By Blake Jackson
Chemical thinning remains one of the most critical yet unpredictable tasks in apple production. Growers often struggle because thinning materials can vary widely in effectiveness, and visible signs of fruit drop may not appear until weeks after an application. By that stage, it is often too late to determine whether another spray is necessary.
For many years, growers have relied on the Fruit Growth Rate (FGR) model to help predict thinning results earlier in the season. The model evaluates the growth rate of individual fruitlets and compares them with the fastest-growing fruitlets on the tree.
Fruitlets growing at 50% or less of the top 10% fastest growers are expected to drop naturally. This approach can provide predictions within five to six days after a thinner application, giving producers enough time to decide on follow-up treatments. The MaluSim platform has helped simplify the use of the FGR model.
Despite its usefulness, the FGR system requires repeated measurements of the same marked fruitlets every few days, making it labor-intensive and limiting its use in commercial orchards.
To address this challenge, researchers Laura Hillmann and Dr. Todd Einhorn at Michigan State University developed the Fruitlet Size Distribution (FSD) model.
Instead of tracking growth over time, the FSD method relies on the size and weight distribution of fruitlets collected during a single sampling event. Fruitlets measuring 50% or less of the size of the largest 10% are predicted to fall off naturally.
The process begins at bloom, when growers record cluster counts, flower numbers, and target fruit levels while tagging representative spurs for later sampling.
After thinning sprays are applied, growers collect and weigh fruitlets from tagged spurs at three, six, and nine days after treatment. Data is then entered into the FS-Predict Excel spreadsheet, which generates thinning predictions automatically.
Research has shown the FSD model delivers a higher prediction success rate than the FGR system while requiring less time and fewer orchard visits, although predictions may arrive slightly later.
Photo Credit: istock-rixipix
Categories: Wisconsin, Crops, Fruits and Vegetables, Education