Optimizing GA₃ fruit set applications in Navel sweet orange to prevent shoot die-back: a case study in South Africa

Following an investigation after shoot die-back on certain navel cultivars such as ‘Cara Cara’ after commercial fruit set foliar application of gibberellic acid (GA₃) at night and at low temperatures was identified as a common denominator. A subsequent study was conducted to test the impact of treatment dosage, low night temperatures and a reduction in drying time GA₃ on Navel sweet orange shoot die-back, using ‘Cara Cara’ Navel as a model cultivar in Kirkwood, Eastern Cape, South Africa. A foliar application of either 10 or 20 ppm GA₃ (ProGibb^®, Philagro SA, Valent Bioscience) + non-ionic wetting agent + buffer + water (6.25 L spray mixture tree^‑1) was applied at night, and at temperatures < and > 10°C (Exp. 1). An additional experiment was performed where 20 ppm GA₃ was applied to inside and outside shoots < and > years, respectively, and covered with plastic for 24 h (Exp. 2). In Exp. 1, symptoms were present 22-30 d after application, with leaves turning light green and wilted. Symptoms appeared sporadic on certain trees and were mostly located inside the tree canopy. In some instances, the 20 ppm GA₃ treatment applied at night and <10°C treatment, had a large branch showing shoot die-back symptoms. In Exp. 2, only 1.6% of all treated shoots developed symptoms within 14 d, however, they were <2 years old. To conclude, the symptoms were mild as opposed to those observed in the commercial applications in the 2020 season, indicating the role of climate (low night temperature) and low rainfall differences between seasons. However, the reason for some shoots being more susceptible to GA₃ remains unclear. An improved understanding of the interaction of GA₃ with citrus trees would enable the update of current recommendations to producers to ideally prevent phytotoxicity in sensitive cultivars after application whilst improving fruit set efficiency. Due to the importance of GA₃ in obtaining fruit set in seedless cultivars, this line of research should be continued.