Pennsylvania Tree Fruit Phenology Report: May 5, 2026

Thinning Considerations for Apple

After a cool stretch last week, temperatures across southeast and central Pennsylvania are off to a warm start this week, with Biglerville and the Adams County area reaching into the low 80s today. That warmth makes for a good thinning window as fruitlets are entering the 8-10 mm sizing range.

However, a notable cooldown is on the way, with Wednesday bringing temperatures down to the upper 60s along with the main rain event of the week. Thursday stays cool in the upper 50s before a modest recovery to the low 60s by Friday. That cooler stretch through the back half of the week is worth keeping in mind as you plan your thinning applications, particularly for products like MaxCel®/6BArrel®/Exilis® (6-BA products) which tend to lose effectiveness below 70°F.

When it comes to deciding how to approach thinning, the starting point is understanding what kind of primary set you are working with.

If you have a strong enough primary set on spur clusters to support a full or near-full crop, this is a good time to remove both the laterals in the spur cluster, as well as the lateral bloom on 1-year-old wood, just as you would in a typical thinning year. Yesterday and today represented good thinning windows for Adams County given where fruitlet sizing currently stands.

If your primary set is more modest, sitting somewhere in the 30-50% range, thinning is still worth doing but with a lighter touch. Carbaryl at 1 quart per acre can help loosen up the clusters, and you can pair that with reduced rates of 6-BA products like MaxCel®, Exilis®, or 6BArrel®, or NAA products like PoMaxa™ or Refine®, to manage the set without overcorrecting. Looking at the carbohydrate thinning model, we are not currently in a significant deficit period, but with the warm temperatures we are seeing now, this could be a good opportunity to begin thinning fruitlets or at least get the first round of thinning sprays out.

If your primary set was completely wiped out by the freeze, the approach is different. In that case, hold off on disturbing the clusters on 1-year-old wood that you would normally thin away. Depending on where you are in the state, those laterals could be anywhere from 5-6 mm in Adams County all the way to full bloom in Central PA (Figure 1). The guidance here is to wait until the lateral king bloom reaches at least 12 mm before making any thinning decisions, which given current conditions will likely push into next week. Once that threshold is reached, you can evaluate the set and thin accordingly.

The most complex situation is when you have a very low primary set but still want to preserve it, while also relying on lateral bloom to piece together a crop. The challenge is that the two sets can easily end up 7 to 10 days apart in development, which makes a single thinning pass difficult to time well for both.

The recommended approach in this scenario is to target thinning when the lateral king bloom is at 12 mm or higher, focusing on removing laterals from the lateral set. Hand thinning will be required as well, since the primary set will likely have grown well beyond 15-18 mm by that point and will be harder to manage chemically, without damaging the lateral set. Additionally, any freeze-ringed fruit, especially on trees that received a Promalin® application, are likely to set and will probably need to be hand thinned off individually.

One broader consideration worth keeping in mind is that lateral blooms, as a general rule, do not size up the way primary bloom does. The fruit quality and size potential is typically lower, so the economics of carrying a heavy lateral set need to be weighed carefully.

Adams County

Apples

Apple fruit diameters were collected on May 4^th at the Fruit Research and Extension Center (FREC) in Biglerville (Table 1). The selection of the king and laterals was random along the rows just avoiding the yellow-pedicel fruitlets that are about to fall.

Among the apple varieties monitored over time, the ones presenting king fruitlets with the fastest rate of growth are Royal Red Honeycrisp® (0.52 mm/day), followed by Premier Honeycrisp® and Buckey Gala® (respectively 0.43 and 0.41 mm/day), while Pink Lady® kings are not growing and about to drop (-0.05 mm/day). Buckeye Gala® and RubyRush™ lateral fruitlets presented the best growth with 0.59 and 0.58 mm/day respectively.

At FREC, chemical thinning day will most likely occur on May 5^th. While the NEWA carbohydrate model does not forecast a deficit, temperatures will be favorable in the first half of the morning, while on Wednesday rain is forecast.

Some growers reported after the cold snap on April 21^st, some fruitlets looked healthy from the outside. But when cut longitudinally, they presented brown tissue with cavities surrounding the seeds, while for some reason the seeds still looked healthy (Figure 2).

This scenario has been previously reported in the literature (Tagliasacchi et al. 2006) as a consequence of a cold event, where the locular tissue around the seeds shows necrosis and cavitations with collapsed or translucent areas, while the seeds look healthy.

Then the question is, will that fruit grow and survive? The cold event clearly damaged the maternal tissue that surrounds the seeds. The ice formation most likely killed the parenchyma and collapsed the vascular continuity in the ovary walls leading to a blockage of reserves usually transported into the seeds. The embryo did not die right away, but it is expected to slowly "starve to death" since the cellular system that normally works to feed it, like an umbilical cord, is disrupted.

Normally, the quick death of seeds leads to rapid fruit drop within 7-14 days (post-bloom drop). The apparent "healthy look" of the seeds in this situation—of damaged ovary tissues with cavities—is due to the hormones inside that are still functioning until the maternal vascular supply from outside the embryo terminates. This will translate into a delayed response for the final destiny of those fruitlets since there is still some partially viable portions of the vascular system. But it's not certain whether it will be enough to guarantee seed survival and, therefore, fruitlet retention. Possible outcomes in this scenario can be:

1. The partial vascular system allows the survival of the fruitlets, and they can turn out as small apples with internal issues (cavities).
2. The vasculature feeding the seeds is totally dead, that fruitlet will slowly drop or hang as a small and/or deformed fruit until harvest.

This type of cold damage inside the fruit can be linked to the specific cold event we experienced. It was previously mentioned that the temperature below 30˚F persisted for several hours: 5-8, depending on the region, orchards, and blocks. However, the dew point plays a role.

Normally, with high dew point, the flowers are better protected from the cold through condensation, while in the case of low dew point, the plant tissues freeze much quicker and reach temperatures below the air temperature. Looking at the microclimate of one of the FREC blocks that suffered severe cold damage during the event of April 21^st, the fruit experienced not the cold air temperature per se (27˚F), but the dew point of 18˚F. That results in rapid plant tissue cooling, no buffering from condensation (insufficient humidity), hard radiation frost, and therefore very high cold damage (Figure 3).

Citations

Tagliasacchi, Maria, A., Giraldi, E., Ruberti, F., Costantino, R., & Forino, L. M. C. (2006). Cytohistological and cytochemical features of the seeds of Malus domestica Borkh exposed to spring frosts. Caryologia, 59(1), 75–87.

Pears

The average caliper maximum fruit diameter in 'Harrow Sweet' at the Fruit Research and Extension Center (FREC) was 13.4 mm (range 11.00 to 15.5 mm), as measured on the morning of Monday, May 4^th.

Berks County

In the sampled locations, the remaining crop is between 0% and 10% of normal on most blocks and cultivars. Some blocks at higher elevations appear to be closer to 20-30% percent, but it is highly location and cultivar-dependent. The higher elevation crop load is not indicative of the crop load for the majority of blocks. Similarly, there is a smattering of secondary bloom—also highly location and cultivar-dependent—but observations suggest it may not be sufficient to be productive.

Centre County

Freeze-damaged fruits from the primary spur or terminal set are now actively abscising, whereas last week they remained attached; the king and lateral fruits from this initial set are non-viable and show no evidence of continued growth, confirming freeze injury. The second wave of blooms (set) is progressing, with stages ranging from pink to petal fall (Figure 4). Overall, most secondary blooms are at petal fall, with a smaller proportion of later-emerging (tertiary) blooms still at pink.

Reference Information

Growing Degree Day Accumulation

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