Written by Mokhles Elsysy, Sara Serra, Phil Schwallier, Stefano Musacchi and Todd Einhorn. Summarized by Bernardita Sallato, WSU, December 2019.
Fruit thinning is essential for sustainable and consistent production in apple (Malus x domestica Borkh.) production. Inadequate flower and fruit thinning can result in excessive crop, compromising fruit size, fruit quality and storability, as well as the following year’s flower development, tree resources, limb integrity and hardiness (Dennis, 2000, Link, 2000). Labor cost and the need for timely effective and precise crop load management remains a challenge. Thus, alternative crop load management have been a priority for the apple industry, especially for organic growers. A novel environmentally friendly approach was evaluated by Kelderer et al. (2014), utilizing exclusion netting in ‘Golden Delicious’ and ‘Golden Rush’ apple trees, with a reduction on fruit set and larger fruit. However, the response to exclusion netting appears to vary with environmental conditions and between cultivars.
In this project, Elsysy et al., 2019 evaluated the effect of exclusion netting at different bloom stages in ‘Honeycrisp’ and ‘Gala’ apple trees during 2017 and 2018. The experiment was conducted on a commercial farm in a 9-year-old ‘Honeycrisp’/M9 ‘Pajam 2’ orchard in Quincy, Washington (WA) and a 10-year-old ‘Brookfield Gala’/M9 ‘Nic 29’ orchard in Sparta, Michigan (MI). The first managed organically and the second conventionally.
The treatments consisted on complete exclusion netting utilizing Alt-carpo, 10% shading, 2.8 mm x4 mm weave (Helios® antihail systems, Bergamo, Italy), enclosing whole trees and preventing bees entry (Figure 1).
In the following summary, we are going to discuss only the portion of the experiment carried out in Washington on organic Honeycrisp.
Five treatments with four replicates were established in Washington (Table 1). In 2018, a new unthinned control (non-netted and non-thinned) was implemented in the trial, since the 2017 control (non-netted and non-thinned) was affected by alternative bearing and marked reduction in 2018.
Table 1. Net enclosure treatments in ‘Honeycrisp’ experiments in 2017 and 2018. Data shown are percent of open king bloom at the time nets were fully closed around multiple tree replicates.
Targeted |
Actual King Bloom |
|||
King Bloom % |
‘Gale’ |
‘Honeycrisp’ |
||
| 2017 | 2018 | 2017 | 2018 | |
| Non-netted | (Non-thinned) | (Non-thinned) | (Non-thinned) | (Thinned 2017) |
| Non-netted | (Hand thinned) | (Hand thinned) | (Non-thinned) | |
| Non-netted | (Hand thinned) | |||
| 0% | 0% | 0% | 0% | 0% |
| 20% | 26% | 20% | 23% | 28% |
| 40% | 60% | 40% | 58% | 68% |
| 80% | 95% | 80% | ||
Below is a summary of the results obtained from the Washington trial.
Results
‘Honeycrisp’ fruit set was similar among netting treatments in both years; however, in 2017, netting at 23% and 58% of full KB reduced fruit set relative to non-netted (non-thinned) control trees by ~42% (Figure 2C). In 2018, 28% KB and 68% KB netting treatments reduced fruit set compared to the non-netted (non-thinned) control trees by ~61% (Figure 2D). The non-netted 2017–2018 non-thinned treatment had 49% fruit set due to biennality. Enclosing canopies before flowering (i.e., ‘pink’) did not reduce fruit set compared to trees netted that had higher percentages of open bloom present before netting, regardless of cultivar or year (Figure 2).
Netting significantly reduced the number of mature seeds per fruit compared to non-netted trees, in both years (Figure 3). Seeds were classified as mature, nondeveloped or nonfertilized. In both years, mature seed counts were significantly higher in the control treatments compared to the netted trees in all bloom stages (0%, 23%, and 58% KB) (Figure 3).
In ‘Honeycrisp’, the highest number of nonfertilized seeds/fruit was observed in netted 0% KB for both years.
Netting significantly reduced yield and fruit number and increased fruit weight compared to non-thinned, non-netted controls in both years (Table 2). Yield of netted trees in 2017 and 2018 was similar to hand-thinned control trees and was commercially acceptable. Hand thinning markedly increased fruit weight compared to net treatments and non-thinned control in 2017 but not in 2018.
Table 3. Effect of netting treatments in 2017 and 2018 on average tree yield and number of fruit, average fruit weight, and fruit quality attributes; red color (percentage red), flesh firmness, soluble solids concentration (SSC), nondestructively predicted dry matter (%), and shape of ‘Honeycrisp’ fruit 1 month after harvest (RA storage).
| Treatments | Avg. Tree Yield | YE | Cropload | Fruit Weight | Red Color | Firmness | Dry Matter | SSC | Shapez | |
| (kg) | (Fruit no.) | kg-cm2 | (g) | (%) | (kg) | (%) | (%) | (%) | (% Misshapen) | |
| ‘Honeycrisp’ 2018 | ||||||||||
| Non-netted (Non-thinned) | 35.1 ay | 366 a | 1.4 a | 14.5 a | 96 c | 53 a | 6.5 | 14.5 d | 11.1 b | 47.6 |
| Non-netted (Hand thinned) | 27.8 ab | 139 b | 1 b | 5 c | 204 a | 46 a | 6.6 | 15.2 bc | 11.4 ab | 33.8 |
| Netted 0% (Pinkw) | 22.3 b | 148 b | 0.9 b | 6.3 bc | 155 b | 44 ab | 6.7 | 15.8 a | 12 a | 45.1 |
| Netted 23% (KB) | 31.7 ab | 213 b | 1.1 b | 7.1 bc | 151 b | 32 b | 6.5 | 15.6 ab | 11.5 ab | 47.7 |
| Netted 58% (KB) | 31.6 ab | 217 b | 1.2 b | 8.4 b | 146 b | 31 b | 6.3 | 14.8 cd | 11 b | 48.4 |
| ‘Honeycrisp’ 2018 | ||||||||||
| Non-netted (Thinned 2017) | 37.7 a | 328 a | 1 a | 11.1 a | 94 c | 55 ab | 7.1 c | 15.1 b | 12.1 d | 45.1 |
| Non-netted (Non-thinned) | 5.1 c | 35 b | 0.18 c | 1.2 b | 152 b | 68 a | 8.3 a | 17.6 a | 14.7 a | 32.9 |
| Non-netted (Hand thinned) | 16.9 b | 74 b | 0.58 b | 2.5 b | 234 a | 52 bc | 7.6 b | 17.3 a | 14.2 a | 48 |
| Netted 0% (Pink) | 12.1 bc | 50 b | 0.47 bc | 2 b | 240 a | 40 c | 7.6 b | 17.3 a | 13.6 b | 54.5 |
| Netted 28% (KB) | 14.9 bc | 59 b | 0.46 bc | 1.8 b | 257 a | 45 bc | 7.5 bc | 17.1 a | 13 c | 56.4 |
| Netted 68% (KB) | 16.9 b | 71 b | 0.58 b | 2.4 b | 242 a | 49 bc | 7.6 b | 17.2 a | 13.3 bc | 51.6 |
| w Pink, pink stage of bud development; KB, king flower bloom. y Mean separation among treatments by Tukey HSD (p<0.05), whereby means associated with different letters are significantly different. Data are means of four replicates. Mean separation among treatments by Tukey HSD (p<0.05), whereby means associated with different letters are significantly different. n = 4 for yield, YE, cropload, and avg. fruit wt; n = 24 and 40 for individual fruit quality attributes for 2017 and 2018, respectively. z Shape was expressed as percentage of the fruit asymmetrical in both horizontal and vertical planes. |
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In terms of fruit quality, apples from the 23% and 58% netted treatments had significantly lower percentage of red color than non-netted fruit one month after harvest in 2017 (RA cold storage at 34 F), while in 2018 the values were not statistically different from the non-netted and hand thinned control (farm standard practice). Probably a combination with reflective fabric will help to increase the overcolor. Trees netted at ‘pink’ had the highest SSC and predicted dry matter content in 2017, one Month after harvest as well as 6 Month after storage. Non netted 2017–2018 (non-thinned) apples had the highest percentage of red color, SSC, and DM% compared to all other treatments. Netting did not affect fruit shape.
Conclusions
Numerous benefits associated with netting include the exclusion of insect pests, hail protection, and mitigation of abiotic stress. In this project, the authors evaluated the potential for netting to reduce pollination and fruit set. The results showed consistent fruit set and crop levels when utilizing exclusion netting. Commercial fruit set was achieved under netting but was not markedly influenced by the percentage of open flowers at the time of canopy enclosure. ‘Honeycrisp’ showed a significantly reduced fruit set when nets were applied at either 23% or 58% KB and 28% or 68% KB (respectively in 2017 and 2018) compared to non-thinned, non-netted trees. In 2018, fruit set of the three netted treatments was comparable to the hand thinned control. In ‘Honeycrisp’, netting may provide an alternative approach to crop load management for organic orchards as previously suggested for ‘Gold Rush’ and ‘Golden Delicious’ (Kelderer et al 2014). With an alternative to hand thinning or chemical thinning, particularly in organically managed systems.
Acknowledgments
Helios® Anti-Hail Systems, Ag-Liner, Inc. S. Musacchi and S. Serra, Stefan Roeder, Brendon Anthony, Ryan Sheick, Alex Goke, and Angela Knerl.
To review the full paper, visit https://www.mdpi.com/2073-4395/9/9/478/htm.
Contact
Stefano Musacchi
Endowed Chair of Tree Fruit Physiology and Management
Washington State University
98801 Wenatchee, Washington State (USA)
Email: stefano.musacchi@wsu.edu
Phone: (+ 1) 509-293-8787
