Apples Diseases Insects Pest Management

What the Crop?! Apple Update: May 18, 2023

Cool temperatures this week bring concerns for frost/freeze damage across the province while dry weather encourages powdery mildew development.

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

Cool nights have been top of mind as of late, especially considering many regions are currently in king to full bloom. Early regions are further ahead at petal fall to early fruit set.


Growers continue to plant and prune throughout the province.

Some growers have used bloom thinners throughout the past week, while many wait to decide their future thinner applications after freeze/frost assessments are complete.

Frost assessments

Cold temperatures have already affected some apple growers, but the full extent of the damage won’t be known until a few days after any frost or freeze event. I encourage growers to wait 24 hours to assess damage and to utilize strategies outlined in the ONFruit Blog post linked here.

Recommendations for apple flower assessments suggest checking the king and side blooms separately. King blooms are more likely to be damaged while the side blooms can remain undamaged.

Assessing dead/injured flower ovaries in 100 buds (10 buds on 10 shoots) provides a good estimate of percentage crop loss. The following websites have some excellent pictures and resources for assessing frost injury for tree fruit:

Some final notes on freeze/frost damage in apples.

  • Often the freeze will only damage some of the flowers such as the most developed ones or flowers in the bottom of the tree
  • In a radiation freeze with clear, calm conditions, fruit on higher sites or in the tops of trees will be less damaged than those at lower sites
  • Be careful not to add insult to injury when it comes to applying pesticides shortly after a cold event. The damaged tissue can be susceptible to the uptake of spray materials for 24-48 hours after a freeze before leaves have a chance to heal. This damage can serve as entry points allowing some products such as oil, copper or captan to be easily absorbed into the plant causing phytotoxicity.


Fire blight

Fire blight infection risk has dropped for the remainder of this week with the cooler average daily temperatures. Ideal conditions for rapid multiplication of the fire blight bacteria is above 18C. Research suggests bacteria are capable of surviving on open flowers for several days so it is possible for infection should a wetting event occur, even with potentially cooler conditions, though likelihood is much less. If there is a history of fire blight in your orchard or neighbourhood, keep a close eye should conditions change.

Sudden, sporadic or isolated showers, heavy dew, fog or spraying can all provide enough moisture to wash bacteria into an open bloom. How much water is actually needed? If you look at the requirements for blossom blight infection based on the Maryblyt model, a wetting event is >0.01” (0.25mm) rain, heavy dew or fog sufficient to wet foliage and provide thin film on tissue surface or rain of >0.1” (2.5mm) the previous day.

In areas of the province that are seeing the end of bloom, growers should be aware of blossoms still open, particularly young trees or rat-tail bloom. Protecting these open blossoms from infection will be critical during times of high risk. Antibiotics will break down quickly in sunlight, likely only providing 24-72 hours of protection before a re-application would be required. This means any antibiotics applied in the last week during the main bloom period will have already degraded away.

If time and labour are available, it may be best to go through the orchard every couple of days and remove the rat-tail blossoms by hand. Be sure to remove blossoms in dry weather to prevent the spread of infection. For many operations, this practice is just not viable. Instead, continue to monitor infection risk and apply protectants if needed.

King bloom petal fall is the ideal timing for applying Apogee (typically when growth is 2.2 – 5.5 cm long) to help with shoot blight management. Studies have found Apogee applied at a high rate on young trees provides good control of shoot blight without significantly impacting growth.


Most areas are still within – though nearing the end of – the primary apple scab maturation period. However, despite the maturing ascospores, little has been released with the dry weather. What this means is that we could see a major infection event with the next rain as massive amounts of ascospores will be released. Watch the forecast very closely and keep covered. 

Powdery mildew

With the high overwintering pressure going into this year for many, it has been no surprise to see early signs of powdery mildew infection following the stretch of warm, dry weather. Unfortunately, when conditions are not necessarily conducive to scab infection, they can often be ideal for powdery mildew.

Powdery mildew on new terminal of Idared.

Mildew thrives in dry weather and high relative humidity. In fact, rain deters powdery mildew development by washing off spores. So, protectant sprays may still be required during dry periods when there is little risk from apple scab. Be careful how long the interval between sprays become, especially in mildew susceptible blocks.

Some key points for effective powdery mildew control to consider:

  • Maintain a tight spray schedule with high rates during primary scab infection period. Powdery mildew does not invade mature leaf tissue, so spread of mildew ceases when trees stop producing new terminal leaves.
  • Getting good mildew control following an outbreak will take several seasons. Mildew infected white shoots from last year’s failure will persist through the season, but does not indicate current fungicides are failing. The current season mildew program is designed to prevent spread that would lead to primary infection for next year.
  • Where they are working, include sterol inhibitors, strobilurins or SDHIs during the critical infection period, generally at pink through petal fall.

Black rot

Frog-eye leaf spot has been found in varieties that tend to be more susceptible to black rot such as Gala and Idared. These early lesions appear as small purple to reddish flecks with light centers.

Frog-eye leaf spot on young apple leaves.

While the optimum temperature for leaf infection is 26⁰C, 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 period earlier this month, this hasn’t been out of the question.

If you are seeing frog-eye leaf spot now, you now know inoculum is present in the orchard or coming from a sporulating nearby source such as dead or decaying wood in an adjacent woodlot. Black rot produces 2 types of inoculum – ascospores (sexual spores), which are wind blown and conidia (asexual spores), which require a wet period and rain splash for release. Both begin to release shortly after bud break; however, the heaviest ascospore release occurs for a 4-6 week period following petal fall and fruit infection can occur at 20-24⁰C with 9 hours of wetting. In other words, if your protectant program considerations haven’t included black rot yet, now is the time!  


Sporulating cedar apple rust galls on the winter, or alternate host (juniper species including red cedar) have been observed over recent weeks. Orange, gelatinous spore masses called telia horns extend from the galls for a period of a few days after a wet period. This usually coincides with apple bloom.

Sporulating cedar-apple rust gall on overwintering host, Eastern red cedar.

Spores are dispersed by wind to nearby apple trees. Golden Delicious, Idared, Mutsu and Russet are fairly susceptible to rust. Most scab fungicides used at this timing will have some efficacy on rust diseases. Refer to the fungicide efficacy view for apples on the Ontario Crop Protection Hub for efficacy ratings of registered fungicides.


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 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 these last couple of 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 gypsy 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.

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. No damage has been reported yet in those areas where fruit are sizing. 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.

Apple leafcurling midge

Apple leafcurling midge eggs have been observed in growing terminals. Signs of leaf curl are likely to begin within the next week or so if temperatures warm up.

Small orange eggs of apple leafcurling midge on newest leaves of growing terminal.

While Movento is registered for suppression, the timing for leafcurling midge is often earlier than for scale and woolly apple aphid so timing 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. Summer oils will also help to smother eggs so long as good coverage is achieved to reach within the newest terminal leaves.


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 with the warmer weather predicted 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.,

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