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Optimizing Crop Load for New Apple Cultivar: ‘WA38’

Written by Brendon Anthony, Sara Serra, and Stefano Musacchi, WSU, March 2019

 

Washington State University’s Apple Breeding Program released a new exclusive cultivar in 2017 to the Washington (WA) State apple industry. This apple is known as ‘WA38’, commercially traded as ‘Cosmic Crisp®.’ In the past couple years, the industry has planted millions of these trees with hopes of mitigating some of the problems associated with the production of Minnesota’s ‘Honeycrisp.’ WA38 is an apple bred specifically for the WA climate (Evans et al., 2012), and so disorders like sunburn and bitter pit, which plague ‘Honeycrisp’ production, are considered not much of a concern with WA38. However, one of ‘Honeycrisp’s greatest dilemmas is its natural tendency for biennial or alternate bearing, producing a lot of fruit one year, with little to none in the subsequent year if not properly thinned. An experimental trial in Wenatchee (WA) evaluating the effect of crop load on vegetative parameters, fruit quality, return bloom, and biennial bearing in ‘WA 38’ was conducted from 2017 – 2018 to investigate about the optimal crop load.

In 2017, in a five year old orchard, four different levels of crop load were induced on 24 ‘WA38’ trial trees: 2, 4, 6, and 8 fruits/cm2 of trunk cross-sectional area (TCSA). Fruit quality, fruit mineral analysis, leaf area and leaf mineral analysis were assessed in 2017, and the trees were left untouched (no crop load regulation) in 2018 to assess for natural return bloom and yield. The trees were trained to a Spindle system, grafted on M9-Nic29, and pruned via a “click” technique that simplifies branches, minimizes “blind wood,” and bends branches to a 45-degree crotch angle (Musacchi and Greene, 2017).

Final actual crop load levels ranged from 2.1 – 7.8 fruits/cm2 of TCSA in 2017. This resulted in a wide spread of production from 29 – 86 bins per acre, with 26 – 95 apples per tree on average in 2017 (Table 1). Fruit size on lower cropped trees were larger, apples were sweeter and more advanced in respect to maturity. Heavier cropped trees observed a slight decrease in quality and were less ripe. The fruit on heavier cropped trees also appeared to showcase more yellow in their coloration, especially on the smaller sized fruit from these trees (Figure 1). Fruit quality appeared to decline when leaf to fruit ratios (L:F) dropped below 30 leaves per fruit. This is important as a full, well-illuminated canopy is required for a high level of carbohydrates to be produced in order to maintain high fruit quality characteristics like sugar content and dry matter.

 

Figure 1. The effect of crop load on fruit size and red coloration of fruit positioned on their sun exposed side (the darkest part of the apple) from ‘WA38 apple’ trees adjusted to various crop loads grown in Rock Island, Washington in 2017. A, B, C, D indicates crop load categories 2.1, 4.0, 6.0, and 7.8 fruits/cm2 of TCSA. Fruit size diameter is indicated below with the associated label in each column. Fruit diameter 75, 80, 85, and 90 mm equate to box sizes 88, 80, 64, and 56.

 

Considering that ‘WA38’ is classified as a “bi-color” cultivar, it is suggested that the fruit receive a considerable amount of light exposure to ensure a dark red hue and full-coloration. This can be achieved through summer pruning (via thinning cuts) to open up the canopy for fruit to intercept more light, and if timed properly (when one-year old shoots reach approx. 12 leaves), this increased light penetration in the canopy can promote flower bud induction, enabling consistent yields from year to year. Mechanical pruning can also achieve this, but subsequent protection like overhead cooling or a net system, may be required to discourage sunburn on the newly exposed fruit.

There were no significant nutritional deficiencies across fruit or leaves as a result of the crop load inductions. Fruit calcium concentration was unaffected by crop load, and very little bitter pit was observed at harvest.

At the higher crop loads adjusted in 2017, 6.0 and 7.8 fruits/cm2 of TCSA, the average difference in the number of flower buds from year to year were 39 and 20, demonstrating the potential for consistent yields. This is contrasted with the lower crop loads having a larger difference in the number of flower buds from year to year, with 149 and 90 on average in the 2.1 and 4.1 categories. This led to a big swing in production, especially for the lowest crop load from 28 bins/acre in 2017, to 92 bins/acre in 2018 (Table 2). Of all the crop load categories, only one level resulted with a potential for biennial bearing: 2.1 fruits/cm2 of TCSA. The other three categories: 4.1, 6.0, and 7.8 fruits/cm2 of TCSA resulted in consistent yields from 2017 and 2018 (Table 2). The highest value of cumulated yield 167 bins for acre has been obtained with 6 fruit/cm2 of TCSA.

In conclusion, when ‘WA38’ is cropped too low, it can lead to biennial bearing, low fruit quality, and advanced maturity at harvest, which could lead to issues in post-harvest storage. Additionally, low crop loads may not be economical for production. Increasing crop loads led to higher cumulative yields across 2017 and 2018, with more than 150 + bins/acre at the 6.0 and 7.8 fruit/cm2 of TCSA (Table 1) with a peak of production at 6 fruit/cm2 of TCSA. Given our data on return bloom and yield, ‘WA38’ requires further investigation on fruit set mechanism and bienniality. It is quite difficult to set crop loads higher than the level of 8 fruit/cm2 of TCSA, given its self-thinning tendency, setting 1 – 2 fruits per cluster. Our results from this trial suggest that at moderate crop load levels, between 6 – 8 fruit/cm2 of TCSA, yields can maintain relatively stable, while also ensuring high marketability and fruit quality.

 

References

  1. Evans, K.M., Barritt, B.H., Konishi, B.S., Brutcher, L.J. and Ross, C.F. ‘WA 38’apple. HortScience, 2012, 47(8),1177-1179.
  2. Hoblyn, T.N. Grubb, N.H. Painter, A.C. Wates, B.L. Studies in Biennial Bearing—I. Pomol. Horticul. Sci. 1937, 14, 39–76.
  3. Musacchi, S. and Greene, D. Innovations in apple tree cultivation to manage crop load and ripening. In Achieving sustainable cultivation of apples; Evans, K. ed.; Burleigh Dodds Science Publishing: Cambridge, UK, 2017; pp. 195–237.

Full paper published: Anthony B., Serra S., Musacchi S.* Optimizing Crop Load for New Apple Cultivar: ‘WA38’. Agronomy MDPI, 9(2), p.107.

Link for initial paper: https://www.mdpi.com/2073-4395/9/2/107

 

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