Apples Diseases Insects Pest Management

What the Crop?! Apple Update: May 29, 2023

Frost injury and warmer days may influence thinning decisions while petal fall pest management kicks off across the province in this week's apple update

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

Most regions are at bloom to petal fall with growers reporting lingering bloom in some cultivars such as Ambrosia and Honeycrisp. Early regions are seeing fruitlet set and sizing.


As May continues to throw unexpected weather at us, growers carry on with frost mitigation strategies and evaluation of damage. There have been reports of frost damage in most regions, although the severity and total impact remain to be seen. Eastern regions and Georgian Bay area had the cold temperatures where some reports of below -6°C were reached.

Burned flower blossoms on Honeycrisp following May 17th frost event.

Growers continue to prune and plant throughout the province and have stated that changes may be made to their initial chemical thinner program due to the frost weeks prior.


District meetings continue where much discussion on thinner applications were top of mind. Below are a few notes to keep in mind when making decisions on thinning:

  • Use extreme caution when thinning after a significant frost.
    • Leaves damaged by frost absorb more of the thinner and fruitlets with reduced seed count abscise more easily
    • Consider the application of lower rates of thinner for many varieties under these circumstances, or on a badly damaged block, do not apply a thinner but rely on hand thinning if necessary.
  • Depending on the severity of the damage, growers may thin areas of the orchard that escaped the worst of the frost damage.
    • Differences in elevation and airflow in the orchard can lead to great variations in frost damage.
    • Trees on lower ground can be exposed to air 2-3°C colder than trees on higher ground in the same frost event.
  • When deciding to apply thinners, the grower may plan to spray some of the block because of what appears to be a fairly normal set on trees on higher ground.
    • Nozzles can be shut off in areas of the block that show significant potential losses in crop load from frost. Nozzles that direct thinning sprays into the lower canopy can be shut off.

Thinning when the king bloom is lost to frost

  • If the king bloom or king fruitlet is lost to frost, size and strength differences among remaining fruitlets can be minimal, which makes them equally weak and vulnerable to thinners.
  • The best approach in this case may be to wait until size differences appear in the cluster, provided the largest fruitlet does not exceed the size range for fruitlet thinning.
    • Size differences may become more apparent several days after full bloom. A few hours’ difference in pollination timing can show up in size differences of side bloom fruitlets as the postbloom period progresses.

Weather conditions

Weather conditions encourage absorption of chemical thinners, which may increase the thinning response. These weather conditions include:

  • prolonged period of high humidity without rain, either before or after application
  • high temperatures after cool periods; frost or near-freezing temperatures before or shortly after application
  • prolonged period of dry weather before or after application
  • excessively warm conditions following fruit set.

A decrease in thinning response can result from sunny and warm daytime temperatures accompanied by relatively cool night temperatures before and after thinning treatments have been applied. This results in a high net accumulation of carbohydrate reserves in the tree.

For more in-depth information please refer to the following resources:

Disease Management


Early signs of scab lesion development have been observed in orchards. Many regions are nearing – or have reached – the end of the ascospore maturation period. However, a good rain is needed to release the last of these spores so be sure to maintain a regular protectant program until this primary infection period is over.

Monitor blocks to look for signs of lesions. 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. At the predicted time, begin looking on the top and underside of leaves for velvety brown lesions with indistinct margins. Be sure to take a good look in the top part of the canopy as coverage can often be inadequate there, especially since some of the best application days in certain regions this year were fairly windy.

If lesions are present, continue a regular protectant program to prevent secondary infection on developing fruitlets.

Number of days required for apple scab lesions to appear following an infection event
Average temperature (C)
Lesion appearance (days)

Fire blight

Blossom blight

The Ontario fire blight prediction maps continue to be updated regularly. With the warmer weather we’re experiencing, conditions have been optimal for fire blight infection.

Regions that are at fruit set should be diligent and monitor their orchards closely for new plantings that may still be blooming or for secondary, or “rat tail” blossoms. There have been a number of reports of fairly substantial straggling bloom or rattail bloom in many varieties including Honeycrisp, Ambrosia, Gala, Empire, Cortland and Crimson Crisp. 

Secondary or rattail bloom in Ambrosia

Take necessary action to remove or protect open blossoms from infection. Generally, continuing a protective program for a couple of weeks following petal fall will help ensure any late blooms are covered. Keep in mind, the antibiotics (Streptomycin, Kasumin) provide control for only 24-72 hours prior to a rain event. So for extended infection periods, a subsequent spray may be needed. Also be mindful of preharvest intervals when using these products. For instance, Kasumin has a preharvest interval of 90 days.

However if there are a few days of dry weather and labour allows, it may be best to go through the orchard and pinch off the rat tail blooms altogether since the fruit produced by the late blossoms are more of a risk than they are an asset.

Shoot blight

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.

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.

The use of LifeGuard, a Bacillus product, can also be picked up again now that most orchards are out of bloom or hives have been removed. This product works by activating the tree’s defense mechanism and is most effective when it is applied before infection occurs. Some level of response can occur shortly after application but ideally 3-5 days are needed for maximum level of protection. It’s important to note that since this is a biological product, it should not be tank-mixed with antibiotics such as Streptomycin and Kasumin.

Soon you will see if the fruits (pun intended) of your labour paid off or if fire blight symptoms develop. Scout your orchard for wilting flower clusters or the characteristic “shepherd’s crook” every week until terminal bud set.

Insect Management

Petal fall insecticide sprays are beginning to go on in the earlier regions of the province. 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.


The spray window for oriental fruit moth is quickly approaching in most early regions. Some growers are opting for insecticides with good efficacy on mullein bug, spring-feeding caterpillars, European apple sawfly and/or apple leafcurling midge if monitoring had indicated they had reached threshold or have high pest pressure.

European apple sawfly have been caught in traps where monitoring for this pest is on-going. However, catch has been relatively low likely due to the cooler weather during bloom. Nonetheless, early petal fall spray timing will be critical for areas with sawfly pressure. Delaying this spray may result in extensive damage. While this is a pest east of Toronto, into Georgian Bay and throughout the GTA, damage has been suspected in regions further southwest.

Spring-feeding caterpillar activity can still be found. Of the caterpillar complex, spongy moth can be found causing significant damage in some parts of the province, especially those orchards that saw high pressure in the last two years.

Larva will feed on leaves for several weeks before pupating. Most damaging activity is only present in orchards until July. While you may see the adults later summer, they do not feed and there is only one generation per year. Unfortunately, that early season larvae can feed pretty voraciously until then.

Products applied for leafrollers in apples such as Bacillus thuringienesis (Bt), e.g., Dipel, Bioprotec and Xentari should also provide control of spongy moth. Other products listed under spring-feeding caterpillars in the Ontario Crop Protection Hub may be registered for this pest, including some pyrethroids (Group 3) and Cormoran (Group 4A+15) or have efficacy against spongy moth when applied for a labelled pest.

For best efficacy, apply within the first few weeks after larval emergence. Once larva reach 2.5 cm or larger and develop yellow markings on the head capsule, few products will offer sufficient control. Also try to target application close to the time larvae are actively feeding to ensure exposure. During the day, most of the larva can be found in sheltered areas or on the underside of leaves.

Apple leafcurling midge

Apple leafcurling midge egg hatch continues. Infested terminals are becoming quite apparent already in orchards with a history of this pest. Emergence predictions for the 1st generation are summarized below.

Predicted emergence date of first generation adult apple leafcurling midge, by region (biofix March 1st, base 9C)
5% Gen 1
(76 DDC)
50% Gen 1
(132 DDC)
95% Gen 1
(235 DDC)
May 7
May 19
June 1
May 9
May 22
June 2
May 11
May 24
June 4
May 10
May 19
June 2
May 13
May 30
>14 days
May 12
May 28
June 8

While Movento is registered for suppression, the timing for leafcurling midge is often earlier than for scale and woolly apple aphid so would have to be chosen based on which pest is of greater concern in your orchard. However, there are other petal fall insecticides such as Delegate, Closer and Exirel (and for that matter Minecto Pro) that may provide some efficacy as well, which could help delay the Movento application. Pyrethroids are registered but have greater efficacy earlier to target adult midges and can be very harsh on the numerous beneficial insects that are now active in the orchard. Summer oils will also help to smother eggs so long as good coverage is achieved to reach within the newest terminal leaves.

Mullein bug

Increasing numbers of mullein bugs are being tapped out in various orchards across the province, exceeding the threshold of 7-9 nymphs per 25 taps. No damage has been reported yet in those areas where fruit are sizing. Growers seeing numbers above threshold are applying (or planning to apply) 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.

The critical time for damage from mullein bug is generally dime to quarter-sized fruit.

The cooler temperatures in May have resulted in delayed emergence of some insects mullein bug may feed on such as mites and aphids. However, I have been seeing a significant number of mullein bugs in leafcurling midge rolls already. These have seemed to be quite the voracious predators of midge in previous years. So, for those sites with leafcurling midge issues, it is possible if there is enough other food available in the orchard, it can keep the mullein bug from feeding on developing fruitlets. Without such food sources though, those fruitlets may be prime targets for mullein bug attention.

Plum curculio

Warmer temperatures may increase activity of plum curculio into the orchard. During cooler weather, these weevils tend to move slower, opting to crawl in from surrounding woodlots; however, on warmer days, they’ll fly in which is often when there is an increase in observed damage during these periods.

This year, OMAFRA is trapping for plum curculio in several high pressure apple and tender fruit orchards. To date, catch has remained low with little damage apparent as of yet.


Leaf curling and puckering typical of rosy and green apple aphid has been observed around bloom clusters and terminals, respectively. Winged aphids have also been found within some infestations. Aphids thrive in cool, wet springs so recent conditions may have encouraged development.

While thresholds for green apple aphid are relatively high (400-600 aphids per terminal on 10% of terminals), the most serious rosy apple aphid damage occurs when a toxin found in its saliva translocates from the leaves to fruit, causing apples to remain small, deformed (pigmy fruit) and unmarketable. Control is recommended if more than 5% of fruit clusters are infested with 20 or more aphids.

Signs of woolly apple aphid can also be found around pruning cuts, cankers and rough bark on the tree trunk. Early intervention is critical for effective management of woolly apple aphid. Once these colonies – which appear as tufts of cotton – can be easily observed in the outer canopy, control can be very difficult.


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 at the end of next week in the earliest regions of the province.

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 a number of Simcoe orchards to validate model predictions.

San Jose scale degree day accumulations based on March 1st biofix and base temperature 10C
Current Degree Day Celsius (DDC)
Predicted Crawler Emergence (278 DDC)
165 DDC
June 9, 2023
149 DDC
June 11, 2023
130 DDC
June 14, 2023
148 DDC
June 13, 2023
108 DDC
14+ days out
105 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.

Oystershell scale on trunk of McIntosh tree.

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.

Ambrosia beetle

Tree collapse following last week’s frost events have been observed in young plantings of Ambrosia. Upon closer look, signs of borer holes with toothpick-sized sticks of sawdust extending out were found. Black stem borer, a species of the ambrosia beetle group was identified. This species is currently a raising concern in North America and has been confirmed in a number of Ontario orchards.

Female ambrosia beetle bore into the heartwood of a young or stressed tree and introduce an ambrosia fungus (hence the name – not associated with the Ambrosia apple!) for her larvae to feed on. This triggers the tree to wall off its vascular system in response, leading to a weakened tree that can then be killed by other abiotic (such as the frost) or biotic stressors (disease, etc).

If you suspect ambrosia beetle in your orchard, we would be interested in collecting samples! Call, text or email Kristy at 519-420-9422 or

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