Apples Diseases Insects Pest Management

Apple Update: May 24, 2017

In this update:

  • Crop Stages
  • Production
  • Fire Blight
  • Apple Scab and Powdery Mildew
  • Summer Diseases
  • Insects

Crop Stages

  • In Southwestern Ontario, apples are at king bloom petal fall to 10 mm fruitlet size.
  • In Grey County and east of Toronto, apple blossoms are at full bloom to king bloom petal fall.


Growers are busy with planting trees and moving bees in and out of orchards.

As bloom starts to wrap across the province, be conscience about Apogee timing as new growth can progress quickly. The label application timing is for 2.5 cm to 7.5 cm of growth, but it’s best to aim for king bloom petal and there will likely be enough growth at that stage of growth.

Discussion on thinning has begun across the province last week and this week as we progress to that window. 6-BA (MaxCel or Cilis Plus) can be applied as early as full bloom to improve fruit size. NAA can also be applied early at petal fall. For optimal results with fruitlet thinners, they should be applied in moderately warm temperatures. For NAA and Carbaryl, optimal temperatures are between 21 and 24˚C. NAA should not be applied below 16 ˚C or above 27˚C. MaxCel or Cilis Plus should be applied in temperatures above 20 ˚C.

For those who follow the Cornell Carbohydrate Model there is a warning on the site that states:

“With the severe drought in 2016, there may be orchards that will have a weakened state coming into 2017 which would likely make the tree abnormally sensitive to thinners. So in those cases the model may suggest a stronger thinner concentration than is appropriate. Conversely, if irrigated last year, with the warm season and lots of sun, those trees may be in better than normal condition and harder to thin.”

The carbaryl (Sevin) label has changed twice within the last year. There is now allowed two applications of carbaryl with a minimum interval of 4 days. This has changed from the previous label, from September 2016, which stated that only one application was allowed per year. The maximum application rate has not changed but keep in mind that even if two applications are allowed, you must not exceed a total application rate of 1.5 kg a.i./ha (1.3 L of product/acre/year) for high density orchards and  for low density orchards do not exceed 1.0 kg a.i./ ha (0.87 L of product/acre/year). For high density orchards the minimum re-entry period is 14 days for hand-thinning and managing hand-line irrigation, and 4 days for pruning, scouting, pinching, tying and training.  For low density orchards, the minimum re-entry period is 10 days for hand-thinning and managing hand-line

Fire Blight

Conditions were ideal for fire blight infection in most areas across the province over the long weekend according to the fire blight prediction maps. These maps use the Cougar Blight forecasting model to determine infection risk. According to this model, some of the most damaging fire blight epidemics have occurred under these optimum conditions, including in orchards that have never experienced fire blight.

Growers running the Maryblyt model also saw the epiphytic infection potential (EIP) exceed the spray threshold of 100. It is important to remember with this model, the EIP resets back to zero when a bactericide has been applied. However, with high temperatures, it does not take long for the EIP to jump back up above 100. A benefit to this model is that it also predicts when fire blight symptoms will appear.

If forecasting models are not being followed, bactericides should be applied from first bloom to petal fall when temperatures exceed 18⁰C with high humidity (>65%), heavy dews, rain or when spraying fungicides. According to Dr. George Sundin from Michigan State University, temperatures conducive for growth can result in fire blight populations multiplying to one million cells per flower within 1-2 days. Imagine how quickly pollinators could spread infection around an orchard with bacterial populations like that! Protective sprays are critical.

Cooler temperatures in the forecast do not mean you are out of the woods entirely for fire blight blossom infection. The bacteria are capable of multiplying in temperatures ranging from 4⁰-32⁰C and will remain on the blossom until a wetting event occurs. As well, there is a possibility the day will be warmer than what was forecasted. In terms of fire blight infection risk, a few degrees can make a significant difference in disease pressure.

Figure 1. Secondary, or “rat tail” bloom can be very susceptible to fire blight.

Despite some of the earliest regions seeing petal fall, growers are still reporting bloom in new plantings and/or secondary, or “rat tail” blossoms (Fig 1). These late blooms are very susceptible to fire blight infection. If pinching off blossoms, this should be done in dry weather. Any moisture on the blossom can push bacteria into the floral base, resulting in infection. Since russeting is not a risk, applying copper on new trees following blossom removal will also help protect against spread of infection to any trauma made.

Many growers are applying Apogee 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.

Apple Scab and Powdery Mildew

As reported in last week’s Apple Update, scab lesions have been found in some high inoculum orchards. Growers and scouts are monitoring for developing lesions from the early May infection periods over the coming weeks.

Most areas of the province are in the final stretch of primary scab with ascospore maturation >80% complete. A significant wetting event will be required to release the remaining ascospores for the season so protectant fungicides are still needed, particularly with the forecasted showers. Coverage should continue for at least 2-3 weeks following the end of primary scab to be sure ascospore release is complete and no visible signs of scab are present.

Powdery mildew thrives in warm (low 20⁰C’s) and humid conditions most areas of the province are currently experiencing. Unlike scab, dry weather is ideal for mildew. Infected new growth can be found in various orchards. Fungicides with good mildew activity should continue until terminal bud set even when there is little risk from apple scab. See Table 3-14. Activity of Fungicides on Apple Diseases in the 2016-2017 Publication 360, Guide to Fruit Production for efficacy ratings for most registered powdery mildew products.

Summer Diseases

Frog-eye leaf spot (black rot) (Fig 2) continues to be found in orchards, particularly on leaves near where mummified fruit remains on the tree. Mummified fruit often harbors overwintering black rot. These spores can be rain-splashed onto leaves or wounds, including pruning cuts.

Figure 2. Frog-eye leaf spot appears as small brown lesion with light brown center.

With petal fall, summer disease management should come into play. Although symptoms of many summer diseases such as fly speck, sooty blotch, black rot and bitter rot don’t appear until later in the season, infection generally occurs starting around petal fall. For more information, see Apple Disease Management: Part 3 – Summer to Harvest and Table 3-14. Activity of Fungicides on Apple Diseases in the 2016-2017 Publication 360, Guide to Fruit Production for activity of registered fungicides on these diseases.


Petal fall insecticide sprays are beginning to go on, particularly in the earlier regions. Table 3-8. Activity of Petal Fall Insecticides Against Orchard Pests in the 2016-2017 Publication 360, Guide to Fruit Production 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.

Oriental fruit moth spray timing is on-going in the earlier regions of the province and quickly approaching elsewhere, but exact timing will depend on product choice. 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. This is a pest east of Toronto and into Georgian Bay. 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.

Orchards in earlier regions that are monitoring for San Jose scale observed the start of adult flight this week. Activity has not begun in other areas yet this year. Degree days for San Jose scale have begun to be accumulated to predict crawler emergence. Sticky tape is also currently being placed on infested branches to monitor crawler activity. Typically, 1st generation crawlers emerge mid- to late June in Ontario and are active for 4-6 weeks. Control is rather targeted as most petal fall insecticides will be too early for scale. One option is back-to-back sprays of Movento starting 10-14 days after petal fall, or early June. This is a slower-acting product and requires a couple of weeks to see control so timing is best before crawler activity begins. Another option is a back-to-back spray of Sivanto Prime, Closer or TwinGuard (pre-mix of Delegate and Closer) starting at crawler emergence.

Apple leafcurling midge egg counts are increasing and can be found near leaf folds or margins of young, developing leaves. Leafcurling caused by cream-colored larvae can be found in orchards with high midge pressure. This damage is expected to increase in the next week or two.

Spring feeding caterpillars are active in most orchards with terminal and cluster damage (Fig 3) as well as small caterpillars being observed. Some orchards have reached threshold and growers will be applying a petal fall insecticide with activity on these pests.

Mullein bug can be easily found in many orchards, exceeding threshold of 7-9 per 25 taps in some blocks. A targeted spray may be required particularly on susceptible varieties, such as Red Delicious, Spartan, Spy, Empire, Cortland, Gala, Jonagold and Golden Delicious. The critical time for damage from this pest is generally from dime to quarter sized fruit. However, mullein bug is also a very effective orchard predator for mites, aphids and leafcurling midge. Interestingly, mullein bug numbers were very high in some orchards last year yet fruit injury was minimal.

Tarnished plant bug, aphid and mite activity continues. Rosy apple aphid numbers are higher than normal in some orchards at this time of year. Plum curculio damage has also been reported in a number of areas, signaling the beginning of movement into orchards.

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