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Ministry of Agriculture, Food and Rural Affairs

Fire Blight - Blossom & Fruit

Infected blooms first appear water soaked and later begin to wilt, shrivel and turn brown or black Infected fruit first appear grey, green or water soaked Infected fruit become shriveled, dark brown, mummified and remain attached to the spurClick to enlarge.

Beginner

Scientific Name
Erwinia amylovora

Identification           
Fire blight attacks many different parts of the tree, and fire blight symptoms are often referred to by the part of the tree attacked – blossom, shoot, fruit, limb and trunk, and collar or rootstock blight.

Blossom blight:

  • Infected blooms first appear water soaked and later begin to wilt, shrivel and turn brown or black.
  • Pollinating insects visiting contaminated flowers spread the disease to non-infected flowers.
  • Once blooms are infected, blossom blight frequently remains 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 blight:

  • Fruit become infected during the growing season through wounds caused by insects and damage by high winds, rain or hail (trauma blight).
  • Symptoms begin appearing within 24-48 hours after wounding occurs such as after a hail event.
  • Infected fruit first appear grey, green or water soaked and later become shrivelled, dark brown and mummified.
  • Infected fruit often remain attached to the spur

Often Confused With

  • Nectria twig blight- Fire blight infections progress from infected blossom clusters or the tips of shoots downwards. In contract Nectria infections progress from the base of shoots upwards.
  • Frost- Blossoms damaged by frost can also be confused with those infected with fire blight. 

Period of Activity
Blossom blight is present in orchards from first bloom through to petal fall.  Occasionally rat-tail bloom can result in infection of secondary blossoms after petal fall.

Scouting Notes
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 CougarBlight models (see the advanced section for more information on these 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.

Thresholds
It is important to have knowledge of prior out breaks of fire blight in orchards. If a history is known, use proper management tools to deal with 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 program, bactericide sprays should be applied when the epiphytic infection potential reaches 100 or more.

Advanced

Scientific Name
Erwinia amylovora

Identification           
Fire blight attacks many different parts of the tree, and fire blight symptoms are often referred to by the part of the tree attacked – blossom, shoot, fruit, limb and trunk, and collar and rootstock blight.

Blossom blight: Blossom blight is observed in the spring when flowers are infected. Infected blooms first appear water soaked and later begin to wilt, shrivel and turn brown or black. Pollinating insects visiting contaminated flowers spread the disease to non-infected flowers. Once blooms are infected, blossom blight frequently remains 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 blight: Fruit become infected during the growing season through wounds caused by insects and damage by high winds, rain or hail (trauma blight). Symptoms begin appearing within 24-48 hours after wounding occurs such as after a hail event. Infected fruit first appear grey, green or water soaked and later become shriveled, dark brown and mummified. Infected fruit often remain attached to the spur

Often Confused With

  • Nectria twig blight- Fire blight infections progress from infected blossom clusters or the tips of shoots downwards. In contract Nectria infections progress from the base of shoots upwards.
  • Frost- Blossoms damaged by frost can also be confused with those infected with fire blight. 

Biology
Fire blight bacteria overwinter at the edges of cankers. In the spring, as temperatures increase above 18ºC, 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 also move into the orchard from nearby infected ornamental and wild hosts.

Infection and disease development depends on three events that must happen simultaneously:

  1. the presence of bacteria
  2. a susceptible host
  3. favourable weather conditions

Fire blight bacteria are capable of existing in the orchard environment 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 4-32ºC, with rapid multiplication leading to infection occurring most frequently when temperatures are between 24-28ºC. 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 they provide an opening for bacterial entry. 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 (hypanthium). 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 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 moves 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, hail or spread from contaminated pruning tools. As the growing season progresses, infections slow down and cankers develop in the bark. They tend to be sunken with indefinite margins at first but later develop cracks and become sealed off from the healthy tissues.

Period of Activity
Blossom blight is present in orchards from first bloom through to petal fall.  Occassionally rat tail bloom can result in infection of secondary blossoms after petal fall.  Fruit blight can be present in orchards from petal fall through harvest.

Scouting Notes
When scouting for fire blight, it is important to recognize the stage of development and what it is important to be monitoring for. While the tree is in dormancy, look for cankers that are over-wintering in the orchard. 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 CougarBlight 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.

Thresholds
It is important to have knowledge of prior out breaks of fire blight in orchards. If a history is known, 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 program, bactericide sprays should be applied when the epiphytic infection potential reaches 100 or more.

Management Notes
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.

  • CougarBlight
    The CougarBlight model was developed by Dr. Tim Smith, Washington State University to predict blossom infections. To use the CougarBlight 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.

    Click here to download the CougarBlight model spreadsheet


    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 www.ncw.wsu.edu/treefruit/fireblight/2000f.htm

Cultivar and rootstock susceptibility
All apple cultivars and rootstocks are susceptible to fire blight, however some are less susceptible than others. When planning new plantings, particularly in southwestern Ontario (below a line from Sarnia to Oakville), consider fire blight susceptibility when deciding what cultivars and rootstocks to plant.  For information on the susceptibility of cultivars and rootstocks to fire blight refer to OMAFRA Publication 310, Integrated Pest Management for Apples.

Cultural management practices
To reduce the risk of fire blight incidence and spread in the orchard, the following cultural management practices are recommended.

  • 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”.
  • 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 apple orchards encourages new shoot growth and extends the susceptibility period for shoot blight. If fire blight is present, disinfect pruning tools between each cut. However, sterilizing pruning tools is not required if the pruning is done well beyond the infection (30 cm beyond 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.
  • Suckers (water sprouts) are good entry points for fire blight into large branches, limbs and trunks. Break them out periodically during the early growing season (i.e. June).
  • Prune infected trees during the dormant season to remove all overwintering cankers and sources of inoculum. Prune well beyond visibly infected areas.  For more information on pruning techniques to minimize the spread of fire blight refer to OMAFRA Publication 310, Integrated Pest Management for Apples.
  • Avoid using overhead irrigating to prevent excessive vegetative growth and reduce the spread of bacteria within the orchard.
  • 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 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.
  • Scout orchards twice a week starting at bloom for blossom infections. Prompt removal of early bloom infections significantly reduces the impact of the disease later on in the season. Remove infected spurs by cutting at least 15 cm beyond the farthest signs of infection. 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.

When fire blight appears year after year in an orchard, use a more aggressive approach to keep this disease managed. For more information on managing fire blight, see OMAFRA Publication 360, Guide to Fruit Production - Chapter 4 Apples (PDF) or Apple Calendar only (PDF):

Chemical Management of Fire blight

  • Bactericides
    Currently the application of well timed bactericide sprays (streptomycin), to susceptible cultivars during during bloom is the best method of managing fire blight.  Streptomycin-resistant strains have been identified in British Columbia, Michigan, New York, California, Washington, and elsewhere. A survey in the early 1990s of apple and pear blocks in southern Ontario indicated all strains of fire blight remained susceptible to streptomycin in Ontario.

    Limiting the number of applications to three per season for each product decreases the selection pressure and slows down the development of resistance to both copper and streptomycin. See labels for pre-harvest interval for streptomycin.

Biological control agents 
Serenade Max, Bloomtime and BlightBan are biological products registered for the suppression of fire blight in apples. For more information on these products, see OMAFRA Publication 360, Guide to Fruit Production - Chapter 4 Apples (PDF) or Apple Calendar only (PDF):

Growth regulators
Apogee is a growth regulator registered in Canada for shoot control in apples. Studies show this product also helps reduce the incidence and severity of fire blight in orchards when applied according to label recommendations. Apogee has no direct affect on the fire blight pathogen, but reduces the vegetative growth of susceptible shoots and stimulates the plants defense system to reduce infections. It is not a silver bullet for fire blight control but another tool to help reduce the risk of disease. For more information on the use of Apogee for fire blight, see OMAFRA Publication 360, Guide to Fruit Production - Chapter 4 Apples (PDF) or Apple Calendar only (PDF)- :