Foliar Spray of Macronutrient Influences Fruit Quality of Sugar Belle® Mandarin Grown in Florida Sandy Soil

Sugar Belle® mandarin is considered tolerant to Huanglongbing (HLB); however, recent reports have raised concerns about its fruit quality, noting issues such as reduced fruit size, thin peel, poor coloration, decreased firmness, and suboptimal juice quality. Two-year field experiments were conducted to improve external and internal fruit characteristics through foliar application of potassium (K) in five-year-old Sugar Belle mandarin grown in Florida sandy soil. The experiment consisted of foliar K supply (17 kg/ha) via Potassium Nitrate (PN, 4.7 kg/ha N), Dipotassium Phosphate (DKP, 12.7 kg/ha P2O5), PN with boron (PNB, 0.84 kg/ha B) at different application times (May, July, September), including one-time Gibberellic acid spray (GA@10 mg/L) and control treatments. PN application during July (PNJ) or two applications of PN with B during May and July (PNBMJ) resulted in a larger fruit size (>65 mm). Results showed that PN application before fall (May or July) resulted in a significantly thicker peel (2.3 mm), 1.15 fold more than the control and GA treatment. Fruit puncture resistance force was significantly higher (33.1 N) with GA treatment (p = 0.07), followed by PNBMJ (32.6 N). Meanwhile, K spray positively influenced juice qualities and peel color, regardless of application time or source. However, GA treatment significantly reduced juice quality and peel color. These findings highlighted the benefits of foliar K supply as PN to improve fruit qualities in HLB-affected citrus grown in sandy soil.