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

Growers across the province are in the thick of the season! The warm weather has pushed most of the apple acreage into bloom, where early sites are at petal fall. Planting, pruning, mowing, and spraying are ongoing across the province.

Growth Stages

Southern regions are pushing quickly past petal fall, while other regions are just hitting bloom. Cooler temperatures are expected next week, so keep this in mind for chemical thinner applications.

• Essex & Chatham-Kent – Petal fall
• Lambton, Middlesex, Elgin – Bloom to Petal Fall
• Norfolk – Bloom to Petal Fall
• Brant, Wellington – Bloom
• Niagara – Bloom
• Grey – Bloom
• Durham, Northumberland and Quinte – Bloom
• Ottawa Valley – Bloom

Terminal Growth

Heat this past week really pushed vegetative growth. Measurements in Ambrosia and Gala in Lambton County ranged between 4 and 6 cm.

Thinning Timing

Chasing the Goldilocks Day

Lots of discussion at various district meetings have brought forward questions regarding thinning, including what the ideal conditions conducive to fruitlet thinning would look like. Generally, the following weather conditions promote thinning efficacy, therefore rate adjustments may want to be made.

• During application: Cloudy, warm weather (18 to 29^oC) with high humidity (>60%) increases thinner uptake
• After application: Rising temperatures, warm night time temperatures

All that being said, there are tree related factors that may suppress or promote thinning.

Promoters of chemical thinners:	Suppressors of chemical thinners:
Lower canopy and shaded fruit spurs with low vigour	Fruit set on spurs in well-lit areas (outer and upper canopy)
Heavy Bloom	Light Bloom
Fruit set in clusters rather than singles	Biennial bearing in ‘off’ year
Inadequate nitrogen	Fruit set in singles rather than clusters
Low moisture	No mineral deficiencies
Weak root systems
Young trees with vigorous upright limb growth
Short bloom period	Long bloom period

For more information about chemical thinner product timing and thinner effectiveness table, check out: What the Crop?! Apple Update: May 2, 2025 – ONfruit

Fungus Amoung Us (*And Bacteria)

Scab

Most areas are still within – though nearing the end of – the primary apple scab 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. For those areas that have been dry, be prepared for large releases of ascospores in the next infection event. With temperatures above 15C, only 6 hours of leaf wetness are required for germination to occur.

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.

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

Fire blight infection risk will be HIGH to EXTREME in many regions over the next few days with the warmer weather and potential for rains. Keep open blossoms protected! For those earlier regions that have reached petal fall, be on the look out for delayed bloom.

If you’re not already watching the Ontario fire blight prediction maps, now is the time to key an eye out.

Fire Blight Prediction Maps

With early bloom, some growers have opted for antibiotic alternatives such as Buran, Serenade, Double Nickel or Cueva, especially with risk being lower until now. This helps to save Streptomycin or Kasumin for full bloom and when risk is high.

As bloom progresses, frequent protective sprays may be needed if risk remains high as new blossoms are opening daily and will not be protected by sprays made while these blooms were closed. As well, antibiotics degrade rapidly in sunlight so are only active for 2-3 days MAXIMUM. Apply these products as close to a wetting event as possible. For resistance management with antibiotics, rotate between Streptomycin and Kasumin.

If there are any concerns about missing a potential infection event or potential trauma during a rain or wind storm, Streptomycin provides kickback activity but should be applied within 12 hours after the event.

Those at king bloom petal fall are also applying Apogee/Kudos (typically when growth is 2.2 – 5.5 cm long) to help with shoot blight management.

Other diseases

On a good note, the cooler spring has kept powdery mildew pressure relatively low. Mildew thrives in dry weather and high relative humidity. Those areas that haven’t seen much rain are starting to report some signs of mildew, especially in Cortland and Ginger Gold.

Very little signs of frog-eye leaf spot (foliar symptoms of black rot) have developed as of yet this year. Optimum temperature for leaf infection is 26⁰C. However, infection can occur any time above 10⁰C but will just require a longer leaf wetting period (more than 24 hours at 10⁰C).

If you are seeing frog-eye leaf spot now, you know inoculum is present in the orchard or coming from a sporulating nearby source such as dead or decaying wood in an adjacent woodlot.

Black rot produces 2 types of inoculum – ascospores (sexual spores), which are wind blown and conidia (asexual spores), which require a wet period and rain splash for release. Both begin to release shortly after bud break; however, the heaviest ascospore release occurs for a 4-6 week period following petal fall. In other words, if your protectant program considerations haven’t included black rot yet, now is the time!

Bugs Behaving Badly

As bloom comes to an end, many growers are thinking about petal fall control options. 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.

Spring-feeding caterpillar

As mentioned in last week’s What the Crop?!, the first signs of caterpillar damage can be found on growing terminals and fruit buds. In general, an insecticide is usually recommended when the action threshold of 12-15% infested terminals or buds is observed. For obliquebanded leafroller, this threshold is lower at 1-2% infested terminals or buds. Management is most effective when larvae are young.

Plum curculio

While it is still a little early for plum curculio in most orchards, this is certainly a pest to keep on the radar. Adults generally start moving back into the orchard from adjacent woodlots and overwintering shelters at bloom, with the greatest migration occurring within 14 days after petal fall. Often movement into the orchard follows a period of warm, sunny weather after a rain.

Monitor for signs of oviposition or feeding scars on the flower bud or developing fruitlet.

Apple leafcurling midge

Adult leafcurling midge flight and egg laying has picked up this week. Emergence predictions for the 1st generation adults are summarized below. Based on Ontario trials, Movento provides good control due to its systemic properties. Other products that have also performed well when timed at early petal fall include Delegate, Closer and Exirel.

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	10-14 days
London	Passed	Today	>14 days
Delhi	Passed	May 19	>14 days
Grimsby	Passed	May 22	>14 days
Clarksburg	Today	>14 days	>14 days
Durham	Passed	>14 days	>14 days

Scale

Based on current degree day accumulations, the 1st generation San Jose scale crawler emergence is predicted to begin across the province in several weeks (predicted emergence of 278 DDC).

Predicted emergence date of first generation San Jose scale, by region (biofix March 1st, base 10C)

Region	Current Degree Day Celsius (DDC)	Predicted Crawler Emergence (278 DDC)
Harrow	126 DDC	>14 days
London	100 DDC	>14 days
Delhi	87.5 DDC	>14 days
Grimsby	80 DDC	>14 days
Clarksburg	54 DDC	>14 days
Durham	75 DDC	>14 days

Once crawler activity (ie., movement to new shoots and developing fruit) begins, this will continue for 4-6 weeks. Registered products including Closer and Sivanto Prime should be applied at the beginning of crawler activity and reapplied 10-14 days later due to the extended generation. Movento is a slow-acting product and should be applied at least 1 week prior to predicted crawler emergence, again followed up 10-14 days later. Depending on the product, this timing may also have efficacy on woolly apple aphid and/or leafcurling midge.

San Jose scale is not the only scale pest that can be found in orchards. Oystershell scale populations have been increasing in a number of blocks across the province. Crawler activity for this species is often about 1 week prior to San Jose scale emergence, occuring 2-3 weeks after petal fall. There is only one generation of oystershell scale per year, but crawler activity can continue throughout the summer. Management options would be similar to San Jose scale.

Mites

European red mite activity has begun though overall populations remain fairly low likely due to the cooler weather. Those orchards that did not apply an early season oil should be monitoring and be aware of the appropriate timing for whichever product you use as some can only be applied within weeks of petal fall (eg., AgriMek, Minecto Pro), require earlier application as populations build (eg., Nealta) or have a slow knock-down (eg., Envidor).