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Ministry of Agriculture, Food and Rural Affairs

Fire Blight - Collar & Rootstock

Rootstock blight occurs in the rootstock at ground level below the graft union
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.

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

  • Phytophthora collar rot - Rootstocks infected with Erwinia (fire blight) do not exhibit the reddening that is characteristic of Phytophthora infected tissue, especially below the soil line.

Period of Activity
Rootstock blight can be present in orchards from bloom through to 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 Cougar blight 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. While the shoots are actively growing, 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. For more information on the timing of sprays to manage fire blight, refer to the blossom blight infosheet.

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.

Collar and rootstock blight: Collar and rootstock blight occur in susceptible rootstocks such as M.9 and M.26. Collar and rootstock blight often occurs at ground level just below the graft union in the rootstock part of the tree. This type of blight frequently kills trees and is difficult to distinguish from Phytophthora collar rot. 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. Invasion of the fire blight bacteria occurs through infected suckers or water sprouts, washing bacteria from infected twigs and fruit down the trunk, or internal translocation (without vascular tissue symptoms of infection) of the fire blight bacteria from the infected cultivar scion above ground to the rootstock. Tree death particularly in young trees is almost certain when rootstocks become infected.

Often Confused With

  • Phytophthora collar rot - Rootstocks infected with Erwinia (fire blight) do not exhibit the reddening that is characteristic of Phytophthora infected tissue, especially below the soil line.

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

Period of Activity
Rootstock blight can be present in orchards from bloom through to 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 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. While the shoots are actively growing, 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. For more information on the timing of sprays to manage fire blight, refer to the blossom blight infosheet.

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 more information on the timing of sprays to manage fire blight, refer to the blossom blight infosheet

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.
  • The use of copper as a green tip spray may help to reduce fire blight inoculum.  Copper applications after this time can be phytotoxic and my result in fruit russetting. Cooper does not kill bacteria in the cankers or even to kill the bacteria as they ooze out of such sites. The copper provides an inhibitory barrier over all bark and bud surfaces in the orchard that will prevent the bacteria from colonizing these areas.  There is little information on how effective copper sprays are in managing fire blight.

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.