Apples Diseases Insects Pest Management

What the Crop?! Apple Update: May 9, 2023

Warm weather has set growth off again with many regions in the province seeing signs of bloom and increased pest activity in the orchard

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

Warm weather over the weekend and the continued sunshine throughout the week has further developed apples across the province. Later regions are at tight cluster to pink, while the earliest areas are seeing king to full bloom. Warm weather is expected to continue, where more southern regions are anticipated to hit full bloom by this coming weekend.


Growers continue to plant and prune throughout the province.

Regional apple study groups continued their bi-weekly meetings. Topics are listed below:

  • One of the most common topics was on prohexadione calcium (Apogee, Kudos 27.5). Some general points regarding this product are listed below:
    • First application should occur when terminals are no longer than 2.5 – 5 cm – no later than petal fall. As the product may require 14 days to slow growth.
    • Depending upon the amount a vigour control required a second application can be made 14 to 21 days following the initial application.
    • There is no direct impact on return bloom, but the product may increase fruit set due to a reduction in June drop.
    • May require more aggressive chemical or hand thinning, as prohexadione calcium may cause the tree to retain more fruit
    • On young trees, consider applying to the top of the canopy only. This may provide added fire blight protection for sensitive cultivars (i.e., Royal Gala), while maximizing growth potential.
    • Consider measuring the length of approximately 100 randomly selected extension shoots per cultivar (from treated and untreated trees) on a 7- or 10-day basis until terminal bud set. This will reveal when trees start and stop growing.
    • Do not thank mix with calcium sprays.

Potential benefits from growth reduction as a result of prohexadione calcium applications:

Production Benefits
Pest Management Benefits
Better light penetration in the canopy
Reduced tree row volume
More light = more colour
Better spray penetration
Less summer pruning
Better air circulation
Less dormant pruning
Reduced fire blight susceptibility
  • Another topic included thinning. Some growers stated that they are trying out blossom thinners, where others are sticking with fruitlet thinners. For information regarding thinning please refer to the Thinning Tree Fruit page on
    • Erika DeBrouwer will be running the Pollen Tube Growth Model and the RIMPro thinning model to compare the two across various orchards in Ontario. If you are interested in more information, please reach out to her directly (Erika DeBrouwer, 226-931-4098,



Most areas experienced multiple apple scab infection events over the prolonged rains last week. Earlier regions of the province are beyond 50% ascospore maturation, while later regions are at about 30-50% of ascospores matured. This means that for future rain events, 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.

With the extended wetting periods last week, coverage was certainly a concern. Many growers opted for kickback products such as Scala, Inspire Super, Luna Tranquility, Cevya and Buran to provide extra protection.

Monitor blocks where coverage may have been inconsistent 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.

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

Fire blight

If you’re not already watching the Ontario fire blight prediction maps, now is the time to key an eye out. With the warm weather, potential rains and the start of bloom for many, fire blight risk is also taking a big jump in the coming days.

The risk of fire blight infection is possible if the following conditions are met:

  1. Flowers are opened and/or are opening – There is no risk of infection if bloom is not present in the orchard despite the fire blight maps stating caution to extreme; however, keep an eye on any early signs of bloom as it can happen quickly.
  2. Enough heat units have accumulate based on forecasting models – The fire blight maps use Cougarblight but this applies to MaryBlyt as well. Similar to degree days, daily temperature is accumulated and must reach a certain point before risk increases.
  3. A wetting event is predicted – Any type of wetting event will wash the bacteria into the open bloom including rain, dew or even the artificial rain shower you create with the sprayer during critical infection periods.

Consider alternatives to antibiotics during the early bloom period, especially if risk is low to moderate but fire blight has been an issue in your orchard before. These include:

  • Biologicals – eg., Blossom Protect, Buran, Serenade, Regalia or Double Nickel
  • Surface sterilant – eg., Oxidate
  • Copper labelled for in-season use – eg., Cueva – note that copper and some fungicides are not compatible with certain biologicals. Check the label before use.

Pink is also an important timing if using the Bacillus product, Lifeguard. This product works by activating the tree’s defense mechanism and is most effective when it is applied before infection occurs or when added to a fungicide rotation. 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. Initial inducement of plant defense response occurs soon after application but 3-5 days are required to attain maximum level of protection. Once bloom hits, however, avoid application as this product is toxic to bees. Applications can pick up again at petal fall after hives have been removed.


Those areas that deal with cedar-apple rust are seeing active galls in the overwintering host (such as Eastern red cedar). These spores will be wind-carried into the orchard which will act as the summer host for this pathogen’s cycle. Look for signs of infection over the coming weeks. This will appear as small yellow spots on the leaves.

Orange telia horns extending from galls on alternate host of cedar apple rust. (Photo: Jennifer Dean)

While captan does not provide effective rust control, mancozeb does. If you know this disease can be a problem in your orchard or neighbourhood, be sure to keep mancozeb as the protectant tank mix particularly through to petal fall. Group 3 fungicides have also historically provided good rust control.


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. 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.

Rosy apple aphid

If this spring continues to be on the cool, wet side, be prepared for rosy apple aphid populations to build. Egg hatch is likely on-going currently but infestations really start to be observed into tight cluster or early pink. Pay particular attention to fruit clusters of susceptible cultivars like Cortland, Idared, Golden Delicious and Ambrosia.

Tarnished plant bug

While tarnished plant bug activity and/or damage has not been reported, some growers who have historically dealt with this pest have applied a prebloom insecticide.

Plum curculio

The first adult plum curculios were found moving into orchards this week with the sunny, warm days that followed the rain. This movement from adjacent overwintering sites in nearby woodlots and 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 begun. With the warm weather this week, trap catch has spiked. Egg laying is likely to begin over the next week in the earlier regions of the province. 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 15
>14 days
May 9
May 18
>14 days
May 10
May 21
>14 days
May 10
May 21
>14 days
May 14
>14 days
>14 days
May 12
>14 days
>14 days


European red mite egg hatch has started. At this point, the use of a delayed dormant oil will no longer be effective. Click here for registered petal fall miticide options for European red mites.

Beneficial insects

Many beneficial insects can be found out in orchards now including lady beetles, assassin bugs, spiders and ground beetles. To support these populations, choose control products that are softer on beneficial insects. This information can be found in the “View Details” section of products listed in the Fruit Crop Protection Search.

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