Summer Applications of NAA and Ethephon Show Little Effect on Return Bloom, Yield, Tree Growth, and Juice Quality of Cider Apple Cultivars

Foster, J. Kingsley-Richards, S.L., and Bradshaw, T. 2022. Summer Applications of NAA and Ethephon Show Little Effect on Return Bloom, Yield, Tree Growth, and Juice Quality of Cider Apple Cultivars. Acta Hortic. 1346, 511-518. Abstract For commonly grown dessert apple cultivars, chemical thinning or the removal of some fruit each season helps maintain fruit size, quality, and annual bearing characteristics. Chemical thinning is often achieved with applications of carbaryl at petal fall alone or in combination with other plant growth regulators (PGRs). This traditional thinning program used for dessert fruit does not adequately thin European-origin cider apples resulting in insufficient return bloom or inconsistent cropping from year to year. On dessert apple cultivars with biennial bearing tendencies, midsummer applications of PGRs are used to enhance fruit bud development for the following year. In 2019, experiments were conducted in two apple orchards in Vermont, US. The primary objective was to evaluate the effects of naphthaleneacetic acid (NAA) and ethephon alone and in combination with carbaryl on return bloom, crop yield, and fruit and juice quality. During the two years of the study, ‘Harry Masters Jersey’ and ‘Kingston Black’ both demonstrated strong biennial production habits producing few flowers and fruit in 2020. Ethephon applications alone and in combination with carbaryl showed advanced ripening and fruit softening in ‘Somerset Redstreak’ during the year of treatment. ‘Kingston Black’ had increased fruit softening with ethephon only applications. Growth regulator treatments did not consistently affect juice quality between cultivars. During the treatment year, 2019, all ethephon treated ‘Somerset Redstreak’ had a higher pH, and juice from trees treated with Ethephon and carbaryl had a lower titratable acidity. ‘Kingston Black’ juice was unaffected by PGR applications.