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

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

Orchard Activities

Across the province growers continue to plant and prune. Chemical thinning strategy is top of mind, along with petal fall pest management.

Growth Stages

We continue to see mixed staging on the tree throughout the province, meaning potential future challenges with secondary bloom, and current decision-making regarding thinning strategies.

Regionally, growth stages are fairly spread out depending on cultivar and proximity to water:

• In Essex, Chatham-Kent, Lambton, Middlesex, Elgin, Norfolk, Brant and Niagara, most cultivars are at late bloom to early petal fall.
• Waterloo and Wentworth are showing king bloom and should be in full bloom this weekend.
• In Grey, most cultivars are at pink and are expected to go into king bloom shortly.
• In Durham, Northumberland and Quinte, early cultivars are in full bloom.
• Further along the St. Lawrence and into the Ottawa Valley, early cultivars getting into full bloom.

Weather Review

Growing Degree Day Accumulation

Lately there has been discussion about where we stand in comparison to previous years regarding GDD. Below (Figure 1) demonstrates the differences we are seeing this year in relation to 2023, 2012 and the 5-year average.

• Ultimately 2024 is further along than 2023 and the 5-year average
• Currently we are following a similar pattern to 2012, although not quite reaching that progression

Precipitation Accumulation

Many growers are noting wet orchards this time of year, precipitation is shown below (Figure 2).

• Precipitation is variable throughout the province, with most areas following a similar accumulation to 2023
• Most locations in 2024 have a higher precipitation accumulation than the 5-year average
• Delhi and Ottawa are the two locations with lower precipitation than 2023, but ultimately are following the 5-year average

Thinning Challenges

.

Specific details about how to effectively strategize your thinning program, please read the following blog post by Dr. John Cline.

Chemical Thinning of Apples in the Response to Frost Damage to Flowers – ONfruit

In the instance that the king-bloom is killed, leaving several viable lateral fruitlets, thinning may become more difficult due to less differentiation in size and greater inter competition between lateral fruitlets. Summary notes from the blog are below given the above circumstance:

• Wait until fruit sizes differ within the cluster, and once fruit set is more apparent
• Apply thinners before the largest fruit is no more than 14-15 mm.
• When the lower part of the tree canopy is more affected, apply 65-90 % of water volume to the top half of the tree canopy and consider turning off the lower nozzles on the sprayer.
• Reliance on some hand thinning may be necessary to selectively remove fruit with frost markings or mis-shappen fruit.
• If the frost has resulted in significant crop reduction, then you may want to seriously consider controlling growth for the growing season using Apogee. Sprays for this begin at petal fall.

Thinner Effectiveness Table

Climate Conditions	Prediction
Warm Conditions (>18oC)	All thinners work best
Dark Cloudy Weather	Greater stress Greater thinning response Greater drop
High Night Temperatures (>18oC)	Great stress High demand and use of energy for night respiration Greater drop
Very High Day-Time Temperatures (>29oC)	Great stress High energy demand Greater drop
Very Cool Temperatures (oC)	Reduced stress Reduced energy demand Greater set
High Light	Increased supply: harder to thin
Low Light	Reduced supply: easier to thin
Low Temperatures	Low demand: harder to thin
High Temperatures	High demand: easy to thin
Low light and warm temperatures	Worst

Disease Development

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 large ascospore release and keep good fungicide coverage on those rapidly growing leaves. With temperatures above 15C, only 6 hours of leaf wetness are required for germination to occur.

The first scab lesions have been observed in the province. Based on average temperature and which leaves were affected, infection likely occurred 2-3 weeks ago. We have had a number of infection events so far this season with the frequent rains and unfortunately, it looks like we may be in for more over the coming week. 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+	–

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.

Fire blight

Fire blight infection risk continues to stay low to moderate with the cooler average daily temperatures. Many growers are opting for antibiotic alternatives such as Buran, Serenade, Double Nickel or Cueva with risk being lower. 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.

If you’re not already watching the Ontario fire blight prediction maps, now is the time to key an eye out. With potential warmer weather, rains and continued bloom for many, fire blight risk may also take a big jump.

Fire Blight Prediction Maps

For more information on ideal infection conditions and management options, see the post from earlier this week, Bloom in the Orchard: What is the Fire Blight Risk?

Bloom in the Orchard: What is the Fire Blight Risk?

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.

Other diseases

On a good note, the rains have kept powdery mildew at bay. Mildew thrives in dry weather and high relative humidity. In fact, rain deters powdery mildew development by washing off spores.

Very little signs of frog-eye leaf spot (foliar symptoms of black rot) have been reported. 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). With the extended rainy periods earlier this month, this hasn’t been out of the question so continue monitoring for symptoms.

Insect

Many growers are considering options for petal fall insecticide sprays. The table Activity of Petal Fall Insecticides Against Orchard Pests on the Ontario Crop Protection Hub may be of some help to determine the best insecticide for control of pests listed on the product label, while managing resistance and avoiding unnecessary sprays for non-target pests. Efficacy will be based on rate used.

Spring-feeding caterpillar

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 larvae per 100 terminals and fruit buds is observed. Management is most effective when larvae are young.

If spring-feeding caterpillar such as leafrollers, spongy moth, budmoth or green fruitworm have been a problem in previous years, keep an eye out for damage and consider management if pressure is high. Click here for registered prebloom control products for spring-feeding caterpillar.

Oriental fruit moth & codling moth

Oriental fruit moth trap catch continues to increase. These numbers can 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. 

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

Aphids

If this spring continues to be on the cool, wet side, be prepared for aphid populations to build. Rosy apple aphid and woolly apple aphid colonies have been observed already this year. For rosy, pay particular attention to fruit clusters of susceptible cultivars like Cortland, Idared, Golden Delicious and Ambrosia. Woolly apple aphids tend to be found around pruning cuts or old cankers but are beginning to move to new shoots and water sprouts.

Mullein bug

Increasing numbers of mullein bugs are being tapped out in various orchards across the province, though not exceeding the threshold of 7-9 nymphs per 25 taps. Growers that have dealt with mullein bug pressure in previous years are planning to apply petal fall insecticides with activity against this pest particularly on susceptible varieties such as Red Delicious, Spartan, Spy, Ambrosia, Empire, Cortland, Gala, Jonagold and Golden Delicious.

It’s recommended to continue monitoring after the insecticide has been applied to determine if a follow-up spray is required. Delegate, Minecto Pro, Altacor and Exirel do not have efficacy on this pest.

Plum curculio

Adult plum curculios continue to be caught in monitoring traps in high pressure blocks. With sunny, warm days that follow a rain, this movement from adjacent woodlots and overwintering shelters will continue into petal fall.

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

Apple leafcurling midge

Adult leafcurling midge flight has picked up this week. Egg laying is likely to begin over the next week or two in the earlier regions of the province. Emergence predictions for the 1st generation adults 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	10-14 days
London	Passed	May 15	>14 days
Delhi	Passed	May 18	>14 days
Grimsby	Passed	10-14 days	>14 days
Collingwood	May 11	>14 days	>14 days
Durham	May 12	>14 days	>14 days

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).