Production and pest update brought to you by the OMAFA Apple Team: 

Erika DeBrouwer, Tree Fruit Specialist and Kristy Grigg-McGuffin, IPM Specialist

Happy Friday the 13th!

Grower Grind

Fruitlets are sizing and some are dropping! Fruitlet growth continues across the province, with some regions pushing quickly. Honeycrisp is showing to be ‘off’ with lower yields anticipated in comparison to the past few years. The usual suspect, Gala, has heavy crop loads, with most growers beginning hand thinning or expect hand thinning – although, drop is still occurring and chemical thinner applications continue.

Growth Stages

Warm dry(ish) weather over the past has progressed fruitlet drop and growth:

• Essex & Chatham-Kent – Fruit set (20+ mm)
• Lambton, Middlesex, Elgin – Fruit set (15-20mm)
• Norfolk – Fruit set (15-20mm)
• Brant, Wellington – Fruit set (15-20mm)
• Niagara – Fruit set (15-20mm)
• Grey – Fruit set (10-15mm)
• Durham – Fruit set (10-15mm)
• Northumberland and Quinte – Fruit set (10-15mm)
• Ottawa Valley – Fruit set (10-15mm)

At the Simcoe Research Station, king fruitlets and terminals are sitting at the following:

	Honeycrisp	Ambrosia	Gala
King Fruitlet Size	26.2mm	20.6mm	22.4mm
Terminal Growth	8.5cm	7.9cm	10.8cm

Cull Me Maybe?

Thinner Update

Warm weather has growers noting fruitlet drop after a week or more following chemical thinner applications. Be sure to check out previous What the Crop?! Apple Updates to stay informed with how to conduct thinning assessments and which thinners are available to you.

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

Below is a table of chemical thinners permitted to be applied with the current staging across the province. 

Chemical Thinner Overview

Active Ingredient(s)	Tradename	Application Timing
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
1-ACC 1-aminocyclopropanecarboxylic acid	Accede	Bloom – 25mm
Carbaryl	Sevin XLR	Petal fall to 25 days after bloom

Induction Junction

Flower induction and initiation

As the season continues, thoughts around next years crop begin. Fruit set for next year has already begun.

Stages of flower development include:

Flower Induction: transition from vegetative to flower (not visible), occurs mid to late spring

Flower Initiation: first visible change (under microscope), occurs during the summer

Flower Differentiation: morphological changes in apex (under microscope), occurs in the fall and into winter

Anthesis (Bloom): pollen and ovule formation, occurs in spring

Flower bud induction begins shortly after bloom, typically in late spring to early summer, and is sensitive to carbohydrate availability, hormonal balance, and environmental stress. To aid in flower induction consider the following:

• Early thinning – crop load management (within 30 days after bloom) allows time for adjustments in carbohydrates and hormones
• Balanced nitrogen fertilizer applications – excessive growth can hinder return bloom
• Light penetration – aids in photosynthesis and bud development
• Irrigation – mitigates water stress, allowing for better flower induction
• Application of plant growth regulators – some products may aid in flower induction

Early thinning has shown to be the most efficacious strategy for return bloom. PGR applications will not work on heavily loaded or late thinned trees

Timing of flower induction and initiation changes based on the cultivar, consider reviewing the following resources for more information:

• Apple Flower Bud Development, Thinning and Return Bloom
• Bountiful Buds for the Next Season

Feelin’ the Burn

Unfortunately, the cool weather hasn’t kept fire blight completely at bay. With the increase in daytime temperatures, signs of both blossom and shoot blight have started to be reported across the province.

A lot of the problems seem to have stemmed from the delayed bloom, vigorous growth and the more conducive conditions conditions we’ve had over the last two week.

Prune or Not To Prune

Now that fire blight is present, when should you prune it out? Unfortunately, there is no easy answer, since the severity of the infection and the weather conditions suitable for infection impact the decision.

Pruning the trees at this time of year can have two different outcomes:

1. Reduce the inoculum and prevent the spread of disease.
2. Increase shoot growth, making the tree even more susceptible to fire blight.

Older, Vigorous Trees

When dealing with older (>10 year old) vigorous trees that have many fire blight strikes, avoid pruning out shoot and blossom strikes early in the growing season. Extensive early pruning stimulates trees to produce succulent shoots, making them more susceptible to infections.

When dealing with high vigour trees that have only a few fire blight infections, pruning out the fire blight may help to provide complete control of this disease.

Older, Low Vigour Trees

In slow growing, low vigour trees or trees that have been sprayed with Apogee/Kudos, the bacteria generally doesn’t move into major limbs or trunks during the growing season. As a result, lesions can be pruned out when the tree is dormant. This may help to save time and money.

Young Trees

In comparison, young vigorous trees are very susceptible to fire blight and the bacteria may move rapidly into the trunk and down to the rootstock, killing the tree. As a result, pruning is recommended on young trees as soon as infection appears to prevent the spread of the disease to adjacent trees.

When Does It Stop?

Keep in mind that once the terminal buds have set on apple trees, fire blight stops spreading within infected trees. Summer pruning to increase fruiting wood in high-density orchards encourages new shoot growth and extends the susceptibility period for shoot blight. However, leaving the disease in the orchard may serve as a source of inoculum, and cause increased infections in the result of hail, severe storm, or high wind damage.

Key Pruning Points

If you decide to prune out fire blight strikes, remember:

• Cut at least 30 cm (12 in) or more beyond the visible infected tissue, preferably into 2nd or 3rd year wood.
  • Look at the cambium after your cut (see photo below) – if it appears brown, you need to cut further (but disinfect your pruner first!)
• Prune out strikes when there are 2-3 consecutive days of low humidity and temperatures below 25°C to allow pruning wound time to heal without conditions conducive to disease spread.
• Pruning flush to another healthy limb or spur may result in more cankers – leave a pruning stub, where possible.
• Disinfect tools between cuts by dipping them in a 65-70% alcohol solution or a solution of 1 part bleach to 5 parts water.

In sections where trees are severely affected or trees are continuously infected year after year, it may be more cost-effective to simply remove the whole tree.

Other Management Tips

Other important points to keep in mind:

• If time and labour allow, remove pruning cuts from orchard as soon as possible and burn – but be sure not to touch healthy trees along the way!
• If wet weather is not predicted, pruning clippings could alternatively be tossed into the row middles and allowed to dry before flail mowing.
• Avoid trauma-causing activities such as hedging, hand thinning, bud pinching, etc during conditions conducive to fire blight, since these activities may help to spread the disease.
• Continue monitoring and managing insects in infected blocks that may spread the fire blight bacteria, such as aphids and leafhoppers.
• Manage excessive succulent growth, where possible.
• Consider products that:
  • Surface sterilize such as Cueva or Oxidate
  • Boost immunity such as Lifegard, Regalia or Double Nickel
  • Dessicate infection such as Buran or Cyclone Plus

For a full list of registered products, see Fire Blight on the Ontario Crop Protection Hub.

Trauma blight

The unsettled weather we are experiencing may continue to bring hail, strong winds or heavy rains that could trigger trauma blight, especially with the amount of bacterial-laden ooze out there.

Be sure to apply Streptomycin (PHI 50 days), 0.5-0.8% Cueva, or Oxidate as soon as possible after the trauma event (ideally 4-12 hours after), especially if foliage damage is extensive or fire blight is known to be active in the orchard (ie., cankers, etc). Anything applied later runs the risk that the bacteria will have already established and begun to grow, reducing the efficacy of the product.

Degree (Days) of Separation

Scale

For more information on registered products or those with efficacy against scale, see the San Jose scale on Ontario Crop Protection Hub.

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	343 DDC	Passed
London	233 DDC	June 17
Delhi	259 DDC	June 15
Grimsby	249 DDC	June 17
Clarksburg	171 DDC	June 25
Durham	201 DDC	June 22
* as of June 12, 2025

We are actively monitoring for crawler activity in orchards in the southwest to confirm emergence. I will include these findings when they are observed.

Apple leafcurling midge

Apple leafcurling midge larva can be found emerging from curls and dropping to the soil to pupate. You may notice this particularly following rainy weather.

Emergence predictions for the 1st and 2nd generation adults are summarized below. See May 23rd What The Crop?! for management options.

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

Region	95% Gen 1 (235 DDC)	5% Gen 2 (430 DDC)	50% Gen 2 (556 DDC)	95% Gen 2 (701 DDC)
Harrow	Passed	June 15	June 24	>14 days
London	Passed	June 25	>14 days	>14 days
Delhi	Passed	June 23	>14 days	>14 days
Grimsby	Passed	June 23	>14 days	>14 days
Clarksburg	June 16	>14 days	>14 days	>14 days
Durham	Passed	>14 days	>14 days	>14 days
*as of June 12, 2025