Ethephon (Ethrel, Ethephon II)
Ethephon is an ethylene-releasing material that can advance fruit maturity, increase red color, increase flowering, and thin fruit in apples. In cherries, it can be used to increase color and help loosen fruit. Ethrel must be applied in spray mixtures that have a pH less than 8.5. At a pH above 8.5, Ethrel does not break down to form ethylene. Do not apply Ethrel on cherries when temperatures exceed 85°F, or on apples when temperatures exceed 90°F. Ethephon may be used as a fruit thinner for apples, but the amount of thinning can be excessive, especially when hot (> 80°F) weather follows the application. Ethephon will thin apples up to 20 mm in diameter, making it valuable as a rescue thinner. To improve fruit loosening and red color in apples, apply 7 days before normal harvest and include a suitable stop-drop material such as NAA. Where Ethrel is applied to bearing trees to increase flowering for the next year, some reduction in fruit size can occur. To avoid a thinning response on apples when trying to increase flowering, delay application until 6 weeks after bloom or June drop, whichever is later. Ethrel is not very mobile within the tree; therefore, to obtain the desired response good spray coverage is essential so that the material reaches the fruit.
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