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The development of novel crop load management techniques will be critical to the adoption and success of high density sweet cherry orchard systems based on new clonal rootstocks. Herein we report on a comparison of potential means of balancing crop load of `Bing' sweet cherry grown on the productive and precocious rootstocks `Gisela 5' and `Gisela 6'. In 2002, thinning treatments were applied to entire trees and consisted of an unthinned control (C), and manual removal of 50% of the blossoms (B) or 50% of 2-year-old and older fruiting spurs (S), throughout the tree. In 2003 all trees were left unthinned to characterize the carry-over effect of thinning treatment in 2002. In 2002, compared to C, thinned trees had 38% to 49% fewer fruit per tree, 22% to 42% lower yield, 8% to 26% higher fruit weight, and 2% to 10% larger fruit diameter. S and B treatments reduced yield by 42% and 22% on `Gisela 5' and by 40% and 31% on `Gisela 6', respectively. `Gisela 5'-rooted trees showed greater improvements in fruit quality than did trees on `Gisela 6'. Compared to C-, S-, and B-treated trees on `Gisela 5' yielded fruit that was 15% and 26% heavier, respectively. Yield of fruit ≥25.5 mm diameter was increased by 240% by S and 880% by B, though yield of this size fruit was still low (1.5 and 5.2 kg/tree, respectively). Neither technique had any beneficial carryover effect in the year following treatment despite S trees bearing about 25% fewer fruit than B and C trees. In both years, `Gisela 5'-rooted trees bore about 15% fewer fruit than trees on `Gisela 6'. Compared to `Gisela 5', `Gisela 6'-rooted trees were about 41%, 46%, and 24% more productive for C, S, and B, respectively. Number of fruit/tree in 2003 was within 4% and 8% of the previous year on `Gisela 6' and `Gisela 5', respectively. Crop value analyses suggest growers would be rewarded for producing high yields of medium size fruit (e.g., 21.5 to 25.4 mm) compared to low yields of high quality fruit.

To initiate photosynthetic studies of sweet cherry (Prunus avium L.) canopy architectures and cropping management under high light and temperature conditions (Yakima Valley, Wash.), we developed a whole-canopy research cuvette system with a variable airflow plenum that allowed different patterns of air delivery (in concentric circles around the trunk) into the cuvette. Air and leaf temperatures (T_air and T_leaf, respectively) were determined at four horizontal planes and four directional quadrants inside cuvette-enclosed canopies trained to a multiple leader/open-bush or a multiple leader/trellised palmette architecture. Air flow rate, air delivery pattern, and canopy architecture each influenced the whole-canopy temperature profile and net CO₂ exchange rate (NCER) estimates based on CO₂ differentials (inlet-outlet). In general, T_air and T_leaf were warmer (≈0 to 4 °C) in the palmette canopy and were negatively correlated with flow rate. The response of T_air and T_leaf to flow rate varied with canopy position and air delivery pattern. At a flow of 40 kL·min^-1 (≈2 cuvette volume exchanges/min), mean T_air and T_leaf values were 2 to 3 °C warmer than ambient air temperature, and CO₂ differentials were 15-20 μL·L^-1. T_air and T_leaf were warmer than those in unenclosed canopies and increased with height in the canopy. Carbon differentials declined with increasing flow rate, and were greater in the palmette canopy and with a less dispersed (centralized) delivery. Dispersing inlet air delivery produced more consistent values of T_air and T_leaf in different canopy architectures. Such systematic factors must be taken into account when designing studies to compare the effects of tree architecture on whole-canopy photosynthesis.

This article reports on the physiological effects and horticultural benefits of chemical blossom thinners on 9-year-old and 12-year-old `Bing'/`Gisela®5′ sweet cherry trees in 2004 and 2005, respectively. Chemical thinning agents were applied at 20% and 80% full bloom (FB) by air-blast sprayer and were comprised of: 2% ammonium thiosulphate (ATS), 4% vegetable oil emulsion (VOE), 2% fish oil + 2.5% lime sulfur (FOLS), 1% tergitol, and an untreated control. Leaf gas exchange, leaf SPAD meter readings, chlorophyll fluorescence parameters, fruit yield, and fruit quality were evaluated. FOLS, tergitol, VOE, and ATS suppressed leaf net CO₂ exchange rate (NCER) by 33%, 30%, 28%, and 18%, respectively, over a variable length recovery period directly after 80% FB treatment. Leaf NCER recovered fully from every thinning treatment. Reductions in leaf NCER were unrelated to g_S. VOE reduced estimated leaf chlorophyll content the greatest, suppressing overall levels by 11% for 23 days after treatment. All blossom thinners reduced constant fluorescence (Fo). No thinning agent reduced fruit set or yield in 2004. ATS, FOLS, and tergitol reduced fruit set in 2005. VOE was ineffective as a thinner yet exhibited significant leaf phytotoxicity. Among thinners, there was no relationship between inhibition of leaf NCER and thinning efficacy.

Intuitively, tree architecture will affect harvest efficiency of tree fruit crops, yet there are no empirical studies that document this. The objective of the current research was to investigate the role of training system on harvest rate of individual pickers in commercial sweet cherry (Prunus avium L.) orchards. We used a real-time labor monitoring system (LMS) with the ability to track and record individual picker efficiency in 11 orchards throughout the Pacific Northwest. Trees were trained to one of five different architectures: 1) upright fruiting offshoots (UFO), a planar architecture comprised of unbranched vertical fruiting wood; 2) Y-trellised, an angled dual planar architecture; 3) Kym Green Bush (KGB), a multileader bush; 4) central leader (CL); and 5) traditional multileader open center (MLOC), trees comprised of three to five main leaders. A consistent picking crew was used to facilitate comparisons among systems and eliminate variability in skill among pickers. The LMS calculated harvest rate, picking cost, weight of harvested fruit, number of harvested buckets, range in fruit weight per bucket/bin, and mean fruit weight per bucket/bin for individual pickers. Tests revealed a significant effect of canopy architecture on labor efficiency. The highest mean (± se) harvest rates (0.94 ± 0.02 kg·min⁻¹ and 0.78 ± 0.03 kg·min⁻¹) were recorded in ‘Cowiche’/‘Gisela^®5’ and ‘Tieton’/‘Gisela^®5’ orchards trained to the UFO system, respectively. High harvest efficiency in these orchards was likely the result of the planar, simplified architecture and that most fruit were accessible from the ground. The third highest picking rate was recorded in the KGB system (0.73 ± 0.04 kg·min⁻¹), a fully pedestrian orchard. Interestingly, harvest rate of slower pickers was improved to a greater extent (+132%) than skilled pickers (+83%) when comparing pedestrian and planar systems (e.g., UFO and KGB) with traditional architecture (MLOC). Furthermore, picking rate of individual pickers varied within 1 day by more than 100%, likely as a result of variability in fruit density within trees, tree size as well as fruit accessibility. We documented variability of more than 35 kg in final bin weight across all orchards and a range in bucket weight between ≈7 and 13 kg. These results suggest that architecture has a major effect on harvest efficiency and that current systems of piece-rate picker reimbursement are beset with inaccuracy.

Canopy fruit to leaf area ratios (fruit no./m² leaf area, F:LA) of 7- and 8-year-old `Bing' sweet cherry (Prunus avium L.) on the dwarfing rootstock `Gisela 5' (P. cerasus L. × P. canescens L.) were manipulated by thinning dormant fruit buds. F:LA influenced yield, fruit quality, and vegetative growth, but there were no consistent effects on whole canopy net CO₂ exchange rate (NCER_canopy). Trees thinned to 20 fruit/m² LA had yield reduced by 68% but had increased fruit weight (+25%), firmness (+25%), soluble solids (+20%), and fruit diameter (+14%), compared to unthinned trees (84 fruit/m²). Fruit quality declined when canopy LA was ≈200 cm²/fruit, suggesting that photoassimilate capacity becomes limiting to fruit growth below this ratio. NCER_canopy and net assimilation varied seasonally, being highest during stage III of fruit development (64 days after full bloom, DAFB), and falling more than 50% by 90 DAFB. Final shoot length, LA/spur, and trunk expansion were related negatively to F:LA. F:LA did not affect subsequent floral bud induction per se, but the number of flowers initiated per bud was negatively and linearly related to F:LA. Although all trees were thinned to equal floral bud levels per spur for the year following initial treatment (2001), fruit yields were highest on the trees that previously had no fruit, reflecting the increased number of flowers initiated per floral bud. Nonfruiting trees exhibited a sigmoidal pattern of shoot growth and trunk expansion, whereas fruiting trees exhibited a double sigmoidal pattern due to a growth lag during Stage III of fruit development. Vegetative growth in the second year was not related to current or previous season F:LA. We estimate that the LA on a typical spur is only sufficient to support the full growth potential of a single fruit; more heavily-set spurs require supplemental LA from nonfruiting shoots. From these studies there appears to be a hierarchy of developmental sensitivity to high F:LA for above-ground organs in `Bing'/`Gisela 5' sweet cherry trees: trunk expansion > fruit soluble solids (Stage III) > fruit growth (Stage III) > LA/spur > shoot elongation > fruit growth (Stages I and II) > LA/shoot. Current season F:LA had a greater influence on fruit quality than prior cropping history, underscoring the importance of imposing annual strategies to balance fruit number with LA.

Traditional sweet cherry (Prunus avium L.) training systems in the United States are based upon vigorous rootstocks and multiple leader vase canopy architectures. The sweet cherry research lab at Washington State University has been investigating the potential of new rootstocks and training systems to improve production efficiency and produce high quality fruit. This paper describes the effects of three rootstocks—Mazzard (P. avium), `Gisela 6', and `Gisela 5' (P. cerasus × P. canescens)—and four training systems—central leader, multiple-leader bush, palmette, and y-trellis—on `Bing' sweet cherry tree vigor, fruit yield and quality over a seven year period. Compared to trees on Mazzard, trees on `Gisela 5' and `Gisela 6' had 45% and 20% lower trunk cross-sectional areas after 7 seasons, respectively. Trees on `Gisela 6' were the most productive, yielding between 13% and 31% more than those on `Gisela 5' and 657% to 212% more than trees on Mazzard, depending on year. Both Gisela rootstocks significantly improved precocity compared to Mazzard, bearing fruit in year 3 in the orchard. Canopy architecture had only moderate effects on tree vigor and fruit yield. Across rootstocks, bush-trained trees were about 25% less productive compared to the other systems, which exhibited similar cumulative yields (102 kg/tree). Fruit weight was negatively and closely (r ² = 0.84) related to tree yield efficiency (kg·cm^–2). Crop value was related positively to fruit yield.

‘WA 38’ is a new apple (Malus domestica Borkh.) cultivar, released by Washington State University (WSU) in 2017. An unknown disorder, ‘‘green spot’’ (GS), dark green halos in the epidermis, with necrotic, corky, and oxidated cortical tissue underneath the damaged epidermis, leads to unmarketable fruit and has become a threat to the adoption and profitability of ‘WA 38’, with young and mature orchards exhibiting up to 60% incidence in 2020. Given the apparent susceptibility of ‘WA 38’ to GS, this research investigated GS relation with nutrient levels in fruit. Research was carried out in 2018 and 2019 in a ‘WA 38’ apple block planted in 2013, on ‘Geneva 41’ (‘G.41’) and ‘M.9-Nic 29’ (‘M.9’) rootstocks. In both years, fruit number per tree, fruit weight, and fruit diameter were evaluated in 18 trees per treatment, from both rootstocks. From each tree, fruit were classified for presence or absence of GS, and subsequently analyzed for nutrient concentration in the peel and in the flesh, nutrient extraction, and total nutrient content, on an individual apple basis. Apples with GS had higher nitrogen (N) and magnesium (Mg) levels in the peel, regardless of year and rootstock. Apples grown on ‘G.41’ rootstock exhibited higher GS incidence and reduced crop load in both years; reduced size and fruit diameter were exhibited only in 2018. Fruit on ‘G.41’ had higher N, potassium (K), and Mg in the flesh and higher N and Mg in the peel, with lower levels of calcium (Ca) in the flesh and peel; however, only in 2018, with no differences in 2019. GS in ‘WA 38’ apples appears to be another Ca-related disorder in which excessive vigor, rootstock, and N and Mg excess are predisposing factors for its development.

Although maximizing fruit size is critical for profitable sweet cherry (Prunusavium L.) production, little is known about the cellular differences among and between cultivars that contribute to fruit size differences. A wide range of fruit size exists among sweet cherries, and, due to cultural and environmental differences, significant variation exists among genetically identical fruit from the same cultivar. To determine the relative contributions of flesh cell number and cell size to final fruit size in sweet cherry, equatorial sections of three cultivars with a wide range in final average fruit size [`New York 54' (NY54; 1.4 g fresh weight, 11.8 mm diameter), `Emperor Francis' (EF; 6.1 g, 21.0 mm), and `Selah' (12.8 g, 25.5 mm)] were created from mature fruit. Cells intersecting a transverse line were counted and average cell length was calculated. The average cell numbers were significantly different (P ≤ 0.05) between `NY54', `EF', and `Selah' (26.7, 47.4, and 83.2, respectively), indicating that flesh cell number is the major contributor to differences in fruit size between cultivars. Flesh cell numbers of `NY54', `EF', and `Selah' were similar at bloom and increased rapidly for a short duration after fertilization, suggesting a key developmental period for fruit size differences. To determine the contribution of cell number differences to variation in fruit size within a cultivar, fruit from `Bing' and `Regina' trees exhibiting a range of size due to cultural and environmental differences were measured. In both cases, average cell number was not significantly different (P = 0.9, P = 0.3, respectively), while average cell size was (P ≤ 0.05), further indicating fruit flesh cell number is a genetically controlled trait.

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 II, 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%. GA₃was more inhibiting than GA₄₊₇at 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.