Ethylene plays a significant role in flower and fruitlet abscission. The use of compounds that generate ethylene for chemical thinning of fruit trees has been in the focus of research for decades. 1-aminocyclopropane-1-carboxylic acid (ACC) is a naturally occurring precursor to ethylene and has recently been registered as a chemical thinner in the US and elsewhere for stone fruit and apple. Ethephon has also been evaluated for chemical thinning of peaches but phytotoxicity and inconsistency in efficacy has hindered its commercial use. With the recent registration of ACC for chemical thinning of peaches, questions arose if similar efficacy can be achieved with the use of ethephon. The objective of the study reported here was to compare thinning efficacy and phytotoxicity of ACC (300 mg/L) and ethephon (428 mg/L; molar equivalent to 300 mg/L ACC) when used in peaches. A field experiment was conducted in Red Top peach variety in Oregon where the two compounds were sprayed to different set of trees every day from pink bud to petal fall to evaluate the effect of daily weather conditions on flower abscission. Phytotoxicty was also recorded in the experiment. ACC and ethephon showed similar thinning pattern throughout the flowering time. While no phytotoxicity was observed for ACC, severe leaf drop and gummosis occurred in the ethephon treatments. The presentation will discuss the potential reasons for the efficacy and difference in level of phytotoxicity between the two compounds.
