Production and pest update brought to you by the OMAFA Apple Team:
Erika DeBrouwer, Tree Fruit Specialist and Kristy Grigg-McGuffin, IPM Specialist
Table of Contents
Grower Grind
Similar to last week, the wet and cool temperatures persist. Growers continue to take advantage of breaks in the weather to scout, monitor and preventatively spray. Some pruning and planting (weather permitting) still ongoing. Many questions are coming in about thinning efficacy, rates and timings, and a lot of growers are looking for warmer weather on the weekend or next week in intentions of applying thinners.
Growth Stages
Slow, but still moving, fruitlets are sizing across most of the province. Growers are noting variation in sizing within varieties, along with some early drop in certain cultivars. Keep this in mind when chemical thinning.
- Essex & Chatham-Kent – Fruit set (10-15mm)
- Lambton, Middlesex, Elgin – Fruit set (8-12mm)
- Norfolk – Fruit set (8-12mm)
- Brant, Wellington – Fruit set (5-10mm)
- Niagara – Fruit set (8-12mm)
- Grey – Petal fall to early fruit set
- Durham – Petal fall to early fruit set
- Northumberland and Quinte – Petal fall to early fruit set
- Ottawa Valley – Petal fall to early fruit set
Cool Weather, Cool Apps, Cooler Mindset
Goin’ in to Thin
There have been back and forth discussions among growers and consultants on thinning; specifically on plans, adjusting rates (because of weather), timing for applications and concerns with determining efficacy.
Early application of chemical thinners in southern regions are just starting to see effects.
If low temperatures continue, expect to see a delayed response in the orchard after a chemical thinner application (potentially 7-10 days).
To reiterate from last week’s What The Crop?! Apple Update: May 23, 2025, chemical thinners are best applied when temperatures are above 18C. 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 20C
- 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
I would note that this doesn’t state at which fruitlet stage these products were applied, but it is good information to know.
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 |
Navigating the Cold Crawl
Not only has the cool, wet weather made for difficult thinning choices, it has been throwing a wrench in pest activity as well. Overall, cool temperatures slow down insect activity or emergence and reduce the risk of disease development. However, it doesn’t stop entirely!
For an update on specific pests currently active in orchards around the province, see last week’s update, What The Crop?! Apple Update: May 23, 2025.
Scab
A common question I’m hearing lately is how does extended wetting periods affect apple scab spore release. Scab infection needs moisture for two reasons:
- Ascospores (overwintering scab spores that cause primary infection) are contained within “pouches” that are under pressure. This pressure builds as the spores mature throughout the spring. When water settles on the surface, it dissolves these pouches, triggering an ejection of ascospores into the tree canopy.
- When ascospores land on wet fruit or leaves, and if they remain wet for a few hours, germination and penetration into the tissue will occur (aka infection!).
As has been mentioned in previous updates, the time required for infection (leaf wetness) will vary depending on temperature – from 40 hours at 0C to only 6 hours at 16C. In addition to that variation in infection time, there will also be a delay before lesions actually become visible. This can be upwards of a few weeks in cooler temperatures.
What’s interesting in this scab conversation is what happens when there isn’t a break in the rain. Mature ascospores are released within the first half hour and depleted within the first few days of an infection event. However, those remaining immature ascospores continue to develop over time. Do they get released during extended rains? A period of drying is likely needed to allow for pressure in the leaves to build back up in order to forcibly release any new ascospores that have matured since the start of an infection event. Unfortunately, I don’t have the answer (yet) of how long of a dry period is actually needed for this to occur.
Regardless, scab is not something you want to risk, especially on susceptible cultivars like Ambrosia. Consider reapplying fungicides during brief dry periods to maintain some protection. Apply fungicides again once conditions are dry to replace residues washed off by rain.
Fire blight
The cooler weather has kept fire blight risk relatively low to moderate over bloom for most areas. However, 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.
Petal fall insects
Cool temperatures can slow down the development of various apple insect pests, including those that overwinter as eggs or larvae such as aphids, mullein bug, oriental fruit moth, or codling moth. This can result in delayed or slow emergence times, potentially impacting the timing of management strategies.
Pay close attention to scouting reports and be prepared to pivot or adjust control timings, where needed, based on pest activity (or lack there of).
What are we seeing this year?
- Mullein bug – The start to “mullein bug season” seems to be delayed with low numbers still being tapped out in orchards. Hatch can be asynchronous in cooler springs. Fruit are at risk of damage up to 18-25mm (dime to quarter sized fruit).
- Aphids – Some rosy apple aphid colonies have started to be found in orchards. In previous wet springs, it was not uncommon to see these pests pre-bloom. Very little signs of woolly apple aphid are present.
- Moths – In some orchards or regions, oriental fruit moth and codling moth activity has remained low. This has created challenges for determining biofix dates due to sporadic trap catch.
- Plum curculio – Adult activity has been limited, even in historically higher pressure blocks. During cool weather, the adults tend to crawl into the orchard from surrounding areas instead of the usual warmer weather mode of flying.
- Mites – European red mite activity has been low. While this is not necessarily a bad thing for pest management, it does mean there is less alternative food sources available to sustain our beneficial orchard insects.
- Scale – Predicted San Jose scale crawler emergence is about 1-2 weeks later than “typical” timing based on degree day predictions.
Rain, Rain, Wash Away
With the rain, you may be interested in a review of the qualities of rainfastness of common insecticides and fungicides. Check out Pesticides & Rain: Ensuring Strength in Storms for more information. While some chemical groups do offer retention and/or redistribution in light rain, it should be generally assumed that after 2 inches (5 cm) of rain, most residues will be washed off.
Degree (Days) of Separation
Scale
The cool weather has slowed degree day accumulations for crawler emergence. However, the earliest regions of the province are approaching management timing. For more information on registered products or those with efficacy against scale, see the 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 | 192 DDC | June 7 |
London | 134 DDC | >14 days |
Delhi | 119 DDC | >14 days |
Grimsby | 138 DDC | >14 days |
Clarksburg | 86 DDC | >14 days |
Durham | 111 DDC | >14 days |
* as of May 29, 2025 |
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.
For orchards with historically high scale pressure, even with dormant oil applied, in-season crawler management may be needed for a number of years until pressure can be reduced. Of the summer management, 1st generation is certainly the most critical timing. Management of the second generation, which generally emerges in early August, would be warranted if signs of scale damage on developing fruitlets is observed in July.
Apple leafcurling midge
Apple leafcurling midge infested terminals are becoming quite apparent in many orchards. Emergence predictions for the 1st generation are summarized below. See last week’s What The Crop?! for management options.
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 | Passed |
London | Passed | Passed | June 6 |
Delhi | Passed | Passed | June 8 |
Grimsby | Passed | Passed | June 5 |
Clarksburg | Passed | June 4 | >14 days |
Durham | Passed | Passed | >14 days |