Apples Diseases Pest Management

1…2…3 Fire Blight Strike “You’re Out!”

Written by: Colleen Haskins, Acting Horticulture IPM Specialist, OMAFRA with input from Michael Celetti, Pathologist-Horticulture, OMAFRA

(Excerpts of this article were taken from the article “Fire Blight: Consider and Aggressive Orchard Management Approach”.  Authors: Kristy Grigg-McGuffin and Wendy McFadden-Smith, OMAFRA IPM Specialists)

Have the rains and hot, humid weather brought out some unexpected brown ”wilted” terminals, leaves or fruitlets?  Perhaps some of the branches are curving at the ends, beginning to look like a “shepherds crook”.    If so, take a closer look – do you see pale yellow or amber coloured “droplets” on the fruitlets, stems or tissue?

If you’re seeing these or similar symptoms, there is a good chance you have…

the early signs of a Fire blight strike or infection.

Last week, reports from surrounding areas indicated a few more fire blight strikes, than what had previously been reported at the end of May and early June.  Hot, humid weather, paired with wetting events are ideal conditions for bacterial growth, and spread of inoculum within an orchard.

The following image collage shows various fire blight infection symptoms on mature Ida Red (M109) trees.

1) Top left – several fire blight strikes on a mature tree (Ida Red). 2) Top middle – infected shoots displaying the classic “shepherd’s crook”. 3) Top right – infected shoot. 4) Bottom left – infected leaves and developing fruit. 5) Bottom right – under side of leaf showing signs of fire blight infection.

Best practices to reduce Fire blight inoculum and the potential for subsequent infections in your orchard:

1. Remove:

a) Infected tissue:

  • Remove sources of inoculum (e.g. pruning out strikes) to significantly reduce secondary infection potential and subsequent spread of the bacterium.
  • Disinfect your pruning equipment between each pruning “cut” to reduce spreading and infecting other tissue or surrounding trees.
  • Cut at least 30 cm beyond any visible infected tissue
  • Leave pruning/infected material in the tractor rows to dry up prior to removing or mulching them.
    • Cautionary note: leaving active inoculum in the orchard increases the risk of infection through tree trauma sites, from incidents such as hail, severe wind or rain during the season. This is known as “trauma blight”.

Trauma Blight:

The weather over the past couple of weeks has caused many areas to experience intense weather systems, delivering heavy rains, high winds and hail (for some).  Conditions like these can lead to broken branches, tissue and fruitlet damage, thus creating trauma sites and an entry point for fire blight.

Note: if you have older trees (>8-10yrs) with numerous strikes, delay pruning until terminal bud set, and choose 2-3 consecutive cooler (below 25oC) non-humid days to do so.

Here is a blog post from Cornell University discussing the relationship between terminal bud set and fire blight susceptibility.

b) Root suckers:

  • Rootstock infection from suckers can be lethal causing tree death within the same year. Some rootstocks such as M9, M26 and Swiss Bartlett have shown to die quickly after this type of infection.
  • These suckers can provide an entry point for systemic herbicides (e.g. glyphosate) which may weaken the tree, making them more susceptible to infection/disease such as fire blight.
  • Suckers provide soft, new, desirable tissue for sucking insects, which could promote subsequent spread of bacterium later in the season.

2. Avoid:

a) Excessive succulent growth

  • Similar to reducing the presence of root suckers, reduce succulent growth that is susceptible to fire blight infection.
  • Succulent growth attracts sucking insects, but is also more susceptible to mechanical or trauma injury, which can lead to entry wounds for the bacteria.

3. Maintain:

a) A good IPM program

  • Fire blight can quickly spread throughout an orchard via many channels; including pests, active inoculum left in the orchard, and contaminated pruning equipment or people working within the orchard.
  • Plant sucking insects such as leafhoppers, aphids and plant bugs can be potential vectors of fire blight. Frequent pest monitoring and appropriate management strategies of pests will help reduce the potential to spread the disease.
  • If you end up with fire blight strikes and infections this season, plan your 2019 seasons fire blight protective program, beginning early spring at green tip to ¼” green, with Copper to kill overwintering bacteria that ooze from  fire blight cankers.

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