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Effect of multi-leader training systems and Prohexadione-Ca applications on apple cultivar ‘WA 38’

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Written by: Md Jebu Mia, Sara Serra, Ryan Sheick and Stefano Musacchi, WSU Horticulture; Tianna DuPont, WSU Extension. March 1, 2023 


Adapting modern technology to the apple orchard has allowed growers to shift towards more planar orchards designed with multi-leader planting systems. Top-working is an innovative approach performed on existing and underperforming mature orchards by grafting new varieties over old ones. With this technique, fruit growers can reduce the initial orchard set-up costs and benefit from early bearing by utilizing the existing root system. Thus, the top-working system can transform an unproductive orchard into a highly productive and efficient one. The ‘WA 38’ apple variety has gained widespread consumer satisfaction, and its appeal provides the opportunity for growers to rapidly modernize their orchards by top-working with this new cultivar. With this system, the grafted scion grows strongly in the years following the grafting due to the abundance of resources stored in the mature root system, which can induce excessive growth of the grafted cultivar early in its development. Sustaining a proper balance between vegetative and reproductive growth in fruit trees is crucial for minimizing competition between fruits and shoots, enhancing light interception, and improving fruit quality (Meintjes et al., 2005; Musacchi and Serra, 2018). Tree growth can be managed by different types of pruning, which can be expensive and time-consuming. Practicing multi-leader training systems, such as bi-axis (2-axis), can reduce tree growth by doubling branches and reducing shoot length compared to spindle trees (Dorigoni et al., 2011). In addition, plant growth retardants like Prohexadione-Calcium (Pro-Ca) can be used to control tree growth and restrict canopy volume. The reduction in shoot vigor resulting from Pro-Ca application is reportedly attributed to the inhibition of active gibberellin synthesis (Rademacher et al., 2006). 

The main objective of this study was to evaluate the effect of Pro-Ca on different horticultural aspects of top-grafted ‘WA 38’ under three innovative training systems. 


The experiment was conducted at Washington State University’s Sunrise research orchard (Rock Island, WA). In March 2016, four bud sticks of ‘WA 38’ were top-grafted on each tree stump in an 8-year-old ‘Granny Smith’/’M.9’ orchard. Three months after grafting (June 2016), healthy grafts were selected and trained to a 1-axis (spindle), 2-axis (bi-axis), or 3-axis system following a modern mid-high-density planting system, and extraneous grafts were removed. Trees were treated with or without Pro-Ca (Apogee®) for two years. Post-bloom application of Pro-Ca was performed at the rate of 250 ppm (12 oz per 100 gal per acre) per spray and three sprays per season with an air blast sprayer to test the effect of Pro-Ca on ‘WA 38’ tree vigor, fruit yield, and quality.  

Results and Discussion

Prohexadione-Calcium reduced vigor 

The results of our study illustrated that Pro-Ca could significantly reduce shoot length by 35% to 42% compared to control trees in two consecutive years. As a result, trees treated with Pro-Ca produced less winter pruned material, ranging from 27% to 58% compared to control trees, demonstrating that Pro-Ca application reduces winter pruning regardless of the training system. The number of flower clusters was positively affected by Pro-Ca treatment, while an inverse trend was found for the percentage of fruit set. Regarding the training system, the number of leaders was positively associated with the number of flower clusters (avg. +47 flower clusters more per tree in 3-axis than 1-axis in 2019) but not with fruit set percentage (data not shown).  

Physiological parameters 

Relevant physiological parameters were also assessed during the study, including photosynthetic carbon assimilation rate, stomatal conductance, transpiration, and intercellular CO2. All physiological parameters except photosynthetic carbon assimilation rate were significantly higher in leaves treated with Pro-Ca. The same trend was also observed for both SPAD (a non-destructive measurement of leaf chlorophyll concentrations) and traditional destructive measurement of leaf chlorophyll content. However, different training systems significantly affected SPAD values, where values measured in the 1-axis system were higher than in the 3-axis system. Leaf mineral elements such as nitrogen (N), chlorine (Cl), potassium (K), and calcium (Ca) were inconsistently affected by Pro-Ca treatment, but none of the elements showed any differences across the training system (data not shown).  

Yield and fruit size 

Generally, the ‘WA 38’ cultivar naturally sheds most of the fruitlets in a cluster within the first few weeks following petal fall, which allows to set a regular crop for this variety without performing any additional thinning. Under typical orchard environments, 51% of the clusters set a single fruit in ‘WA 38’ (Serra et al., 2021). However, there was no effect of Pro-Ca on either fruit number or fruit yield per tree in both years of trial. The crop yield in the first year of bearing was very low, ranging from 0.8 kg to 1 kg per tree in both Pro-Ca and control trees, respectively (Table 1). The crop yield was significantly improved in the following year under both treatments, ranging from 12.8 kg to 15.9 kg per tree, although the difference between treatments was not statistically significant (Table 1). 

Regarding the number of leaders, there was a general positive correlation between the number of leaders and their corresponding fruit number and yield. The 3-axis trees produced a significantly higher fruit number and yielded more fruit mass per tree in both seasons than the single-axis system (Table 1). Analyzing the interaction effect between treatment and training systems, trees from the 3-axis were 88% more productive than single-axis trees under the control treatment (data not shown). On the contrary, we found statistically insignificant yield differences across the training system under the Pro-Ca treatment, where all 3 training systems produced relatively reduced fruit yields, indicating that Pro-Ca can reduce fruit yield. Overall, the 3-axis system recorded a significantly higher fruit number and yield per tree in both years of trial. ‘WA 38’ fruit size distribution, especially in 2019, was significantly impacted by Pro-Ca, where 25% and 47% of apples were obtained in the extra-large size class (56 to <48 apples/box) under control and Pro-Ca-treated trees, respectively (data not shown). Pro-Ca combined with 3-axis trees produced larger fruits than the same training system in control (Figure 1). Overall, all training systems demonstrated a more consistent average fruit size when combined with Pro-Ca compared to the corresponding control combination, where fruit size categories were more distributed (Figure 1), highlighting the positive effect of Pro-Ca on ‘WA 38’ fruit size.  

Table 1: Productivity data for a ‘WA 38’ top-grafted orchard in Rock Island, WA: Comparison between Pro-Ca treatment and control (no spray) and between the 3 training systems in 2018 and 2019. Different letters within comparisons indicate statistical differences between means (p = 0.05) by SNK. Significance: * = p ≤ 0.05; ** = p ≤ 0.01; *** = p ≤ 0.001; NS = not significant.  

Treatment and training system  Fruit
(N/ tree)
Yield (kg/ tree) 2018  Avg. apple weight (g) 2018  Production (Mton/A) 2018  Fruit
(N/ tree) 2019 
(kg/ tree) 2019 
Avg. apple weight (g) 2019  Production (Mton/A) 2019 
Control  3  a  1.0    335  2  61  a  15.9    269  b  24   
Pro-Ca  2  b  0.8    350  1  41  b  12.8    313  a  19   
Significance trt  *    NS    NS  NS  **    NS    ***    NS   
1-Axis  2  b  0.7  b  361  1  41  b  12.1  b  301    18  b 
2-Axis  2  b  0.7  b  343  1  50  ab  13.8  ab  290    21  ab 
3-Axis  4  a  1.3  a  323  2  63  a  17.0  a  283    26  a 
Significance training  ***    **    NS  **  *    *    NS    *   
Significance trt*training  NS  NS  NS  NS  *    *    NS    *   


graph proportion of graded fruit to apple size
Figure 1. ‘WA 38’ fruit size distribution at harvest 2019 in the top-grafted orchard in Rock Island, WA: comparison between 6 combinations of Pro-Ca treatment and control (no spray) and the 3 training systems. Apple size classes (in x-axis) are reported in mm and in N apples/US box. Each distribution represents the whole apple production harvested from 3 representative trees (total 18 trees). 


In conclusion, our study presents the effects of top-working ‘WA 38’, a new, vigorous cultivar, on existing trees in the orchard, which can be a cost-effective alternative to a full orchard removal and replanting. The results of this study confirmed that repeated application of Pro-Ca effectively reduces excessive tree vigor and the amount of wood removed during winter pruning, and delivers a larger and more uniform fruit size compared to the control. Moreover, the productive performance of untreated control trees in the 3-axis system was 88% higher than the single axis (spindle) training system, which suggests implementing multi-leader systems in the orchard may improve yield over single-leader systems.  


  1. Dorigoni, A., Lezzer, P., Dallabetta, N., Serra, S., Musacchi, S., 2011. Bi-axes: An alternative to slender spindle for apple orchard. Acta Hortic. 903, 581–588.  
  2. Meintjes, J. J., Stassen, P. and Theron, K. I., 2005. The effect of different rates of prohexadione-calcium and girdling on shoot growth and fruit quality when applied to different pear cultivars. Acta Hortic. 671, 539-546. 
  3. Musacchi S., Serra S., 2018. Apple fruit quality: overview on pre-harvest factors. Sci. Hortic. 234, 409-430. 
  4. Rademacher, W., Spinelli, F., Costa, G., 2006. Prohexadione-Ca: modes of action of a multifunctional plant bioregulator for fruit trees. Acta Hortic. 727, 97-106 
  5. Serra S., Sheick R., Roeder S. Musacchi S., 2021. WA 38 abscission and fruit development in an open pollination scenario. Acta Hortic. 1346, 129-138.


professional photo Stefano Musacchi

Stefano Musacchi

Additional information 

This paper has been published in Scientia Horticulturae.  

Stefano Musacchi, Ryan Sheick, Md Jebu Mia, Sara Serra (2023). Studies on physiological and productive effects of multi-leader training systems and Prohexadione-Ca applications on apple cultivar ‘WA 38′. Scientia Horticulturae. Volume 312, 15 March 2023, 111850. []

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