With huanglongbing (HLB) causing a reduction in fine root mass early in disease progression, HLB-affected trees have lower nutrient uptake capability. Questions regarding the uptake efficiency of certain fertilizer application methods have been raised. Therefore, the goals of this study are to determine if nutrient management methods impact nutrient translocation and identify where in the tree nutrients are translocated. Destructive nutrient and biomass analysis were conducted on field grown HLB-affected grapefruit trees (Citrus × paradisi) grafted on ‘sour orange’ (Citrus × aurantium) rootstock under different fertilizer application methods. Fertilizer was applied in the form of either 100% soluble granular fertilizer, controlled release fertilizer (CRF), or liquid fertilizer. After three years, the entire tree was removed from the grove, dissected into eight different components (feeder roots, lateral roots, structural roots, trunk, primary branches, secondary branches, twigs, and leaves), weighed, and then analyzed for nutrient contents. Overall, application methods showed differences in nutrient allocation in leaf, twig, and feeder root; however, no consistent pattern was observed. Additionally, leaf, twig, and feeder roots had higher amount of nutrients compared to the other tree components. This study showed that fertilization methods do impact nutrient contents in different components of HLB-affected trees. Further research should be conducted on the impact of different fertilizer application methods and rates on HLB-affected trees.

Rhizosphere interactions are an understudied component of citrus production. This is even more important in Florida flatwood soils, which pose significant challenges in achieving sustainable and effective fruit production due to low natural fertility and organic matter. Citrus growers apply soil amendments, including oak mulch, to ameliorate their soil conditions. Thus, the aim of this research was to evaluate the effects of oak mulch on citrus nutrient uptake, soil characteristics, and rhizosphere composition. The plant material consisted of ‘Valencia’ sweet orange (Citrus × sinensis) trees grafted on ‘US-812’ (C. reticulata × C. trifoliata) rootstock. The experiment consisted of two treatments, which included trees treated with oak mulch (300 kg of mulch per plot) and a control. The soil and leaf nutrient contents, soil pH, cation exchange capacity, moisture, temperature, and rhizosphere bacterial compositions were examined over the course of one year (spring and fall 2021). During the spring samplings, the citrus trees treated with oak mulch resulted in significantly greater soil Zn and Mn contents, greater soil moisture, and greater rhizosphere bacterial diversity compared to the control, while during the fall samplings, only a greater soil moisture content was observed in the treated trees. The soil Zn and Mn content detected during the spring samplings correlated with the significant increases in the diversity of the rhizosphere bacterial community composition. Similarly, the reduced rates of leaching and evaporation (at the soil surface) of oak mulch applied to Florida sandy soils likely played a large role in the significant increase in moisture and nutrient retention.

Citrus greening disease, also known as huanglongbing (HLB), is the most serious vector-borne bacterial disease of citrus worldwide and an emblematic case of perennial crop decline caused by insect vector–borne disease. There is an immediate global need to provide the citrus industry with relief from HLB and a return to profitable citrus production. Discovery of HLB treatments typically involve various laboratory-based assays, which then advance to greenhouse and eventually field testing in a workflow that takes multiple years. We present a field-forward experimental framework, “Grove-First,” that reverses the traditional laboratory-to-field pipeline by prioritizing outcome-based screening. Grove-First rapidly screens and validates treatments with regulatory-friendly profiles in commercial citrus groves using trunk injection to select treatments that improve tree health and fruit yield over the course of a single growing season. The Grove-First framework represents a generalizable systems approach for translational agricultural research, applicable to perennial crops where disease impact, long development timelines, and real-world variability demand in-field, outcome-based screening. Using this framework, we identified candidate treatments with effects comparable to or better than the standard oxytetracycline on visual tree-health and/or yield indices in an initial screen of HLB-positive 8-year-old ‘Valencia’ sweet orange trees. Expanded trials in commercial citrus groves allowed us to validate the initial screening results at other locations and in other citrus varieties. Grove-First rapidly accelerated the identification and large-scale field testing of HLB therapies, some of which are available for growers to use immediately and others that require further field testing and/or regulatory actions.

Huanglongbing (HLB) disease has caused a severe decline in citrus production globally over the past decade. There is a need for improved nutrient regimens to better manage the productivity of HLB-affected trees, as current guidelines are based on healthy trees. The aim of this study was to evaluate the effects of different fertilizer application methods and rates with different planting densities on HLB-affected citrus root and soil health. Plant material consisted of ‘Ray Ruby’ (Citrus × paradisi) grapefruit trees grafted on ‘Kuharske’ citrange (Citrus × sinensis × Citrus trifoliata). The study consisted of 4 foliar fertilizer treatments, which included 0×, 1.5×, 3× and 6× the University of Florida Institute of Food and Agriculture (UF/IFAS) recommended guidelines for B, Mn and Zn. Additionally, 2 ground-applied fertilizer treatments were used, specifically controlled-release fertilizer (CRF1): 12−3−14 + B, Fe, Mn and Zn micronutrients at 1× UF/IFAS recommendation, and (CRF2): 12−3−14 + 2× Mg + 3× B, Fe, Mn and Zn micronutrients, with micronutrients applied as sulfur-coated products. The planting densities implemented were low (300 trees ha⁻¹), medium (440 trees ha⁻¹) and high (975 trees ha⁻¹). The CRF fertilizer resulted in greater soil nutrient concentrations through all of the time sampling points, with significant differences in soil Zn and Mn. Grapefruit treated with ground-applied CRF2 and 3× foliar fertilizers resulted in the greatest bacterial alpha and beta diversity in the rhizosphere. Significantly greater abundances of Rhizobiales and Vicinamibacterales were found in the grapefruit rhizosphere of trees treated with 0× UF/IFAS foliar fertilizer compared to higher doses of foliar fertilizers.

Citrus greening disease, also known as huanglongbing (HLB), is the most serious vector-borne bacterial disease of citrus world-wide ^1,2 . There is an immediate global need to provide the citrus industry with relief from HLB and a return to profitable citrus production. Standard screening methods for HLB therapeutic treatments typically involve various laboratory-based assays to select treatments with antimicrobial properties ³ , which then advance to greenhouse and eventually field testing in a workflow that takes multiple years. Unlike traditional lab-first screening, we present a design of experiments framework ^4,5 , referred to as Grove-First, that rapidly screens treatments with regulatory-friendly profiles in commercial citrus groves using trunk injection to select treatments that improve tree health and fruit yield over the course of a single growing season. Using this framework, we identified candidate treatments with effects comparable to or better than the standard oxytetracycline (OTC) on visual tree-health and/or yield indices in an initial screen of HLB-positive 8-year-old Valencia sweet orange trees. Expanded trials in commercial citrus groves allowed us to validate the initial screening results at other locations and in other citrus varieties. Grove-First rapidly accelerated the identification and large-scale field testing of HLB therapies, some of which are available for growers to use immediately and others that require further field testing and/or regulatory actions.