Penn State Extension released the Pennsylvania Tree Fruit Phenology Report on May 5, providing updates on apple and pear development across several counties in the state. The report outlines how recent weather patterns are affecting thinning strategies and fruit set following a period of cool temperatures and a subsequent warm spell.
The timing of thinning applications is important for growers, especially as temperatures shift from highs in the low 80s to cooler conditions later in the week. The report notes that products such as MaxCel, Exilis, or other six-benzyladenine (6-BA) formulations lose effectiveness below 70°F. “That warmth makes for a good thinning window as fruitlets are entering the 8-10 mm sizing range,” according to Penn State Extension.
Growers with strong primary sets are advised to thin both laterals in spur clusters and lateral blooms on one-year-old wood. For modest sets, lighter thinning using carbaryl at one quart per acre combined with reduced rates of other thinners is recommended. In areas where primary sets were lost due to freeze events, growers should wait until lateral king bloom reaches at least 12 mm before making decisions about thinning.
The Adams County section reports that apple fruit diameters were collected at the Fruit Research and Extension Center (FREC) in Biglerville on May 4. Among monitored varieties, Royal Red Honeycrisp showed the fastest king fruitlet growth rate at 0.52 mm per day. Chemical thinning was expected to occur around May 5 while temperatures remained favorable early in the morning.
The report also discusses cold damage observed after an April freeze event. Some apples appeared healthy externally but showed internal cavities when cut open—a phenomenon previously documented after spring frosts—which may result in delayed or incomplete seed death and potential fruit drop over time.
In Berks County, remaining crop loads ranged from zero to ten percent of normal levels depending on location and elevation; some higher-elevation blocks had up to thirty percent crop load but this was not typical statewide. Centre County experienced active abscission of freeze-damaged fruits from primary sets while secondary blooms progressed through petal fall stages.
According to the official website, Penn State Extension delivers science-based education throughout Pennsylvania’s counties via local offices and digital tools, focusing on agriculture, nutrition, environmental stewardship, food safety, resource management, workshops and consultations while collaborating with government agencies and organizations.
