2025 Pennsylvania Apple and Peach Phenology Report - May 5

Phenology This Week

Areas reporting in this week have concluded data collection from peach orchards. Peach trees are now at the shuck split stage or beyond. As discussed previously, the status of the peach crop depends heavily on variety, location, and measures taken to preserve the crop. To learn more about the status of peach crops in your area, consider attending one of the remaining Spring Orchard Meetings occurring this week and next. A review of how to estimate crop potential and discussions about thinning, disease potential, and pest monitoring and prevention will be held in person at these meetings.

2025 Spring Orchard Meeting Schedule

Visit Extension's website to register for these meetings in advance, or register on-site at any of these locations. Three (3) PDA pesticide license renewal credits in categories 02, 18, and PC will be available at each of these meetings.

Tuesday, May 6, 6:00 - 8:30 p.m.

Berks County
Brecknock Orchards
390 Orchard Road
Mohnton

Wednesday, May 7, 6:00 - 8:30 p.m.

Central Susquehanna / Northeastern Region
Greens Fruit Farm
574 Sleepy Hollow Road
Elysburg

Thursday, May 8, 6:00 - 8:30 p.m.

Appalachian Region / Centre County
Harner Farm
2191 West Whitehall Road
State College

Wednesday, May 14, 6:00 - 8:30 p.m.

Western Region
Rivendale Farms
1357 Valleyview Road
Bulger

Thursday, May 15, 10:00 - 11:30 a.m.

Erie / Northwestern Region
Burch Farms
9210 Sidehill Road
Northeast

Note that all meetings begin at 6:00 p.m. except for the Erie location, which begins at 10:00 a.m. Signage will direct visitors to where the programs begin. Extension greatly appreciates the willingness of the hosts to open their farms and markets to the public for these events.

Phenology Data

Growing Degree Day Accumulation

Data taken from NEWA using the Degree Day Type of "Base 43 ℉" and "Base 50 ℉" between the dates of 1/1/2025 and 5/5/2025. The data listed for Franklin was acquired from the station at Kuhn Orchards in Cashtown, closer to most Franklin County growers than FREC.

Apple Fruitlet Sizes

Southcentral Region (Adams and Franklin Counties)

At the Fruit Research and Extension Center (FREC) in Biglerville, fruitlets are growing quickly with Honeycrisp at 15 mm, Golden Delicious at 14 mm, Fuji and Pink Lady at 13 mm, and Red Delicious around 10–11 mm.

This week is a critical and perhaps one of the final opportunities for effective chemical thinning, as we are in the 200–250 GDD (base 4 C) post-bloom window — ideal for thinning. Forecasted temperatures in the 60s ℉ and low 70s ℉ provide less than ideal conditions for thinning. NAA is preferred more than 6-BA in sub-70 ℉ conditions, although 6-BA will still have a positive impact on fruit size and is recommended for small-fruited cultivars like Gala.

Recommended Action

NAA (Fruitone® / Refine® / PoMaxa®) + Carbaryl (Sevin®) will be the workhorse thinner this week. 6-BA thinners such as MaxCel® / Exilis® / RiteWay® can still be used in combination with Carbaryl (Sevin®) in small-fruited varieties such as Gala with medium crop loads - they offer a modest size boost even if thinning efficacy is reduced under cooler temperatures.

Rain caution: With rain forecasted Thursday, Carbaryl's rewetting capability may enhance fruit removal. Be prepared for a slightly stronger thinning response. To reduce rewetting induced Carbaryl thinning, 1 pt/acre can be used instead of 1 qt/acre that is commonly used by growers in Pennsylvania.

This is likely your last effective thinning window. Missing this may limit thinning options to rescue sprays next week, which tend to be less predictable.

Cloudy days and warm temperatures increase thinning response by reducing carbohydrate availability and enhancing hormone uptake, respectively. Conversely, cooler days reduce thinning efficacy, as tree metabolism slows and the effect of hormone thinners is diminished. Keep this in mind when timing sprays - conditions this week may shift the impact depending on the day and weather.

Rescue Thinning Options – 15-20 mm Size

Once your fruit grows past 15 mm in size, they become harder to thin, NAA + Sevin® can still be used. If using MaxCel®+ Sevin®, please add oil to the mix as well, Ethrel® is also a good option for fruits, even if past 20 mm. ACC (Accede®) is also a good option for rescue thinning at the 15-20 mm stage. It is recommended that ACC be combined with a non-ionic surfactant (LI 700® or Regulaid®), and tank mixed with MaxCel® for effective thinning at the 15-20 mm stage.

Sprayer and Nozzle Settings

A common challenge in chemical thinning is that the bottom half of the tree often becomes over-thinned, while the top remains under-thinned. This imbalance arises because the upper canopy receives more sunlight, resulting in higher carbohydrate levels that help fruitlets resist thinning, whereas the shaded lower canopy is more prone to fruit drop. Traditionally, sprayers are set up to deliver about two-thirds of the spray volume to the top and one-third to the bottom. However, this conventional approach often continues to overthin the lower canopy and insufficiently thin the top. In multiple trials, directing a greater proportion of the spray—80% to 100%—to the upper canopy resulted in more balanced thinning across the tree. Based on these findings from Cornell University, it is recommended to maintain the traditional 2/3 (top) to 1/3 (bottom) spray pattern during bloom and petal fall stages. However, for 12 mm fruitlet sprays, adjusting the pattern to 80% top and 20% bottom is advised, and by the time fruitlets reach 18 mm, directing 100% of the spray to the top has shown the most effective results.

Berks, Centre, and Lawrence Counties (Southeast, Central, and Western PA)

Southeastern, Central, and Western Regions (Berks, Centre, and Lawrence Counties)

Fruit sizes across these regions are still in the early thinning window, with Gala, Honeycrisp, Fuji, and Pink Lady® all in the 4–6 mm range.

Recommended Action

The thinning window is open now through next week.

For fruitlets 5–6 mm, apply NAA (Fruitone® / Refine® / PoMaxa®) while daytime highs remain below 70 ℉. When highs exceed 70 ℉ (Wednesday onward), consider switching to 6-BA (MaxCel® / Exilis® / RiteWay®) for small-fruited varieties. Carbaryl (Sevin®) is a mild thinner at petal fall and provides some insect control at higher rates (2 pt / 100 gal), with the added effect of re-wetting that can enhance thinning. 6-BA is also mild on its own but helps increase fruit size in small-fruited varieties. When combined with Carbaryl, 6-BA becomes more effective - especially in warm weather - but less reliable in cool conditions. Use high rates of 6-BA (4–8 pt/acre) and limit Carbaryl to 1 pt / 100 gal to avoid excessive thinning. NAA (Fruitone® / Refine®) combined with Carbaryl is effective, improves return bloom in Honeycrisp, but should be avoided in Fuji and Delicious due to pygmy fruit risk. Avoid high NAA rates (above 3 oz / 100 gal) and surfactants, as both can increase uptake and stress the tree. NAD is a safe thinner at petal fall, even at high rates, and does not negatively affect fruit size or photosynthesis, while also aiding return bloom in Honeycrisp.

If there is worry about using Carbaryl when there are still bees in the orchard, then use non-carbaryl options before petal fall. The combination of BA (6-BA) with NAA (3 oz / 100 gal) provides thinning results similar to BA + Sevin®. Adjust the BA rate (4–8 pt/acre) based on the variety. Similarly, combining BA with NAD (8 oz / 100 gal) thins in a manner comparable to BA + Sevin®, with the BA rate again varying between 4–8 pt/acre depending on the variety. Both combinations are effective for thinning, and the rate of BA should be adjusted based on the specific cultivar being treated. Carbaryl is still recommended as a synergist and especially effective in this week's scattered rain. Be mindful of its rewetting potential, which could lead to stronger thinning outcomes.

In Centre and Lawrence counties, where fruitlets are closer to 4–5.5 mm, you have more flexibility. Consider waiting until next week when fruit size increases and we expect another stretch of ideal thinning weather, especially in sites with low to medium crop loads.