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Ministry of Agriculture, Food and Rural Affairs

Fire Blight - Shoot & Trauma

Infected shoots form an “inverted J” or a shepherds’ crook at the tip Severely infected shoots Bacterial oozing out from the stem of an infected shoot
Click 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.

Shoot blight:

  • Infected shoots first appear water-soaked but quickly turn brown or black.
  • As the bacteria move down the shoot, stem tissues become discoloured and shrivelled.
  • The infected shoots form an “inverted J” or a shepherds’ crook at the tip.
  • Apple and pear trees with several severely infected shoots appear scorched by fire.
  • Initial leaf symptoms appear as brown or black discolouration along the petiole and midvein, while the rest of the leaf tissue remains green.
  • Bacteria ooze along the midvein of infected leaves and the stem of infected shoot is also common.
  • Eventually, infected leaves turn brown starting with the tissue along the midvein.
  • Dead leaves often cling to the shoot throughout the growing season and into winter.
  • Usually the shoot tip is the first part affected, and occasionally the base of the shoot is the primary infection point.
  • Injuries on leaves and shoots caused by sand blasting, wind whipping or hail provide an entrance for the bacteria into the susceptible tissue.

Trauma blight:

  • An unusual, very destructive phase of fire blight that develops as the result of infections following injuries associated with late frosts, hail storms or high winds that damage leaves, fruit and shoots.
  • Injuries caused by these events provide an entrance for the pathogen.

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.
  • Scorched trees- Trees that are located near a burn pile may exhibit similar symptoms to shoot blight.
  • Flagging from oriental fruit moth (OFM)- Frass, and tunnelling in the shoot is always associated with OFM injury.

Period Of Activity
Fire blight can be present in orchards from bloom through to harvest.  Generally after terminals harden off in July there is less risk of new shoot infections, and exist infections tend to spread less.

Scouting Notes
When scouting for fire blight, monitor for terminals that appear to have the characteristic shepherds crook. These deformations of the terminals are indicative of a fire blight infection.

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.

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 or rootstock blight.

Shoot blight: Succulent shoots and suckers (water sprouts) can also become infected resulting in shoot blight. Infected shoots first appear water-soaked but quickly turn brown or black. Shoot blight develops very quickly once infection takes place. 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. Injuries on leaves and shoots caused by sand blasting, wind whipping or hail provide an entrance for the bacteria into the susceptible tissue. The bacteria move into the shoot within a few days after an infection. As the bacteria move down the shoot, stem tissues become discoloured and shrivelled. The infected shoots form an “inverted J” or a shepherds’ crook at the tip – often the first symptoms are observed several weeks after bloom. Apple and pear trees with several severely infected shoots appear scorched by fire. Initial leaf symptoms appear as 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 shoot is also common.

Eventually, infected leaves turn brown starting with the tissue along the midvein. Dead leaves often cling to the shoot throughout the growing season and into winter. Usually the shoot tip is the first part affected, and occasionally the base of the shoot is the primary infection point. Fire blight bacteria also move through the vascular tissue of the tree from a canker into the shoot resulting in similar symptoms.

Trauma blight: Trauma blight is an unusual, very destructive phase of fire blight that develops as the result of infections following injuries associated with late frosts (temperatures less than or equal to 3ºC), 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 defence mechanisms. Trauma blight can result in fruit or shoot infections. 

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.
  • Scorched trees- Trees that are located near a burn pile may exhibit similar symptoms to shoot blight.
  • Flagging from oriental fruit moth (OFM)- Frass, and tunnelling in the shoot is always associated with OFM injury.

Biology
Refer to the blossom blight infosheet for information on the biology of Erwinia amylovora, the bacteria that causes fire blight.  

Period of Activity
Fire blight can be present in orchards from bloom through to harvest. Generally after terminals harden off in July there is less risk of new shoot infections, and exist infections tend to spread less.

Scouting Notes
When scouting for fire blight, monitor for terminals that appear to have the characteristic shepherds crook. These deformations of the terminals are indicative of a fire blight infection.

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.

After bloom management focuses on minimizing shoot blight and the development of cankers.

Management Notes
Chemical management
From bloom through petal fall management focuses on the use of timed sprays to manage blossom blight.  For information on the use of models for timing sprays for fire blight refer to blossom blight.  After bloom management focuses on minimizing shoot blight and the development of cankers that overwinter and serve as next years inoculum.

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):

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