Pg. 5-10 The Long-term Effect of Growth Regulators and Pruning on the Size, Yield and Fruit Quality of Litchi Trees

Methods to control the size of adult litchi trees were investigated at Friedenheim (25°S) over a five-year-period. The treatments consisted of pruning fruit-bearing branches by 30 cm at harvest; applying foliar sprays of paclobutrazol (PP333} onto postharvest flush and young flower panicles; drenching soil with PP333 and a combination of pruning and
soil applications with PP333 and Chlormequat-chloride (CCC) sprays of post-harvest flush and young flower panicles. Annual yields and fruit quality were recorded. Residue analysis of PP333. in fruit was also determined. Tree height and canopy diameter of trees of the different treatments were measured before commencement, during and after termination of the project.

Effective size control and manipulation of litchi trees were achieved by means of pruning, soil application of PP333 and a combination of these two treatments. These treatments, however, require more intensive and higher inputs. Higher yields are therefore required to economically justify their use, but this was not always achieved and a wider variation in yield occurred than in control trees. There was as much variation in fruit quality from season to season as yield, which made interpretation of results in this case difficult. Residues were present in fruit of the trees that were treated with PP333. The detection of traces of PP333 residues mitigated against a recommendation for its use on a commercial scale, particularly if the crop is to be exported.

Pruning of bearing branches can be recommended as a method of tree size control. It controlled tree growth without affecting yield significantly, in the long run, while being an environmentally friendly manipulation technique. CCC cannot be recommended from the results of this study. Yields over the five seasons of this trial were not improved substantially, and probably not economically, if used in conventionally spaced orchards.

Summary:

• Objective: The study investigated long-term effects of growth regulators and pruning on adult litchi tree size, yield, and fruit quality over five years.
• Treatments tested: Various pruning methods, foliar sprays of paclobutrazol (PP333), soil application of PP333, combination of pruning and soil PP333, and chlormequat chloride (CCC) sprays.
• Key results:
  • Effective tree size control was achieved using pruning, soil-applied PP333, or a combination of these; foliar sprays less effective.
  • Yield outcomes: Treatments required higher inputs and labor; only higher yields can justify their use economically, but this was not consistently achieved. Yield and fruit quality varied widely between seasons.
  • Fruit quality: Showed similar seasonal variation to yields, making interpretation difficult. Presence of PP333 residues observed in fruit arils for soil-applied treatments, raising concerns for export.
  • CCC treatments did not improve yield or were economically beneficial in conventionally spaced orchards and are not recommended.
• Pruning: Can be recommended as an environmentally friendly, effective tree size control technique with minimal negative impact on long-term yield.
• PP333 (soil application): Controlled tree size but did not consistently increase yield in standard orchards; traces found in export-quality fruit may restrict commercial use.
• General insights: Proper timing and application are crucial for effectiveness of treatments. Overdosing and drought stress can significantly reduce success.
• Conclusion: Pruning is best for controlling tree size without affecting yield; soil-applied PP333 is effective but not recommended for commercial/export due to residue. Further research suggested, especially into mechanical pruning for better yield-size balance.