Apples Diseases Insects Pest Management

Apple Pest Update: April 26, 2021 (with audio)

Tight cluster to early pink in apples across the province signals the start of new pest development.

Listen to the audio article here:

Things were off to a fast start this spring prior to last week. Since the cooler temperatures (and snow!), growth has seemed to be at a stand still around tight cluster for most regions. Earlier parts of the province are seeing pink in some varieties.

By the looks of things, temperatures will be returning more seasonable over coming days with potentially higher than average nighttime temperatures. This could move growth along quickly as well as pest development.



It’s too cold, right?

Following cooler spring temperatures, I often get questions around risk of apple scab infection and survival of disease during cold weather. As if there isn’t enough to worry about already with freezing temperatures, unfortunately, this does not kill scab spores. Spores will continue to mature and release so long as there is a wetting period long enough for infection to occur, says Dr. Kari Peter (Penn State University) and Dr. George Sundin (Michigan State University). While mature spores are released more during warm rains than in colder rains, they can still cause infection nonetheless if they land on green tissue.

How snow plays into this in terms of leaf wetting, is still relatively unknown. My concern would be snow melt. While melted snow results in very little actual water, it can still provide enough moisture to release ascospores from the overwinter leaves on the orchard floor if there were any that had matured. Whether the leaf wetting hours with snow melt would be long enough to result in infection would be the question.

More information on temperature and leaf wetness requirements for scab infection can be found in Table 3-9. Relationship of Temperature and Moisture to Apple Scab Infection of the 2021 Publication 360, Crop Protection Guide for Apples or on Ontario AppleIPM website.

What’s in the scab forecast?

The long-term forecast seems to suggest warmer weather is on its way with the potential of scattered showers. As growing degree-days continue to accumulate and temperatures become warmer, the rate of ascospore maturity will increase. This could result in large amounts of spores being released during infection periods and with temperatures above 15C, only 6 hours of leaf wetness are required for germination. Those without adequate fungicide protection could find themselves in a bad situation.

Early season scab management

The following are some key points to consider for effective scab control this time of the year:

  • Keep an eye on the forecast for freezing temperatures.
    • EBDC fungicides (Manzate, Penncozeb, Dithane, Polyram) should be used.
    • Applying captan, copper or oil to cold injured leaves could make the injury worse due to uptake of these products into the tissue.
  • Tight cluster to petal fall is a time of critical infection period with higher daily temperatures, large amounts of lush growth and rapid maturation of spores.
  • Be sure leaves are covered before any potential infection event. Most of the available mature spores are discharged within 2 hours after the start of a wetting event.
  • Management programs at this point should begin incorporating systemic fungicides from Group 1, 3, 7 and 11.
    • Unlike protectant fungicides which have multi-site activity and low resistance potential, systemic fungicides are typically single site with high risk of resistance development.
    • Tank mix with at least half rate protectant fungicides.
    • Rotate between chemical groups for consecutive applications.
  • If using protectant fungicides alone, remember these are contact fungicides and do not provide effective post-infection or anti-sporulant activity.

Fire blight

Putting the cold temperatures behind us, with early areas approaching bloom, growers will need to start considering their fire blight management program. The Ontario fire blight prediction maps are available again this year to help with that decision making.

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. With cool weather in early bloom, prediction maps will stay on low or caution.
  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 OPTI, Regalia Maxx or Double Nickel
  • Host resistance inducer – eg., Lifegard. Note: Timing for this product is pink and then again at petal fall. Do not apply during bloom.
  • Surface sterilant – eg., Oxidate
  • Copper labelled for in-season use – eg., Cueva

This will allow you to save your limited uses of antibiotics for later bloom (50-80% bloom) when fire blight risk is greatest.

Powdery mildew

Healthy (left) and powdery mildew infected (right) apple buds.

High powdery mildew pressure last year followed by a mild winter has some growers concerned about management this season. A low rate (3-5 kg/ha) of sulfur can be incorporated into an early season protectant scab program in orchards with a history of powdery mildew until systemic fungicides with mildew activity are used. Some orchards with historically low powdery mildew pressure are using early season and summer oil sprays to help with suppression of this disease.

Peak infection often occurs from tight cluster to terminal bud set. Most systemic fungicides from Group 3, 7 and 11 applied at this time for scab have good efficacy on powdery mildew. However, keep in mind that dry, warm weather is conducive to powdery mildew unlike the wet weather needed for scab infection. So coverage is still important during those dry spells.


Reports from our neighbours to the south are spotting overwintering galls on the alternate hosts for cedar-apple and quince-apple rust beginning to sporuate. These orange, gelantinous masses can be quite obvious. Those areas that have high rust pressure should be prepared for infection risk to increase in this area shortly, often around tight cluster to pink in apples. EBDC fungicides provide good efficacy on rust as well as most products containing Group 3.

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


Spring feeding caterpillar

Small caterpillars have been found feeding on new terminal growth and developing fruit buds. While not a stand alone control tactic, supporting beneficial insects in the orchard such as ladybugs and lacewings will help suppress some pests such as spring feeding caterpillars. Consider this when deciding if a prebloom insecticide is worthwhile.

Apple leafcurling midge

Adult catch in pheromone traps has begun in regions that are monitoring for apple leafcurling midge. While the freezing temperatures may have slowed activity, it has been only temporary. There are still no signs of egg laying but I imagine this will begin this week.

Apple leafcurling midge monitoring traps.


Trying to find a window for dormant oil targeting scale timing was difficult in areas areas this year. Another prebloom timing to consider is a tight cluster to pink application of Sivanto Prime or Closer to target the maturing scale. The high rate of Closer at this timing will also have efficacy on leafcurling midge, woolly apple aphid, tarnished plant bug and mullein bug.

Upcoming Events

Looking for a chance to refresh your knowledge of apple IPM? New to growing? Join me for an Apple IPM webinar on May 6th from 10-11:30 am where we will discuss orchard scouting including monitoring techniques for various insect and disease, tools of the trade, safety protocols and tips for success.

Pre-registration is required

Time will also be available to answer any pest-specific questions regarding the pre-recorded presentations available on the ONhortcrops YouTube channel.

Other IPM workshops are also being offered over the coming weeks. Check out ONfruit for more information.

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