Citrus greening (huanglongbing, HLB) is a bacterial disease caused by Candidatus Liberibacter asiaticus (CLas) and vectored by the Asian citrus psyllid (Diaphorina citri) (ACP). No cure is yet available, and as a result, increased fertilizer applications continue to be a major management method used to prolong the productive life of affected trees. However, questions remain regarding proper fertilizer rates and in what form these nutrients should be applied to increase tree health and yield. Therefore, the goal of this study was to identify optimum micronutrient application forms and rates to increase root growth, canopy size and fruit yield as well as reduce leaf and root CLas cycle threshold (Ct) value of HLB-affected grapefruit trees (Citrus × paradisi Macfad). A large-scale field study consisting of six-year-old HLB-affected ‘Ruby Red’ grapefruit trees grafted on ‘sour orange’ (Citrus × aurantium) rootstock was conducted in the Florida Indian River District. Eight different treatments consisting of four granular and four liquid fertilizers with 1×, 2×, and 4× the current University of Florida, Institute of Food and Agricultural Sciences recommended rates of boron, zinc, manganese, and iron were applied for three times a year (granular), biweekly (liquid), or weekly (liquid), for three years. Root density, length and diameter, root, and leaf CLas Ct value, canopy volume, visual HLB symptoms, and fruit yield were measured throughout the study. Regardless of treatment, tree health declined over time, and no significant impact on severely HLB-affected grapefruit tree health was detected within the three-year time frame of the study. This was the first study to solely compare the impact of ground applied micronutrient application rates and methods on HLB-affected grapefruit tree health in Florida’s Indian River District. More time would be needed to determine the potential benefits of increased micronutrient performance of HLB-affected grapefruit trees.

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.

Growers in Florida face unique challenges regarding maintaining proper citrus nutrition. Poor draining soils with low fertility, low C.E.C., and high rates of leaching are common in this region. In response to these challenges, interest has grown in products labeled as soil conditioners. Using a completely randomized experimental design, this greenhouse study tested the effects of 5 different combinations of a traditional fertilizer (TF) and a new soil conditioner (SC) on lemon and orange seedling physiology. Eight-month-old ‘Bearss’ lemon and ‘Valencia’ sweet orange grafted on sour orange rootstocks were employed, and five repetitions were used for each treatment. Plant biomass (dry weight), height, stem diameter, chlorophyll content, stomatal conductance and nutrient uptake were analyzed after 120 days of treatment. The results show that SC has a positive impact upon both chlorophyll levels and stomatal conductance values in both orange and lemon seedlings. However, based on dry weight growth data, we can only conclude that the SC was effective for orange seedlings at 50% TF and 0.5% SC. Based on this short 120-day evaluation, the SC achieved positive growth promotion for orange (50% TF) but not for lemon seedlings.

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.

Cover crops are increasingly used by growers in the United States as an effective approach to enhance soil health. The interactions between cover crops and soil microbiomes have been studied extensively; however, research evaluating the impact of cover crops on rhizosphere microbial communities in tree crop systems, particularly under nutrient-limited field soils in Florida, remains limited. To address these knowledge gaps, a three-year field study was conducted to evaluate the effects of cover crop mixtures on soil bacterial and fungal communities in orchards of different citrus species and cultivars across seasons. The trial included ‘Star Ruby’ grapefruit trees grafted on ‘US-942’, ‘Bearss’ lemon trees grafted on ‘Sour Orange’, and ‘OLL-8’ sweet orange trees grafted on ‘US-942’ rootstock in a randomized complete block design comparing conventional management (no cover crop-control) with cover crop treatments. Bacterial community composition showed consistent taxonomic patterns across cultivars and treatments, with Proteobacteria consistently the most abundant phylum across grapefruit, lemon, and orange orchards, followed by Actinobacteriota, Acidobacteriota, and Firmicutes. Despite this overall stability, cultivar was the strongest determinant of bacterial community structure. Additionally, functional analysis revealed significant treatment-associated shifts in predicted guilds related to nitrogen transformations, carbon metabolism, sulfur oxidation, and manganese oxidation. In contrast, fungal community composition and predicted functional profiles were primarily structured by season, with limited effects of cultivar and cover cropping. Fungal communities were consistently dominated by Ascomycota across orchards, sampling dates, and treatments.