Apple Thinning and PGR Update

A full apple update will be coming soon. This update will be on thinning and growth control with apogee. 

Crop Stages

  • Essex County: fruitlets are greater than 6 mm
  • Kent and Elgin County: along the lake, apples are at petal fall to early fruit set
  • Norfolk County: fruitlets are at 7-10 mm
  • Niagara Region and GTA: fruitlets range in size between 6 and 15 mm
  • Grey County: apples are at king petal fall to full petal fall
  • In areas East of Toronto, along Lake Ontario up until the Ottawa Valley: apples are at full bloom to petal fall


Fruitlet thinning meetings have been occurring in different regions in Ontario. A common theme to two meetings that I attended is to apply thinners early to get the most benefit for size or to manage biennial bearing of Honeycrisp. There were a few growers who have blossomed thinned some blocks with a caustic product such as lime sulphur or ATS to get earlier thinning to achieve even more benefit. For those fruit that you want larger size on, like Gala, early applications of 6-BA (Maxcel or Cilis Plus) starting at petal fall can enhance size during this period of cell division. NAA is recommended for Honeycrisp for early thinning, the timing is as early as petal fall on the Fruitone label.  NAA has been used as a blossom thinner as a less harsh alternative than ATS or Lime Sulpur in New York and is recommended by Dr. Poliana Francescatto from Cornell. For those fruit that you want larger size on, like Gala, early applications of 6-BA starting at petal fall can enhance size during this period of cell division.

Temperature is a very important factor for thinning, getting enough heat in the last week hasn’t been a problem for thinning, though questions have arose on how hot is too hot. With NAA, do not apply if temperatures are above 26.7˚C. For 6-BA (Maxcel or Cilis Plus) and carbaryl, as temperatures rise, these products will become “hotter” and thin more. Above 25˚C you will want to want to reduce the product rate. This week’s forecast has some cooler days below 20˚C, remember that it is better to wait for the right temperature conditions to apply fruitlet thinners rather than waiting for the perfect fruitlet size. For NAA and carbaryl, optimal temperatures are between 21 and 24˚C. NAA should not be applied below 16 ˚C. MaxCel or Cilis Plus should be applied in temperatures above 20 ˚C.  For more information on rates and optimal conditions see the OMAFRA Thinning of Tree Fruit, information on this webpage was previously included in Publication 360: Guide to Fruit Production.

There have been recent label changes to Sevin XLR (carbaryl) as of February 2017. The allowable number of applications are 2. See Table 1 for maximum application rates and restricted entry intervals (REI) for Sevin XLR.

Table 1:  Maximum application rates and restricted entry intervals for Sevin XLR

Orchard System
High Density, Trellised
Tradition Medium – Low Density, Not Trellised
Maximum Application Rate
1.5 kg a.i./ ha
1.0 kg a.i./ha
Maximum Product Rate
3.22 L/ ha
2.15 L/ha
REI for hand thinning
14 days
10 days
REI for hand pruning, scouting, pinching, tying and training
4 days
0.5 days

 Growth Control

First prohexadione-calcium (Apogee or Kudos) applications have been applied in Southwestern Ontario for growth control and some growers may be getting ready to apply a second spray, which should be made 14 -21 days after the first application.  Kudos is a new registered product in Canada with the active ingredient prohexadione-calcium (27.5%). The rates are similar or the same as Apogee.  For more information on applying prohexadione-calcium for growth control see the OMAFRA Plant Growth Regulators page.

If you have missed applying Apogee when shoots are 2.5 to 5 cm long and you want the fire blight suppression benefits, BASF, the registrant of Apogee, has sent out notification that Apogee can provide suppression when applied later. Their recommendation for applying Apogee without significantly impacting growth is to apply when new shoots have reached more than 15-20 cm of new growth.

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