Apple Fruit Set and Thinning: It's Complicated!
Frost and freeze events caused uncertainty about the initial fruit set in apples across Pennsylvania in 2020. Early cold injury to the primary class of flowering spurs enhanced the later flowering of weaker spurs and lateral blossom clusters on 1-year-old wood. This resulted in a wide range of bloom stages from pink to petal fall all on the same tree, with an atypical wide range of sizes of developing fruit as the outcome.
Uncertainty about the initial fruit set combined with unfavorable cold temperatures resulted in a later start to chemical thinning. Fruit development was slowed by the cool weather making it still more difficult to assess fruit set and the response to chemical thinners.
As May comes to an end, the weather pattern seems to have shifted back to more normal warm temperatures and setting fruits are responding to the warmth with rapid growth. Given that growing fruit are setting fruit, growers may find that some orchards now need a follow-up chemical thinner spray to remove smaller undesirable fruit.
Once fruit have grown beyond 17 mm in diameter, the effective tools left in the toolbox are carbaryl and ethephon (see pages 73 and 76 of the Penn State Tree Fruit Production Guide). At this stage, the shoot leaves begin contributing carbohydrates to the developing crop, so carbon stress is less of a factor for thinning response. However, the efficacy of both late thinners is temperature-dependent, so the forecasted high temperatures for the day of application and the following three-to-four days should be the primary consideration for setting the timing and dosage. Daytime highs between 75-85°F are ideal. Use caution and reduce rates if temperatures are forecast to exceed this range.
