Understanding citrus tree root development and dynamics are critical in determining crop best nutrient management practices. The role of calcium (Ca) and magnesium (Mg), manganese (Mn), Zinc (Zn), and boron (B) on huanglongbing (HLB) affected citrus trees’ root growth and lifespan in Florida is not fully documented. Thus, the objective of this study was to determine the impact of foliar and ground-applied essential nutrients on seasonal fine root length density (FRLD; diameter (d) < 2 mm) and coarse roots (d > 2 mm), FRLD dynamics, root survival probability (lifespan), and root-zone soil pH of HLB-affected sweet orange trees. Results indicated that Ca treated trees budded on Cleopatra (Cleo) and Ca and Mg combined treatments on Swingle (Swc) rootstocks significantly increased seasonal FRLD of fine (< 2 mm) and coarse roots. The highest median root lifespan of Ca treated trees was 325 and 339 days for trees budded on Cleo and Swc rootstocks, respectively. In the second study, the coarse roots showed a significantly higher reaction to the nutrition applied than the fine roots. Meanwhile, the 2× (1× foliar and 1× ground-applied) treated trees showed a significantly higher median root lifespan compared to the other treatments. Thus, the current study unwraps future studies highlighting the combined soil and/or foliar application of the above nutrients to stimulate FRLD and improve root lifespan on HLB-affected sweet oranges with emphasis on root-zone soil pH.

Understanding the role of Huanglongbing (HLB) caused by Candidatus Liberibacter asiaticus on citrus ( Citrus sp.) water use is critical for determining if changes in water management of commercial citrus orchards affected by this disease is necessary. Reference evapotranspiration (ET _o ) is the most used methodology for irrigation scheduling, particularly in light of reports that indicate the lack of water for irrigation will become a major problem in many places around the world including Florida. The objectives of this study were to determine citrus tree water relationship [water use, sap flow, and stem water potential (Ψ)] as affected by ET _o and HLB. The study was initiated on Jan. 2014 on two sweet orange ( Citrus sinensis ) cultivars included Hamlin and Valencia grown in a fine sandy soil texture (sandy, siliceous, and hyberthermic Arenic Alaquods) in a greenhouse. Twelve weighing lysimeters of each cultivar (six HLB-affected and six nonaffected trees) were used to determine selected water relation parameters. Results indicated a significant reduction in water use, Ψ, and sap flow for affected trees when compared with nonaffected trees. Citrus water use of healthy trees during the 2-year study was 31% and 27% greater than HLB-affected trees for ‘Hamlin’ and ‘Valencia’, respectively. Lower water use of HLB-affected trees resulted in 10% and 15% greater available soil water content than healthy ‘Hamlin’ and ‘Valencia’ trees, respectively. Therefore, reducing the water supply for HLB-affected trees than healthy trees may be a desirable practice that would lead to significant water savings.

The decrease in the rate of inflow and outflow of water—and thereby the uptake of plant nutrients as the result of Huanglongbing (HLB or citrus greening)—leads to a decline in overall tree growth and the development of nutrient deficiencies in HLB-affected citrus trees. This study was conducted at the University of Florida, Southwest Florida Research and Education Center (SWFREC) near Immokalee, FL from January 2017 through December 2019. The objective of the study was to determine the effect of rootstocks, nutrient type, rate, and frequency of applications on leaf area index (LAI), water relations (stomatal conductance [g_s], stem water potential [Ψ_w], and sap flow), soil nutrient accumulation, and dynamics under HLB-affected citrus trees. The experiment was arranged in a split-split plot design that consisted of two types of rootstocks, three nitrogen (N) rates, three soil-applied secondary macronutrients, and an untreated control replicated four times. LAI significantly increased in response to the secondary macronutrients compared with uncontrolled trees. A significantly greater g_s, and thus a decline in Ψ_w, was a manifestation of higher sap flow per unit LA (leaf area) and moisture stress for trees budded on Swingle (Swc) than Cleopatra (Cleo) rootstocks, respectively. The hourly sap flow showed significantly less water consumption per unit LA for trees that received a full dose of Ca or Mg nutrition than Ca + Mg treated and untreated control trees. The soil nutrient concentrations were consistently higher in the topmost soil depth (0–15 cm) than the two lower soil depths (15–30 cm, 30–45 cm). Mobile nutrients: soil nitrate–nitrogen (NO₃-N) and Mg²⁺ Mg²⁺, Mn²⁺, Zn²⁺, and B leached to the lower soil (15–30 cm) depth during the summer season. However, the multiple split applications of N as Best Management Practices (BMPs) and optimum irrigation scheduling based on reference evapotranspiration (ET_o) maintained soil available N (ammonium nitrogen [NH₄-N] and NO₃-N) below 4.0 mg kg⁻¹, which was a magnitude 2.0–4.0× less than the conventional N applications. Soil NH₄-N and NO₃-N leached to the two lower soil depths during the rainy summer season only when trees received the highest N rate (280 kg ha⁻¹), suggesting a lower citrus N requirement. Therefore, 224 kg ha⁻¹ N coupled with a full Ca or Mg dose could be the recommended rate for HLB-affected citrus trees.

The following study was conducted to determine the impact of frequent foliar Cu applications on water relations of Huanglongbing (HLB)-affected Citrus sinensis cv. ‘Valencia’. HLB in Florida is putatively caused by Candidatus Liberibacter asiaticus that is vectored by the Asian citrus psyllid. The experiment was conducted in a psyllid-free greenhouse with trees grown in Immokalee fine sand soil with the trees well-maintained to promote health. Cu was applied to the foliage at 0×, 0.5×, 1×, and 2× the commercially recommended rates, which were 0, 46, 92, and 184 mM, respectively, with applications made 3× in both 2016 and 2017. Previous studies indicate that HLB causes roots to decline before the canopy develops symptoms, which increases the ratio between the evaporative surface area of the canopy to the uptake surface area of roots and increases the hydraulic strain within the tree. In the current study, overall growth was suppressed substantially by HLB and Cu treatments but the ratio between evaporative surface area (leaf surface area) and the uptake surface area of roots (feeder root surface area) was not affected by either treatment. Stem water potential (Ψ_xylem), which was used as a measure of plant water deficits and the hydraulic strain within the tree, was significantly 13% lower for HLB-affected trees than the non-HLB controls but were not affected by Cu treatments. All Ψ_xylem measurements were in a range typical of well-watered trees conditions. Stomatal conductance (k_s) and root and soil resistances (R_r+s) were not affected by HLB and Cu. The results of this experiment suggest that tree leaf area and feeder roots are reduced when the trees are affected by HLB or are treated with foliar Cu applications such that plant water deficits are not significantly different over that of the controls.

Because of the decline in production and negative economic effects, there is an urgent need for strategies to reduce the impact of Huanglongbing (HLB) on citrus [ Citrus × sinensis (L.) Osbeck]. The objective of this study was to evaluate the impact of different irrigation schedules on total available soil water (TAW) and water uptake characteristics of citrus trees affected by HLB in central and southwest Florida. The study was initiated in Jan. 2014 for 2 years on 5-year-old sweet orange trees located in three commercial groves at Arcadia, Avon Park, and Immokalee, FL. Each grove had three irrigation scheduling treatments including the University of Florida, Institute of Food and Agricultural Sciences (UF/IFAS) recommendations, Daily irrigation, and an Intermediate treatment. All groves received similar volumes of water per week based on evapotranspiration (ET _o ) reported by the Florida Automated Weather Network. Sap flow (SF) measurements were taken for two trees per treatment for at least 10 days per site (twice/year). During those periods, leaf area, leaf area index (LAI), and stem water potential (Ψ) were determined. Also, TAW was determined using drainage curve and capacitance soil moisture sensors installed at incremental soil depths of 0–15, 15–30, and 30–45 cm. Results showed significant differences in average SF, LAI, Ψ, and TAW measurements among treatments. Diurnal SF value under daily irrigation treatment increased by 91%, 51%, and 105% compared with UF/IFAS irrigation in Arcadia, Avon Park, and Immokalee, respectively. Soil water contents (WCs) under daily treatment increased by 59%, 59%, and 70% compared with UF/IFAS irrigation treatment in Arcadia, Avon Park, and Immokalee, respectively. Our results indicated that daily irrigation improved tree water dynamics compared with IFAS or Intermediate irrigation scheduling treatments and reduced tree stress with the same volume of water.

Citrus yields have declined by almost 56% since Huanglongbing (HLB) was first found in Florida (2005). That reduction forced citrus growers to replant trees at much higher densities to counter-balance tree loss. The current project aims to determine how much water is required to grow citrus trees at higher planting densities without reducing their productivity. The study was initiated in November 2017 on eight-month-old sweet orange (Citrus sinensis) trees grafted on the ‘US-897′ (Cleopatra mandarin × Flying Dragon trifoliate orange) citrus rootstock planted in the University of Florida, Southwest Florida Research and Education Center (SWFREC) demonstration grove, in Immokalee, FL (lat. 26.42° N, long. 81.42° W). The soil in the grove is Immokalee fine sand (Sandy, siliceous, hyperthermic Arenic Alaquods). The demonstration grove included three densities on two rows of beds (447, 598, and 745 trees per ha) replicated four times each and three densities of three rows of beds (512, 717, 897 trees per ha) replicated six times. Each density treatment was irrigated at one of two irrigation rates (62% or 100%) during the first 15 months (2017–2019) then adjusted (2019–2020) to represent 26.5, 40.5, 53, and 81% based on recommended young citrus trees evapotranspiration (ETc). Tree growth measurements including trunk diameter, height, canopy volume, leaf area, and root development were evaluated. During the first year, reducing the irrigation rate from 100% to 62% ETc did not significantly reduce the young citrus tree growth. Conversely, the lower irrigation rate (62% ETc) had increased citrus tree’s leaf area, canopy volume and tree heights, root lifespan, and root length by 4, 9, 1, 2, and 24% compared with the higher irrigation rate (100%), respectively. Furthermore, the root lifespan was promoted by increasing planting density. For instance, the average root lifespan increased by 12% when planting density increased from 447 to 897 trees per ha, indicating that planting young trees much closer to each other enhanced the root’s longevity. However, when treatments were adjusted from April 2019 through June 2020, results changed. Increasing the irrigation rate from 26.5% to 81% ETc significantly enhanced the young citrus tree growth by increasing citrus tree’s canopy volume (four fold), tree heights (29%), root lifespan (86%), and root length (two fold), respectively. Thus, the application of 81% ET_c irrigation rate in commercial citrus groves is more efficient for trees from two to four years of age.