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.

Some lingering bloom can still be found out there in some cultivars such as Ambrosia, Gala, Honeycrisp and Crimson Crisp.

Growers are starting to take note of how thinning is shaping up. For more information on thinning, see the previous What the Crop?! update on May 17th:

What the Crop?! Apple Update: May 17, 2024

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. The significant rainfall most areas welcomed last week likely brought the end of the primary infection period by releasing the last of the matured ascospores.

However, it is important to wait a few weeks after the last infection event before backing off of a scab program. Monitor for signs of scab lesions. Be sure to take a good look in the top part of the canopy as coverage can often be inadequate there. Before reducing fungicide rates, be sure your orchard is completely free from primary scab.

Powdery mildew

Some orchards continue to see powdery mildew and the warm, dry 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 will not cause damage to fruit at this point in the season. Fruit damage is a result of early infection that occurred in the flower buds and during bloom. However, infected shoots will be the inoculum source for next year. Prune out any strikes that are present.

Fire blight

The rattail bloom still found in some blocks across the province remain at risk of blossom blight infection.

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). With the warmth and rains, 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. Scout your orchard for wilting flower clusters or the characteristic “shepherd’s crook” every week until terminal bud set.

In terms of shoot blight management, prohexadione-calcium, or Apogee/Kudos is an important management tool to help slow the growth of these vigorous shoots and reduce the spread of fire blight.

Shoot blight can also be managed with a variety of registered summer products including Cueva, Double Nickel, Buran, Regalia, Lifegard, Serenade and Oxidate. For a full list, see the Fire Blight section of the Ontario Crop Protection Hub.

Insect

Scale

Crawler emergence for 1st generation San Jose scale can be predicted using a degree day model with a March 1st biofix and base temperature 10C. Based on current degree day accumulations, crawler emergence is predicted to begin across the province over the coming weeks.

The table below highlights the predicted timings for various locations using the 14 day forecast for the respective regions. These dates will continue to be updated as timings approach. We are also actively monitoring for crawlers in Simcoe to validate model predictions.

San Jose scale degree day accumulations as of May 30, 2024

Region	Current Degree Day Celsius (DDC)	Predicted Crawler Emergence (278 DDC)
Harrow	292 DDC	Started
London	218 DDC	June 7, 2024
Delhi	197 DDC	June 9, 2024
Grimsby	226 DDC	June 6, 2024
Clarksburg	152 DDC	14+ days out
Durham	167 DDC	14+ days out

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.

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.

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.

European apple sawfly

Damage caused by European apple sawfly can be found in some orchards within regions where this pest is present. This damage can sometimes be confused with that caused by codling moth. However, the important thing to note is the timing this damage is appearing. If you are seeing damage now, that is caused by sawfly as codling moth egg hatch is only just beginning.

Mullein bug

Mullein bug stings on developing fruitlets have been reported in orchards. However, overall damage from this pest has been fairly low. This is especially surprising in some orchards especially given the high numbers of mullein bug nymphs were tapped out during weekly monitoring. At this point in the season, damage is not a concern as most mullein bugs are adults and fruit is larger than dime- to quarter-sized. In fact, mullein bugs are very effective predators of many orchard pests so supporting these populations from this point on in the season is encouraged.

Plum curculio

Low levels of plum curculio damage has been found on developing fruitlets. 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.

Apple leafcurling midge

Adult flight for the 2nd generation of leafcurling midge has begun and egg laying can be found in growing terminals. New leaf curls will likely start developing in the coming week or so. Damage will continue until terminal set.