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.Plants 2020, 9, 483 2 of 20 to 2.02 cmcm -3 for trifoliate orange (Poncirus trifoliata). Although the citrus root system is estimated to account for more than 65% of above-ground dry mass [12,13] the root system of HLB-affected citrus trees is usually poorly developed and new root growth can also be inhibited [14]. Studies indicated that 30%-50% of roots of HLB-affected citrus trees are impaired before canopy symptoms appear and 70%-80% of root loss could be prevalent as citrus deprived of intensive cultural management to pacify abiotic and biotic stress [10,15]. The optimum distribution of the root system is mainly restricted by water and nutrients available in soil profiles [16,17]. However, it has been reported that HLB develops an imbalance of nutrient concentrations, which cause nutrient toxicity for some nutrients, including Cu or deficiency for others such as Ca, Mg, Mn, and Zn [18]. Therefore, nutrient supply is an imperative aspect of disease control because nutrients influence plant resistance, pathogen vigor, growth, and associated factors [19,20].Ground-applied fertilizers are subject to unfavorable soil processes such as precipitation as forms that are unavailable to plants, leaching, and runoff [21,22]. Therefore, split ground and foliar-applied fertilizer improve nutrient use efficiency and reduce both nutrient leaching and application cost [23][24][25]. Moreover, HLB-affected trees are limited in soil nutrient uptake because of the associated decline in FRLD [12,16,26]. The role of adding essential nutrients in improving citrus growth in general a...

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 [gs], stem water potential [Ψw], 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 gs, and thus a decline in Ψw, 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 (ETo) 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 fate of foliar and ground-applied essential nutrients is the least studied topic under citrus greening or Huanglongbing (HLB)-affected citrus, which is inherently suffering from severe root decline because of HLB-associated problems. The objective of this study was to evaluate if ground-applied coupled with foliar spray of essential nutrients can reverse the decline in tree growth and understand the fate of the nutrients in the soil-root-tree interfaces. The treatments were arranged in a split-split plot design in which nitrogen (N) was ground-applied in 20 splits biweekly and Mn, Zn, and B were foliar and /or ground-applied in three splits following the spring, summer, and late summer flush seasons. Soil nutrients in three depths (0–15, 15–30, and 30–45 cm), root, and leaf nutrient concentrations of the essential nutrients, leaf area index (LAI), and tree canopy volume (TCV) data were studied twice (spring and summer) for two years. A significantly higher soil NH4-N and NO3-N concentrations were detected in the topsoil depth than the two lower soil depths (15–30 and 30–45 cm) indicating lesser nutrient leaching as trees received moderate (224 kg ha−1) N rate. Except for soil zinc (Zn) concentration, all the nutrient concentrations were significantly higher in the topsoil (0–15 cm), compared with two lower soil depths indicating that Zn was intricate by changes in soil environmental conditions, root acquisition, and/or leaching to lower soil depth. Leaf N concentration significantly increased over time following seasonal environmental fluctuations, tree growth, and development. Thus, leaf N concentration remained above the optimum nutrient range implying lower N requirement under irrigation scheduling with SmartIrrigation, an App used to determine the daily irrigation duration to meet tree water requirement and split fertigation techniques. Root Manganese (Mn) and Zn concentrations were significantly higher in the root tissues of the treated than the control trees and translocated to the leaves accordingly. Meanwhile, a significantly higher LAI for trees budded on Swingle (Swc) rootstock however, larger TCV for trees budded on Volkameriana (Volk) rootstocks. The trees had significantly larger TCV when the trees received a moderate N rate during early study years and under foliar 9 kg ha−1 coupled with the ground 9 kg ha−1 Mn and Zn treatments during the late study years. Therefore, split ground application of 224 kg ha−1 of N, foliar applied 9 kg ha−1 coupled with ground-applied 9 kg ha−1 Mn and Zn were the suggested rates to sustain the essential leaf nutrient concentration within the optimum ranges and improve the deterioration of vegetative growth associated with HLB-induced problems of citrus trees.