Prohexadione-Ca reduces russet and does not negate the efficacy of GA4+7 sprays for russet control on 'Golden delicious' apples

A series of four experiments were undertaken to evaluate the effects of individual and combined applications of prohexadione-Ca (P-Ca) and GA(4+7) Primarily on fruit russet, but also on fruit set, fruit weight, early season shoot growth, and fruit maturity of 'Golden Delicious' apples (Malus x doniestica Borkh.). A single application of P-Ca (138 to 167 mg(.)L(-1)) at petal fall (PF) reduced the severity of russet in three of the four experiments; however, multiple applications of 20 ppm GA(4+7) at 10-day intervals beginning at PIT generally reduced russet more effectively than P-Ca. P-Ca did not reduce the efficacy of GA(4+7) sprays for russet reduction. However, GA(4+7) sprays reduced the inhibitory effects of P-Ca on shoot growth measured 30 days after PF. A single application of P-Ca at PF had no effect on mean fruit weight at harvest. Fruit size was lowest for the combined P-Ca and GA(4+7) treatment in every experiment, although there was a significant interaction between P-Ca and GA(4+7) sprays on mean fruit weight in only one experiment. There were no consistent effects of P-Ca and GA(4+7) sprays, alone or in combination, on fruit maturity parameters at harvest. These data show that a single application of P-Ca at PF reduced russet severity, and the effects of P-Ca and GA(4+7) sprays on russet can be additive. The economic benefits resulting from a reduction in russet severity after combined P-Ca and GA(4+7) sprays will need to be balanced against their occasional negative effect on fruit size. Chemical names used: prohexadione-calcium [3-oxido-4-propionyl-5-oxo-3 cyclohexene carboxyl ate formulated as Apogee (27.5% a.i.)].