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

Grower Grind

The province is divided in apple staging, where Western areas are advancing quickly (roughly 10 days ahead of normal), while Eastern regions are following typical phenological staging. Growers are pruning, applying pre-emergent herbicides, and trying to stay ahead of scab. Thankfully rainfall has slowed over the past week, but low temperatures earlier this week may have caused some frost damage in Southwestern regions.

Growth Stages

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

The Horrid F Word

Crop development is progressing at different rates across the province, and with time still remaining before the frost‑free period, frost risk remains part of the conversation.

Frost Kill Table

	Silver Tip	Green Tip	1/2 inch Green	Tight Cluster	First Pink	Full Pink	First Bloom	Full Bloom	Post Bloom
10% kill	-9.4	-7.8	-5.0	-2.8	-2.2	-2.2	-2.2	-2.2	-2.2
90% kill	-16.7	-12.2	-9.4	-6.1	-4.4	-3.9	-3.9	-3.9	-3.9

The Frost Files

Before taking action on frost mitigation, identifying the type of frost using on‑site monitoring could aid you as certain tools are more effective than others.

Radiation “White” Frost:
occurs on clear, calm nights when temperatures drop to around 0 °C and water vapour in the air condenses and freezes on plant surfaces, forming a visible white frost – often less damaging in comparison to other frosts.

Radiation “Black” Frost:
occurs under similar clear, calm conditions but with lower humidity and colder temperatures, so no visible frost forms on plants.

Advective Frost:
occurs when a large mass of cold air moves into an area, bringing stronger winds, cloud cover, and sustained sub‑freezing temperatures. Driven by weather systems rather than local heat loss, making it more severe and harder to protect against.

Passive frost‑mitigation methods (pruning, orchard floor management, cold air drainage, orchard location) are often the best approach because they lower overall frost risk, even if their effectiveness varies with terrain and weather.

Active frost‑protection approaches can be effective for preventing crop loss when properly matched to frost type, correct timing, and suitable weather (e.g. presence of an inversion for wind machines), meaning performance can drop quickly if conditions fall outside limits.

Tools at your disposal that could aid you during cold nights:

• Wind machines: most effective during radiational frost events when a temperature inversion is present; they mix warmer air from above down into the orchard to raise canopy temperatures.
• Overhead irrigation: protects buds and blossoms by releasing latent heat as water freezes on plant surfaces, maintaining tissue temperature near 0 °C when properly managed.
• Helicopters: used during strong inversions to aggressively mix warm air aloft with colder surface air.
• Orchard heaters or fires: provides direct heat within the orchard and can slightly improve air mixing; effectiveness depends on heater density, fuel, and weather conditions.
• Cold air drains: involves maintaining open areas or drainage channels to allow cold air to flow out of the orchard, reducing frost risk in low‑lying areas.
• Fogging or smoking: creating a smoky or misty layer intended to reduce heat loss from the orchard by limiting radiative cooling, but in practice it provides little reliable protection
• Chemical applications: can offer limited, short-term tolerance during light radiation frost events, but have highly variable effectiveness. It may be best to utilize this strategy alongside other frost mitigation tools and to consider expense associated with applications.

Method	Radiation “White” Frost clear, calm, temps near 0°C	Radiation “Black” Frost clear, calm, low humidity, colder temps	Advective Frost cold air mass, windy, colder temps
Overhead irrigation	Effective	Effective	Limited effectiveness
Wind machines	Effective	Limited effectiveness	Ineffective
Helicopter	Effective	Limited effectiveness; dependent on inversion	Ineffective
Orchard Heaters	Effective	Effective	Effective
Wind machine + orchard heaters	–	More effective than either alone	–

Chemical Frost Protection?

Discussion around frost protection products was raised this week, which prompted me to briefly outline their role and limitations in managing spring frost risk in apple orchards.

Unlike physical frost‑protection methods (wind machines, irrigation, heaters), these products do not raise ambient air temperature. Instead, they aim to modify plant physiology or the freezing process itself. Their effectiveness is highly variable, and they should be viewed as supplemental tools, not stand‑alone frost protection.

• Copper products: can reduce populations of ice‑nucleation‑active bacteria that promote earlier freezing. Any frost benefit is short‑lived and highly inconsistent and limited to very light radiational frosts.
• Zinc products: important role in overall tree nutrition and enzyme function, but there is no consistent field evidence showing that zinc applications protect buds or blossoms from frost injury.
• Cryoprotectant products: typically contain sugars, amino acids, glycols, or polymers intended to stabilize cells during freezing. Research and field experience show products provide highly variable and unreliable protection.
• Pristine® and Merivon® (fungicides): both products consist of pyraclostrobin which has been associated with plant health and stress-response, although there has not been evidence that they provide frost protection.
• Promalin® and Perlan® (plant growth regulators): role is post frost, where timely application after injury (once tissues are thawed) may help improve fruit retention or shape when viable flowers have been reduced. Promalin has been studied and proven to show positive affects, although fruit tend to have low or no seeds which impacts fruit quality and the longevity in long-term storage.

Keep in mind, any application of a fungicide, such as Merivon and/or Pristine for frost protection counts toward the maximum number of applications per season. In addition, resistance management practices, such as rotating products with different modes of action should still be considered.

Additional information on frost mitigation and assessments:

• Evaluating Frost Damage on Buds in Tree Fruit: learn how to identify frost affecting plant tissue
• Reducing Frost Damage in Tree Fruit: learn about the types of frost and methods to decrease its impact on tender fruit and apples
• Freeze Protection Methods for Crops: learn about the types of frost and methods to decrease its impact on fruit and vegetables

GDD Update

For reference, across Ontario an average day in April will accumulate 3.5 GDDs in early regions and 1.5 GDDS in later regions.

GDD Over Time (base 5^oC, starting January 1 as of Julian Day 111)

	2026	2025	2024	2012	5-year average	10-year average
Brantford	113	61	106	N/A	88	72
Goderich	94	71	103	159	91	69
Guelph	104	49	84	136	73	57
Harrow	190	103	171	220	126	111
London	139	72	122	184	97	83
Oshawa	77	30	87	134	68	51
Ottawa	51	35	67.5	126	65	43
Vineland	125	86	112	162	96	86

Mapping Fire Blight Plans

With bloom quickly approaching, the ONTARIO FIRE BLIGHT PREDICTION MAPS are now available for the 2026 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.

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 defense 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.

Don’t overlook powdery mildew! During warm, humid periods, powdery mildew is active even when scab risk is low. Consider mildew activity when selecting fungicides. Check out the efficacy breakdown of products registered for scab and powdery mildew for more details.

Have You Registered?

This year’s Apple IPM Workshops kick off in another week! Now’s your chance to secure your or your scout’s spot if you haven’t already.

• VIRTUAL – Monday, May 4th from 9:00AM – 12:00PM
• IN-PERSON (Simcoe) – Tuesday, May 5th from 9:30AM – 3:30PM

CLICK HERE TO REGISTER

We will discuss common orchard pests, scouting techniques for various insects and disease, tools of the trade, safety protocols and tips for success. Time will also be available to answer any pest-specific questions regarding pre-recorded presentations available on the Apple IPM Resources page.

Apple IPM Resources

Other IPM workshops are also being offered over the coming weeks, starting next Tuesday, April 28th with Introduction to IPM. Click below for more information.

Register Now! FREE IPM Training Workshops