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Chemical thinning, the most common and cost-effective thinning method, is conducted during early apple fruit development over a 3- to 4-week period using multiple applications of plant growth regulators. It is critical to provide apple growers with tools to assess the efficacy of chemical thinners quickly and accurately because visible responses are not apparent for up to 2 weeks after application. The objective of this study was to build a model to predict apple fruitlet abscission following a chemical thinner application with in situ reflectance data obtained with a portable visible and near infrared (Vis/NIR) spectrophotometer. Developed models were compared with the currently available fruitlet growth model (FGM). ‘Honeycrisp’ fruitlet diameter and reflectance were measured on dates around a chemical thinner application across a 2-year period. After June drop, measured fruitlets were determined to have either persisted or abscised. Random forest, partial least squares regression, and XGBoost classification models were used to predict fruitlet abscission from reflectance data. Each classification model was developed with 2021, 2022, and combined 2021 + 2022 data. For each dataset, 5-fold cross validation was used to assess three model performance metrics: 1) overall accuracy, 2) recall, and 3) specificity. Datasets tested were either unbalanced, majority class down-sampled, or minority class up-sampled with synthetic minority oversampling technique. In both years, the FGM reliably estimated chemical thinner efficacy 9 days after application. Before this time point, the FGM had low prediction accuracy of the minority class in both years—persisting fruitlets in 2021 and abscising fruitlets in 2022. For reflectance spectroscopy, the developed random forest models that were balanced with synthetic minority over-sampling technique were found to be the best combination in predicting chemical thinner efficacy. The combined 2021 + 2022 dataset overall model accuracy ranged from 84% the day before to 93% at 9 days after thinner application. These results show that Vis/NIR is a promising tool to predict chemical thinner efficacy. This technology had high prediction accuracies over a range of fruitlet abscission potential and two growing seasons. Further development and testing of the model over cultivars, chemical thinner timings, and growing regions would facilitate commercialization of the technology.

Apple (Malus ×domestica L. Borkh.) growers need tools to predict the efficacy of chemical thinners that are applied to induce fruitlet abscission to aid in crop load management decisions. Recently, reflectance spectroscopy-based models to predict fruitlet abscission rates were developed. Using spectroscopy, persisting fruitlets had lower reflectance in the red-light (∼600 nm) and near infrared (∼950 nm) regions than abscising fruitlets. The goal of this study was to better understand how reflectance models distinguished between fruitlets that ultimately persisted or abscised. Individual models for the difference and ratio of each combination of wavelengths were developed to identify key wavelengths for abscission prediction from reflectance models. Accuracy for wavelength difference and ratio models was improved for all model prediction dates when reflectance (R) from R_640–675 was subtracted from or divided by R_675–696. This spectra region indicates differences in chlorophyll content between persisting and abscising fruitlets. Calculation of the chlorophyll concentration index (R_522–579:R_640–700) from nondestructively measured spectra supported this result. Chlorophyll concentration index was higher for fruitlets that ultimately persisted than abscised fruitlets (P < 0.01) for all measurement dates –1 to 9 days after thinner (DAT) in both years, except –1 DAT in 2021 (P = 0.468). Plant water index (R_950–970:R_890–900) was lower for persisting than abscising fruitlets for 3 to 9 DAT in 2021 (P < 0.001) and on –1 (P < 0.01) and 9 DAT (P < 0.001) in 2022. The relationship of fruit size and plant pigment (anthocyanins or chlorophyll) content in fruitlets to reflectance spectra between persisting and abscising fruitlets was also followed. Fruitlet persistence or abscission was predicted from developed models for destructively sampled fruitlets using measured reflectance spectra. Whole-fruit chlorophyll content was numerically higher in fruitlets predicted to persist than abscise for all collection dates. Higher total chlorophyll was correlated to a larger fruit size in persisting than abscising fruitlets. This higher chlorophyll content led to a lower reflectance of red light and was a key factor in model development. These results indicate that chlorophyll and water content can distinguish physiological parameters between persisting and abscising fruitlets.

Multistep chemical thinning programs have been widely recommended in the eastern United States; however, adoption of bloom thinners is limited. With caustic blossom thinners, narrow effective application timings and concerns related to spring frost damage are barriers for commercial use in this region. If effective and safe, use of hormonal blossom thinners for apple would be an attractive alternative. We evaluated the effects and interactions of bloom thinners [6-benzyladenine (BA) and lime sulfur (LS, or calcium polysulfide) + stylet oil (LS+SO)] and a postbloom thinner (NAA) in the context of a multistep, carbaryl-free thinning program across three locations. Experiments were conducted in 2017 and 2018 on mature ‘Gala’ in North Carolina, Massachusetts, and Pennsylvania, USA. In four of six studies, BA at bloom increased the efficacy of postbloom NAA and reduced crop density (P < 0.08). Postbloom NAA generally increased fruit relative growth rate (RGR) and reduced crop density. However, where NAA failed to reduce crop load, there was a negative influence on RGR. BA and LS+SO increased RGR in one of six studies; however, BA was generally ineffective as a blossom thinner, whereas LS+SO was more effective. Nevertheless, BA applied at bloom may have utility as part of a multistep thinning program. As a part of a multistep thinning program, BA applied at bloom may be useful in increasing efficacy of postbloom applications, particularly when use of caustic blossom thinners is not permitted.