The use of plant growth regulators to increase apical extension growth of first-leaf apple trees

In regions with inadequate winter chilling, apple trees often do not develop optimally in terms of leader extension growth due to basal dominance. Plant growth regulators (PGRs) are used as summer foliar applications to induce lateral branching in nurseries, but not to increase apical extension growth in the orchard. Field trials were conducted on 'Cripp's Red' on MM109 in 2013 and 2014 in Elgin, South Africa (34.1486°S; 19.0428°E). The effects of five 2-weekly apical foliar PGR applications starting from November 26, 2013 were compared. The PGRs used were; 800 mg L^-1 6-benzyl adenine + gibberellin (GA)₄₊₇ (Promalin^®), 800 mg L^-1 GA₄₊₇ (Regulex^® 10%), 400 mg L^-1 GA₃ (Progibb^® 40%). In addition, a single foliar application of 280, 420 or 560 g L^-1 prohexadione-calcium (ProCa) (Regalis^TM 10%) was made to the lower tree canopy on November 26, 2013. ProCa at 280 and 420 g L^-1 did not increase apical extension growth significantly and ProCa at 420 g L^-1 did not differ from GA₃. Promalin and GA₄₊₇ increased apical extension growth by 50%. The treatments in 2014 were ten, weekly apical foliar PGR applications starting from November 12, 2014 of 400 mg L^-1 GA₄₊₇ and 400 mg L^-1 GA₃, as well as five 2-weekly applications of 800 mg L^-1 Promalin, 600 and 800 mg L^-1 GA₄₊₇, 600 and 800 mg L^-1 GA₃. All the treatments increased apical extension growth significantly with the biggest increase (±100%) obtained with the weekly application of 400 mg L^-1 GA₄₊₇ in 2014 which differed significantly from the 2-weekly application of 600 mg L^-1 GA₄₊₇ and 800 mg L^-1 Promalin. Weekly applications of GA₄₊₇ at 400 mg L^-1 or 2-weekly applications at 800 mg L^-1 appeared to result in the best apical extension growth.