Dormant Pruning for Fire Blight Management

With the clear, sunny days lately making for excellent pruning weather, I’ve had several questions around pruning out fire blight cankers – what to look for and when the best timing is to make those cuts.

The first line of defense for fire blight management in an orchard is prevention. There is no silver bullet to eradicate this disease once it becomes well established. This means management is a year-long process to reduce inoculum in the orchard and prevent the spread of bacteria to healthy trees. Once infection has spread to the rootstock, there is nothing that can be done to eliminate the pathogen.

Winter and early spring is a good opportunity to clean up any remaining infected limbs, cankers or – in the case of rootstock blight – trees that will act as sources of inoculum and get as much fire blight out of the orchard as possible. These overwintering cankers will be the source of disease in the coming year, oozing in the spring as the tree begins to push.

Dormant pruning is effective for many reasons. Firstly, E. amylovora is not active in the cold temperatures and therefore will not be spread on pruning shears, tools or infected tissue. There will be no need to disinfect tools between cuts. Secondly, the cankers are easier to see in the orchard without foliage. 

What To Look For When Pruning

Scout thoroughly for cankers while pruning. It is recommended to do this more than once during the early spring and at different times during the day. While this does take additional time, that extra work may save you trees down the road by removing inoculum sources.

Shoot blight can be easily identified by the characteristic shepherd’s crook. Since the limb is dead, blighted leaves often remain on the limb throughout winter.

Cankers may not be as obvious. They can vary in size on twigs, limbs or trunks of trees (Figure 1). Typical characteristics of fire blight canker include: Sunken or wrinkled area with a narrow ridge along the margin

  • Rough or darkened bark often located around a spur, wound or pruning stub
  • May develop cracks within or around the margins
  • Inner bark of canker may appear reddish brown and ooze a sticky amber substance during the spring and early summer, especially in humid conditions
  • Black sooty mould can grow on the sugary substances, giving the canker a black appearance
  • May become colonized by other fungi such as the black rot fungus, Botryosphaeria obtuse

Pruning cuts should be made at least 30 – 45 cm (12 – 18 inches) beyond the canker or visible infected tissue into 2nd year wood. For young trees (less than 3 years old), trees that appear to be infected perennially or trees that have the canker established in the trunk should be removed altogether. Mulch limbs in the row middles in the early spring. An application of urea applied in the early spring for apple scab may help with degradation as well.

Trees with rootstock blight should be removed as soon as possible to reduce any chance of further spread. Cut off and remove the scion from the orchard, making sure to leave enough to allow the roots to be pulled out. Leave the remaining trunk to let the roots dry out prior to removal.

Figure 1. Fire blight cankers can appear as rough, dark regions with cracked margins often around pruning stubs, infected limbs or wounds.

Plan ahead

Winter is also a good time to start to prepare for in-season management as bloom can come quickly. Having products ready and at your fingertips will allow you to act fast should conditions for infection occur. Don’t get caught unprepared!

  • Be prepared to apply copper at silver tip to help protect the spread of bacteria from any oozing cankers that were missed during dormant pruning.
  • Consider an immune-boosting product such as an SAR (Systemic Acquired Resistance) like Lifegard or Regalia Rx at late tight cluster to pink.
  • Have enough control product – antibiotic (Streptomycin, Kasumin), copper (Cueva) and/or biologicals (Blossom Protect, Serenade, Buran, Double Nickel, Oxidate) to cover all rows of susceptible blocks every 2–3 days during bloom.
  • Become acquainted (or refresh your memory) with forecasting models such as Cougar Blight or MaryBlyt if you will be running these for your farm. Otherwise, bookmark the Ontario Fire Blight Prediction Maps in your browser for quick reference during bloom.
  • Make a plan for management – what products you will use (antibiotics, coppers, biologicals, Apogee/Kudos, etc) and how (biologicals early bloom? antibiotics full bloom to petal fall?). Don’t forget to consider your action plan in case of trauma blight.
Kristy Grigg-McGuffin
Kristy Grigg-McGuffin

Horticulture IPM Specialist, OMAFRA