Effective crop load management programs are critical to maximize apple orchard profitability. A series of coordinated experiments were conducted at four locations in the United States (Michigan, North Carolina, New York, and Washington). The studies aimed to determine the relationship between crop load and yield, fruit size, color, and return bloom of ‘Honeycrisp’. Mature and uniform ‘Honeycrisp’ apple trees grafted on ‘M.9’ rootstock were selected at each location. Crop load, expressed as fruits/cm 2 trunk cross-sectional area (TCSA), was used to standardize trees and provide a physiological context for interpreting fruit size and yield efficiency. Fruit size, independent of crop load, among the four locations was positively correlated to growing degree hours from green tip to full bloom and average daytime temperature from 6 weeks after bloom until harvest. The percentage of marketable red blush area and return bloom were negatively correlated with the final crop load at all locations and years. Fruit red color, independent of crop load, among the four locations was positively correlated to growing degree hours from green tip to full bloom, and average solar radiation the week before harvest negatively correlated to average daytime temperature the week before harvest. Return bloom the following year was also negatively correlated to crop load. Return bloom, independent of crop load, among the four locations was positively correlated to growing degree hours from full bloom to 6 weeks after full bloom and negatively correlated to average of radiation from green tip to full bloom. The market-preferred fruit size for ‘Honeycrisp’ (200–250 g) was achieved with different crop loads depending on the region and weather conditions (between 4.4 and 13 fruits/cm 2 ). However, a general range of 4.5 to 6 fruits/cm 2 TCSA is recommended to balance size, color, and return bloom.

Bitter pit is a physiological disorder of pome fruit associated with calcium deficiency that renders fruit unsalable for fresh consumption. Unlike other macronutrients, calcium is exclusively mobile in xylem; however, within the fruit of apple, xylem becomes progressively more dysfunctional with maturity, effectively limiting calcium delivery to fruit tissues. Auxins promote the differentiation of xylem, and abscisic acid (ABA) is linked with calcium mobility. Therefore, we hypothesized that early-season applications of auxins (native and synthetic) or ABA would impart greater longevity and functionality to xylem tissue and/or improve the transport of calcium and mitigate bitter pit incidence. Exogenous indole-acetic acid, naphthaleneacetic acid (NAA), and ABA were applied individually at 30, 45, and 60 days after full bloom in 2021 and 2022 to crop load-adjusted trees. The primary vascular bundle function was three-fold higher for NAA and ABA compared with the control and negatively correlated with bitter pit (−0.87) at harvest in 2021. Treatment differences in the number of vascular bundles were not observed in 2022, when bundle function was higher and fruit size was smaller than those in 2021. Peel calcium was unaffected by any treatments in either year; however, the most efficacious rates of auxins and ABA markedly and significantly reduced the bitter pit incidence of control fruit by 60% to 71% at harvest and by 54% to 65% after postharvest in 2021, and by 32% to 44% at harvest and by 36% to 45% after postharvest in 2022. More research is warranted to elucidate the mechanisms by which these plant growth regulators mitigate the incidence of bitter pit.

Pollination is essential for producing temperate-zone tree fruits like apples (Malus × domestica). While traditionally considered insect-dependent, this view may result from orchard designs tailored to European honeybees. Previous research showed that low-seed apples could develop in insect exclusion nets, suggesting wind as an alternative pollinator. This study investigated the paternal origin of seeds and fruit set under nets compared to open canopies. Netted canopies of ‘Gala’, Fuji’, and ‘Honeycrisp’ set commercial fruit numbers without manual thinning. To determine the parental source of seeds, genotyping was performed using 16 SNP markers tailored for distinguishing apple cultivars, with primer design and genotyping conducted via the KASP™ system. Results showed significant genetic overlap between seeds from netted and non-netted fruits and nearby pollinizers, ruling out self-pollination. Netted canopies retained fruits with similar or fewer seeds compared to abscised fruits in open canopies, indicating fruit set depends on the population’s seed content rather than individual fruit seed count. These findings supporting the hypothesis that apple trees are adapted to utilize both wind and insect pollination. While wind pollination offers a sustainable approach, it requires adjustments in orchard design to ensure sufficient pollen transfer for reliable fertilization and yield.

All branches of single-axis apple trees in high-density orchards are subject to limb renewal. Renewal limb management aims to replace overly vigorous and yield inefficient limbs with weaker, balanced limbs of higher yield potential. Recently, producers have adopted the application of long stubs, i.e., 10 cm, when renewing apple limbs despite a dearth of research on the topic. The current multi-year project evaluated the effect of stub length (between 2.5 to 10 cm in 2.5 cm increments) on the number, length and orientation of replacement limbs. The experiment was conducted on 3 rd and 4 th leaf 'Honeycrisp', 'Gala' and 'Fuji' trees on Budagovsky 9 rootstock, trained to tall spindle. We demonstrate a significant, positive relationship between increasing stub length and limb renewal response (expressed as the number of shoots and total vegetative growth per stub) but observed varying levels of the response in relation to scion vigor. Moreover, stub length did not affect the orientation of replacement limbs nor decrease the length of the longest limb on the stub. The data suggest that different stub lengths should be considered on the basis of cultivar and site vigor and not uniformly applied across cultivars.

Bitter pit, a physiological disorder of pome fruit, causes substantial annual losses of apple fruit, especially for the commercially important cultivar 'Honeycrisp.' Despite being classified as a calcium deficiency disorder, methods for increasing fruit calcium do not fully prevent bitter pit. Bitter pit is a complex disorder which has been connected with fruit xylem dysfunction, mineral ratios, water stress, and cell membrane stability. Each of these factors are influenced by hormones and plant growth regulators (PGRs), notably auxins, gibberellins (GAs), and abscisic acid (ABA), though other compounds are likely involved. Before auxins were determined to promote xylem function and, therefore, calcium transport, two experimenters in the 1950's applied several auxins to apple trees to remedy boron deficiency and bitter pit. GAs have been demonstrated to exacerbate bitter pit by reducing membrane stability and calcium translocation. GAs also increase vegetative growth resulting in greater competition for calcium between vegetative and reproductive sinks. ABA is a GA antagonist that mitigates bitter pit by reducing transpiration, improving xylem function and, ultimately, facilitating calcium transport to fruit. The connection between other classes of hormones and PGRs such as cytokinins, ethylene, jasmonates, and salicylic acid to bitter pit has not yet been firmly established, but their effects on plant growth and cellular function suggest potential for bitter pit mitigation. The objective of this review is to synthesize the physiological, anatomical and biochemical effects of endogenous and exogenous PGRs on xylem function, mineral transport and calcium-related disorders of apple and other species. We suggest that hormone concentrations and interactions ultimately control xylem dysfunction that results in the often characterized nutrient imbalances linked to bitter pit.

In 2015, an orchard trial of ten apple rootstocks was established at ten locations in the United States and Canada using 'Modi (R)' as the scion cultivar. Trees were managed in accordance with United States organic standards to expose these rootstocks to the nutrient conditions and biome typically associated with organic tree-fruit production. Rootstocks included nine named Cornell-Geneva clones [Geneva (R) 11 (G.11), Geneva (R) 30 (G.30), Geneva (R) 41 (G.41), Geneva (R) 202 (G.202), Geneva (R) 214 (G.214), Geneva (R) 222 (G.222), Geneva (R) 890 (G.890), Geneva (R) 935 (G.935), and Geneva (R) 969 (G.969)] and M.9 NAKBT337. All trees were spaced 1 x 3.5 m and trained using the tall spindle system. After 5 years, the greatest mortality was for trees on M.9 NAKBT337 (14%). Rootstocks separated into size classes from large semi-dwarf to small dwarf. G.890 resulted in large semi-dwarf trees, and G.202 produced moderate semi-dwarfs. G.41 and G.30 resulted in small semi-dwarf trees, and trees on G.935 were large dwarfs. G.11, G.214, G.969 and M.9 NAKBT337 resulted in trees that were moderate dwarfs, and G.222 resulted in small dwarf trees. The most yield efficient (cumulatively, 2016-19) trees in the trial were on G.935, G.11, and G.969, and the least efficient trees were on G.202 and G.890. The largest fruit (2016-19) were harvested from trees on G.30, G.41, G.890, and M.9 NAKBT33, and the smallest were harvested from trees on G.202.

The number of fruits that remain on an apple tree directly affects yield, fruit size and the quality of fruit that are harvested, which largely determines crop value. If the number of apples can be precisely managed the final crop value can be maximized. We are conducting a USA national SCRI project to develop precision crop load management strategies and machines to manage the number of fruits per tree to exactly the economic optimum. We have done physiological experiments to define the biological potential of yield and fruit size of 'Gala' and 'Honeycrisp' apple cultivars in 4 sites (West, Mid-West, North-East and South-East USA) to estimate the economic optimum number of fruits per tree. To determine the optimum fruit number per tree we employed: 1) precision pruning to remove flower buds at pink bud stage to various pre-determined flower bud loads followed by hand thinning to single fruitlets at 10mm fruit size; and 2) precision hand thinning to remove fruitlets on uniformly pruned trees to establish different crop loads at 10mm fruit size. Our results showed that the dry, high light climate of WA generally can support a higher crop load than the eastern USA growing regions. Our multi-location experiments demonstrated that leaving too many flower buds during pruning results in lower crop value than the optimum flower bud number. Optimum flower bud number in our studies of 'Gala' and 'Honeycrisp' was between 1.5 and 2.0 flower buds per final target fruit number. To simplify counting of flowers or fruitlets, we are developing computer vision systems to streamline the counting of buds, flowers and fruitlets. The information from each tree is geo-referenced and cloud-stored therefore can be communicated to human workers to guide their work in reducing crop load to the optimum level.

Ethylene is an endogenous plant hormone associated with natural senescence and abscission of plant organs. Ethylene release from the degradation of 2-chloroethylphosphonic acid, the a.i. of the plant growth regulator ethephon, facilitates the mechanized harvest of sour cherry fruit. Methods to abscise immature sour cherry fruit, however, have not been explored but recent carryover inventories, inexpensive sour cherry imported products, and outbreaks of the invasive insect, spotted wing drosophila, have combined to reduce crop value below the cost of production thereby necessitating crop elimination. We assessed the ethylene emission and abscission rates of untreated, ethephon-treated and AVG-treated ‘Montmorency’ sour cherry flowers and fruitlets at several phenology stages. The natural ethylene emission from untreated flowers was < 1 μL kg−1 h−1 with a small peak between full bloom (FB) and petal fall (PF). AVG reduced, but did not eliminate, ethylene biosynthesis. Ethylene emission was linearly related to ethephon concentration and remained elevated for ~ 5 days following application. Ethephon application at FB induced the highest ethylene production (10–100 μL kg−1 h−1) depending on year, timing, and rate. Application at the shuck split (SS) stage resulted in very low ethylene production (~ 1 μL kg−1 h−1). Abscission rates were unaffected by AVG suggesting that natural levels of endogenous ethylene biosynthesis do not limit fruit set. Abscission rates increased, however, with increasing ethephon dose and were also highest at FB timing compared to advanced phenology stages; however, at the highest ethephon concentration (800 ppm), ~ 90% abscission occurred for all developmental stages, despite the comparatively low ethylene production at SS. Thus, ethylene emission of ‘Montmorency’ sour cherry cannot be used to predict abscission. Because temperature affects ethylene production and the maximum ambient temperatures were variable following ethephon applications, we incubated ethephon-treated flowers and immature fruitlets over several temperatures (10, 21, 30 and 35 °C). Ethylene emission was, again, highest at full bloom and increased with ethephon dose and temperature, but tthe activation energy (Ea) for ethylene emission over the temperature range was similar among ethephon rates or phenology stages (~ 76 kJ mol−1 in all cases). Together the data suggest that ethephon can eliminate sour cherry crops, but ambient temperature, phenology stage and rate interact to regulate efficacy.

Tree fruit species and cultivars have discrete thresholds of chill and heat, quantified with environmental models that relate temperature accumulation over time to tree fruit development. The accumulation of heat in the spring progressively leads to the deacclimation and development of reproductive buds of Prunus sp. Phenology scales have historically been used to estimate and relate this developmental progression with lethal temperatures to facilitate orchard management practices. For sour cherry, however, ecodormancy release is accompanied by an approximate 20 degrees C loss of hardiness before clearly distinguishable changes in external bud phenology occur. For this reason, we aimed to characterize the physiological changes of 'Montmorency' sour cherry floral buds during their transition from endo- and eco-dormancy through growth resumption to determine how well phenology relates to shifts in critical survival temperature of buds. We present a developmental timeline of the first preanthesis changes in floral buds as they relate to lethal temperature (LT50) changes. A simple Growing Degree Hour (GDH) model predicted freeze sensitivity explaining 93% of the variation in LT50 where overlapping phenological changes were difficult to separate. Our results can aid producers to manage frost protection and enable a better prediction of freeze susceptibility.

Thinning is a challenging practice in organic apple production. We previously demonstrated the use of insect exclusion netting to reduce fruit set and thinning when applied at different stages of bloom; however, fruit set of different cultivars responded differently to bee exclusion with nets. The objective of the current experiment was to evaluate the technique on the apple cultivars 'Honeycrisp', 'Fuji' and 'SweeTango' in a northern production climate (Michigan, USA). In two consecutive years (2019 and 2020), whole canopies were enclosed with nets at different percentages of open flowers from 0% bloom [i.e., pink stage] to 80% king bloom (KB). Fruit set of netted canopies was positively related to percentage of open KB at the timing of netting. 'Honeycrisp' required more open bloom to set a full commercial crop, without the need for thinning, than 'SweeTango' or 'Fuji'. Netted 'Fuji' canopies produced higher fruit set and yield than netted 'SweeTango' and could achieve a relatively high fruit set even when netted at pink. In all cases, fruit set, yield, and seed number of netted trees were significantly reduced compared to the non-netted controls. Individual fruit weight was significantly increased under netting due to reduced crop load. Bee exclusion with nets may constitute an alternative, viable strategy to manage fruit set but will require fine-tuning depending on the cultivar and/or climate.