Foliar micronutrient applications enhance growth and yield of huanglongbing (HLB)-affected sweet orange

Florida citrus production has declined by more than 70% because of Huanglongbing (HLB), and a major cause of tree decline is nutrient uptake limitations caused by HLB. This study was conducted on 6-9-year-old "Valencia" sweet orange [Citrus sinensis (L.) Osbeck] on Swingle rootstock in the central and southwest Florida to evaluate the effects of macronutrient fertilization and foliar application of Mn and Fe on growth and yield of HLB-affected citrus trees in Florida. Seven treatments, including a control (standard fertilization), two levels of macronutrients (1x Macro and 2x Macro), and three levels of foliar micronutrients (F-1x, F-2x, and F-4x), were arranged in a randomized complete block design in two locations for 3 years. Trees subjected to 1x Macro + F-4x showed the highest average leaf Mn concentration for both sites through all years. The treatment that contained 1x Macro + F-2x showed at least 4% increase in canopy volume for 2020 and 2021 at the central Florida site. However, trees subjected to 1x Macro + F-2x showed at least 12% increase in canopy volume in 2020 in southwest Florida. In 2021, the trees that were subjected to 2x Macro + F-1x resulted in 15% greater canopy efficiency than the control at the central Florida site. In 2021, citrus trees treated with 2x Macro + F-1x increased yield at both sites. In general, the growth parameters and yield results showed that treatments subjected to supplemental fertilization over current recommendations have strong potential to increase citrus growth and yield.