Pg. 23-32 Optimising the use of ethephon for cooler mid-season production areas

In mid-season production areas such as Nelspruit and Hazyview, flower induction in ‘Mauritius’ litchi takes place between mid-April and mid-May. Although water stress is a good way to suppress leaf flush before and during flower induction, late autumn rains often make it impossible to apply water stress properly. Therefore, the only way to control autumn/winter flush for mid-season areas is the application of ethephon to burn off the unwanted flush. Currently, ethephon is used as spot spray to control autumn/winter flush, but such applications generally have to be repeated several times due to re-occurring flush. In order to reduce the number of applications, trials were initiated to determine whether a once-off full-cover ethephon application after the last post-harvest flush has hardened (i.e. dormant trees), could inhibit new leaf flush long enough until temperatures become conducive for flower induction. The trials were conducted in the Nelspruit area on ‘Mauritius’ trees during the 2014 to 2016 litchi seasons. Ethapon^® (a.i. 48% ethephon) was applied as a once-off full-cover spray after hardening of the last desired post-harvest flush at concentrations of 0, 500, 750, 1000 and 1250 ppm at the end of March, mid- and end of April. The control trees were treated with spot sprays as per current industry practice (at 1000 ppm whenever young shoots appeared). Shoot control on the control trees started at the end of March and needed to be repeated two to three times for sufficient flush control. In all three years, concentration and timing of full-cover ethephon applications affected growth of leaf flush and time of flower panicle emergence. The higher the concentration applied, the better was flush inhibition during April/May and the later was flower panicle emergence. Climatic conditions during April/May also influenced tree reaction, with higher temperatures reducing the dormancy period and vice versa. Drought conditions prolonged the dormancy period, even in the untreated trees. Temperature, stress conditions and flush maturation time will determine what ethephon concentration is most effective.

Summary:

• The study focuses on controlling leaf flush in Mauritius litchi trees in mid-season cooler production areas like Nelspruit and Hazyview where flower induction occurs from mid-April to mid-May.
• Water stress is an effective flush suppressant but unreliable due to late autumn rains in this region, so ethephon is used to control unwanted leaf flush by killing young shoots.
• Current industry practice involves spot spraying ethephon multiple times to control recurring flush.
• The study tested if a once-off full-cover spray on dormant trees after the last post-harvest flush could suppress flush long enough for flower induction temperature conditions.
• Trials took place from 2014-2016 with full-cover ethephon at 0, 500, 750, 1000, and 1250 ppm applied at end March, mid-April, and end April.
• Key findings:
  • Higher ethephon concentrations and later application timings provided better flush inhibition and delayed flower panicle emergence.
  • Climatic conditions influenced flush and flowering; higher temperatures shortened dormancy, drought prolonged it.
  • Full-cover single application could reduce the need for multiple spot sprays, being cost-effective.
  • Concentrations of 750-1000 ppm were most effective for longer dormancy and flush control, especially when applied before mid-April.
  • Applications after mid-April required concentration adjustment based on temperature; lower concentrations were better in cool autumns.
  • Flower panicle emergence delay reduced leafy panicles and increased flowering with more but smaller flower panicles per branch.
  • Yield correlated with good flush control and flowering percentage; fruit size was unaffected but fruit maturity was advanced by ethephon.
  • The highest concentration (1250 ppm) provided no added benefit over 1000 ppm and is not recommended.
• The study concludes a single full-cover ethephon application to dormant trees up to mid-April is equal or superior to current multiple spot sprays for flush control and flower timing in mid-season cooler areas.
• Future research is recommended to explore the physiological mode of action of ethephon with respect to bud dormancy, hormone interactions, and gene expression.
• The study was supported by the South African Litchi Growers Association and Agricultural Research Council.