Pennsylvania Tree Fruit Phenology Report: May 12, 2026

Due to the variability observed throughout the state due to geographical location, site-specific topography, and varietal differences, consider reviewing the summaries for each of the regions covered in this report before making decisions. This is especially true for orchards in regions not covered this week, and for orchards a significant distance from Adams and Northumberland counties.

Adams County

Temperatures in southern Pennsylvania this week are expected to be variable, with highs in the low-to-mid 60s Tuesday through Wednesday before dropping to the low 50s Thursday with a chance of rain. The week closes on a warmer note, with Friday rebounding to the mid-60s and Saturday forecast to reach the upper 70s to near 80°F under mostly dry conditions.

Most blocks with a crop are going to be targeted for thinning this weekend, as it represents the last chemical thinning window for most of Adams County with fruitlets reaching 15 to 20 mm and above as the weekend approaches. While the carbohydrate thinning model does not currently indicate a deficit, temperatures this weekend will be favorable for efficacious thinning.

Apple fruitlets across Adams County are currently ranging from 7 to 15 mm, depending on which fruit positions survived earlier in the season. Blocks where kings or first laterals came through will trend toward the larger end of that range, while blocks where third or fourth laterals took over as the primary cluster will be slightly smaller. In any case, this weekend represents a prime opportunity.

Thinning product selection should be guided by fruitlet size at the time of application. 6-BA products such as MaxCel® or 6BArrel® have peak efficacy between 8 and 15 mm and can be tank-mixed with carbaryl and, if additional thinning pressure is needed, spray oil.

As fruitlets approach 15 to 16 mm, 6-BA with carbaryl and oil should be sufficient; however, for fruitlets closer to 20 mm, tank-mixing with Accede® would be advisable to achieve adequate thinning. With the forecasted high temperatures, Accede® alone could also drive significant fruit drop and is worth considering as a standalone option for fruitlets in the 15 to 20 mm range.

Metamitron (Brevis™) is labeled up to 20 mm and remains an option, though efficacy may be reduced under the bright, sunny conditions forecast for the weekend and into early next week. It is nonetheless worth trialing if it has not been evaluated in your operation previously and serves as a useful addition to the late-season thinning toolbox.

Ethephon is another option, particularly for blocks where fruitlets are already at or above 20 mm heading into the weekend, but growers should exercise significant caution. With high temperatures forecast not just on Saturday but into Sunday and Monday as well, the risk of over-thinning is real. If using ethephon, rates should be reduced to account for the sustained heat that will follow the application. That said, ethephon remains one of the more reliable thinning options once fruitlets have passed 20 mm.

Dealing with Two Distinctly Sized Fruitlets

In blocks where spur flowers were largely killed, the bloom on one-year-old wood becomes the primary crop and should be preserved. To protect this set, thinning applications should be delayed until the king fruitlets on those one-year-old wood clusters reach at least 12 mm, by which point spur fruitlets will already be approaching 16 mm.

At 12 mm, king fruitlets on one-year-old wood clusters are well set and less susceptible to chemical thinning, while the lateral fruitlets within those same clusters remain responsive and can still be selectively removed. A reduced thinner rate is advisable at this stage to avoid over-thinning the smaller one-year-old wood set.

Fruitlets that set with the aid of Promalin® or Perlan® can be just as difficult to thin as undamaged fruit in a normal year, and hand thinning in mid-to-late May and June will likely be required to finish the job regardless of the chemical program applied.

Growers should also be aware that fruitlets exhibiting frost ring, russet, or misshapen development do not respond well to chemical thinners and can only be removed by hand. These should be accounted for separately when assessing crop load and planning hand thinning crews.

Central Pennsylvania (Northumberland County)

In apple trees, a primary cluster develops on older spur wood and is the first to bloom and set fruit each season, making it the most desirable from a fruit quality and size standpoint. A secondary cluster develops on younger, one-year-old shoot wood and blooms slightly later, resulting in fruitlets that are smaller and later to develop at the time of thinning (Figure 1).

Because primary clusters have a head start in cell division, fruit retained from primary positions will almost always size better and pack out at higher grades than fruit carried from secondary positions, which is why thinning programs prioritize retaining primary kings first and removing secondary fruitlets where crop load allows.

Recommendations for Central and Northern Pennsylvania

For growers in central and northern Pennsylvania who are relying primarily on bloom from one-year-old wood for their crop this season, this weekend's thinning window may not be the right time to spray. Based on the sizes observed in Table 1, applying thinners too early risks removing the very fruit you are depending on.

The recommended approach is to wait until king fruitlets on those one-year-old wood clusters reach at least 12 mm before making any chemical applications. At that size, kings are well enough set to withstand thinning pressure while the surrounding laterals remain responsive and can still be selectively removed.

However, in blocks where the primary spur set survived adequately and the crop load is close to full, this weekend presents a good opportunity to apply thinners to manage lateral fruitlets and reduce overall density. Block-by-block assessment is essential this season, given the variability in frost damage across the region.

Remember to factor in the fruit's quality before making decisions, even if it appears healthy on the outside. A longitudinal cut through the center of the developing fruitlet may reveal decay in the cavity. Longitudinal cuts are preferred over lateral cuts because they better reveal the extent of the damage (Figure 2).

For growers who want to take advantage of the favorable temperatures this weekend but are not yet ready for a full thinning application, a pint of carbaryl per acre can be applied to loosen clusters and prime trees for a more targeted thinning program later next week, once secondary kings have reached 12 mm or larger.

Reference Information

Growing Degree Day Accumulation

Current growing degree days for Adams, Berks, and Centre counties are provided in Table 2.