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Understanding the growth pattern of fibrous, orange tree [Citrus sinensis (L.) Osbeck] roots enables proper fertilizer placement to improve nutrient uptake efficiency and to reduce nutrient leaching below the root zone. The objective of this study was to develop relationships defining citrus fibrous root length density (FRLD) as a function of soil depth, distance from the tree trunk, and tree size. Root systems of 18 trees with tree canopy volumes (TCV) ranging from 2.4 to 34.3 m³ on two different rootstocks and growing in well-drained sandy soils were sampled in a systematic pattern extending 2 m away from the trunk and 0.9 m deep. Trees grown on Swingle citrumelo [Citrus paradisi Macf. × Poncirus trjfoliata (L.) Raf.] rootstock had significantly greater FRLD in the top 0.15 m than trees on Carrizo citrange (C. sinensis × P. trifoliata). Conversely, Carrizo citrange had greater FRLD from 0.15 to 0.75 m below the soil surface. FRLD was significantly greater for ‘Hamlin’ orange trees grown on Swingle citrumelo rootstock at distances less than 0.75 m from the tree trunk compared with those on Carrizo citrange. Fibrous roots of young citrus trees developed a dense root mat above soil depths of 0.3 m that expanded both radially and with depth with time as trees grow and TCV increased. Functional relationships developed in this study accounted for changes in FRLD with increase in tree size.

Huanglongbing (HLB) causes citrus root systems to decline, which in turn contributes to deficiencies of essential nutrients followed by decline of the canopy and yield. This study was conducted on a 6-year-old ‘Valencia’ [Citrus sinensis (L.) Osb.] on Swingle rootstock (Citrus paradisi Macf. × Poncirus trifoliata (L.) Raf.) trees in a commercial grove near Immokalee, FL, to evaluate the effects of foliar applications of selected essential nutrients (N, K, Mn, Zn, B, and Mg) on growth and productivity of citrus trees infected with Candidatus Liberibacter asiaticus (CLas), the pathogen putatively associated with HLB in Florida. Mn, Zn, B, and Mg were applied in all experiments to drip at 0×, 0.5×, 1.0×, and 2.0×/spray of what has been traditionally recommended in Florida to correct deficiencies. Treatments were applied foliarly 3×/year with the sprays occurring during each growth flush for 5 years (2010–14). Thus, the 0×, 0.5×, 1.0×, and 2.0×/spray treatments resulted in 0×, 1.5×, 3.0×, and 6.0×/year to correct deficiencies. MnS0₄ and ZnSO₄ were applied with or without KNO₃ and in separate experiments were compared with Mn₃(PO₃)₂ and Zn₃(PO₃)₂, respectively. Disease incidence, foliar nutrient content, canopy volume, and yield were measured. At the beginning of the experiment, foliar N, P, Ca, Mg, Cu, and B were in the sufficient range and K, Mn, Zn, and Fe were slightly low. Disease incidence was very high with 83% and 98% of trees testing positive for CLas in 2010 and 2014, respectively. Nutrients that are not mobile or have limited mobility in plants, namely Mn, Zn, and B, demonstrated an increase in foliar concentration immediately after spray and in the annual averages. Foliar K increased from the deficient to the sufficient level by KNO₃ sprays, but the mobile nutrients N and Mg did not show an increase in foliar levels, indicating that intraplant transport occurs in the presence of HLB. Foliar KNO₃ application had a stronger effect on growth than yield. Yield was most strongly affected by application of MnSO₄ where yield of the 3×/year treatment was 45% higher than that of the unsprayed control, but yield declined by 25% for the 6×/year treatment. Yield within 95% of the maximum occurred with foliar Mn concentrations of 70–100 µg·g⁻¹ dry weight when Mn was applied as MnSO₄, which is at the high end of the traditionally recommended 25–100 µg·g⁻¹ dry weight range. The phosphite form of Mn [Mn₃(PO₃)₂] depressed yield by an average of 25% across all application concentrations. Zn, B, and Mg did not significantly impact yield. Canopy volume demonstrated concave relationships across application concentrations for MnSO₄ and ZnSO₄ without KNO₃ and Mn₃(PO₃)₂, Zn₃(PO₃)₂, Boron, and MgSO₄ with KNO₃, with the minimum occurring near the 3×/year application concentration. These data indicate a complex interaction in the amount of nutrients applied and their corresponding effects on foliar concentration, growth, and yield for HLB-affected trees. The results of this study at least partially explain the current confusion among scientists and the commercial industry in how to manage nutrition of HLB-affected citrus trees. The traditionally recommended approaches to correcting nutrient deficiencies need to be reconsidered for citrus with HLB.

No calibrated phosphorus (P) soil test exists to guide Florida citrus fertilization. Applying P fertilizer to citrus when it is not needed is wasteful and may cause undesirable P enrichment of adjacent surface water. The objective of this study was to establish guidelines for P management in developing Florida grapefruit (Citrus paradisi Macf.) and orange (Citrus sinensis L. Osb.) orchards by determining the effect of P fertilizer rate on soil test P and subsequently calibrating a P soil test for citrus yield and fresh fruit quality. Two orchards were planted on sandy soil with 3 mg·kg⁻¹ (very low) Mehlich 1 soil test P. In Years 1 through 3, P fertilization increased soil test P up to 102 mg·kg⁻¹ (very high). In Years 4 through 7, canopy volume, yield, and fruit quality did not respond to available soil P as indexed by soil testing. As tree size and fruit production increased, leaf P was below optimum where soil test P was below 13 mg·kg⁻¹ (grapefruit) or 31 mg·kg⁻¹ (oranges). Total P in the native soil at planting was ≈42 mg·kg⁻¹, which was apparently available enough to support maximum tree growth, fruit yield, and fruit quality for the first 7 years after planting. Trees were highly efficient in taking up P from a soil considered very low in available P. Citrus producers can likely refrain from applying P fertilizer to young trees on Florida sandy soils if soil test P is very high or high and probably medium as well.

The following study was conducted in 2016 and 2017 to determine the impact of frequent foliar copper (Cu) applications on Huanglongbing (HLB)-affected Citrus sinensis cv. Valencia orange. The experiment was conducted in a psyllid-free greenhouse with HLB-positive and non-HLB control trees grown in an Immokalee fine sand soil. The trees were well-maintained to promote health. Cu was applied to the foliage at 0x, 0.5x, 1x, and 2x the commercially recommended rates, which were 0, 46, 92, and 184 mm, respectively, with applications made 3x in both 2016 and 2017. The impact of HLB and Cu treatments on leaf and root Cu concentrations, vegetative growth, Candidatus Liberibacter asiaticus (CLasiaticus) genome copy number, and acquisition of other essential nutrients were determined. HLB caused the roots to acidify the soil more than non-HLB controls, which promoted Cu availability and promoted greater Cu concentrations in leaves and roots. HLB and Cu application treatments suppressed leaf area and total root length observable in rhizotron tubes such that, by the end of the experiment, leaf, stem, root, and whole-plant dry weights were reduced. HLB reduced foliar concentrations of calcium (Ca), magnesium (Mg), manganese (Mn), zinc (Zn) and possibly iron (Fe), but HLB did not affect root concentrations of these same essential nutrients. Cu application treatments did not affect leaf or root concentrations of other essential nutrients except foliar concentration of Fe, which may have been suppressed. Foliar applications of Cu are used to suppress Xanthomonas citri ssp. citri (Xcc) the causal agent of citrus canker, and the frequency of its use may need to be reconsidered in commercial groves.

The abscission compound CMNP (5-chloro-3-methyl-4-nitro-1H-pyrazole) was applied to fully mature sweet orange trees at different spray volumes using a vertical, multiple-fan air-blast sprayer to determine distribution of fruit loosening throughout the canopy and subsequent effects on mechanical harvester efficiency. CMNP was applied at 0, 935, 1871, and 2806 L·ha⁻¹ in three ‘Valencia’ and one ‘Hamlin’ grove. Spray coverage was measured using water-sensitive paper and fruit loosening was measured by fruit detachment force (FDF). Spray coverage and FDF were measured at 1-, 2-, and 4-m height within the canopy and inside the canopy near the trunk and on the periphery of the canopy. Spray coverage increased with volume of CMNP applied. Spray coverage was higher at 4 m than 1 and 2 m, which were similar. Spray coverage within the canopy was decreased almost half compared with that of the periphery. FDF was unaffected by spray volume at the different heights except in one trial where fruit had higher FDF at 4 m. Fruit inside the canopy did not loosen as much as fruit outside the canopy in three of the four trials. FDF inside the canopy averaged 52 to 84 N, whereas fruit on the periphery of the canopy averaged 50 to 74 N. CMNP promoted fruit drop, but only in two trials was the amount over 5% of the total yield for the 2806-L·ha⁻¹ treatment. The fruit were harvested by canopy shakers that captured fruit on catch frames, except one of the ‘Valencia’ trials in which the canopy shaker did not have a catch frame. The percent of the total crop removed by the harvesters increased when CMNP was applied at higher spray volumes except in the ‘Hamlin’ trial in which there was no difference among volume treatments. The percent of the total crop removed by the harvester but not captured by the catch frame increased at higher volumes of CMNP applied for two of the three trials in which catch frames were used. Fruit loss with greater volume of CMNP applied was promoted by peripheral canopy contact with the front shield of the harvester that knocked fruit down before the catch frame moved under that portion of the canopy. Recovery percentage, or the percentage of total yield that was caught and conveyed to bulk collection by the harvester catch frame, averaged 78.1% to 87.8% of total yield. Higher CMNP volume with increased removal rate compensated for higher catch frame loss, providing overall higher recovery percentage. Based on the goals of minimizing fruit drop and maximizing fruit recovery, the range of FDF that should be reached by harvest is 40 N to 65 N for canopy shakers equipped with catch frames. These trials underscore the importance of adequate CMNP coverage for reducing in-canopy variation of fruit loosening and maximizing fruit removal.

Essential nutrients for citrus [‘Bingo’ (Citrus reticulata, Blanco)] production are important for different functions, including photosynthesis, resistance to disease, and productivity. During the past 15 to 20 years, citrus production in Florida has significantly declined as a result of the devastating citrus greening disease also called huanglongbing (HLB). Therefore, a greenhouse study was conducted for 2 years, starting in 2018, at the University of Florida/Institute of Food and Agricultural Sciences Citrus Research and Education Center in Florida to evaluate the effect of varying rates of iron on the growth and development of 2-year-old HLB-affected ‘Bingo’ (Citrus reticulata, Blanco) trees on Kuharske citrange rootstock. Four treatments were used in a randomized complete block (HLB status) design with seven single tree replicates for each treatment. The treatments applied were 0.0 (control), 5.6 (standard fertilization, lx), 11.2 (2x), and 22.4 (4x) kg⋅ha⁻¹ iron on HLB-affected and healthy (non-HLB) citrus trees. Data including trunk diameter, tree height, and leaf samples were collected, processed, and analyzed at 3-month intervals for 2 years. At the end of the second year, trees were destructively sampled and processed as above-ground and below-ground biomass. Tree heights were different among iron rates of HLB-affected trees (P < 0.001); however, they were similar for non-HLB trees for both years. Higher average trunk diameters (P < 0.001) were observed for HLB-affected trees that received the 2x rate compared with the 1x rate and the control. In 2019, non-HLB trees showed 13% to 40% higher iron concentrations in leaves than HLB-affected trees. However, leaf iron concentrations were comparable for HLB-affected and non-HLB trees in 2020. Above-ground biomass for HLB-affected trees had between 33% and 44% more biomass (P < 0.01) than below-ground biomass for the corresponding iron fertilization. Iron accumulation correlated positively with all studied nutrients in the above-ground parts for both HLB-affected and non-HLB trees. A 95% confidence interval at which total biomass was nearly maximum corresponded to an iron rate of 9.6 to 11.8 kg⋅ha⁻¹, which was close to the 2x rate. Therefore, soil iron application using the aforementioned rates may be appropriate for better growth and development of young HLB-affected trees.

Most strategies used to determine crop N fertilizer recommendations do not address potential environmental issues associated with agricul-tural production. Thus, a more holistic approach is required to reduce N loading associated with vegetable crops production on soils that are prone to N leaching. By linking fertilizer N uptake efficiency (FUE) with irrigation management, root interception capacity, and N uptake dynamics, we aim to improve FUE. Nitrogen uptake for peppers, tomato, potato, and sweet corn followed a logistic N accumulation patterns. Up to 80-85% of N uptake occurred between 4 to 7 weeks (sweet corn) vs. 6 to 12 weeks (other crops), while N uptake during initial growth and crop maturation was relatively low. Maximum daily N accumulation rates occurred at 5 weeks (sweet corn) vs. 8-10 weeks (other crops) and maximum daily N uptake rates were 4-8 kg N/ha. Overall FUE for most vegetables may range between 23% and 71%, depending on production practices, soil type, and environmental conditions. Maximum root interception capacity was typically attained 3 to 5 weeks prior to crop maturity. It is concluded that, during initial growth, root interception may the most limiting factor for efficient N use. Although recent uptake studies have shown that FUE may be highest toward the end of the growing season, this may not coincide with the greatest crop demand for N, which occurs during the onset of the linear growth phase. As a result, yield responses to N applied later in the season may be limited. Integration of these results into best management practices and expert systems for vegetable production can minimize the externalities associated with commercial vegetable production on vulnerable soils in the southeastern United States.

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.

This study was conducted on well-watered citrus to determine changes in water relations during cold acclimation independent of drought stress. Potted sweet orange and Satsuma mandarin trees were exposed to progressively lower, non-freezing temperatures down to 10/4 °C, light/dark temperatures, respectively, for 9 weeks in environmental growth chambers to promote cold acclimation. The trees were watered twice daily and three times on the day water relations data were collected to minimize drought stress. Although soil moisture was higher and non-limiting for plants in the cold than in the warm chamber, cold temperatures promoted stomatal closure, higher root resistance, lower stem water potential (Ψ_stem), lower transpiration, and lower leaf ψ_S. Leaf relative water content (RWC) was not different for cold-acclimated trees compared with the controls. Cold acclimation promoted stomatal closure at levels only observed in severely drought-stressed plants exposed to warm temperatures and where Ψ_stem and RWC are typically much lower than what was found in this study. Ψ_stem continued to decline the last 4 weeks of the experiment although air temperature, leaf ψ_S, RWC, stomatal conductance (g _S), and transpiration were constant. The results of this experiment indicate that water relations of citrus during cold acclimation vary from those known to occur as a result of drought stress, which have implications for using traditional measures of plant water status in irrigation scheduling during winter.

Fertilizer material costs, particularly nitrogen (N), have increased substantially over the past 5 years. Increased costs, along with increased awareness of the impact of fertilizer leaching on the environment in humid regions, have increased interest in use of slow-release fertilizer (SRF) or controlled-release fertilizer (CRF) materials. The goals of SRF and CRF use are that no nutrient should be limiting for crop uptake, there should be improved nutrient uptake efficiency, and nutrient-leaching potential should be reduced. These considerations are particularly important for crops grown on sandy soils with relatively low nutrient and water holding capacities. Release rates of biodegradable, or slow-release materials, such urea formaldehyde, isobutylidene diurea, and methylene urea are proportional to soil microbial activity and are therefore soil temperature dependent. These materials are N sources and depend on soil biological activity, thus, soil temperature during specific crop growth phenology must be considered and release may be delayed by soil fumigation. Whereas CRFs depend on diffusion through coatings and not biodegradation, both are soil moisture and temperature dependent. Examples of coated materials are sulfur-coated urea, polymer-coated urea, and polymer/sulfur-coated urea. The advantage of these materials is that leachable fertilizer elements other than N can be incorporated within the coating. However, this comes at an increased cost. The use of any single or combination of these materials depends on time of year, the length of crop cycle and crop nutrient demand patterns, and the use of soil fumigants.