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

Grower Grind

Rain and cool temperatures have slowed things in the orchard, but growers continue to move and take advantage of breaks in the weather. Growers are talking about the cold nights anticipated in the forecast with some prevention applications taking place. Regions are evening out regarding staging given the cool weather over the last week.

Growth Stages

• Essex – Pink to early bloom
• Chatham-Kent – Tight cluster to pink
• Lambton, Middlesex, Elgin – Tight cluster to pink
• Norfolk – Tight cluster to pink
• Brant, Wellington – Tight cluster to pink
• Niagara – Tight cluster to 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

F This Weather

Cold temperatures are expected over the weekend in a few regions with temperatures potentially dropping below -4°C. Based on your growth stage please take note of the frost damage that can occur based on temperature.

	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

Forecast Face-Off

Given the weather forecast, discussion on where to get weather data was discussed. Sean Westerveld (Ginseng & Herb Specialist, OMAFA) has a great article comparing the accuracy of various weather platforms (Forecasting Frost Events). Common weather platforms that Sean specifically looked at are below:

• Environment Canada (the official weather forecasting service of Canada): 7 day high and low temperature and condition forecasts, official weather advisories and warnings and hourly forecasts for 24 hours for temperature, conditions and wind speeds (click on “Today” in the local forecast). Also provides official historical records for many locations.
• Weather Network: Same as Environment Canada but also 36 hours of dewpoint, wind speed, temperature and condition forecasts. Also provides 14 day forecasts (read further for comments on long range forecasts)
• Weather Underground: Provides 10 days of hourly temperature, wind speed, cloud cover, conditions, rainfall, dew point and humidity forecasts (click on “Table” and then the day you want to view for the details).
• Meteoblue: Provides 7 days with 3-hour forecast periods for temperature, precipitation, conditions and wind speeds.
• Accuweather: Most of the same information as Weather Underground but hourly forecasts beyond three days require a subscription. Daily high and low temperature forecasts extend to 90 days (read further for comments about long-range forecasts).

Ultimately, Sean stated that the best strategy for growers to do when trying to forecast an upcoming frost is to:

1. Look at several different forecast sites and look at the range of forecast low temperatures.
2. Err on the side of caution and assume the worst case scenario.
3. Examine specifically the wind speeds, cloud cover and dew points to give an idea of how low temperatures may go.
4. Keep in mind that temperatures can be significantly lower on the ground than the actual air temperature, and the drier the air, the larger this difference can be. This is important for crops that are close to the ground.
5. Keep track of forecasted low temperatures and your actual low temperatures so you can adjust forecasts to more reliably match your local microclimate over time.

He also noted that the dewpoint is the most important measurement for determining frost risk. The dewpoint is the temperature the current airmass would have to cool to before dew or fog would form.

“At this time of year, the dewpoint is often a good indication of how low temperatures could get at ground level on calm and clear nights. If the dewpoint remains above freezing, then the risk of frost is low… Dewpoints tend to rise slightly at night when winds calm and the air near the ground absorbs some moisture from the ground and any nearby vegetation.” (Comments on Frost Risk for Ginseng)

It is best to check the dewpoint in late afternoon to give you a better idea of the risk overnight.

Resources below can aid in detecting the type of frost, prevention tools available and detecting damage following cold temperatures.

• To determine the type of frost
• Tools to prevent frost damage
• To protect against frost
• To evaluate frost damage

Chill Diseases

With much of the province sitting in cooler conditions, disease development is progressing differently than many were perhaps thinking a couple of weeks ago. With growth stages ranges from half-inch green through bloom, it’s important to recognize that risk is currently low to moderate – but highly weather-dependent. Small shifts in temperature or extended wetting could quickly change the picture.

Not Too Scabby

Cool temperatures slow down ascospore maturity and infection progress, but it doesn’t eliminate risk entirely. What becomes more important under these conditions is prolonged leaf wetness.

When temperatures are low, the fungus develops more slowly and requires longer periods of continuous free moisture to infect. The issue is that the current weather – drizzly, intermittent rain and overcast days can easily provide those extended wetting periods needed. Low-intensity, long duration wetting events can be just as conducive to infection as shorter, heavier rains under warmer conditions.

For more information on how temperature and leaf wetness affect scab infection, check out Relationship of Temperature and Moisture to Apple Scab Infection.

Why isn’t the risk zero?

• Prolonged leaf wetness (24+ hours) can meet infection thresholds even at cooler temperatures.
• Infections may occur during these extended wetting periods but symptom development will be delayed, not appearing for weeks down the road (and creating a false sense of security).
• Actively expanding tissue from tight cluster through bloom means new, susceptible and unprotected tissue is constantly emerging.
• With the unsettled weather, conditions haven’t be ideal for spraying (too rainy, too windy, etc) and coverage could be a concern.

Keep protectant fungicides on through this critical window. Be cautious of stretching intervals too far just because the temperature is cooler. If coverage is questionable after prolonged drizzle or missed timing, consider post-infection, or kickback activity – but avoid relying on it as a primary strategy.

Check out the What the Crop?! Apple Update from April 17th for more tips on kickback products.

A Blight Idea

For those areas entering into bloom, cool weather is generally keeping fire blight risk low. Daily temperatures have to be high enough to accumulate the required heat units needed to trigger infection. At lower temperatures, especially below 10C, multiplication of the bacterial cells slow significantly. That said, while their growth may be limited, the pathogen still survives on plant surfaces and risk can increase quickly with even short periods of warmth.

In blocks not yet at bloom, risk remains minimal. However, open bloom plus a modest increase in current temperature (even 1-2 days) can quickly elevate risk, particularly in blocks with a history of fire blight or young / susceptible trees.

Managing the risk

• Continue to monitor forecasting models closely as bloom progresses. If you’re not running your own on-farm model, use the ONTARIO FIRE BLIGHT PREDICTION MAPS to help in decision-making.
• In low to moderate risk situations, consider non-antibiotic options such as biologicals or low-risk products. For more information on why biologicals may be a good fit in early blooms, refer back to last week’s What the Crop?! Apple Update: April 24, 2026.
• Hold onto antibiotics (e.g., Streptomycin, Kasumin) for higher-risk infection events (high to extreme, or EIP >100) in later bloom (50-80%).
• Be prepared to pivot quickly – conditions can shift fast!

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

Fire Blight Prediction Maps

Hate to “Bug” You

Very Hungry Caterpillar

For the most part, insect activity has been low. However, this week there have been a few signs of early spring-feeding caterpillar damage, in particular leafrollers, fruitworm, and budmoth. Look for signs of leaf feeding, rolled terminal leaves and webbing.

When scouting between tight cluster to petal fall, check 5 terminal shoots and 5 fruit buds in 10 trees (50 terminals and 50 fruit buds in total). Control is typically warranted when 12-15 terminals and buds are infested. Click here for registered prebloom control products for spring-feeding caterpillar.

Spring Watch List

If this spring continues to be on the cool, wet side, be prepared for rosy apple aphid populations to build. Egg hatch is likely on-going currently but infestations really start to be observed into tight cluster to bloom. Pay particular attention to fruit clusters of susceptible cultivars like Cortland, Idared, Golden Delicious and Ambrosia.

Another insect pest we commonly start to see an increase in activity at tight cluster to pink is apple leafcurling midge. Despite the cooler weather, leafcurling midge flight has historically been active even in temperatures between 7-9C. There are still no signs of egg laying but I imagine this will begin in the coming weeks. I will continue to provide updates on egg laying as well as regional degree day predictions.

Oriental fruit moth trap catch has started this week in early regions of the province. This will be used to determine peak flight or to kick off degree day modelling (first sustained moth catch) for targeting first generation timing. Terminal flagging, especially on young plantings can be an issue early season by this pest.