Plant growth regulators do not reduce biennial bearing of two cider apple cultivars in Vermont, USA

Crop load management is a critical component of commercial apple production. For commonly-grown dessert cultivars, removal of a portion of fruit each season via chemical thinning helps to maintain fruit size, quality, and annual bearing characteristics. Commonly-used thinning protocols in northeastern USA orchards include application of carbaryl at petal fall alone or in combination with other plant growth regulators (PGRs). European-origin cider apple cultivars typically do not respond similarly to chemical thinning programs used in production of dessert fruit. In the past two decades, application of PGRs at a later timing in midsummer has increasingly been used to increase fruit bud development for the following year on certain dessert apple cultivars with pronounced biennial bearing tendencies. In 2016 and 2017, multiple PGR programs were applied to 'Kingston Black' and 'Ellis Bitter' trees in a commercial orchard in Vermont, USA. Trees were evaluated for crop yield and juice quality in each season. During the two years of the study, both cultivars exhibited a strong biennial production habit. Within each year, PGR treatments had inconsistent effects on crop yield, but no effects of practical significance on juice quality. Despite a lack of PGR effects on substantially reducing biennial bearing, interactions of specific treatments and cultivars may inform continued research on this topic.