Abstract
Florida citrus growers apply gibberellic acid (GA[sub3]) in the late summer or fall to reduce senescence-related peel disorders of fresh fruit and to increase juice yield of processing oranges. Rainfall that may occur daily during this time could reduce the efficacy of GA[sub3] sprays by removing or diluting the compound before it has been fully absorbed. Thus, we conducted experiments in 1998-99 and 1999-2000 to test the effects of simulated rainfall ("wash-off" water sprays) on GA[sub3] efficacy by subjecting 'Hamlin' orange [Citrus sinensis (L.) Osb.] trees that were previously treated with GA[sub3] to timed wash-off sprays. We then used peel color (hue angle) and peel puncture resistance (PPR) of fruit as an indicator of GA[sub3] uptake. In Oct. 1998 and 1999, the canopies of 14- or 15-year-old trees were sprayed to runoff (ca. 2.5 gal) with the a.i. GA[sub3] at 0.6 acre[sup-]) and a surfactant (Silwet, 0.05%, v/v). For the next 4 hours (1998-99) or 5 hours (1999-2000), three different GA[sub3]-treated trees each hour were sprayed with about 5 gal of tap water. Three additional trees did not receive a GA[sub3] treatment and three more were sprayed with GA[sub3] but not with wash-off water. Fruit were harvested in Nov., Dec., or Jan. In 1998-99, peel hue angle and PPR increased linearly for ca. 1 hours until wash-off and then reached a plateau, while in 1999-2000, peel hue angle and PPR increased linearly for ca. 2 hours until wash-off before reaching a plateau. Therefore, rainfall within 1 to 2 hours of application may reduce GA[sub3] effectiveness, even when a surfactant is used.
