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

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

Growth Stages

Fruitlets are sizing well across the province! Depending on the cultivar and location, sizing varies but most are near or past the 20mm stage.

Weather Review

Growing Degree Day Accumulation

About a month ago (What the Crop?! Apple Update: May 10, 2024), we showed how the 2024 season was shaping up regarding GDD in comparison to 2023, 2012 and the 5-year average.

What the Crop?! Apple Update: May 10, 2024

In early spring, 2024 was sitting between the 2023 and the 2012 season. This still seems to be the case – noting that 2021 was added due to the similarities at certain locations (Figure 1).

• 2024 is mostly comparable to the 2021 season
• 2024 is further along than 2023 and the 5-year average
• This season is shaping up to be earlier than last year by roughly a week depending on location, certain areas are seeing even earlier timings – keep this in mind for growth-oriented orchard tasks i.e. hedging

Precipitation Accumulation

Most growers have had a wet spring, but more recently some dry days have occurred (Figure 2). Keep this in mind for any future irrigation decisions.

• Precipitation is variable throughout the province, with most areas following a similar accumulation to 2023
• All locations in 2024 have a higher precipitation accumulation than the 5-year average
• London and Delhi have had considerable amounts of rainfall in comparison to 5-year

Over-Winter Temperatures

Looking at average temperatures across Ontario (using the locations listed above in previous charts), we can see that over-winter temperatures have been more variable in recent years (Figure 3).

• 2023 going into the 2024 season has been considerably milder than the past few years and the 5-year average (except for November)
• February of 2024 has shown the most extreme difference between the 5-year average with a difference of roughly 4 °C
• May of 2024 is the second most mild, showing a temperature difference of roughly 2.5°C

Disease Development

Scab

Low levels of apple scab infection can be found in some orchards across the province. Though overall, many orchards still appear to be clean at this time. You may want to look back at weather, infection events and spray records to see if there was a missed timing or if the issue was with product choice / application.

For those blocks with active scab lesions on fruit and/or leaves, it is important to keep in mind that although some fungicide groups such Group 3 may have curative or post-symptom properties, it is not advisable to use these products due to resistance management concerns. Applying single site fungicides to actively sporulating lesions creates a very risky resistance development situation.

Powdery mildew

Some orchards continue to see powdery mildew and the warm, humid weather will favour development. Continue to protect susceptible varieties such as Gala, Honeycrisp, Cortland, Idared, Crimson Crisp, Goldrush, Russet, Fuji and Spy.

Powdery mildew requires actively growing tissue to complete its cycle so fungicides with efficacy against this disease should continue until terminal growth stops. Infected shoots can be pruned or removed during other orchard activities; however, you can also wait until these infected terminals dry out and remove during summer or dormant pruning to prevent inoculum for next season. The spores will overwinter in infected fruit and vegetative buds.

Fire blight

With high daytime temperatures, some orchards are beginning to see signs of blossom and shoot blight.

Shoot vigour can be managed with prohexadione-calcium (Apogee/Kudos). Some products registered for summer fire blight control or suppression can be used to desiccate infected tissue. This may help slow spread. For a full list of registered products, see the Fire Blight section of the Ontario Crop Protection Hub.

However, once the bacteria is in the tree, it will continue to move until terminal growth stops. In highly vigorous or young trees, this movement can happen very quickly. It’s best to remove strikes as they develop to prevent more severe infection from occurring.

The unsettled weather we are experiencing may continue to bring hail, strong winds or heavy rains that could trigger trauma blight. These could be very isolated events, occurring in one area but not another nearby. Assess for signs of damage in all blocks following any storm.

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

But is it black rot?

Various spots and lesions have been appearing on leaves across the province. While frog-eye leaf spot (black rot) is often the common cause for these lesions, it is important to properly identify the issue as there are a number of other pathogens and abiotic factors that look similar.

Frog-eye leaf spot first appears as tiny purple flecks, which eventually enlarge into circular lesions about 4-5 mm in diameter. As the lesion enlarges, the margin remains purple while the centre turns tan or brown with a light centre (“frog-eye” appearance). You can often find the source of inoculum nearby from dead limbs or trees, fruit mummies still in the tree or adjacent woodlots.

Other possible causes for spots and lesions on the leaves could include (but not limited to):

• Marssonina – “the new pathogen on the block”. For more information, see the Dec 2021 article, Marssonina (Diplocarpon) Blotch Confirmed in Ontario Apples.
• Alternaria – certain species can cause minor leaf issues but is not typically an issue in Ontario. For more information, see the Dec 2022 article, Apple Alternaria – Another Annoyance?
• Necrotic leaf blotch – a physiological disorder common to Golden Delicious and related cultivars. For more information, see the Aug 2022 article, (Common) Uncommon Orchard Issues.
• Spray burn – product incompatibility (tank-mixes, surfactants, oils), poor drying conditions and other spray-related situations can cause unusual patterns, blotches or spotting.
• Herbicide drift – often tends to be in lower canopy or perimeter rows
• Nutrient imbalance – all the more reason to stay on top of nutrient testing!

Insect

Scale

San Jose scale crawler activity for the 1^st generation has begun. To spot these small yellow-orange nymphs, you’ll need to get close to the trunk with a hand lens as you can see in the picture below. While emergence may be slow at first, activity can pick up quickly. This generation will be active, however, for 4-6 weeks as crawlers move to new shoots and developing fruitlets. Once attached and starting to feed, these immature scales will form waxy coverings that protect them from insecticides.

The table below highlights the predicted timings for various locations using the 14 day forecast for the respective regions.

San Jose scale degree day accumulations as of June 13, 2024

Region	Current Degree Day Celsius (DDC)	Predicted Crawler Emergence (278 DDC)
Harrow	419 DDC	Crawlers active
London	321 DDC	Crawlers active
Delhi	300 DDC	Crawlers active
Grimsby	327 DDC	Crawlers active
Clarksburg	256 DDC	June 16, 2024
Durham	258 DDC	June 16, 2024

Targeting sprays during the vulnerable time when crawlers are exposed will maximize the efficacy of products such as Closer, TwinGuard and Sivanto Prime. Since Movento is slow-acting, ideal application timing is prior to crawler emergence. If planning to use this product and it has not gone on yet, applying as soon as possible may still provide some efficacy for peak emergence due to the extended pest activity. While Purespray Green Spray Oil is registered for summer use on mites and aphids only, trials in Ontario have found weekly oil applications may provide some suppression of scale during the crawler stage.

Caterpillars

Codling moth

Egg hatch for 1^st generation codling moth is on-going. There have been no reports of fruitlet damage yet.

Following egg hatch, codling moth larvae search out the fruit where they begin to tunnel in. This can happen as quickly as 24 hours after egg hatch which is why timing of control strategies are so important. After entering fruit, the larvae will feed for approximately 3 weeks before leaving the fruit to pupate (usually on tree trunk or larger branch of the tree). Pupation generally lasts 14-21 days. Some larvae do not pupate at this time, but remain as larvae until the next year. Second generation adults begin to appear in orchards as early as July and lay eggs over two months.

It’s important to note for resistance management, if re-application is required for control of a pest with discrete generations, use the same product within a generation. For instance, codling moth is still in the first generation window. Second generation larvae activity will not begin for another few weeks – typically early August. Rotate to another insecticide class at that time to ensure each generation is only exposed to one active ingredient.

Oriental fruit moth

Control timing for 2nd generation oriental fruit moth is still a couple weeks away. This generation more commonly feeds on developing fruit. I have been in a few orchards in the province with terminal damage caused by oriental fruit moth. The 1st generation larva often enter growing terminals when other fruit sources (fruit) are still not present. Once terminal set occurs, this risk is reduced but can impact growth of new plantings. This damage can also be misidentified with shoot blight. Look for presence of brown frass and an entry hole at terminal tip.

Obliquebanded leafroller

Obliquebanded leafroller biofix has been set for some but not all regions. Control timing will begin over the next couple for weeks across the province.

Plum curculio

Low levels of fresh plum curculio injury has been found on developing fruitlets. This can be determined by presence of sap oozing from damaged area. As the injury ages, it scabs over to give a russeted appearance.

Often movement into the orchard follows a period of sustained warm weather and rain. Activity is believed to be linked to weather patterns, so could be as long as 6 weeks during cooler, more dry conditions.