Tree Fruit Newsletter — May 27, 2026
Thinning Update
Apple and pear trees are close to petal fall today at Highmoor Farm. Most chemical thinners become effective at this time. Here is a link to the Guide’s section on thinning: Specific Apple Variety Thinning Recommendations : New England Tree Fruit Management Guide : UMass Amherst
- BA-type thinners (Maxcel, Rite-Way, Exilis) are not effective at temperatures below 70 °F. With predicted high temps in the 60’s, this type of thinner is not likely to be effective until high temps return to the 70’s next week. BA thinner is typically combined with NAA or carbaryl for an effective response.
- NAA thinners (Fruitone, PoMaxa, Refine, Amid-thin) are temperature dependent and will thin less when applied during a cool spell, but too much when high temps are above 85 °F which is not in the forecast. For better thinning if applied in the next few days, combine with carbaryl and adjust the rate according to variety (2 to 6 oz. with 1 pint carbaryl in 100 gallons dilute equivalent); 3 oz. for Honeycrisp, 6 oz for Golden Delicious.
- Carbaryl is toxic to bees, so wait till trees are actually at or past petal fall (90% petal drop). Avoid applying to pears because carbaryl is not labeled for pears.
- Protone, an ABA-based thinner, becomes effective at thinning after petal fall. OMRI listed.
- Accede can be used from bloom to fruit set on apples. It remains effective at thinning for a longer time than other thinners. It can also thin stone fruit when applied at pink to petal fall. Not labeled for pear.
The long-range forecast indicates cool temperatures starting tomorrow and lasting until June 3. If thinners are applied now, the NEWA thinning model suggests increasing the rate of chemical for NAA, BA, Protone or Accede). Carbaryl is not rate-dependent so for more aggressive thinning, combine it with NAA or BA. Experience with cabaryl combined with Protone or Accede is limited. Most of us have trees at petal fall, so waiting till June 3 is possible.
Peaches and plums can be hand thinned anytime. I am finding that some peach varieties do not need much this year: Desiree, Summer Serenade, PF15A, Autumnstar. Winter bud kill took care of most of it. Other varieties have excessive fruit set (Blazingstar, Garnet Beauty, Glowingstar, Messina) and will need most of the fruit removed in order to get adequate fruit size.
Renae Moran
University of Maine Cooperative Extension: Tree Fruits
PO Box 179
Monmouth, ME 04259
(207) 933-2100
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