Production and pest update brought to you by the OMAFRA 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 prepare to take advantage of breaks in the weather. Whenever there is a window to get into the orchard, growers are taking advantage, especially given concerns of scab and chemical thinner efficacy. Planting, pruning, mowing etc. (when the weather permits), but the focus is on spray timing to mitigate future challenges.

Growth Stages

Fruitlets are sizing in many regions, with chemical thinning weather being less than ideal. We are seeing strong terminal and leaf growth on trees across the province, terming this bloom season a ‘green bloom’ (Thanks Cathy!). Fruitlets continue to size, even in this cool weather, meaning it is important to note that things are still progressing in the orchard.

• Essex & Chatham-Kent – Fruitlet sizing (4-6mm, Gala)
• Lambton, Middlesex, Elgin – Fruitlet sizing (6-10mm)
• Norfolk – Fruitlet sizing (6-8mm, Gala)
• Brant, Wellington – Petal fall
• Niagara – Petal fall to early fruitlet sizing
• Grey – Petal fall
• Durham, Northumberland and Quinte – Bloom to petal fall
• Ottawa Valley – Bloom to petal fall

Weathering the Thinning Window

Much discussion on chemical thinning over the past week, where contemplation on when to apply thinners and if it’s worth it given the cool and wet weather.

Did We Mist Our Mark?

In short, yes, but also no. There are a few timings still available this season to apply thinners, and although we are currently in a less than ideal weather situation, ultimately it is out of your control, which makes utilizing the remaining season more critical. Plant growth regulators are best applied when temperatures are above 18 and although there isn’t a lot of research regarding cold daytime temperatures during application, Dr. James Schupp from PennState Extension noted the following:

• 6-BA is not very effective when temperatures are below 20
• Carbaryl is somewhat effective, but can increase chance of pygmies and mummies
• Mummies and pygmy fruit can occur with NAA and 6-BA thinners if applied post-bloom, with risk rising when rates are increased and timing becomes later

Remember that temperature, light and fruit diameter all play a factor in the efficiency of chemical thinner application.  

This season is a prime example of why understanding and using different crop load management tools throughout the season to get to your target is critical. Information on different tools available to you can be found and explained in more detail in the Bud to Bin – Targeting Your Crop Load article found in the 2023 February Issue of ONcore.

Breaking Down Chemical Thinners

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

Fruit in Progress: Your Growth Stage Voice

If you have time, please consider aiding Kristy and Erika in developing models for you in the future by filling out the survey below throughout the season.

2025 Apple Growth Staging Survey

Fungus Amoung Us (*And Bacteria)

Scab

Many regions are nearing the end of the ascospore maturation period. This means that for future rain events, still expect that primary infection is a risk and keep good fungicide coverage on those rapidly growing leaves – and more importantly now, developing fruitlets!

It’s too cold for infection though, right?

If only it were that simple! While mature spores are released more during warm rains than in colder rains, they can still cause infection nonetheless if there is a wetting period long enough for infection to occur. With the prolonged rains we’ve had this week, most areas have met those conditions despite the cool weather.

How do I prevent infection during extended rains?

In prolonged wetting events, it is better to re-apply fungicides during a break in the rain to provide temporary protection, than to not spray at all. Apply again once conditions are finally dry to replace residues washed off by rain.

If there is concern about coverage during a potential infection period, consider including a product with good kick-back. Post-infection activity of registered scab products can be found in the table, Characteristics of Apple Scab Fungicides.

Seeing scab?

Early signs of scab lesion development have been found. The table Relationship of Temperature and Moisture to Apple Scab Infection (also summarized below) provides the number of days after infection before lesions will become visible.

Now is the time to start monitoring blocks for signs of lesions. Take a good look at the top AND underside of leaves for velvety brown lesions with indistinct margins. Overwintering spores are coming from the orchard floor, so most early season infections typically start on the underside first. Things have also been fairly windy this season which could impact spray coverage, particularly in the upper canopy. Look at all parts of the canopy to ensure there are no escapes.

If lesions are present, continue a regular protectant program to prevent secondary infection on developing fruitlets.

Number of days required for apple scab lesions to appear following an infection event

Average temperature (C)	Lesion appearance (days)
0-5	–
6-9	17
10	16
11	15
12-13	14
14-15	12-13
16-24	9-10
25+	–

Fire blight

Blossom blight

The Ontario fire blight prediction maps continue to be updated regularly. Risk has stayed low to moderate with the cooler average daily temperatures. 

Fire Blight Prediction Maps

Of particular concern this year is the staggered or irregular bloom that some growers are seeing. With different bloom stages occurring within an orchard (or even on a tree), the window of infection risk is extended until the last blossom falls. This also includes delayed bloom on young trees and secondary, or rat-tail bloom. Protecting these open blossoms from infection will be critical during times of high risk. We have seen situations in previous years where blocks have been severely affected by fire blight as a result of rat-tail bloom infection.

Shoot blight

Getting through bloom with no sign of infection, unfortunately, does not mean you are out of the woods! Symptoms of shoot blight can develop without blossom infection occurring if the pathogen is present in or around the orchard (ie. neighbouring farms). Once (if?) things warm up, trees will be going through extensive growth with rapidly expanding leaves. During this time of rapid growth, microscopic tears in the leaf tissue can be enough of an entry point for fire blight bacteria to get into the shoot and cause an infection.

In terms of shoot blight management, prohexadione-calcium, or Apogee/Kudos is an important management tool to help slow the growth of these vigorous shoots and reduce the spread of fire blight.

For registered control options, see the Ontario Crop Protection Hub.

Bugs Behaving Badly

Petal fall insecticide sprays are beginning to go on in the earlier regions of the province. For more information on efficacy of registered products for common petal fall pests, refer to Activity of Petal Fall Insecticides Against Orchard Insects (Table 2). Efficacy will be based on rate used.

Caterpillars

The spray window for oriental fruit moth is quickly approaching in most early regions. Some growers are opting for insecticides with good efficacy on mullein bug, plum curculio, spring-feeding caterpillars and/or apple leafcurling midge, if monitoring had indicated they had reached threshold or have high pest pressure.

Codling moth biofix is being set in earliest regions of the province.

Spring-feeding caterpillar activity can still be found. Of the caterpillar complex, leafroller and spongy moth can be found causing significant damage in some parts of the province, especially those orchards that saw high pressure in the last two years.

Apple leafcurling midge

Apple leafcurling midge egg hatch continues. Infested terminals are becoming quite apparent already in orchards with a history of this pest. Emergence predictions for the 1st generation are summarized below.

Predicted emergence date of first generation adult apple leafcurling midge, by region (biofix March 1st, base 9C)

Region	5% Gen 1 (76 DDC)	50% Gen 1 (132 DDC)	95% Gen 1 (235 DDC)
Harrow	Passed	Passed	May 29
London	Passed	Passed	June 5
Delhi	Passed	Passed	>14 days
Grimsby	Passed	Passed	>14 days
Clarksburg	Passed	June 1	>14 days
Durham	Passed	May 29	>14 days

While Movento is registered for suppression, the timing for leafcurling midge is often earlier than for scale and woolly apple aphid so would have to be chosen based on which pest is of greater concern in your orchard. However, there are other petal fall insecticides such as Delegate, Closer and Exirel (and for that matter, Minecto Pro) that may provide some efficacy as well, which could help delay the Movento application. Pyrethroids are registered but have greater efficacy earlier to target adult midges and can be very harsh on the numerous beneficial insects that are now active in the orchard. Summer oils will also help to smother eggs so long as good coverage is achieved to reach within the newest terminal leaves.

Mullein bug

Low numbers of mullein bugs are being tapped out in various orchards across the province. Controls are warranted when threshold reaches 7-9 nymphs per 25 taps. Continue to monitor as populations can jump quickly, especially on susceptible varieties such as Red Delicious, Spartan, Spy, Ambrosia, Empire, Cortland, Gala, Jonagold and Golden Delicious.

The critical time for damage from mullein bug is generally dime to quarter-sized fruit.

Plum curculio

Plum curculio have been found in monitoring traps within high pressure orchards, indicating they are beginning to migrate from surrounding overwintering sites. Keep an eye out for damage to the developing fruitlets, especially once sunny days come back. During cooler weather, these weevils tend to move slower, opting to crawl in from surrounding woodlots; however, on warmer days, they’ll fly in which is often when there is an increase in observed damage during these periods.

Aphids

Leaf curling and puckering typical of rosy apple aphid has been observed around fruit clusters. This is not surprising given aphids thrive in cool, wet springs so recent conditions may have encouraged development.

While thresholds for green apple aphid are relatively high (400-600 aphids per terminal on 10% of terminals), the most serious rosy apple aphid damage occurs when a toxin found in its saliva translocates from the leaves to fruit, causing apples to remain small, deformed (pigmy fruit) and unmarketable. Control for rosy apple aphid is recommended if more than 5% of fruit clusters are infested with 20 or more aphids.

Scale

The cool weather has slowed degree day accumulations for crawler emergence. Currently, all regions of the province are still a few weeks before activity is predicted to begin.

San Jose scale degree day accumulations based on March 1st biofix and base temperature 10C

Region	Current Degree Day Celsius (DDC)	Predicted Crawler Emergence (278 DDC)
Harrow	171 DDC	>14 days
London	117 DDC	>14 days
Delhi	107 DDC	>14 days
Grimsby	110 DDC	>14 days
Clarksburg	71 DDC	>14 days
Durham	93 DDC	>14 days