Apple fruit buds in pink growth stage
Apples Diseases Pest Management

Apple Pest Update: May 22, 2020

The latest update on insect and disease activity in apple orchards across the province.

Listen to the audio article here:

It looks like wet weather may be in the forecast for the southwest in the coming days. The warmer temperatures this week have moved growth quite quickly and earlier regions are starting to see some bloom. This means disease risk could be high during this upcoming rainy stretch.

Apple Scab

It’s very likely there will be a significant scab infection period during this time. The rate of ascospore maturity will have increased in the warm weather this week which means a large amount of spores could be ready to be released in the next wetting event. With temperatures above 15C, only 6 hours of leaf wetness are required for germination to occur.

Be sure leaves are covered with a protectant fungicide before any potential infection event. Ideally, these products are better absorbed in dry conditions and become rainfast over several days after application. If that timing is just not possible, do try to avoid spraying within several hours before a rain as much can be lost to wash-off regardless of formulation.

If there is any concern about coverage, reapply during a break in the rain and consider a product with post-infection activity. Keep in mind though, this kickback is generally only effective if applied within 48-72 hours of an infection depending on the temperature. Anything after this and there will be no efficacy. Remember, post-infection activity is not the same as post-symptom activity. If lesions are present, do not apply a post-infection product.

Fire Blight

If you’re not already watching the Ontario fire blight prediction maps, now is the time to key an eye out. With the warm weather, predicted rains and the start of bloom for many, fire blight risk is also taking a big jump in the coming days. Most areas in the southwest could potentially see high to extreme infection risk beginning May 23rd.

Fire blight prediction risk across southwestern Ontario on May 23 2020

Incorporating Biologicals

In last week’s apple pest update, I had mentioned the use of antibiotic-alternatives in early bloom. The early timing for products like biopesticides is important for a number of reasons.

  • Firstly, because most of these products work by competitive exclusion, inhibiting growth or triggering a defense mechanism in the plant, they need time to colonize and become established.
  • Many of these products provide only limited control under high pressure or during certain environmental conditions.
  • Using these suppression products when there are few blooms and risk is lower, allows you to save your uses of antibiotics like Streptomycin and Kasumin for peak bloom and high infection risk events.

Research trials by Quan Zeng (Connecticut Agricultural Experiment Station [CAES]), Dan Cooley (UMass) and Neil Schultes (CAES) looked at non-antibiotic control methods such as Double Nickel, Blossom Protect, Bloomtime, Cueva and Oxidate over 4 years on Red Delicious. Blossom Protect consistently out-performed the other products as a stand alone program and was not significantly different from Streptomycin control in some years. This efficacy was only improved with the addition of the surface sterilant, Oxidate. The recipe for the most effective non-antibiotic method they found were:

  • Early to full bloom – 2 applications of Blossom Protect
  • Full bloom – 1 application of Oxidate
  • 24 hours after bloom – 1 application of Oxidate, if pressure is high or if there is a history of fire blight in the orchard

Rethinking Apogee

For those orchards still approaching bloom, you may want to think about the potential benefits a pink application of Apogee could have on fire blight management. Following the label, Apogee can be applied between 2.5-5.5 cm of new shoot growth. That early range can easily occur prebloom depending on the spring.

The use of Apogee has been known to reduce shoot blight when applied at king bloom petal fall largely due to the thickening of the cell walls within the shoot, preventing infection from occurring. Recently published research by Anna Wallis and Kerik Cox from Cornell University proposed Apogee could do the same to reduce blossom blight by thickening the cell walls of blossom pedicels. With their trials on mature Gala, they saw over 70% control of blossom and shoot blight with a pink application of Apogee and minimal effect on tree growth.

Burning the Midnight Oil

The last food for thought on fire blight management I’d like to leave you with are the recent findings from George Sundin’s team at Michigan State University. While there is an optimal temperature range for the fire blight pathogen to multiply, cooler temperatures when preceded by warmer temperatures can still be conducive to infection. If the cells are in the flowers from that warmer period, they can grow even if temperatures then cool down so risk still remains high.

What’s even more interesting, Sundin found most growth actually happens overnight between 10:00 pm and 2:00 am. This is likely because the drop in temperature at night results in the formation of dew, which encourages the growth of the fire blight bacteria.

So what does this mean? When there is a potential high risk of infection, you may want to consider spraying the evening before rather than in the morning to avoid a night of significant growth.


Sundin, G. International Fruit Tree Association, Grand Rapids, MI February 2020.

Wallis & Cox. 2020. Management of Fire Blight Using Pre-bloom Application of Prohexadione-Calcium. Plant Disease: 104. DOI

Zeng, Q., Cooley, D. and Schultes, N. 2019. Fire Blight IPM Using Non-Antibiotic Control Methods. Northeastern IPM Center: The IPM Toolbox Webinars:

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