580 GA Application Timing in Fall and Winter Influences Bloom Period, Fowering Intensity, and Final Crop in Florida `Navel' Orange

Abstract

In Florida's subtropical climate, citrus floral induction is primarily stimulated by cool weather in the fall and winter. Frequently, inductive periods are separated by warm conditions conducive to bud development, resulting in prolonged and multiple bloom. Large variability in date of fruit-set creates an array of problems, and prolonged bloom contributes to severity of postbloom fruit drop. GA applied during the inductive period inhibits bloom in citrus. This study was conducted to determine whether different GA application timings could shift bloom intensity and duration to reduce problems associated with prolonged bloom. GA was applied via airblast sprayer to mature `Navel' on sour orange rootstock near Ft. Pierce, Fla., at 49.4 g GA/ha, 0.05% Silwet L-77, and 2340 L/HA spray volume. Six single-tree experimental units per treatment were blocked by size and vigor in a randomized complete block design. Treatments were time of GA application: 23 Nov. 1998; 17 Dec. 1998; 6 Jan. 1999; 25 Jan. 1999; 23 Nov. and 17 Dec.; 23 Nov. and 25 Jan.; 6 Jan. and 25 Jan.; and a control. Bloom began one month earlier on trees that received the two January applications compared to trees that received GA on 23 Nov. and 17 Dec.. Trees sprayed 25 Jan. (or 6 Jan. and 25 Jan.) had 2 weeks shorter bloom duration compared to controls. Total flowering on GA-treated trees ranged from 13% to 55% of the controls. Single GA applications on 17 Dec. and 6 Jan. increased fruit yield at harvest by 42% and 25%, respectively, while GA on 6 and 25 Jan. yielded 27% less than controls. All other timings had no effect on yield.