delayed dormant copper spray

Blossom stage

  • monitor temperature during blossom
  • apply preventative sprays during blossom if blight-favourable conditions are forecast. For example, apply a biological treatment in early bloom followed by an antibiotic if the weather remains favourable for fire blight.
  • consider Apogee for shoot blight suppression in blocks at risk of fire blight. Must be applied early.

Post bloom

  • scout for and remove strikes at regular intervals
  • disinfect tools between cuts
  • control aphids, leafhoppers and pear psylla

Post harvest

  • apply copper sprays if fire blight is active
  • continue to remove new infections

Further Information

  • Fact Sheet – Integrated management of fire blight on apple and pear in Canada – Agriculture and Agri-Food Canada
  • Fire blight – Washington State University

Updated July, 2018

FIREBLIGHT OF APPLES AND PEARS

SYMPTOMS OF FIREBLIGHT

The symptoms of fireblight are hard to miss even at the initial stages of infection. First the blossoms are infected then new shoots, fruit and finally the main branches can be affected. The key symptoms are:

  • Blossoms quickly die off turning a dark brown colour
  • New shoots then become infected a week or so later and they too wither and blacken although they tend to remain on the tree. Some shoots may randomly be completely unaffected.
  • If the weather is warm and wet the affected shoots begin to weep a sticky goo which is laden with bacteria
  • Developing fruits then wither and die
  • This year’s wood growth is then infected and this turns black and begins to die
  • If the tree or shrub is not treated the infection then affects wood from previous years’ growth and at this stage the tree will almost certainly die.

Fireblight does not attack plum, greengage, cherry, apricot or peach trees.

With pear trees in particular, the infection of blossom stage can sometimes not occur, the first signs being noticed on new shoots. This is because pear blossom tends to appear a week or two earlier than on apple trees when the conditions are not yet warm enough for the bacteria to flourish.

Picture of Fireblight from Stark Bros

LIFE CYCLE OF FIREBLIGHT

Fireblight affects apple and pear trees as well as other plants. The most important of those are hawthorns because these are so common in the UK that the bacteria can spread large distances using hawthorn hedges as their main vehicle. The bacteria first arrived in the UK in 1957 from North America. Currently in the UK fireblight is common on southern and warmer areas and less common in northern areas where cooler temperatures don’t suit the lifecycle of this bacteria.

Infected branches from the previous year begin to weep bacterially infected goo which is transferred to blossoms and new shoots by bees and other insects. The infections early in the year are spread by bees feeding on infected blossoms. Later in the year when new shoots weep infected goo, the bacteria is spread by insects and rain. Blossom and new shoots very quickly show signs of infection although older wood takes longer to be infected. However it is on old wood that the bacteria overwinter ready to restart the infection cycle next year. The diagram below is a graphical representation of the lifecycle.

HOW TO TREAT FIREBLIGHT OF APPLES AND PEARS

There is no magic cure for fireblight and with trees 4 years old or younger it is probably best to dig them up, destroy them and start again with a resistant variety. The list below describes the key prevention and damage limitation measures that can be used:

  • If you are in an area which is known to have fireblight, by far the most effective course of action is to grow varieties with some resistance. See the list at the end of this article for apple and pear trees which show resistance although there are no varieties which are totally immune
  • There are no chemical sprays available in the UK to treat fireblight. Copper fungicide sprays were available a few years ago but they have been withdrawn from sale.
  • As soon as you confirm that there is fireblight remove all infected shoots and blossom. Cut away any infected wood at least 15cm / 6in further back from the infection. This may result in a badly shaped tree but if not done the tree will almost certainly die. Scrupulously remove all prunings and infected material well away from the infected tree and burn it.

APPLE AND PEAR TREES WITH RESISTANCE TO FIREBLIGHT

No apple or pear trees are fully resistant to this bacteria but some varieties do show some resistance.

PEAR TREES RESISTANT TO FIREBLIGHT

There are no pear tree varieties available in the UK which are totally resistant to fireblight. Concorde does show some resistance however. Varieties normally grown outside the UK which are resistant to fireblight include:

  • Seckel – an old American variety which shows some resistance
  • Moonglow – although not resistant to fireblight it seems to respond to treatment better than others.

APPLE TREES RESISTANT TO FIREBLIGHT

There are no apple tree varieties available in the UK which are resistant to fireblight. Anyone who tells you differently is pulling the wool over your eyes, so beware! Common belief is that early flowering trees do better than later flowering ones although we have found no evidence of scientific trials to prove or disprove this.

Fireblight

Fireblight is one of the most serious diseases affecting apple and pear trees (as well as related species such as Medlars, Quince, Cotoneaster, and Hawthorn). Whilst most diseases of fruit trees are caused by fungal infections, fireblight is a bacterial disease. Infection usually occurs in spring, and the bacteria are spread by rain and by insects.

Apple tree on M7 killed by fireblight

The most common method of infection is through the blossom, with the bacteria being carried by pollinating bees. (For this reason it is unusual to see fireblight in very young trees since they will not have developed blossom, although the infection can also get in through aphid attacks, or even through holes created by heavy rainfall or hail).

The infection progresses rapidly and in a severe case leaves the tree looking as if it has been set on fire. The photo shows a mature apple tree killed by fireblight. Note the browned branches, and the attempts made by the owner to beat the infection by sawing off the main stem and the limb on the right.

Although fireblight can be a devastating problem for commercial apple and pear growers in affected areas, it is very dependent on warm weather and rain in early spring during the blossom season. It also tends to occur in cycles with heavy infections every 5-10 years. (Spring 2014 appears to have been a bad year in much of the north-eastern and south-eastern states of the USA).

Fireblight is rare in cool temperate climates such as the north west and north east of North America, and north-western Europe (including the UK), where spring weather is too cool for the bacteria to flourish.

For the same reason, late-blossoming apple and pear varieties (including most European cider apple varieties) are often more at risk than earlier-blooming varieties – there will be more bacteria in the air, and the weather is likely to be warmer.

History of fireblight

Fireblight is an interesting disease because it highlights the importance of international phytosanitary controls. It appears to have arisen spontaneously in the eastern USA in the 17th or 18th centuries, and progressed southwards. It is notable that some old local apple varieties in these areas appear to have been selected by the settlers for their fireblight resistance.

Because of poor controls between North America and Europe, infected plant material arrived in Europe in the 1950s, and it is now endemic in most of Europe. Some islands (e.g. Ireland, Isle of Man, Channel Islands, and Corsica) are still free of the disease.

Fireblight is also endemic in most of the UK. According to Defra, the UK government department responsible for plant health, it spread through the UK in the second half of the 20th century, living on hawthorn hedges that are commonly found along roadsides and railways.

However as noted above, although the fireblight bacteria is prevalent in large areas of northern Europe, it is not virulent in these areas and is of little consequence for gardeners and small orchards in the UK.

Fireblight resistance

Most of the work on fireblight resistance in apples and pears has taken place in the USA, where the disease is particularly aggressive in orchards in the eastern and southern states. There are essentially two approaches, the development of disease resistant varieties, and the development of resistant rootstocks.

The development of fireblight resistant rootstocks has proved to be one of the most effective ways to limit the disease. Fireblight usually only becomes fatal once the disease gets into the roots, and resistant rootstocks prevent this phase of the disease, giving the tree owner more options to cut out the infected parts of the main tree. Fireblight resistance has been the central goal of the Geneva series of rootstocks developed by Cornell University. The first rootstocks – G.11 and G.16 – showed good resistance and more recent releases such as G.41, G.30, G.935 and G.202 have shown further improvements.

In contrast the Malling-series rootstocks were developed in England in the early 20th century, before fireblight existed there, so they are not resistant. The one exception is M7, which by chance has quite good resistance and as a result is still widely used in the USA.

Resistant varieties are an eclectic mix of traditional varieties from the southern states such as Arkansas Black, and new disease-resistant varieties such as Honeycrisp. Contrary to the popular belief that supermarket apple varieties are not as disease-resistant as heirloom varieties, Red Delicious is well-known for its fireblight resistance. However note that “resistant” does not mean “immune”.

Similar approaches have been followed with pear trees. It was found that two pear varieties, Old Home and Farthingdale were largely immune to fireblight, and most pear trees grown in the USA are now grafted on a combination rootstock developed from Old Home and Farthingdale – known as OHxF. This rootstock is rarely used in the UK, since fireblight is not a significant issue here.

If you live in an area where fireblight is active it is best to choose resistant varieties grafted on resistant rootstocks.

Treatment

Prompt action is the key with a fireblight strike. The approach is similar to dealing with canker – but note that fireblight is more aggressive and spreads more rapidly. Cut back well behind the infected area of the shoot, and if in doubt cut back even further. You may feel this will ruin the attractive shape of your tree, but work on the basis that if left unchecked the infection will kill the tree, so a mis-shaped tree is better than no tree at all – and if the tree survives it will soon produce replacement branches.

As with canker, an infected tree is also a threat to all other nearby trees, so carefully remove debris from the orchard.

Copper-based sprays and bacteriacides are the main control method, applied in early spring. Insecticides are also very effective in controlling the disease (since it is mainly spread by insects) but unfortunately this can include the bees needed to pollinate the blossom!

Fireblight is more likely to affect young fast-growing trees than older more mature ones, so avoid hard-pruning of young trees during the dormant season (particularly in the 2nd and 3rd years) and do not use fertilizers which might encourage excessive rapid growth. If you have applied too much fertilizer, hoe in sawdust (which will cause soil bacteria to use up the Nitrogen in the fertilizer) and allow grass and weeds to grow in competition with the tree.

In short, whilst the disease can be controlled (particularly in the garden or backyard orchard), this is a case where prevention is better than cure, and it is best to plant fireblight-resistant varieties in areas where fireblight is known to occur.

More resources about Fireblight

These articles are from US university extension services in areas where fireblight is common, and provide useful insights into indentifying and managing the disease.

University of Missouri Extension.

North Carolina State University Extension.

Cornell University Hudson Valley.

Colorado State University Extension.

Transmission and Disease Cycle

Figure 5. Pathogen cells (1) multiply on the flower stigma (2) and if rain or dew occur are washed into the floral cup (3).

Erwinia amylovora overwinters within diseased plant tissue (e.g. cankers). In 20 to 50% of cankers active cells survive the winter (van der Zwet and Beer 1991) and when humidity is high in the spring the pathogen oozes out of these cankers. This ooze is attractive to bees, flies and other insects who transfer the blight pathogen to flowers. Pathogen cells can also be moved from old cankers to flowers by splashed and wind-blown rain. Pathogen cells multiply quickly on nutrient-rich floral stigmas when temperatures are warm (70-80 F is optimal for the pathogen) (Ogawa and English 1991). Bacterial colonies can then be washed down the style into the floral cup by water (usually from rain or heavy dew) where they can invade flowers through the nectaries. Once initial blossoms are infested, insects and rain can move the pathogen to additional flowers (Pattemore et al. 2014, Johnson et al. 1993). If the pathogen is successful in infecting the developing fruit-let, the disease spreads into the cambium (just between the bark and the wood) of the tree, killing young host tissues as it progresses (Momol et al. 1998) creating characteristic strikes and cankers. Pathogen cells migrate inside the tree well ahead of visible symptoms where they can accumulate in other susceptible tissue such as one-year-old shoot tips and susceptible rootstocks causing infections distant from the original infection point. Erwinia amylovora can also infect susceptible one and two-year-old tissue directly through wounds (e.g. insect feeding and hail) causing shoot blight infections.

Cultural Controls

Plant on resistant rootstock. Resistant rootstocks (e.g. Geneva series for apples) do not make the scion less susceptible, but will help prevent tree death from rootstock blight.

Sanitation. In winter, cut out old blight cankers as thoroughly as possible. Ideally, cut blight before you prune for tree structure so that the blighted cuttings can be removed from the orchard. Compared to cuts made in summer, winter removal cuts can be made closer to the visible canker edge. In winter the pathogen is confined to the cankered area. Cut at the next “horticulturally sensible” site below the canker. You do not need to sterilize tools when you are cutting on fully dormant trees. Late dormant copper applications may also provide orchard sanitation, reducing inocula levels going into spring (Elkins et al. 2015). During the summer, cut out blight when you see it. Make summer cuts AT LEAST 12-18” below the edge of the visible canker. Cut more aggressively in young, vigorous trees or susceptible varieties. Removing a strike can greatly reduce further damage to the tree if cut early.

Manage the orchard environment. In addition to warm temperatures moisture is required to create infections. As little as two to three hours of wetting is sufficient to trigger infections. Manage weeds/cover crops to limit relative humidity. Do not irrigate during bloom.

Blossom removal in young blocks. Blossom removal in young blocks and removal of late blooms limits the numbers of flowers and thus reduces potential points of infection.

Keep vigor of the tree moderate. Moderating vigor will not prevent infection, but it can reduce damage to the tree.

Temperature Risk Models

The risk of fire blight infections during bloom can be calculated based on the temperature and moisture. In Washington, the best prediction model is CougarBlight available at WSU Decision Aid System for Tree Fruit (DAS). This model calculates fire blight risk based on the temperature of the previous four days using the documented growth rate of the bacteria, e.g. higher risk with multiple hours above 70 F. (Pusey and Curry 2004). The model then projects risk for the next three days based on predicted temperatures. Growers can use model information to decide when to spray. If trees are likely to be blooming during an upcoming high-risk period, protective sprays are recommended (Smith and Pusey 2010).

Chemical Control Programs

There is a risk of fire blight infection any time there are flowers on the tree, the weather is warm, and wetting occurs. Watch for and protect secondary blossoms during the three weeks after petal fall, which is the most common time of fire blight infection. Most sprays only protect the blooms that are open. Protect new blooms as they open. In warm weather, follow-up sprays are needed every few days.

Conventional Management

Prebloom. Fixed-copper sanitation, but only if fire blight was in the orchard last year.

Early bloom. Apply biologicals (Blossom Protect) during early bloom. If fire blight was in the orchard last year, apply two applications of the biological. Re-apply biologicals a second time if lime sulfur was applied. Lime sulfur applied during early bloom is also antimicrobial and reduces blight pressure.

Early bloom to petal fall. Watch the model. After a period of warm weather, best results are obtained when antibiotics are applied within the 24-hour window before flower wetting during a high infection risk period. Products used must contact the interior of the flowers in sufficient water and approved wetting agent to completely cover the interior. Repeated antibiotic sprays may be necessary during extended high or extreme risk periods. One pound of any 17% oxytetracycline product per 100 gallons gives a 200-ppm solution. Kasugamycin is another effective antibiotic. Some trials have shown that a full rate of Kasugamycin and a full rate of oxytetracycline provides excellent control. Antibiotics plus Actigard (2oz) has increased control in WA and OR trials by 10%. Applications of less than 100 gal/A can be effective on small trees if flower interiors are well covered, but do not drop the ppm below 200 (oxytetracycline). Application by ground equipment on each row is highly recommended (aircraft is NOT recommended). Many fire blight bacteria in the Pacific Northwest are resistant to streptomycin, another registered antibiotic.

Petal fall to two weeks after. Continue protective programs one to two weeks post petal fall. Warm conditions during late bloom increase fire blight risk for late blooms still present.

Organic Management

Prebloom. Fixed copper sanitation if fire blight was in the orchard last year.

Early bloom. Lime sulfur plus oil (apples only). One to two applications of biologicals (Blossom Protect). Reapply biological after lime sulfur, which is antimicrobial.

Full bloom to petal fall. Depending on the cultivar russet risk and the CougarBlight model risk follow with Bacillus subtilis (Serenade Opti) (most fruit safe) every 2-5 days during flower/petal fall or copper hydroxide/octanoate (e.g. Cueva, Previsto) every 2 to 6 days (less fruit safe for russet). Coppers have had higher efficacy than biologicals during bloom in Washington trials. Do not follow coppers with any products with acidifiers. Good drying conditions are important to avoid russet risk.

Petal fall to two weeks after. Continue protective programs one to two weeks post petal fall. Warm conditions during late bloom increase fire blight risk for late blooms still present.

Figure 6. Relative control from coppers WSU Trials 2013 to 2017.

Strategies for Improving Protective Programs

Coverage. Product efficacy is based on thorough coverage of flowers. Use tree row volume to apply appropriate volumes to cover the tree architecture in your orchard. Products applied every other row or at high speeds may have insufficient coverage and lower efficacy.

Timing. Antibiotics have the highest efficacy when applied shortly before a moisture event. Nonetheless, Kasumamycin and Streptomycin can also be applied up to 12 hours after a moisture event, but with reduced effectiveness. Streptomycin has locally systemic activity and Kasumamycin is effective on bacteria which have been washed into the floral cup but not yet invaded the flower.

pH of spray tank water. It is important to appropriately acidify spray tank water when using antibiotics (especially oxytetraclycline and kasugamycin). Antibiotic efficacy reported in WSU trials is with spray tank water acidified to pH 5.6. At higher pH antibiotic degradation rate is higher and thus efficacy is often lower. For example, in one trial Kasugamycin reduced bacteria by 86 to 96% at pH 5.1 but only 21 to 35% at pH 7.3 (Adaskaveg, Forster, and Wade 2011).
Use appropriate rates. Quantity of active ingredient is important to obtain efficacy. For example, recent work looking at rates of copper products is demonstrating that as metallic copper content increases, copper product efficacy increases up to approximately 0.2 lb metallic copper per 100 gal per acre.

Mixtures. A full rate of kasugamycin (100 ppm) with a full rate of oxytetracycline (200 ppm), as well as streptomycin (100 ppm) mixed with a full rate of oxytetracycline (200 ppm) have provided improved efficacy in some trials (Oregon 2015-2018).

Actigard (2oz) plus an antibiotic applied during bloom has improved the efficacy of antibiotics an average of 10% in trials in Washington and Oregon (Smith and Johnson 2011-2014).

Chemical Control Products

Figure 7. Blossom Protect in WSU Trials 2013, 2014, 2016, 2017.

Biological Products

When applied to open flowers, these micro-organisms produce colonies on the stigma surfaces and nectary. With biological materials (e.g., Blossom Protect), spray treatments need to be initiated relatively early in the bloom period before high fire blight risk has developed.

Blossom Protect is a combination of two strains of Aureobasidium pullulans, a yeast that occurs naturally in Pacific Northwest pome fruit flowers. This organism grows on the nectary and stigmas of treated flowers and competes directing with the fire blight pathogen for the nutritional resource available on these surfaces. Blossom Protect is applied with a companion buffer, Buffer Protect, which reduces the pH of the sprayed suspension and helps the yeast grow faster than the pathogen. In Pacific Northwest trials, Blossom Protect has been the most effective bio-control organism to date (Johnson et al. 2014). If this product is used, it is important to spray every row at least once.

Antibiotics

Figure 8. Percent control from antibiotics in WSU Trials 2006, 2009, 2010, 2011.

Kasugamycin (tradename: Kasumin) is a recently labeled antibiotic that provides good levels of control (~80%). All Erwinia amylovorastrains are currently sensitive to this material but there is an intermediate risk of resistance developing to this antibiotic (Adaskaveg, Forster, and Wade 2011). Kasumin controls streptomycin-resistant strains of E. amylovora. Kasumin provides forward control for two to four days prior to rain events (on flowers open when applied) and will be partially effective for blossom blight control if applied within 12 hours after a rain event. Kasumin is not locally systemic like streptomycin. Thus, Kasumin will not penetrate into the nectaries and will not be able to control an infection once the fire blight pathogen reaches the nectaries. Acidifying spray tanks (target 5) is important to reduce antibiotic break down and extend activity.

Oxytetracycline (tradenames: Mycoshield, FireLine) generally provide good levels of control in Washington trials and has a low risk of resistance development. Oxytetracycline products should be applied within one day prior to a rain event for best results. Oxytetracycline is considered bacteriostatic (inhibits bacterial growth). Thus, it has to be applied prior to rains where it can prevent growth on stigmas. Oxytetracycline is also sensitive to UV degradation and much of the activity is lost within one to two days after application. Acidifying spray tanks (target 5) is important to reduce antibiotic break down and extend activity.

Streptomycin (tradenames: Agri-Mycin, FireWall): Streptomycin-resistant strains of the fire blight pathogen have been present in Washington orchards since 1975 (Coyier and Covey 1975, Loper et al. 1991). Recent tests have indicated that the proportion of the pathogen population resistant to this antibiotic has dropped, and expected control levels have improved (Forster et al. 2015). This product should only be used in combination with oxytetracycline, and should not be used unless a high to extreme risk infection period is expected. Limit use to once per season. Remaining pathogen colonies in the orchard should be assumed to be streptomycin-resistant.

Figure 9. WSU Trials 2013, 2014, 2016, 2017. Cueva (copper octanoate) 3-5 qt; Previsto (copper hydroxide) 3-5 qt; Instill (copper sulfate pentahydrate) 1-1.85 qt; Mastercop (copper sulfate pentahydrat) 0.25-1.25 qt; Basic coppers Champ, Kocide (0.5 lb), Badge (1.25 pt). Rates 100 gall/Acre.

Coppers

Copper materials vary in the form and amount of metallic copper (the active ingredient). “Fixed” (copper hydroxides, copper oxychlorides) –copper products have a longer residual time and are generally used for delayed dormant (green tip) in bearing orchards and summer shoot blight protection in non-bearing (young) orchards. In fixed coppers, most of the copper is insoluble with soluble copper ions released slowly over time. Application of low-pH materials (e.g., Buffer Protect) to trees treated recently with a fixed copper can cause a large release of copper ions and increase the potential for phytotoxicity (Rosenberger 2011). Copper is toxic to plants when a sufficient concentration of ions penetrates tissue. Growers should avoid spray additives such as foliar nutrients and surfactants when applying coppers. Fixed-coppers should not be used with Imidan, Sevin, Thiodan, Captan, or phosophorus acid compounds (Fostphite, Prophyt, Phostrol, Agri-Fos, Alliete) (Shane and Sundin 2011).

Soluble coppers. Newer copper formulations are designed to reduce copper phytotoxicity and fruit russeting potential by introducing far few copper ions to the plant surface and adding safeners that also reduce injury potential. Examples are Cueva (copper octanoate), which is a salt of copper and a fatty acid (copper soap), and Previsto, which is copper ions in a matrix with alginate (polymer from seaweed). Both Cueva and Previsto have shown little phytotoxicity in semi-arid Washington trials but have shown some risk of russeting in wetter areas of Oregon and California. Cueva is compatible in tank-mixes with Bacillus-based biopesticides, while Previsto is not due to its high pH.

SARs

Acibenzolar-S-methyl (ASM, Actigard 50 WG), is a synthetic inducer of systemic acquired resistance (SAR). Its mode of action is to mimic the plant hormone, salicylic acid, which is responsible for priming the plant’s defense system. The level of protection is smaller compared to an antibiotic but it lasts longer, approximately a week (Maxson-Stein et al. 2002).

Biorationals and Biopesticides

Serenade Optimum is an apparently ‘fruit safe’ material, made by fermenting a strain of Bacillus subtilis. The antimicrobial activity of Serenade comes primarily from biochemical compounds produced by the bacterium during fermentation, and not because of the bacterium’s colonization of flowers in the orchard.

Apple Materials

Excerpt from the WSU Crop Protection Guide. For timings at which each pesticide can be used refer to the Crop Protection Guide.

Pear Materials

Excerpt from the WSU Crop Protection Guide. For timings at which each pesticide can be used refer to the Crop Protection Guide.

Cutting Fire Blight Infections in Season

Cut hard, cut fast

An infected shoot has many millions to billions of pathogen cells. The highest concentration will be near the tip of the branch or infected floral cluster. By cutting a branch we hope to remove many of these cells so that they cannot flow through the tree where they may concentrate in other susceptible tissue and create new infections. Cut AT LEAST 12 to 18 inches below the noticeably infected area into two year or older wood in order to remove the highest concentration of pathogen cells. Young, vigorous or susceptible varieties will require cutting further. Removing infected tissue quickly increases the likelihood of removing more pathogen cells before they invade deeply into the tree. Some recommendations suggest an ‘ugly stub cut’ where growers make cuts 20-30 cm below visible symptoms into two-year-old or older wood (where resistance is greater due to carbohydrate reserves (Suleman and Steiner 1994)leaving a 10 to 12 cm naked stub. While small cankers will form on many of these cuts, these cankers can be removed during winter pruning (Steiner 2000).

Use of concentrated Actigard during blight clean-up

New research has shown that treatment of trees with the chemical, Acibenzolar-S-methyl (ASM, Actigard 50 WG), may reduce re-occurrence of blight after cutting out infected strikes. Re-occurrence happens when the act of cutting out the disease does not completely remove the pathogen cells that have moved ahead of the expanding canker.

Plants have defense systems. If something stimulates the plant’s defense response before the symptoms develop (or re-develop), the plant will be in an active defense mode and will be less affected by disease when it occurs (or re-occurs). Actigard is a compound that has been found to trigger induced resistance. Its mode of action is to mimic the plant hormone, salicylic acid, which is responsible for priming the plant’s defense system.

For more than five years, Dr. Ken Johnson of Oregon State University has found that painting a concentrated solution of Actigard on trees after cutting out infection reduced the severity of re-occurring fire blight cankers in pears. For example, he found that without treatment after cutting out fire blight cankers in young Bosc pear trees, the disease came back 50% of the time and began to run through the tree. With Actigard applications, both the proportion of trees in which fire blight came back and the rate of canker expansion was reduced (Johnson and Temple 2016).

During the summer, cut out blight when you see it. Removing a strike can greatly reduce further damage on the tree, especially if you catch the strike early. Apply concentrated Actigard with an up and down motion to a ½ meter length of the central leader or major scaffold near where the fire blight infection was removed. Use the labeled rate of 1 oz/ 1 quart with 1% silicone-based penetrant. One quart will treat approximately 500 cuts.

Additional Resources

Decision Aid System

Visit for the recent model projections of blossom blight risk at your site.

Crop Protection Guide

Crop Protection Guide recommendations are updated on an annual basis.

Organic Fire Blight Management in Western US

eOrganic article.

Dealing with Fire Blight Once it is in the Orchard.

WSU Newsletter article July 2017.

Tips for Using Blossom Protect

WSU Newsletter article April 10, 2017.

Remember last year’s infections are this year’s risk.

WSU Newsletter article April 2018.

Canker Management

WSU Newsletter article February 2017.

Literature Cited

Johnson, K.B. 2000. Fire blight of apple and pear. Plant Health Instructor. doi: DOI: 10.1094/PHI-I-2000-0726-01.

Johnson, K.B., and T N. Temple. 2016. Comparison of methods of acibenzolar-S-methyl application for post-infection fire blight suppression in pear and apple. Plant Disease 100.

Kleinhempel, H., M. Nachtigall. 1987. Disinfection of pruning shears for the prevention of fire blight transmission. Acta Horticulturae 217.

Lecomte, P. 1989. Risk of fire blight infection associated with pruning of pear trees. Acta Horticulturae 273.

Ogawa, J., and H. English. 1991. Diseases of temperate zone tree fruit and nut crops. Publication 3345. University of California Division of Agriculture and Natural Resources. Oakland, California.

Ostenson, H., and D. Granatstein. 2013. Critical Issues Report: Fire Blight Control Program. The Organic Center.

Rosenberger, D., 2011. Spring copper sprays for fruit diseases. In Scaffolds Fruit Journal. Cornel University.

Shane, B. and G.W. Sundin. 2011. Copper formulations for fruit crops. edited by Department of Plant Pathology. Michigan State University Extension.

Steiner, P.W. 2000. Integrated orchard and nursery management for the control of fire blight.” In Fire Blight: The Disease and its Causative Agent Erwinia Amylovora, edited by Joel L Vanneste. CABI Publishing, New York, NY, USA.

Suleman, P., and P.W. Steiner. 1994. Relationship between sorbitol and solute potential in apple shoots relative to fire blight symptom development after infection by Erwinia amylovora.” Phytopathology 84:1244-1250.

Teviotdale, B.L. 2011. Fire Blight. UC Statewide Integrated Pest Management Program UC ANR Publication 7414.

Toussaint, V., and V. Philion. 2008. Natural epidemic of fire blight in a newly planted orchard and effect of pruning on disease development. In Proceedings of the Eleventh International Workshop on Fire Blight, edited by K. B. Johnson and V. O. Stockwell, 313-+. Leuven 1: Int Soc Horticultural Science.

van der Zwet, T., and S.W. Beer. 1991. Fíre Blight—Its Nature, Prevention and Control: A Practical Guide to Integrated Disease Management. U.S. Department of Agriculture, Agriculture Information Bulletin 631.

Use pesticides with care. Apply them only to plants, animals, or sites listed on the labels. When mixing and applying pesticides, follow all label precautions to protect yourself and others around you. It is a violation of the law to disregard label directions. If pesticides are spilled on skin or clothing, remove clothing and wash skin thoroughly. Store pesticides in their original containers and keep them out of the reach of children, pets, and livestock.

YOU ARE REQUIRED BY LAW TO FOLLOW THE LABEL. It is a legal document. Always read the label before using any pesticide. You, the grower, are responsible for safe pesticide use. Trade (brand) names are provided for your reference only. No discrimination is intended, and other pesticides with the same active ingredient may be suitable. No endorsement is implied.

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Purdue expert: Trees can get ‘burned’ again by fire blight

Fire blight infections, such as this canker on a crabapple branch, must be pruned out for the tree to survive. Pruning should take place 12-15 inches away from visible canker symptoms. (Purdue Agricultural Communication photo/Janna Beckerman)
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There is no cure for fire blight; however, some trees can be successfully pruned. Severely damaged trees may have to be removed. In some cases, the disease may have spread because homeowners were taken in by the fraudulent claims for a cure.

“We want people to be aware that no cure exists, so they are not misled into an expensive treatment,” Purcell said.

Homeowners can get reputable advice or service from an International Society of Arboriculture certified arborist. A directory of certified arborists is available on the Indiana Arborist Association website at ag.purdue.edu/fnr/Associations/IAA or contact the IAA office at 765-494-3625.

Before pruning a tree themselves, homeowners should review steps outlined in Fire Blight on Fruit Trees in the Home Orchard, a Purdue Extension publication authored by Beckerman. Improper pruning can further damage trees or spread the disease. The publication is available for free download from Purdue Extension’s The Education Store at http://www.The-Education-Store.com

Homeowners who have questions should contact the Purdue Plant and Pest Diagnostic Laboratory at 765-494-7071 or go to www.ppdl.purdue.edu.

Writer: Olivia Maddox, 765-496-3207, [email protected]

Sources: Janna Beckerman, 765-494-4628, [email protected]

Lindsey Purcell, 765-494-3625, [email protected]e.edu

Ag Communications: (765) 494-2722;
Keith Robinson, [email protected]
Agriculture News Page

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