The Influence of Prohexadione-Calcium Sprays on Apple Tree Growth, Chemical Fruit Thinning, and Return Bloom

Abstract

P-Ca [formulated as BAS-125 (10% P-Ca) or Apogee™, (27.5% P-Ca + 56.1% (NH ₄) ₂S0 ₄+ 16.4% other proprietary additives)] applied to ‘Fuji’/M.9 trees in 3 applications at 250 mg•L ^-1(a.i. P-Ca), increased fruit set compared with the unsprayed control. Chemical thinning sprays reduced crop load and promoted tree growth in both P-Ca and non-P-Ca sprayed trees. P-Ca did not affect chemical fruit thinning when chemical thinners were applied between the first and second of three P-Ca applications. Carbaryl + oil, carbaryl + Accel + oil, oxamyl + Accel caused fruit thinning, but ethephon or shading trees for 3 days did not. Shading + P-Ca sprays caused additional shoot growth suppression, which suggested that P-Ca may be light labile. In a second experiment, three sprays of P-Ca [BAS 125: 10% P-Ca; 90% (NH ₄) ₂SO ₄] at a low rate of 63 mg•L ^-1or ethephon at 135 mg•L ^-1applied to ‘Fuji’/M.9 did not affect shoot growth; however, the combination of P-Ca + ethephon at these rates gave good control of tree growth without influencing flowering, fruit set, or fruit abscission. In a third experiment, GA ₃alone did not promote shoot growth when compared with the unsprayed control, but GA ₃sprays partially counteracted the shoot growth suppression of P-Ca + (NH ₄) ₂SO ₄+ Regulaid. Flower bud formation was inhibited by GA ₃in the next season, but P-Ca had no effect on return bloom or return fruit set.

Chemicals used: Prohexadione-calcium (P-Ca) formulated as BAS-125 (10% P-Ca + 90% (NH ₄) ₂S0 ₄) or Apogee™, (27.5% P-Ca + 56.1% (NH ₄) ₂S0 ₄+ 16.4% other proprietary additives), Regulaid® (a surfactant mixture, polyoxyethylenepolypropoxypropanol, alkyl 2-ethoxethanol, and dihydroxy propane; Silwet L-77 (polyalkyleneoxide modified heptametyltrisiloxane, silicon surfactant), oxamyl (Vydate; methyl N’, N’ dimethyl-N[methylcarbamoyl] (oxy)]-l-thiooxamimidate); superior oil (Drexel Damoil 70 sec delayed dormant spray oil), ethephon (Ethrel, 2-chloroethyl phosphonic acid), (NH ₄) ₂SO ₄; carbaryl (Sevin XLR, 1-naphthy N-methylcarbamate); Accel (mixture of 6-benzyladenine and gibberellin A _{4 + 7}), and gibberellin A ₃.