Fire Blight Risk: Are You Out of the Woods?

The fire blight prediction maps for Ontario have been updated for May 27-June 2, 2016. With the warm weather expected to continue into the weekend and early next week, the conditions are optimal for fire blight infection. Forecasted rain in many regions this weekend can wash the bacteria into the blossom which begins the infection process. According to Dr. George Sundin from Michigan State University, temperatures conducive for growth can result in fire blight populations multiplying to one million cells per flower within 1-2 days. Imagine how quickly pollinators could spread infection around an orchard with bacterial populations like that!

The forecasted weather conditions have triggered the Cougar Blight model to predict high to extreme risk of fire blight infection in orchards with open blossoms in most apple and pear growing regions of the province over the next 7 days. Some of the most damaging fire blight epidemics in the past have occurred under these weather conditions, including in orchards that have never experienced fire blight before. For early regions that are at petal fall, growers should be diligent and monitor their orchards closely for new plantings that may still be blooming or for secondary, or “rat tail” blossoms. Take necessary action to remove or protect open blossoms from infection. Keep in mind, the antibiotics (Streptomycin, Kasumin) provide control for only 2-3 days prior to a rain event. So for extended infection periods, a subsequent spray may be needed.

Soon you will see if the fruits (pun intended) of your labour paid off or if fire blight symptoms develop. Scout your orchard for strikes, or the characteristic “shepherd’s crook” (Fig 1) every week until terminal bud set.

Fig 1. Fire blight infected shoot forms an inverted J or a “shepherd’s crook” at tip

Fig 2. Visible margin of fire blight infection below which tissue appears healthy

Not sure what to do if you find fire blight?

Michael Celetti, OMAFRA’s plant pathologist posted some helpful tips for removing fire blight strikes in 2015. Some key points include:

• Prune out infections early, but only if there are just a few strikes per tree.
• Excessive pruning can stimulate growth, potentially making the situation worse.
• Cut at least 30 cm beyond the water soaked margin of the infected shoot (Fig 2).
• Only prune out strikes when there are 2-3 consecutive days of low humidity and temperatures below 25⁰C.
• Prevent the spread of fire blight by not dragging infected material through the orchard.

Where do you to start once fire blight has hit your orchard?

The “triage” method proposed by Dr. David Rosenberger from Cornell is a good decision-making guide which lists areas of your orchard from highest to lowest priority:

1. Young orchards (<8 years old) with few strikes
2. Young orchards (<8 years old) with many strikes
3. Older orchards with few strikes
4. Orchards with so many strikes most of tree would be removed (the “walk away” group)