Tree Fruit Newsletter — May 29, 2025
Apple Thinning Update
McIntosh, Triumph, and Gala apples were showing signs of fruit set Wed. The king fruit were swelling and had sepals closing over the calyx. However, there were also unpollinated blossoms dropping. This early assessment indicates that fruit set may not be heavy this year, but enough to justify applying chemical thinners.
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- King fruit; Photo by Renae Moran, UMaine.
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- King fruit; Photo by Renae Moran, UMaine.
Chemical thinning will be necessary based on the good pollination weather during early king bloom and late in the bloom phase, but I anticipate applying carbaryl by itself or with a low rate of an additional thinner, depending on variety. Another option would be NAA or BA at a moderate rate without the carbaryl. As of May 28, king fruit diameter was 5 mm in Honeycrisp and McIntosh. Golden Delicious and Fuji were not yet at petal fall. Fruit diameter should be about 10 mm by next week. This is an ideal fruit size for applying thinners.
The cool temperatures this weekend will not favor good thinning if thinners are applied at this Friday. After Sunday, the warmer weather will be better for chemical thinning of apples and pears. The high temperatures need to be above 65 °F for thinners to work. High temperatures will be above 70 °F for at least a few days and may reach above 80 °F in some locations.
More options for chemicals are available for thinning than in the past, so check label rates.
| Variety | Rate per 100 gallons dilute equivalent |
|---|---|
| Cortland and Evercrisp | 1 to 2 pt carbaryl |
| Easy to thin varieties such as McIntosh, Gingergold, Pink Lady | 2 oz NAA* + 1 pt carbaryl or 32 oz. 6-BA** + 1 pt carbaryl |
| Hard to thin varieties such as Honeycrisp and Ambrosia |
3 oz NAA* + 1 pt carbaryl or 48 oz. 6-BA** + 1 pt carbaryl |
| Organic | ABA (Protone) 6.6 to 33.1 oz. Variety specific rates have not yet been tested. |
| When king fruits are past 14 mm diameter | Accede 23 fluid ounces for mild thinning 46 fluid ounces for aggressive thinning |
| *Fruitone-L, PoMaxa, Refine 3.5 WSG **Maxcel, Riteway (Rate for Excelis is different from Maxcel) |
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For Specific Apple Variety Thinning Recommendations, see the New England Tree Fruit Management Guide: UMass Amherst
Thinning Pears
48 to 96 oz Maxcel or RiteWay /100 gallons TRV dilute
or
4 to 8 oz NAA (Fruitone, Refine, PoMaxa) /100 gallons TRV dilute
or
1.6 to 8 ox per 100 gallons Amidthin TRV dilute
Carbaryl is not labeled for thinning pears because it can lead to fruit being misshapen.
Champlain Valley Fruit Set Webinar Today at 3:00 PM
Mike Basedow, CCE-ENYCHP, Northern NY
Join us for our virtual Champlain Valley fruit set webinar at 3:00 PM today, May 29, 2025.
Mike and Dr. Robinson will give their observations and recommendations for thinning during the 10-13mm window.
We will then hear from Dr. Andres Antolinez, Dr. Kerik Cox, and Dr. Anna Wallis for their IPM suggestions.
No registration required, simply click on the zoom link for Champlain Valley Fruit Set Webinar here.
Renae Moran
University of Maine Cooperative Extension: Tree Fruits
PO Box 179
Monmouth, ME 04259
(207) 933-2100
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