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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. [ABSTRACT FROM AUTHOR]

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 R640-675 was subtracted from or divided by R675-696. This spectra region indicates differences in chlorophyll content between persisting and abscising fruitlets. Calculation of the chlorophyll concentration index (R522-579:R640-700) 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 (R950-970:R890-900) 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. [ABSTRACT FROM AUTHOR]

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 downsampled, 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. [ABSTRACT FROM AUTHOR]

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.

Abstract Metamitron is a relatively new postbloom thinning compound for pome fruits that inhibits the photosystem II (PS II) pathway of photosynthesis. Reduced assimilation of carbohydrate by metamitron action may lead to a carbohydrate deficit that promotes fruit abscission. We have evaluated the thinning efficacy of metamitron rate and, to a limited extent, application timing for ‘Bartlett’ pear in five separate trials over 3 years (2015–17) in northern Oregon. Comparisons were made to a nontreated control and, depending on the trial, a commercial standard thinning compound [benzyladenine (6-BA) or abscisic acid (ABA)]. Application timings ranged between 7- and 16-mm fruit diameter depending on the trial. Metamitron markedly inhibited photosynthesis (PN), typically for a duration of 2 to 3 weeks, although longer persistence was observed in two trials. Generally, PN was reduced linearly with increasing metamitron rate, but the effect varied by rate and year and may have been enhanced by high temperatures. Metamitron effectively thinned in four of five trials whereby pear fruit set was negatively and linearly related to rate. Rates of 200 to 300 ppm were efficacious and produced target crop loads. In only one trial, increasing concentrations (600 ppm) led to greater thinning. Metamitron significantly thinned ‘Bartlett’ pears when applied between 10 and 13 mm. In contrast, early application timing (≈7 mm) had little effect on fruit abscission. Fruit size increased with decreasing crop load and thus was significantly improved by metamitron. Interaction between biological and environmental factors likely contributed to year-to-year variability in efficacy.

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Metamitron is a relatively new postbloom thinning compound for pome fruits that inhibits the photosystem II (PS II) pathway of photosynthesis. Reduced assimilation of carbohydrate by metamitron action may lead to a carbohydrate deficit that promotes fruit abscission. We have evaluated the thinning efficacy of metamitron rate and, to a limited extent, application timing for 'Bartlett' pear in five separate trials over 3 years (2015-17) in northern Oregon. Comparisons were made to a nontreated control and, depending on the trial, a commercial standard thinning compound [benzyladenine (6-BA) or abscisic acid (ABA)]. Application timings ranged between 7- and 16-mm fruit diameter depending on the trial. Metamitron markedly inhibited photosynthesis (PN), typically for a duration of 2 to 3 weeks, although longer persistence was observed in two trials. Generally, PN was reduced linearly with increasing metamitron rate, but the effect varied by rate and year and may have been enhanced by high temperatures. Metamitron effectively thinned in four of five trials whereby pear fruit set was negatively and linearly related to rate. Rates of 200 to 300 ppm were efficacious and produced target crop loads. In only one trial, increasing concentrations (600 ppm) led to greater thinning. Metamitron significantly thinned 'Bartlett' pears when applied between 10 and 13 mm. In contrast, early application timing (...7 mm) had little effect on fruit abscission. Fruit size increased with decreasing crop load and thus was significantly improved by metamitron. Interaction between biological and environmental factors likely contributed to year-to-year variability in efficacy. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The objectives of this experiment were to test the efficacy of a mechanical string thinner (Darwin PT-250; Fruit-Tec, Deggenhauserertal, Germany) on apple and to identify an optimal range of thinning severity as influenced by spindle rotation speed. Trials were conducted in 2010 and 2011 at the Pennsylvania State University Fruit Research and Extension Center in Biglerville, PA, on five-year-old 'Buckeye Gala'/M.9 apple trees that were trained to tall spindle. A preliminary trail on five-year-old 'Cripps Pink'/M.9 was conducted to determine the relationship between string number and thinning severity. As the number of strings increased, the level of thinning severity increased. A range of spindle speeds (0 to 300 rpm) was applied to the same trees for two consecutive years. As spindle speed increased, blossom density (blossom clusters per limb cross-sectional area) was reduced as was the number of blossoms per spur. In 2010, leaf area per spur was reduced 9% to 45%. In 2011, the fastest spindle speed reduced leaf area per spur 20%. Although increased spindle speed reduced cropload, injury to spur leaves may have inhibited increases in fruit size. The largest gain in fruit weight was 28 g (300 rpm) compared with the control. In both years, the most severe thinning treatments reduced yield by more than 50%. There was no relationship between spindle speed and return bloom. Severe thinning treatments (240 to 300 rpm) caused significant reductions in spur leaf area, yield, and fruit calcium and did not improve fruit size or return bloom. Spindle speeds of 180 and 210 rpm provided the best overall thinning response and minimized injury to spur leaves, but cropload reduction was insufficient in years of heavy fruit set. Therefore, mechanical blossom thinning treatments should be supplemented with other thinning methods. Mechanical string thinning may be a viable treatment in organic apple production, where use of chemical thinners is limited. [ABSTRACT FROM AUTHOR]

The risk of spreading fire blight in apples after mechanical thinning with a rotating string blossom thinner was evaluated in field and potted-tree experiments. In the field experiment, using the mechanical thinner on noninoculated trees immediately after operating the equipment on inoculated trees significantly (P < 0.01) increased fire blight incidence resulting in 90 ± 20.01 (mean ± SE) infected shoots compared with 23.5 ± 8.97 diseased shoots in similar trees that were not thinned mechanically. A similar result was obtained in greenhouse experiments whereby healthy apple plants positioned adjacent diseased plants before the group was subjected to the mechanical thinner developed more than twice the number of infected shoots as that on similar plants that were not thinned. These results indicate that under conditions conducive to infection, the mechanical blossom thinner significantly increases the risk of spreading Erwinia amylovora. The use of the thinner should therefore be limited to orchards with no history of disease in the last 3 years and on days when predicted weather is not suitable for tree infection by E. amylovora; otherwise, a severe fire blight epidemic could develop in the orchard. [ABSTRACT FROM AUTHOR]

This paper reports on the potential of gibberellic acid (GA3 and GA4+7) to reduce sweet cherry (Prunus avium L.) floral bud induction and balance fruit number and improve fruit quality in the season following application. In 2003, GA3 was applied to 'Bing'/'Gisela 1' trees at 50 and 100 mg-L-1 at the end of stage I of fruit development, end of stage 11, and on both dates. These treatments were compared to the industry standard application of 30 mg⋅L-1 applied at the end of stage II and an untreated control. Fruit quality was evaluated in the year of application (i.e., nontarget crop) and return bloom, fruit yield and quality were assessed in the subsequent season (2004). In 2003, GA3 delayed fruit maturity proportional to rate. In 2004, bloom density and fruit yield were related negatively and linearly to GA3 concentration. GA3 reduced the number of reproductive buds per spur and did not affect the number of flowers per reproductive bud. Nonspur flowering at the base of 1-year-old shoots was more inhibited by GA3 than flowering on spurs. Double applications significantly reduced bloom density and yield versus single applications. Trees treated with two applications of 50 and 100 mg-L-1 yielded fruit with 7% and 12% higher soluble solids, 15% and 20% higher firmness, and 7% and 14% greater weight, respectively. However, no treatment improved crop value per tree. In a separate isomer trial, GA3 and GA4-7 were applied to 'Bing'/'Gisela 1' trees at 100 and 200 mg⋅L-1 at both the end of stage I and II in 2004. GA3 and GA4+7 applied at 100 mg⋅L-1 reduced bloom density similarly by 65%. GA3 was more inhibiting than GA4+7 at 200 mg⋅L-1, reducing bloom density by 92% versus 68%. We observed a 4- to 5- day delay in flowering from both GA formulations at 200 mg⋅L-1. At both concentrations, GA3 reduced yield by 71% and 95% versus 34% and 37% reduction by GA4+7. Fruit weight and soluble solids were unaffected but fruit firmness was increased by all treatments (6% to 17%). However, crop value per tree was highest from untreated control because improvements in fruit quality were insufficient to offset reductions in yield. GA3 shows potential as a novel crop load management tool in productive 'Bing' sweet cherry orchard systems. [ABSTRACT FROM AUTHOR]