Key strategies for managing crop load and reducing alternate bearing
Alternate bearing, or biennial bearing, is a common challenge in citrus production. It refers to the natural cycle where a tree produces an abundant fruit load in one season (“on-year”), followed by a significantly reduced yield in the next season (“off-year”). This pattern is observed in various citrus cultivars, particularly mandarins, tangelos, and tangors. The issue affects fruit quality and marketability, leading to financial consequences for growers.
1. Pruning
Winter pruning is a valuable tool for managing crop load and reducing alternate bearing. When anticipating an “on-year”, heavier pruning should be done to remove some bearing positions, ensuring a more balanced yield across seasons.
Summer pruning, followed by regrowth management, helps maintain a balance between sunlight penetration and the development of new fruit-bearing shoots. Selectively pruning branches with small fruitlets during an “on-year” can improve fruit size and quality.
2. Fruit thinning
Thinning is a critical practice in managing alternate bearing. Excessive fruit load in an “on-year” depletes tree resources, leading to a weak yield in the following season. Identifying a sustainable yield per hectare for each variety is essential for effective thinning.
Chemical thinning
When fruit set is high, synthetic auxins such as 2,4-D (Dichlorprop, Corasil-P®) and 3,5,6-TPA (Maxim®) can be applied at specific fruitlet sizes to encourage controlled fruit drop and improve fruit size. Application timing is crucial:
- Earlier application (smaller fruit size): Leads to more thinning and increased fruit size.
- Later application (larger fruit size): Improves fruit size without significant thinning.
These products should be applied carefully, considering factors like soil type, rootstock selection, and nutrient levels to avoid negative effects such as fruit granulation.
Summer hand thinning
Selective hand thinning of smaller, damaged, or misshapen fruit helps improve the overall pack-out percentage. Research on Nadorcott mandarins has shown that summer hand thinning can effectively increase the proportion of large and premium-sized fruit while minimizing alternate bearing effects.
3. Timely harvesting
Harvesting fruit as soon as it reaches maturity during an “on-year” prevents trees from depleting energy reserves. Delaying harvest to extend the commercial picking window can intensify alternate bearing by reducing the return bloom for the next season.
4. LB Urea sprays for flower induction
In “off-years,” applying foliar LB Urea at the right time (typically July–August) can stimulate a more intense bloom. Consulting a crop advisor for correct dosages and application timing is essential to maximize its effectiveness.
5. Controlled water stress
Flower induction can be enhanced by controlled water stress during the autumn and winter months (April–July in South Africa). This practice mimics natural drought conditions, promoting a stronger bloom and improving yield potential in an “off-year.”
6. Autumn girdling
Girdling, a technique that involves removing a strip of bark from branches, has been shown to intensify bloom and reduce alternate bearing in some citrus cultivars. This practice increases carbohydrate reserves in the tree, supporting better flower development. However, it should be tested on a small scale before widespread adoption, as results may vary by orchard.
7. Gibberellic acid applications
Applying gibberellic acid between 50% and 90% petal drop can improve fruit set in an “off-year.” When combined with girdling, this approach has been shown to increase fruit set and final yield in some citrus cultivars.
Conclusion
Maintaining a balanced crop load is the key to mitigating alternate bearing in citrus orchards. Since fruit production is hormonally linked to vegetative growth and flowering, managing crop load through pruning, thinning, timely harvesting, and strategic chemical applications is essential.
Growers should prioritize quality over quantity by reducing excessive fruit loads in “on-years” through pruning and thinning. This approach ensures that fruit meets market specifications while sustaining tree health and productivity over multiple seasons. By implementing these crop load management strategies, citrus producers can achieve more consistent yields, larger fruit sizes, and improved long-term profitability.
Acknowledgements: Thank you to Dr OPJ Stander (Product Manager at Philagro) for his input.
For more information, read the full article in ROOTED magazine (Issue 4) at https://duroinursery.co.za/media/
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