Precision crop load management of apple requires counting fruiting structures at various times during the year to guide management decisions. The objective of the current study was to evaluate the accuracy of and compare different commercial computer vision systems and computer applications to estimate trunk cross-sectional area (TCSA), flower cluster number, thinning efficacy, and yield estimation. These studies evaluated two companies that offer different vision systems in a series of trials across 23 orchards in four states. Orchard Robotics uses a proprietary camera system, and Pometa (previously Farm Vision) uses a cell phone camera system. The cultivars used in the trials were ‘NY1’, ‘NY2’, ‘Empire’, ‘Granny Smith’, ‘Gala’, ‘Fuji’, and ‘Honeycrisp’. TCSA and flowering were evaluated with the Orchard Robotics camera in full rows. Flowering, fruit set, and yield estimation were evaluated with Pometa. Both systems were compared with manual measurements. Our results showed a positive linear correlation between the TCSA with the Orchard Robotics vision system and manual measurements, but the vision system underestimated the TCSA in comparison with the manual measurements (R²s between 0.5 and 0.79). Both vision systems showed a positive linear correlation between nubers of flowers and manual counts (R²s between 0.5 and 0.95). Thinning efficacy predictions (in June) were evaluated using the fruit growth rate model, by comparing manual measurements and the MaluSim computer app with the computer vision system of Pometa. Both systems showed accurate predictions when the numbers of fruits at harvest were lower than 200 fruit/tree, but our results suggest that, when the numbers of fruits at harvest were higher than 200 fruit/tree, both methods overestimated final fruit numbers per tree when compared with final fruit numbers at harvest (R²s 0.67 with both systems). Yield estimation was evaluated just before harvest (August) with the Pometa system. Yield estimation was accurate when fruit numbers were fewer than 75 fruit per tree, but, when the numbers of fruit at harvest were higher than 75 fruit per tree, the Pometa vision system underestimated the final yield (R² = 0.67). Our results concluded that the Pometa system using a smartphone offered advantages such as low cost, quick access, simple operation, and accurate precision. The Orchard Robotics vision system with an advanced camera system provided more detailed and accurate information in terms of geo-referenced information for individual trees. Both vision systems evaluated are still in early development and have the potential to provide important information for orchard managers to improve crop load management decisions.

Carbohydrates play a key role in apple fruit growth and development. Carbohydrates are needed for cell division/expansion, regulate fruitlet abscission, and influence fruit maturation and quality. Current methods to quantify fruit carbohydrates are labor intensive and expensive. We quantified carbohydrates throughout a growing season in two cultivars and evaluated the use of near infrared spectroscopy (NIR) to predict apple carbohydrate content throughout changes in fruit development. Carbohydrates were quantified with high performance liquid chromatography (HPLC) at five timepoints between early fruitlet growth and harvest in ‘Gala’ and ‘Red Delicious’ apples. NIR spectra was collected for freeze-dried fruit samples using a benchtop near infrared spectrometer. Sorbitol was the major carbohydrate early in the growing season (~40% of total carbohydrates). However, the relative contribution of sorbitol to total carbohydrates rapidly decreased by 59 days after full bloom (<10%). The proportion of fructose to total carbohydrates increased throughout fruit development (40–50%). Three distinct periods of fruit development, early, mid-season, and late, were found over all sampling dates using principal component analysis. The first (PC1) and second (PC2) principal components accounted for 90% of the variation in the data, samples separated among sampling date along PC1. Partial least squares regression was used to build the models by calibrating carbohydrates quantified with HPLC and measured reflectance spectra. The NIR models reliably predicted the content of fructose, glucose, sorbitol, sucrose, starch, and total soluble sugars for both ‘Gala’ and ‘Red Delicious’; r² ranged from 0.60 to 0.96. These results show that NIR can accurately estimate carbohydrates throughout the growing season and offers an efficient alternative to liquid or gas chromatography.

In the eastern USA and several other apple-growing regions, apple blossom thinning using lime sulfur is a relatively new crop load management strategy. This study sought to evaluate how application timing of lime sulfur + stylet oil blossom thinning sprays would influence thinning efficacy and crop safety of ‘Gala’ apples. This project occurred at two locations in the USA, Winchester, Virginia, and Mills River, North Carolina, during the 2019 growing season. Two main timing strategies were assessed: (1) model-guided sprays with the pollen tube growth model (PTGM), (2) fixed spray intervals with the first spray applied at a specified percentage of open bloom (20% vs. 80%), and the second spray applied at a reapplication interval (48 h vs. 72 h). Model-guided and 20% open bloom + 48-h treatments reduced fruit set and increased fruit weight, diameter, and length at both sites. Treatments with a delayed first spray at 80% open bloom or a more extended second reapplication of 72 h were generally ineffective. There was no conclusive evidence that lime sulfur + stylet oil blossom thinning spray timing influenced russet incidence/severity or leaf phytotoxicity. This study demonstrated that effective and safe blossom thinning can be obtained from applying two sprays at 20% open bloom and 48 h thereafter or using the PTGM.

Reducing apple crop load during bloom can increase fruit size and promote annual bearing when compared with crop reduction at later timings. In this study, we compared the efficacy of chemical blossom-thinning strategies on ‘Golden Delicious’ and ‘Gala’ apple trees. Several blossom-thinning treatments were evaluated, including 1) unthinned control (control), 2) hand-thinned (HT) at bloom, 3) liquid lime sulfur + Stylet-Oil (LS + SO), 4) ammonium thiosulfate (ATS), 5) endothall (ET), and 6) naphthaleneacetamide (NAD). Chemical treatments were applied twice during bloom, using a predictive model to determine application timings. Blossom thinner effects on pollen tube growth, fruit set, and yield responses were evaluated. LS + SO and ATS reduced the number of pollen tubes that entered the style for ‘Golden Delicious’ by 75% and 63%, respectively. ET and NAD did not affect the number of pollen tubes that entered the style. In one of 2 years, LS + SO resulted in a near-ideal crop load and increased fruit weight. ATS was effective in reducing initial fruit set in ‘Golden Delicious’ and ‘Gala’, but did not reduce whole-tree crop density. ET reduced crop load in all experiments but caused excessive spur leaf injury and negatively affected fruit size of ‘Gala’ but not ‘Golden Delicious’. NAD had limited efficacy on ‘Golden Delicious’ at the concentrations and application timings used in this trial. When used as the sole method of crop load management, none of the chemistries evaluated over-thinned or increased fruit injury. However, ET caused excessive thinning when evaluated as part of a commercial crop load management program on ‘Gala’. Of the products evaluated, LS + SO provided the best overall thinning response.

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.

For some apple cultivars, inadequate red fruit color development can reduce crop value. The use of reflective groundcovers has been demonstrated to improve red coloration in apples in other regions, but evaluation in the southeastern USA has been limited. To address this, we compared the performance of multiple reflective groundcovers in 2018 and 2020 on mature ‘Fuji’ trees in Edneyville, NC, USA. Woven reflective (Extenday^® DayBright, Lumilys^® WH100, Beltech PD2911, and Belton experimental), mylar, and sod groundcovers were deployed ~5 weeks before anticipated harvest. The effects of the treatment on light reflectance (photosynthetically active and UV radiation), fruit color, fruit quality, and crop value were determined. Across both years of evaluation, reflective groundcovers were consistent in increasing the reflectance of photosynthetically active radiation. However, only Extenday^® DayBright consistently increased reflected UV radiation (250–400 nm), red fruit coloration at commercial harvest, and crop value. Fruit maturity and sunburn incidence were not influenced by any treatment in both years. Reflected UV light quality was not characterized, but it is clear that UV_250–400nm reflectance intensity is critical to enhance ‘Fuji’ fruit color development. Growers in the southeastern US can use reflective groundcovers to enhance red fruit coloration to meet market demands.