Put Out the Fire – Part 1: Managing Rootstock Fire Blight

Since the outbreak year in 2016, fire blight has been a hot topic for many growers across the province. Many have been battling to prune out cankers and shoot blight in susceptible blocks each year. Unfortunately for some, with the higher inoculum present in the orchard, tree death from rootstock blight has caused (and is continuing to cause) extensive loss in some blocks.

What Is Rootstock Blight?

Rootstock blight often occurs at ground level just below the graft union in susceptible rootstocks such as M.9 and M.26. Infections initially appear water-soaked, purplish to black discolorations with cracking or crevices (Figure 1). Fresh ooze can sometimes be seen during hot, humid conditions or signs of dried ooze from previous activity (Figure 2). Poor vigour is typically the first indication of infection, with little to no growth and sparse, yellowing leaves (Figure 3). Depending on the cultivar, branches may have a distinct yellow or golden colour. Tree collapse can occur rather suddenly in some situations but usually occurs the following season.

Removing the bark, the affected area shows red-brown streaking in the internal tissues. It is common to mistake tree decline caused by rootstock blight for Phytophthora root and collar rot; however, fire blight does not cause reddish-orange necrosis below the soil line and roots that is characteristic of Phytophthora (Figure 4). The fire blight bacteria, Erwinia amylovora also moves through the vascular cambium leaving a black ring if looking at a cross section of the affected area (Figure 5). For accurate identification – and to prevent an expensive mistake – send a sample of the diseased rootstock to a qualified pest diagnostic lab.

Rootstock infection can occur through infected suckers or water sprouts, as well as washing E. amylovora bacteria from infected twigs and fruit down the trunk in an existing wound. However, a very important avenue of infection is internal translocation of the pathogen from the infected cultivar scion above ground to the rootstock. In young, vigorous trees, E. amylovora can move from infected blossoms or shoots to the roots in one month under favourable weather conditions.

First Line of Defense

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 6). 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 obtusa
Dark, cracked cankers on trunk of apple tree caused by fire blight
Figure 6. Fire blight cankers can appear as rough, dark regions with cracked margins often around pruning stubs, infected limbs or wounds.

Pruning cuts should be made at least 30 cm (12 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.

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!

  • Have enough control product to cover all rows of susceptible blocks every 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, etc.) and how (biologicals early bloom? bactericides full bloom to petal fall?). Don’t forget to consider your action plan in case of trauma blight.
  • 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.

Stay tuned for Part 2 of this fire blight series in the next issue of Orchard Network Newsletter.

Kristy Grigg-McGuffin
Kristy Grigg-McGuffin

Horticulture IPM Specialist, OMAFRA