Discoloration of developing fruitlets indicating they will be dropping. Photo: Rob Crassweller, Penn State
In most areas of the state, the Memorial Day weekend weather was hot and humid with scattered downpours. Unlike last year, when we rarely had any warm temperatures that were in the mid 70’s, this year we have had a string of temperatures in the 70’s for the last ten days. This is the first year I can remember in quite a few years when the weather conditions were conducive to thinning. By all accounts, you should see fruit dropping in response to applications made in the last 10 to 14 days. At the beginning of the thinning window we recommended centering initial applications on the time when the fruit is in the 10–12 mm diameter range and then looking for the optimal weather that we experienced.
If you do not see fruit drop or you think you still have too many fruit on the trees, there is still the late window for thinning when fruit are above 16 mm. Given all the warm weather we have been experiencing I suspect that many apple varieties blew quickly through the smaller diameter, and you may need additional thinning. At this stage, the use of an ethephon product can be utilized according to recommended label rates.
Review your application procedures if your thinner applications do not seem to work. Remember that the best response to thinning applications is when they are applied in at least 100 gallons of water per acre. This gallonage is needed to ensure adequate coverage of the foliage and sufficient time to allow the leaves to absorb the materials. Also, consider the vigor and density of the foliage; old poorly pruned canopies are not conducive to getting adequate coverage and subsequent response.
Remember to keep detailed records by variety and block as to how the trees responded to thinner applications. Make multiple copies of the Plant Growth Regulator Evaluation Checklist on page 74 of the 2018-19 Commercial Tree Fruit Production Guide.
Apple Carbohydrate Thinning Models for Pennsylvania Counties
How to Use the Carbohydrate Model
Where to Find Penn State's Recommendations on Chemical Thinning Materials