Production and pest update brought to you by the OMAFA Apple Team: Erika DeBrouwer, Tree Fruit Specialist and Kristy Grigg-McGuffin, IPM Specialist

Orchard Activities

Across the province growers are early planting and are in the later stages of pruning. Fertilizers are being applied and spray season continues; where early areas have multiple applications, and later districts are beginning as tree growth progresses and warm temperatures move in. Thinning strategies are top of mind as growth staging progresses.

Growth Stages

This time of year is showing some variation across the province in growth staging due to the ever-changing weather. Cool temperatures have slowed progression of growth, although with warm conditions anticipated over the next week things may pop.

• Essex & Chatham-Kent – Tight cluster to early bloom
• Lambton, Middlesex, Elgin – Tight cluster to early pink
• Norfolk – Tight cluster to early pink
• Brant, Wellington – Tight cluster
• Niagara – Tight cluster to early pink
• Grey – half inch green to early tight cluster
• Durham, Northumberland and Quinte – half inch green to early tight cluster
• Ottawa Valley – Green tip to half-inch green

Chemical Thinning

Considerations for chemical thinning are creeping up quicker than we think. Please take into account the products available to you along with looking ahead at the upcoming weather forecast to aid in any decision-making (Thinner Effectiveness Table).

Chemical thinners are listed in the table below, along with timings related to each thinner. For more detailed information about volumes and rates, please refer to the following links:

• The Apple Section of the Thinning of Tree Fruit webpage at Ontario.ca
• The Thinning & Plant Growth Regulator Section found in the Apple Landing Page on the Crop Protection Hub
• The Pesticide Label Search for specific product searches

Chemical Thinner Overview

Active Ingredient(s)	Tradename	Application Timing
1-ACC (1-aminocyclopropanecarboxylic acid)	Accede	Bloom – 25mm
6-BA & GA4,7 (6-benzyladenine & gibberellins A4A7)	Perlan, Promalin	King bloom – early petal fall
Ethephon	Ethrel1	Bloom – only non-fruiting trees
Ammonium thiosulphate	ATS2	Bloom
Lime Sulphur	Lime Sulphur2	Bloom
1-NAA (1-naphthaleneacetic acid)	Maintain, Fruitone-L	Bloom – fruitlet sizing, bloom to 30 days after bloom
6-BA (6-benzyladenine)	Cilis Plus, Maxcel	Petal fall – fruitlet sizing, up to 20mm
Carbaryl	Sevin XLR	Petal fall to 25 days after bloom

Thinner Effectiveness Table

Climate Conditions	Prediction
Warm Conditions (greater than 18oC)	All thinners work best
Dark Cloudy Weather	Greater stress Greater thinning response Greater drop
High Night Temperatures (greater than 18oC)	Great stress High demand and use of energy for night respiration Greater drop
Very High Day-Time Temperatures (greater than 29oC)	Great stress High energy demand Greater drop
Very Cool Temperatures (less than 18oC)	Reduced stress Reduced energy demand Greater set
High Light	Increased supply: harder to thin
Low Light	Reduced supply: easier to thin
Low Temperatures	Low demand: harder to thin
High Temperatures	High demand: easy to thin
Low light and warm temperatures	Worst

Mapping Fire Blight Plans

With bloom quickly approaching, the ONTARIO FIRE BLIGHT PREDICTION MAPS are now available for the 2025 growing season. Cooler weather can keep the risk low despite forecasted rains. However, use the prediction maps to help in decision-making.

Fire Blight Prediction Maps

These interactive maps show the 7-day predicted risk based on the Cougar Blight model. Growth stage is not considered in this prediction – if there are no open blossoms in the orchard, there is no risk of infection!

Risky Business?

The risk of fire blight infection is possible if the following conditions are met:

1. Flowers are opened and/or are opening – There is no risk of infection if bloom is not present in the orchard despite the fire blight maps stating caution to extreme; however, keep an eye on any early signs of bloom as it can happen quickly.
2. Enough heat units have accumulate based on forecasting models – The fire blight maps use Cougarblight but this applies to MaryBlyt as well. Similar to degree days, daily temperature is accumulated and must reach a certain point before risk increases.
3. A wetting event is predicted – Any type of wetting event will wash the bacteria into the open bloom including rain, dew or even the artificial rain shower you create with the sprayer during critical infection periods.

So far this season, we have had warmer temperatures to move the model along. However, during stretches of cooler average daily temperatures, infection risk remains low to moderate. Ideal conditions for rapid multiplication of the fire blight bacteria is above 18C. Research suggests bacteria are capable of surviving on open flowers for several days so it is possible for infection should a wetting event occur, even with potentially cooler conditions, though likelihood is much less. If there is a history of fire blight in your orchard or neighbourhood, keep a close eye should conditions change.

What Sparks Infection?

Fire blight bacteria multiply rapidly in a very short period of time when temperatures are above 18⁰C and relative humidity above 90%. Bacteria can be transferred to new blooms by rain, wind or insect. Bees and other pollinators are very active in the orchard right now and can easily spread the bacteria around.

Sudden, sporadic or isolated showers, heavy dew, fog or spraying can all provide enough moisture to wash bacteria into an open bloom. How much water is actually needed? If you look at the requirements for blossom blight infection based on the Maryblyt model, a wetting event is >0.01” (0.25mm) rain, heavy dew or fog sufficient to wet foliage and provide thin film on tissue surface or rain of >0.1” (2.5mm) the previous day.

Does It Ever End?

Susceptibility to fire blight decreases as the blossom ages and calyx begins to close, so following petal fall, blossom blight risk ends. Staggered bloom can extend the risk until the last blossom falls.

Fight The Blight Before It’s A Thing

For a full list of products that have efficacy against fire blight, click here.

Now is the time to apply prebloom fire blight protection products such as Lifegard or Regalia Rx, if these are being considered. These products work by boosting the immune response of the tree to better fight off disease, also known as System Acquired Resistance (SAR) or Induced Systemic Resistance (ISR). Because of this mode of action, these products should be proactively applied in advance to conditions conducive for disease development to allow time for the immune response to kick in. Initial inducement of plant defence response occurs soon after application, but 3–5 days are required to attain maximum level of protection.

Note: Lifegard is toxic to bees so cannot be applied during bloom or if hives have already been brought into the orchard.

Early bloom, consider alternatives to antibiotics (Streptomycin, Kasumin) if risk is low to moderate but fire blight has been an issue in your orchard before. These include:

• Biologicals – eg., Blossom Protect, Buran, Serenade, Regalia or Double Nickel
• Surface sterilant – eg., Oxidate
• Copper labelled for in-season use – eg., Cueva – note that copper and some fungicides are not compatible with certain biologicals. Check the label before use.

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. A good approach to timing for biologicals is when risk is coming in the next 3-4 days.
• 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 (50-80% bloom) and high infection risk events.

Breaking Scab

Most areas have experienced at least one apple scab infection event over the last week or so. Earlier regions of the province are about 20-40% of ascospores matured. This means that for future rain events, expect large ascospore release and keep good fungicide coverage on those rapidly growing leaves. With temperatures above 15C, only 6 hours of leaf wetness are required for germination to occur.

With the extended wetting periods, coverage can certainly be a concern. Consider the kickback of a product to provide extra protection.

Use Characteristics of Apple Scab Fungicides for reference. Note, not all products have been evaluated or have information available for post-infection activity, including Aprovia, Excalia, Kenja, Merivon and Pristine.