Ethephon as a bloom-delaying agent for frost-sensitive tree crop flowers at risk from a changing climate

Background Climate change increases the risk of late spring frosts, which damage flowers and developing fruitlets in tree crops. Ethephon, a plant growth regulator, may delay flowering and potentially reduce frost damage. Methods We investigated the effects of ethephon on flowering time and fruit set in two sweet cherry (Bellise and Kordia) and two apple cultivars (Elstar and Kanzi). Treatments included two ethephon concentrations (500 and 750 ppm) applied either in early or late autumn. Results Ethephon significantly delayed flowering in cherry cultivar Kordia, but not cultivar Bellise, with a lower proportion of Kordia floral buds in bloom in two of four treatments compared to controls. Specifically, on 9 April 88% of floral buds on control trees were in bloom, compared to 56% and 63% on trees treated with high ethephon in early autumn and low ethephon in late autumn, respectively. No significant flowering delay was observed in the apple cultivars. Initial and final fruit set were generally unaffected, although Kordia showed a reduction in final fruit set from 16 to 6% after the late-high treatment. Conclusions Our results indicate that ethephon can effectively delay bloom without major yield loss, offering a practical tool to mitigate spring frost damage under climate change. However, its effectiveness was crop- and cultivar-dependent.