Apples Diseases Insects Pest Management

Apple Pest Update: May 25, 2021 (with audio)

Warm, dry weather favours powdery mildew and insect development as apples reach petal fall across the province.

Listen to the audio article here:

Mother Nature has cranked up the furnace recently, going quickly from a cool spell to the heat now moving things along quickly. Trees are full bloom to early petal fall in the later areas to the earlier regions now chemically thinning and applying petal fall insecticide sprays.


The warm, dry weather has not necessarily been conducive to most diseases commonly found this time of the year, with the exception of one…

Powdery mildew

Quite a number of reports of powdery mildew have been coming in. Unfortunately, when conditions are not necessarily ideal for scab infection, they can often be prime for powdery mildew. 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.
  • Include a mildewcide, such as sulphur in all sprays beginning at ½” green until temperatures are greater than 25°C or when applying oil. Sulphur lacks post-infection activity, so must be applied early season.
  • If pressure was low last year, oil applied for mites may provide suppression of powdery mildew. Use a 2% solution (20 L/1,000 L water) for dormant sprays or a 1% (10 L/1,000 L water) solution for summer sprays. Do not use captan- or sulphur-containing products within 14 days of an oil application.
  • Where they are working, include fungicides from Groups 3, 7 and 11 during the critical infection period, generally pink to petal fall. Tank mix these products with a Group M, where possible.
  • Alternative or rotational options providing suppression of powdery mildew include Diplomat, Serenade, Buran, Regalia Maxx and Oxidate. Note, Buran may provide control under low to moderate pressure with the addition of 0.1% v/v non-ionic surfactant.


Most regions in the province have reached – or are very near – 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.  

Fire blight

Fire blight infected pear blossom.

The hot temperatures over the last week have been ideal conditions for rapid multiplication of the fire blight bacteria, Erwinia amlyovora. Windy days and increasing pollinator activity this week will have moved bacteria to open blossoms, particularly if fire blight has been an issue in the orchard or neighbourhood.

Just because there is no rain in the forecast does not mean you are necessarily out of the woods. 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 neeed? 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.

While bloom has passed in many regions of the province, 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 last week during the main bloom period will have already degraded away.

Secondary, or rat-tail bloom on Idared apple. Note the developing fruitlets in background.

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.

These warm, humid days can also bring unsettled or extreme weather including heavy rain, hail and high winds. These events can cause trauma blight. If there is concern, Streptomycin or Cueva and/or Double Nickel can protect against spread of infection if used immediately following a trauma event.

Many growers have been applying Apogee or Kudos starting at king bloom petal fall (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.


Petal fall insecticide sprays are beginning to go on in the earlier regions of the province. Table 3-11. Activity of Petal Fall Insecticides Against Orchard Pests in the 2021 Publication 360A, Crop Protection Guide for Apples 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 will be over in most early regions in the next 10 days. Some growers have selected 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 had high pest pressure.

Codling moth biofix has been set in some areas but the spray window is still a ways out. Obliquebanded leafroller traps are going up.

European apple sawfly have been caught in traps where monitoring for this pest is on-going. This is a pest east of Toronto, into Georgian Bay and throughout the GTA. However, damage has been suspected in regions further southwest. Early petal fall spray timing will be critical for areas with sawfly pressure. Delaying this spray may result in extensive damage.

Spring-feeding caterpillar activity can still be found including leafroller larvae in rolled foliage.

Of the spring-feeding caterpillar complex, gypsy moth can be found causing significant damage in many parts of the province. The Ontario Ministry of Natural Resources and Forestry (MNRF) are estimating defoliation to be severe in many regions this year. For more information on particular high pressure regions, survey information can be found on the MNRF website.

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.

Young gypsy moth larva feeding on apple leaf.

Products applied for leafrollers in apples such as Bacillus thuringienesis (Bt), e.g., Dipel, Bioprotec, Foray and Xentari should also provide control of gypsy moth. Other products listed under spring-feeding caterpillars in Publication 360A, Crop Protection Guide for Apples may be registered for this pest, including some pyrethroids (Group 3) and Cormoran (Group 4A+15). Always refer to the label for registered pests. For best efficay, apply within the first few weeks after larval emergence and close to the time larvae are actively feeding to ensure exposure. Good coverage of leaf surfaces is also important.

For more information, check out ‘Is This The Year For Gypsy Moth?’ in the April 2021 issue of Orchard Network Newsletter.


Leaf curling and puckering typical of rosy and green apple aphid has been observed around bloom clusters and terminals, respectively. Aphids thrive in cool, wet springs so earlier conditions this year may have encouraged development. While thresholds for green apple aphid are relatively high (400-600 aphids per terminal on 10% of terminals), this is not the case for rosy apple aphid. 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.

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. Growers seeing numbers above threshold are applying 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 early May have resulted in delayed emergence of some insects that mullein bug may feed on such as mites, aphids and leafcurling midge. It is possible if there is enough other food available in the orchard, it can keep the mullein bug from feeding on developing fruitlets. However, without such food sources or delayed activity of those food sources, the developing fruitlets in the orchard may be prime targets for mullein bug attention.

Apple leafcurling midge

Apple leafcurling midge damage.

Apple leafcurling midge egg hatch continues. Infested terminals are becoming quite apparent already in orchards with a history of this pest.

While Movento is registered for suppression, the timing for leafcurling midge this year may be earlier in some regions than for scale and woolly apple aphid. However, there are other petal fall insecticides such as Delegate, Closer, Twinguard and Exirel 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.

Publication 360A Errata

There are a few errors/omissions in the 2021 Publication 360A, Crop Protection Guide for Apples that have been brought to my attention:

  • In addition to Decis 5 EC, the formulation Decis 100 EC is now registered for aphids, mullein bug, leafcurling midge, codling moth, tentiform leafminer, obliquebanded leafroller and oriental fruit moth. As with other Group 3 (pyrethroid) insecticides, Decis is highly toxic to beneficial insects.  
  • In Table 3-7. Characteristics of Apple Scab Fungicides (page 129), the post-infection activity for Buran is 350 degree hours with base 0˚C.


The 2021 Apple IPM webinar that was held on May 6, 2021 is now available on the ONhortcrops YouTube channel.

This is part of an IPM series offered in lieu of an in-person full-day workshop. All videos in the Apple IPM series can be viewed here. Please reach out ( if you would like a pdf version of any presentation within this series.

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