The Pennsylvania Tree Fruit Phenology Report was released on May 12, providing updates and recommendations for growers in Adams and Northumberland counties as well as central and northern regions of the state. The report highlights variability in crop development due to geography, topography, and varietal differences.
The information is important for orchard managers making thinning decisions at a critical point in the growing season. The report emphasizes that growers should review region-specific summaries before acting, especially if their orchards are outside the areas covered this week or located far from Adams and Northumberland counties.
In Adams County, temperatures are expected to fluctuate throughout the week. Most blocks with a crop will be targeted for thinning during the weekend, which represents the last chemical thinning window for most of the area as fruitlets reach between 15 to 20 millimeters. The report outlines specific product recommendations based on fruitlet size: “6-BA products such as MaxCel or 6BArrel have peak efficacy between 8 and 15 mm and can be tank-mixed with carbaryl and… spray oil.” For larger fruitlets approaching or exceeding 20 mm, Accede is advised either alone or in combination with other thinners. Metamitron (Brevis) remains an option up to this size but may be less effective under sunny conditions forecasted for the weekend.
Growers are cautioned about using ethephon when high temperatures follow application: “If using ethephon, rates should be reduced to account for the sustained heat that will follow… ethephon remains one of the more reliable thinning options once fruitlets have passed 20 mm.” Special attention is given to blocks where two distinctly sized sets of fruitlets exist due to frost damage; hand thinning may still be necessary even after chemical applications.
In central Pennsylvania’s Northumberland County region, primary clusters develop first on older wood while secondary clusters bloom later on younger wood. Thinning programs prioritize retaining primary kings because they produce better-sized fruits: “fruit retained from primary positions will almost always size better… than fruit carried from secondary positions.” Recommendations advise waiting until king fruitlets on one-year-old wood reach at least 12 mm before applying chemicals unless there is sufficient spur set surviving frost damage.
Block-by-block assessment is recommended due to uneven frost impact across regions. Growers are also encouraged to check internal quality by making longitudinal cuts through developing fruitlets since freeze injury can cause hidden decay leading to premature drop later in spring.
Current growing degree days were reported for Adams (Biglerville), Berks (Manheim), and Centre (Rock Springs) counties using data from NEWA weather stations between January and May.
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