Crop Load Overwhelms Effects of Gibberellic Acid and Ethephon on Floral Initiation in Apple

Potential strategies against biennial bearing in apple [Malus x sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] include promotion of return bloom with an on-year application of ethephon or inhibition of return bloom with an off-year application of gibberellic acid (CA), but the influence of initial crop load on the efficacy of these bioregulators is poorly understood. In 2004 and 2005, six total trials were initiated in which whole trees were manually adjusted shortly before anthesis to one of three levels of crop load (100%, 50%, 0%) in 'Cameo', 'Honeyerisp', and 'Fuji'; GA(4) (+ 7) was overlaid on trees of each crop level in four trials and ethephon in two. In all trials, initial crop load was the primary determinant of return bloom; proportional influence on flower density, fruit density, and yield was generally most pronounced at the 50% crop level. GA(4 + 7) consistently reduced floral initiation, whereas ethephon promoted it. Flowering responses from a historically alternating 'Cameo' trial site showed greater sensitivity to ethephon and less sensitivity to GA(4 + 7) than did responses from parallel trials established in an annually bearing 'Cameo' block, suggesting a predilection of nascent buds to a specific fate before the influence of exogenous bioregulators or gibberellins from seeds produced in developing fruit. Light crop loads and GA(4 + 7) applications generally promoted shoot extension, whereas heavy crops and ethephon had the opposite effect.