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Ministry of Agriculture, Food and Rural Affairs

Fireblight

Fireblight on pear fruit Fireblight on pear fruitlet Characteristic burning of shoot Oozing infected pear fruit Fireblight on pear Fireblight on pear shoot Fireblight on pear shoot Fireblight on pear spur Fireblight in pear orchard
Click to enlarge.

Beginner

This pest affects:

Pear

Scientific Name
Erwinia amylovora

Identification           
Blossoms:

  • Infected blooms first appear water soaked and later begin to wilt, shrivel and turn brown or black
  • Blighted blossoms frequently remain attached to the tree allowing the disease to progress into the flower stem and then into the spur
  • If the spur on a main branch or the trunk becomes infected, the disease can move from the spur into the trunk or branch eventually developing into a canker which girdles the branch or limb

Fruit:

  • Infected fruit first appear grey, green or water soaked and later become shrivelled, dark brown and mummified
  • Under high humidity, droplets of bacterial ooze may develop
  • Infected fruit often remain attached to the spur

Shoots:

  • Succulent shoots and suckers (water sprouts) can become infected
  • Usually the shoot tip is the first part affected
  • Infected shoots first appear water-soaked but quickly turn dark brown to black and form an “inverted J” or a shepherds’ crook at the tip
  • Leaves on infected shoots initially have brown or black discolouration along the petiole and midvein, while the rest of the leaf tissue remains green. Bacterial ooze along the midvein of infected leaves and the stem of infected shoots is also common
  • Eventually, infected leaves turn brown
  • Dead leaves often cling to the shoot throughout the growing season and into winterx
  • Trees with several severely infected shoots appear scorched by fire

Trunks and limbs:

  • Cankers first appear brown to purple
  • Eventually they become sunken with cracked margins
  • Tissue under the bark initially appears water soaked, then with red streaks and finally turns brown
  • During rainy or very humid periods, bacteria ooze onto the surface of the cankers
  • At first, bacterial ooze appears a milky white drop but quickly becomes brown upon exposure to air. The ooze remains as a gummy substance or completely dries to an amber resin-like substance on canker surfaces where it remains for several months

Collar and rootstock blight:

  • Often occurs at ground level just below the graft union in the rootstock part of the tree
  • Infections initially appear dark, water soaked and purplish – the margins are indefinite or raised and blistered at first, but become definite and marked by a crack or crevice later
  • Upon removal of the bark, the affected area shows red-brown streaking in the internal tissues
  • Tree death particularly in young trees is almost certain when rootstocks become infected

Often Confused With
Scorched trees- Trees that are located near a burn pile may exhibit similar symptoms to shoot blight

Frost injury – no shepherd’s crooks, cankers or bacterial oozing; occurs after a cold event after bud break

Blossom blast - no shepherd’s crooks, cankers or bacterial oozing; often occurs after a cold event after bud break

Herbicide drift – mainly occurs on lower limbs, affecting both leaves and fruit with localized necrotic lesions

Scorch by high winds or temperatures – no cankers or bacterial oozing;  usually occurs much later in the season than blossom blight

Period of Activity
Fire blight can be present in orchards from bloom through to harvest. Blossom blight can occur in orchards from first bloom through to petal fall.  Occasionally rat tail bloom can result in infection of secondary blossoms after petal fall.  The first symptoms are often observed several weeks after bloom. Generally after terminals harden off in July there is less risk of new shoot infections, and existing infections tend to spread less.

Scouting Notes
Start monitoring for active cankers between tight cluster and white bud.  Scout orchards twice a week starting at bloom for blossom infections. Concentrate on blocks with susceptible cultivars and a history of fire blight   When scouting for fire blight, it is important to recognize the stage of development of the disease and which symptoms might be present. While the tree is dormant, look for cankers that are over-wintering in the orchard as well as terminals with the characteristic shepherd’s crooks with brown leaves still attached. During the bloom period to first cover, record precipitation, temperature and stage of development in those varieties most likely to become infected. By using Maryblyt or Cougar blight models, timing of infection often can be ascertained.  It is important to scout for cankers that appear to be oozing and blossoms that appear to be infected.

Monitor orchards after severe weather.  Symptoms become evident within 24-48 hours after wounding occurs such as after a hail event.

Thresholds
If the orchard has a history of fire blight, use proper management tools to deal fire blight during weather periods that are conducive to possible outbreaks.

Bactericide sprays should be applied to susceptible varieties from first bloom through petal fall if any of the following occurs – temperatures exceeding 18°C with high humidity (>69%), heavy dews or rainfall.  For growers with access to the Maryblyt or CougarBlight programs, follow the risk levels and apply  bactericide sprays when recommended by the program.

Management after bloom focuses on minimizing shoot blight and the development of cankers.

Advanced

This pest affects:

Scientific Name
Erwinia amylovora

Identification           
Blossoms:

  • Infected blooms first appear water soaked and later begin to wilt, shrivel and turn brown or black
  • Blighted blossoms frequently remain attached to the tree allowing the disease to progress into the flower stem and then into the spur
  • If the spur on a main branch or the trunk becomes infected, the disease can move from the spur into the trunk or branch eventually developing into a canker which girdles the branch or limb

Fruit:

  • Infected fruit first appear grey, green or water soaked and later become shrivelled, dark brown and mummified
  • Under high humidity, droplets of bacterial ooze may develop
  • Infected fruit often remain attached to the spur

Shoots:

  • Succulent shoots and suckers (water sprouts) can become infected
  • Usually the shoot tip is the first part affected
  • Infected shoots first appear water-soaked but quickly turn dark brown to black and form an “inverted J” or a shepherds’ crook at the tip
  • Leaves on infected shoots initially have brown or black discolouration along the petiole and midvein, while the rest of the leaf tissue remains green. Bacterial ooze along the midvein of infected leaves and the stem of infected shoots is also common
  • Eventually, infected leaves turn brown
  • Dead leaves often cling to the shoot throughout the growing season and into winter
  • Trees with several severely infected shoots appear scorched by fire

Trunks and limbs:

  • Cankers first appear brown to purple
  • Eventually they become sunken with cracked margins
  • Tissue under the bark initially appears water soaked, then with red streaks and finally turns brown
  • During rainy or very humid periods, bacteria ooze onto the surface of the cankers
  • At first, bacterial ooze appears a milky white drop but quickly becomes brown upon exposure to air. The ooze remains as a gummy substance or completely dries to an amber resin-like substance on canker surfaces where it remains for several months

Collar and rootstock blight:

  • Often occurs at ground level just below the graft union in the rootstock part of the tree
  • Infections initially appear dark, water soaked and purplish – the margins are indefinite or raised and blistered at first, but become definite and marked by a crack or crevice later
  • Upon removal of the bark, the affected area shows red-brown streaking in the internal tissues
  • Tree death particularly in young trees is almost certain when rootstocks become infected

Often Confused With
Scorched trees- Trees that are located near a burn pile may exhibit similar symptoms to shoot blight.

Frost injury – no shepherd’s crooks, cankers or bacterial oozing; occurs after a cold event after bud break

Blossom blast - no shepherd’s crooks, cankers or bacterial oozing; often occurs after a cold event after bud break

Herbicide drift – mainly occurs on lower limbs, affecting both leaves and fruit with localized necrotic lesions

Scorch by high winds or temperatures – no cankers or bacterial oozing;  usually occurs much later in the season than blossom blight

Biology
Fire blight can occur on apple, pear, hawthorn, crabapple and Japanese or flowering quince, mountain ash, cotoneaster, raspberry, serviceberry and spirea.

The bacteria that cause fire blight overwinter at the edges of cankers on trunks and limbs. In the spring, as temperatures rise above 18ºC, bacteria in cankers become active and droplets containing high numbers of bacteria ooze out of infected bark tissue. The ooze can be rain-splashed or carried by insects to open blossoms and tender developing shoot tips. The bacteria can also move into the orchard from nearby infected ornamental and wild hosts.

Fire blight bacteria are capable of existing on host tissue surfaces without infecting or causing disease symptoms. Once favourable environmental conditions occur, bacteria multiply rapidly and enter susceptible tissue resulting in infection and disease. The bacteria grow over a range of temperatures from 4 to 32ºC, with rapid multiplication leading to infection occurring most frequently when temperatures are 24 to 28ºC. Hot, wet weather for an extended period of time favours the multiplication of the pathogen and infection, and encourages the succulent growth of susceptible plant tissue.

Open blossoms are the most susceptible tissues since. Bacteria are carried to blossoms by wind, rain and insects. Once bacteria are introduced to an open blossom, they multiply very rapidly on stigmas. Free moisture (rain, dew) moves the bacteria into the flower cup. The bacteria pass through the natural openings directly into the host tissue. Further spreading of the pathogen occurs when pollinating insects carry it from infected to non-infected blossoms.

Once infection occurs, the disease moves quickly through the vascular tissue into spurs and other succulent tissues (one- to two-year-old wood), especially if accompanied by warm temperatures. In young nursery plantings and two- and three-year-old trees, the pathogen can move from infected blossom to the root in one month under favourable weather conditions (20-28ºC).

Secondary infections occur throughout the growing season caused by the spread of the bacteria from infected blossoms and oozing cankers to developing shoots and wounds made by insects, wind or hail.  Fire blight can also be spread by contaminated pruning tools. As the growing season progresses, infections slow down and cankers develop in the bark.

Shoot blight symptoms are observed throughout the spring and summer seasons, especially when fire blight is present in the orchard in infected blossoms, other blighted shoots and overwintering cankers.  Shoot blight develops very quickly once infection takes place. The bacteria move into the shoot within a few days after an infection.  Fire blight bacteria also move through the vascular tissue of the tree from a canker into the shoot resulting in similar symptoms.

The most severe injury from both blossom and shoot blight occur when the disease progresses into older wood. Fire blight progresses into the main limbs and trunk of the tree from infected spurs or shoots when warm temperatures with high humidity combine to form ideal conditions for fire blight. The cankers that form in the older wood girdle the branch, killing healthy wood from that point outward by cutting off the transport of nutrients and water. In the winter, cankers usually dry up and become inactive. In the spring, renewed activity at the margins of cankers results in the extension of the cankers. Infections extend and spread into adjacent water sprouts, shoots and limbs resulting in canker blight. Canker extension may begin before, during or shortly after bloom, depending on the environmental conditions experienced during the spring in the orchard.

Trauma blight is an unusual, very destructive phase of fire blight that develops as the result of infections following injuries associated with late frosts while the tree is actively growing, hail storms or high winds that damage leaves, fruit and shoots. Injuries caused by these events provide an entrance for the pathogen and shock or traumatize trees, reducing their natural defense mechanisms. Trauma blight can result in fruit or shoot infections.

Period of Activity
Fire blight can be present in orchards from bloom through to harvest. Blossom blight can occur in orchards from first bloom through to petal fall.  Occasionally rat tail bloom can result in infection of secondary blossoms after petal fall.  The first symptoms are often observed several weeks after bloom. Generally after terminals harden off in July there is less risk of new shoot infections, and existing infections tend to spread less.

Scouting Notes
Start monitoring for active cankers between tight cluster and white bud.  Scout orchards twice a week starting at bloom for blossom infections. Concentrate on blocks with susceptible cultivars and a history of fire blight. When scouting for fire blight, it is important to recognize the stage of development of the disease and which symptoms might be present. While the tree is dormant, look for cankers that are over-wintering in the orchard as well as terminals with the characteristic shepherd’s crooks with brown leaves still attached. During the bloom period to first cover, record precipitation, temperature and stage of development in those varieties most likely to become infected. By using Maryblyt or Cougar blight models, timing of infection often can be ascertained.  It is important to scout for cankers that appear to be oozing and blossoms that appear to be infected.

Monitor orchards after severe weather.  Symptoms become evident within 24-48 hours after wounding occurs such as after a hail event.

Thresholds
If the orchard has a history of fire blight, use proper management tools to deal fire blight during weather periods that are conducive to possible outbreaks.

Bactericide sprays should be applied to susceptible varieties from first bloom through petal fall if any of the following occurs – temperatures exceeding 18°C with high humidity (>69%), heavy dews or rainfall.  For growers with access to the Maryblyt or CougarBlight programs, follow the risk levels and apply  bactericide sprays when recommended by the program.

Management after bloom focuses on minimizing shoot blight and the development of cankers.

Management Notes
Plant resistant cultivars and rootstocks.  Cultivars Harovin Sundown, Harrow Crisp, Harrow Delight, Harrow Gold, Harrow Sweet and Harvest Queen are considered resistant to fire blight; Kiefer, Seckel and Spartlett are moderately susceptible and Anjou, Bartlett, Bosc, Cascade, Clapp's Favorite and Flemish Beauty are very susceptible.  Bartlett rootstock is susceptible to fire blight; Old Home x Farmingdale clones are highly resistant.

Avoid over fertilizing with nitrogen. Excess nitrogen stimulates succulent growth that is susceptible to fire blight infection. Apply nitrogen fertilizer only where warranted by annual leaf analysis. Consider a split application of nutrients, half in the spring before growth starts and half again after petal fall. If severe blossom blight occurs, withhold the later application. Similarly, avoid late cultivation that can make nitrogen available to the tree resulting in late succulent growth.

In healthy trees, avoid excessive winter pruning which stimulates vegetative growth the following growing season.  Regular annual pruning and minimizing the number of cuts made keeps the tree “calmer”.

After harvesting operations are complete, walk several times through the orchard and identify cankered trees, marking them with bright-coloured paint or tape in order to be able find them again during the winter. Prune infected trees during the dormant season before sap flow begins to remove all overwintering cankers and sources of inoculum. Prune well beyond visibly infected areas. Remove all prunings from the orchard and burn them.

If collar rot develops on susceptible rootstock and if fire blight is a significant problem, remove the entire tree from the orchard immediately and to burn it.

Delay summer pruning until terminal bud set has occurred (i.e. terminals “hardened off”), generally by early to mid August. Summer pruning to increase fruiting wood in high-density orchards encourages new shoot growth and extends the susceptibility period for shoot blight.

Remove infections (blossom and shoot blight) as soon as they are found.  Avoid pruning during wet weather or when severe weather (hail, heavy rain, storms) is expected in the next 24 hours.

Disinfect pruning tools between each cut. However, sterilizing pruning tools is not required if the pruning is done well below the infection (30-40 cm below the visible symptom) such that the pruned section does not have dark concentric sections. In older orchards that are severely infected with fire blight, avoid summer pruning altogether. Remove spurs on the main trunk and scaffold limbs to eliminate their potential of infection.

Break out suckers (water sprouts) periodically during the early growing season (i.e. June).  They are good entry points for fire blight into large branches, limbs and trunks.

Avoid using overhead irrigating to prevent excessive vegetative growth and reduce the spread of bacteria within the orchard when blossoms are present and temperatures are favourable to fire blight infection.

Maintain good integrated pest management practices to minimize the spread of the fire blight bacteria by insects and to reduce insect-caused wounds to leaf and shoot tissue, which act as entry points for the bacteria. Good control of plant-sucking insects such as psylla, mites, leafhoppers, aphids and plant bugs is particularly important. Conduct frequent (weekly) insect pest monitoring and use appropriate control measures where warranted.

Closely monitor non-orchard sources of fire blight such as susceptible ornamental plants or abandoned apple trees, hawthorn, mountain ash, cotoneaster and quince that are close to commercial orchards for signs of fire blight and cankers. Overwintering cankers are one of the primary sources of bacteria for subsequent infections. If possible, remove these host tree and shrubs from the vicinity of the orchards.

Remove infected spurs by cutting at least 15 cm beyond the farthest signs of infection. Prompt removal of early bloom infections significantly reduces the impact of the disease later on in the season.  If continual blossom blight is observed in a particular area of the orchard, scout for an active canker in the vicinity and remove it promptly.

Management with bactericides:
When fire blight appears year after year in an orchard, use a more aggressive approach to keep this disease managed. From bloom through petal fall, management focuses on the use of timed sprays to manage blossom blight.  After bloom management focuses on minimizing shoot blight and the development of cankers that overwinter and serve as next years inoculum. For more information on managing fire blight, seeOMAFRA Publication 360, Fruit Crop Protection Guide:

Chapter5 - Pear or Pear Calendar only: Recommendations for fire blight at Bloom.

Decision support models
Fire blight prediction models (Maryblyt and CougarBlight) help growers forecast conditions that lead to the infection of apple blossoms and time their product applications. For best results using these models, collect and record information separately from different blocks and cultivars. Prediction models are extremely valuable management tools and their use is encouraged for apple growers that have experienced fire blight in their orchard.

  • Maryblyt
    The Maryblyt model (developed in Maryland), is a computer software program used to determine the risk of fire blight in an orchard based on environmental conditions and the susceptible growth stage. To use the Maryblyt model, record the maximum and minimum daily temperature, rainfall or dew events, and the growth stage of trees in the orchard. Based on the information input into the computer model, Maryblyt predicts when blossom infection will occur and provides information on when a bactericide is required. The model also predicts when fire blight symptoms will appear to help growers schedule time for pruning if necessary. If the following conditions develop in sequence, and fire blight bacteria are present, the Maryblyt model predicts blossom infection:
    • blossoms are open with petals intact
    • passage of 110 degree hours above 18.3°C from first open bloom
    • a wetting event of at least 0.25 mm of rain or a heavy dew, or more than 2.5 mm rain the previous day
    • an average daily temperature of 15.6°C

For more information on using Maryblyt refer to OMAFRA Publication 310, Integrated Pest Management in Apples or refer to Maryblyt website at http://www.caf.wvu.edu/kearneysville/maryblyt/

  • CougarBlight2010
    The CougarBlight model was developed by Dr. Tim Smith, Washington State University to predict blossom infections. To use the CougarBlight 2010 model growers must record leaf wetness and temperature data from their orchard and enter this information into an excel spreadsheet. As with the Maryblyt model, leaf or blossom wetness, as well as minimum and maximum daily temperatures during the time blossoms are present is recorded along with information about the history of fire blight in and around the orchard.  Start collecting weather data within the orchard at least three to five days before the first blossoms open. Follow these instructions to use the CougarBlight model.

    Entering the current day’s high and low temperatures and the predicted temperatures into the spreadsheet allows growers to assess the potential of fire blight risk in the orchard before infection occurs and take action prior.  
    • A caution risk assessment indicates conditions may be favourable for fire blight development. Be vigilant if open blossoms are present and the flowers were wet either from dew or rain.
    • If the risk assessment for the forecasted days indicates a high risk and active fire blight is in the region, an application of streptomycin is required immediately. 
    • If the risk assessment indicates an extreme fire blight risk based on the predicted weather data, the conditions for fire blight bacteria to multiply and infect are approaching and an application of streptomycin is required before that predicted date.

For more information on using CougarBlight refer to OMAFRA Publication 310, Integrated Pest Management in Apples, or visit the Washington State University website at http://ext100.wsu.edu/chelan-douglas/agriculture/treefruit/pestmanagement/fireblight/cb2010fireblightriskmode.

Some information above has been excerpted from: