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Inadequate lateral branch development can lead to decreased apple (Malus ×domestica Borkh.) orchard productivity and profitability in modern high-density orchard systems. Although plant growth regulator applications are used to increase lateral branching on leaders of young apple trees, inconsistent responses have been observed in the southeastern United States. In North Carolina and Washington, three experiments were conducted to identify effective leader management strategies to increase lateral branching. Effects and interactions of leader bagging, 6-benzyladenine (6-BA), and 6-BA + gibberellic acid (GA4+7) on lateral branch development of 1-year-old leaders were evaluated. Across all experiments, leader bagging was an influential factor. When compared with unbagged trees, leader bagging increased lateral branch number (20% to 48%), number of feathers (74% to 125%), average branch length (28% to 34%), and total linear bearing surface (428%) of the treated section of the leader. Blossom cluster density and final fruit set were increased in bagged trees, 65% and 36%, respectively. At the rates and timings tested, 6-BA and 6-BA + GA4+7 were generally ineffective in stimulating lateral branching and interactions among the factors evaluated were not influential. Leader bagging was an effective lateral branch induction strategy, although the mechanism of action is poorly understood. Future research to characterize the bagged environment and/or physiological responses to bagging may aid in the development of future environmentally sustainable technologies to stimulate lateral branching of apple trees.

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.

Organic apple (Malus ×domestica Borkh.) growers lack effective strategies to manage preharvest drop. For susceptible cultivars, yield losses caused by preharvest drop can exceed 30% at the beginning of harvest. To address this issue, a formulation of aminoethoxyvinylglyine (AVG) designed for organic use was developed and compared with a commercially available AVG formulation. We evaluated the effects and interactions of the AVG formulation and application number on preharvest drop and fruit maturity in 2017 and 2018. We selected 30 pairs of mature ‘Oregon Spur II Red Delicious’/‘M. 111’ trees planted at the Mountain Horticultural Crops Research and Extension Center in Mills River, NC, USA. Trees were planted with spacing of 2.7 × 6.1 m, trained to a central leader, and received plant protectant sprays that adhered to local recommendations throughout the growing season. Both AVG formulations were applied at 132 mg⋅L−1 at 3 or 3 + 1 weeks before the anticipated harvest. An untreated control was also included for comparison. The experiment had six replicates and a randomized complete block design with a 2 × 2 augmented factorial treatment structure. A one-way analysis of variance was performed and single degree of freedom contrasts were used to compare treatment groups of interest. During both years, organic and conventional AVG were equally effective for reducing preharvest drop and delaying fruit softening and starch hydrolysis at harvest. During one year, increasing the number of applications of AVG reduced cumulative fruit drop, delayed fruit softening at harvest, and reduced internal ethylene concentrations. Inconsistencies in responses across years may be explained, in part, by abnormally warm temperatures observed in 2018. AVG approved for organic use appears to be a promising preharvest drop management technology with efficacy similar to that of conventional AVG.

Aminoethoxyvinylglycine (AVG) is widely used in commercial apple (Malus ×domestica Borkh.) production to reduce preharvest fruit drop (PFD) and delay ripening for harvest management. Recently, the maximum allowable concentration of AVG was doubled (up to 264 mg⋅L−1). Reports of the relationship between the AVG concentration and fruit growth, size, and quality have been contradictory. We evaluated the relationship between the AVG concentration and PFD, fruit size, fruit quality, and expression of ethylene signaling-related and cell wall modification-related genes. Experiments were conducted in 2019 and 2020 using mature ‘Red Delicious’ in western North Carolina. The AVG treatments [0 and 132 (AVG-1x) and 264 mg⋅L−1 (AVG-2x)] were applied 3 weeks before the expected harvest. The AVG treatments reduced fruit drop and internal ethylene concentration relative to the control in both years. There was no difference in drop between AVG-1x and AVG-2x applications. Only in 2020 did AVG treatments delay fruit softening and starch hydrolysis and reduce soluble solids concentration. There were no effects on red fruit color development. Fruit size was unaffected by AVG in 2019, but it was reduced in 2020 with the AVG-2x application. AVG reduced ethylene synthesis and altered signaling, evidenced by decreased relative expression of genes related to ethylene signaling (ARGOS1, ARGOS2). AVG applications also reduced the expression of EXPA8;1, suggesting that reduced cell wall disassembly was associated with a reduction in fruit softening. These results indicate that preharvest applications of 132 mg⋅L−1 AVG effectively reduced PFD via altering ethylene evolution and signaling. Use of a higher AVG concentration was of limited benefit.

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.

Blossom thinning can confer significant benefits to apple growers, including increased fruit size and annual bearing. However, current blossom thinning practices can damage spur leaves and/or fruit. We evaluated the use of short duration forced heated air treatments [thermal shock (TS)] as a blossom thinning strategy for ‘York Imperial’. Using a variable-temperature heat gun, TS treatments were applied to solitary blossoms 24 hours after pollination. Effects of output temperature (five levels) and treatment duration (four levels) were evaluated using a completely randomized design with a factorial treatment structure. Short duration treatments (0.5 and 1.0 seconds) were ineffective for arresting pollen tube growth in vivo. TS temperature required to inhibit stylar pollen tube growth was inconsistent across years. In 2014, TS temperatures ≥56 °C inhibited pollen tubes from reaching the style base at 2.0 and 4.0 second durations. However, in 2015, TS temperatures ≥81 °C at 4.0 seconds prevented pollen tubes from reaching the style base. Inconsistent effects of TS across years were attributed to treatments being applied too late due to optimal conditions for pollen tube growth during the intervening 24-hour period after pollination. Excessive injury to spur leaf tissue was observed at temperatures higher than 84 °C and 70 °C (2.0 and 4.0 seconds, respectively). Pollen tube growth was reduced or arrested at temperature and duration combinations that caused minimal visible injury to spur leaves. Identifying and exploiting structural differences between apple blossoms and vegetative spur leaves may provide insight for the future development of TS or other attempts at developing selective thinning technologies.

The use of short-duration applications of thermal energy (thermal shock; TS) as an apple blossom thinning strategy was investigated. Effects of TS temperature and timing on stigmatic receptivity, pollen tube growth in vivo, and visible leaf injury were evaluated in multiple experiments on ‘Crimson Gala’. TS treatments were applied to blossoms and spur leaves using a variable temperature heat gun. TS temperatures ≥86 °C had a strong inhibitory effect on pollen tube growth on the stigmatic surface and in the style. TS temperatures >79 °C reduced average pollen tube length to less than the average style length. Timing of TS treatment (0 or 24 hours after pollination) was not an influential factor, indicating that effective TS temperatures reduced pollen tube growth up to 24 hours after the pollination event. The onset of thermal injury to vegetative tissues occurred at similar TS temperatures that inhibited pollen tube growth in vivo. Excessive leaf injury (>33%) was observed at 95 °C, suggesting relatively narrow differences in thermal sensitivity between reproductive and vegetative tissues. Inconsistent TS temperatures and/or responses were observed in some experiments. Ambient air temperature may have influenced heat gun output temperatures and/or plant susceptibility. While results suggest some promise, additional work is required to validate and further develop this concept.