São Paulo, Brazil, and Florida, USA, were the two major orange production areas in the world until Huanglongbing (HLB) was discovered in São Paulo in 2004 and Florida in 2005. In the absence of resistant citrus varieties, HLB is the most destructive citrus disease known because of the lack of effective tools to reduce spread of the vector, Diaphorina citri (Asian citrus psyllid), and transmission of the associated pathogen, Candidatus Liberibacter asiaticus. In both countries, a three-pronged management approach was recommended and begun: planting only disease-free nursery trees, effective psyllid control, and removal of all symptomatic trees. In Brazil, these management procedures were continued and improved and resulted in relatively little overall loss of production. In contrast, in Florida the citrus industry has been devastated with annual production reduced by approximately 80%. This review compares and contrasts various cultural and pest management strategies that have been used to reduce infection by the pathogen and increase tolerance of HLB in the main orange-growing regions in the world.

Plant genomes are comprised of nuclear, plastid and mitochondrial components characterized by different patterns of inheritance and evolution. Genetic markers from the three genomes provide complementary tools for investigations of inheritance, genetic relationships and phenotypic contributions. Plant mitochondrial genomes are challenging for universal marker development because they are highly variable in terms of size, gene order and intergenic sequences and highly conserved with respect to protein-coding sequences. PCR amplification of introns with primers that anneal to conserved, flanking exons is effective for the development of polymorphic nuclear genome markers. The potential for plant mitochondrial intron polymorphisms to distinguish between congeneric species or intraspecific varieties has not been systematically investigated and is possibly constrained by requirements for intron secondary structure and interactions with co-evolved organelle intron splicing factors. To explore the potential for broadly applicable plant mitochondrial intron markers, PCR primer sets based upon conserved sequences flanking 11 introns common to seven angiosperm species were tested across a range of plant orders. PCR-amplified introns were screened for indel polymorphisms among a group of cross-compatible Citrus species and relatives; two Raphanus sativus mitotypes; representatives of the two Phaseolus vulgaris gene pools; and congeneric pairs of Cynodon, Cenchrus, Solanum, and Vaccinium species. All introns were successfully amplified from each plant entry. Length polymorphisms distinguishable by gel electrophoresis were common among genera but infrequent within genera. Sequencing of three introns amplified from 16 entries identified additional short indel polymorphisms and nucleotide substitutions that separated Citrus, Cynodon, Cenchrus and Vaccinium congeners, but failed to distinguish Solanum congeners or representatives of the Phaseolus vulgaris major gene pools. The ability of primer sets to amplify a wider range of plant species’ introns and the presence of intron polymorphisms that distinguish congeners was confirmed by in silico analysis. While mitochondrial intron variation is limited in comparison to nuclear introns, these exon-based primer sets provide robust tools for the amplification of mitochondrial introns across a wide range of plant species wherein useful polymorphisms can be identified.

Xanthomonas citri pv. citri (Xcc) (X. citri subsp. citri) type A is the causal agent of citrus bacterial canker (CBC) on most Citrus spp. and close relatives. Two narrow-host-range strains of Xcc, Aw and A*, from Florida and Southwest Asia, respectively, infect only Mexican lime (Citrus aurantifolia) and alemow (C. macrophylla). In the initial stage of infection, these xanthomonads enter via stomata to reach the apoplast. Herein, we investigated the differences in chemotactic responses for wide and narrow-host-range strains of Xcc A, X. euvesicatoria pv. citrumelonis (X. alfalfae subsp. citrumelonis), the causal agent of citrus bacterial spot, and X. campestris pv. campestris, the crucifer black rot pathogen. These strains of Xanthomonas were compared for carbon source use, the chemotactic responses toward carbon compounds, chemotaxis sensor content, and responses to apoplastic fluids from Citrus spp. and Chinese cabbage (Brassica pekinensis). Different chemotactic responses occurred for carbon sources and apoplastic fluids, depending on the Xanthomonas strain and the host plant from which the apoplastic fluid was derived. Differential chemotactic responses to carbon sources and citrus apoplasts suggest that these Xanthomonas strains sense host-specific signals that facilitate their location and entry of stomatal openings or wounds.

Citrus Black Spot (CBS), caused by the ascomycete, Phyllosticta citricarpa, is a fruit, foliar, and twig spotting fungal disease affecting the majority of commercial cultivars of citrus. The disease causes cosmetic lesions, may cause fruit drop and P. citricarpa is considered a quarantine pathogen by some countries, impacting domestic and international trade of citrus fruit. Regulatory requirements affecting fruit trade exist even though there is no documented case of disease spread via infected fruit into previously disease-free areas. To clarify the risk of fruit as a potential pathway for the spread of CBS, we developed a quantitative, probabilistic risk assessment model. The model provides an assessment of all steps in the fruit pathway, including production, packinghouse handling, transportation, export-import distribution channels, and consumer endpoints. The model is stochastic and uses Monte Carlo simulation to assess the risk of P. citricarpa moving through all steps in the pathway. We attempted to use all available literature and information to quantitate risk at each point in the potential pathway and by sequentially linking all steps to determine the overall quantitative risk. In addition, we assessed climatological effects on incidence of diseased fruit at production sites and on fungal reproduction and infection, as well as criteria for establishment at endpoints. We examined ten case studies between exporting and importing locations/countries. Model results indicated fruit to be an epidemiologically insignificant means for CBS spread, even between producing countries where CBS occurs and CBS-free importing countries with disease-conducive climates. We created a second model to examine the introduction of infected plant material from countries where CBS occurs. This model demonstrated significant probability of introduction via such infected material. However, pathogen establishment and disease development was still restricted only to areas with conducive climatological conditions. We created a tool to quantitatively explore the viability of various potential pathways via combinations of CBS-present production sites and corresponding pathway endpoints, including environments conducive and non-conducive to CBS. The tool is provided to aid decision makers on phytosanitary risk relative to international trade of citrus fruit.

Proper timing of copper sprays for protection of citrus fruit from infection by Xanthomonas citri subsp. citri, the cause of citrus canker, is important because early season lesions induce premature fruit drop. The objective of the trials reported herein was to evaluate early season sprays of copper for control of fruit infection and drop in 3- to 5-year-old Citrus sinensis 'Hamlin' trees in a south central Florida citrus grove. Soluble and fixed copper formulations were applied with an airblast sprayer at 21-day intervals. In 2011, early season infection occurred due to periodic rains in late March and early April before the initiation of copper sprays. Subsequently, nine sprays of copper formulations from April to September only marginally reduced the incidence of fruit lesions compared to the untreated checks (UTC). Fruit drop ranged from 69% of the tree crop in the UTC to an average of 45% in the copper treatments. In 2012 in the same location, five copper sprays of 5-year-old trees began before rains in March-April. Incidence of fruit lesions was substantially lower and fruit drop due to canker was 10 fold less than in 2011. In 2014, copper sprays of 3-year-old trees were initiated before below average March-April and above average May rainfall. In this season, incidences of fruit disease and fruit drop were very low. In 2015 in the same location, copper sprays of 4-year-old trees were initiated before above average April and below average May rainfall and the incidence of fruit disease and fruit drop were significantly reduced by copper sprays. Timing of sprays in advance of rains in late March and early April is critical for protecting fruit from 0.5-1.0 cm (0.25-0.50 inch) in diameter. Inoculum in form of infected leaves and stems from the previous season is always present in the spring. Early fruit infection resulting in fruit drop depends on coincidence of March-April rains with the most susceptible fruit stage. In June-July, infections of fruit larger than 4.0 cm (1.5 inch) result in smaller lesions that do not induce fruit abscission and premature drop.

The management of citrus canker, caused by Xanthomonas citri subsp. citri, has been widely studied in endemic areas because of the importance of the disease in several citrus-producing countries. A set of control measures is well established, but no study has investigated the efficiency of each measure individually and their combination for disease suppression. This study comprised a 3-year field study to assess the relative contribution of three measures for the control of citrus canker and reduction of crop losses. Windbreak (Wb), copper sprays (Cu), and leafminer control (Lc) were assessed in eight different combinations in a split-split plot design. The orchard was composed of 'Valencia' sweet orange trees grafted onto 'Rangpur' lime. Casuarina cunninghamiana trees were used as Wb. Cu and Lc sprays were performed every 21 days throughout the year. Individually, Cu showed the highest contribution for canker control, followed by Wb. Lc had no effect on reducing citrus canker. Wb+Cu showed the highest efficiency for control of the disease. This combination reduced the incidence of diseased trees by approximately 60%, and the incidence of diseased leaves and fruit by ≥90% and increased the yield in 2.0- to 2.6-fold in comparison with the unmanaged plots. Cu sprays were important for reducing disease incidence and crop losses, whereas Wb had an additional contribution in minimizing the incidence of cankered, non-marketable fruit. The results indicated that the adoption of these measures of control may depend on the characteristics of the orchard and destination of the production.

Nowhere in the U.S. is Huanglongbing (HLB) under adequate control due to the lack of effective tools to reduce spread of the vector, Diaphorina citri (Asian citrus psyllid), and transmission of the associated pathogen, Candidatus Liberibacter asiaticus, in the absence of disease resistance in commercial citrus varieties. In Florida, Asian citrus psyllid was well established by the time of HLB discovery and growers did not remove trees to eliminate inoculum, so there was no chance of controlling the epidemic. Based on the Florida experience, Texas proactively controlled the vector by implementing area-wide management and disease spread was slower than in Florida, however, they also did not remove infected trees. California controlled both ACP and tree inoculum and have delayed the development of an epidemic in the Central Valley. This situation is aided by topographic isolation from the Los Angeles basin where ACP and HLB are endemic and a climate distinctly different from other citrus growing regions in the U.S.

Xanthomonas citri subsp. citri (Xcc) causes citrus bacterial canker (CBC) on several Citrus species. Xcc requires bacterial aggregation and biofilm formation to colonize the host plant. Those biofilms are dependent on the environmental condition and their structures differ according to the host range of the Xcc strain. Herein, we have visualized and evaluated the extracellular structures produced by Xcc strains with different host range at early stages of biofilm formation, in mature biofilms and during planktonic growth. Moreover, the presence of these structures was related to the transcription of quorum sensing, flagellar and fimbrial genes. Our results demonstrate that the variation in biofilm formation between Xcc strains is associated with expression of fimbrial and flagellar genes which in some cases is dependent on a quorum sensing system.

The effects of drought stress on phospholipase D (PLD) gene expression and enzymatic activity were investigated in ‘Pineapple’ sweet orange. PLDs are tissue-specific, with overlapping functions, and in response to stress they may interact with ABA signaling. Tissue specificity for expression of PLDs and their regulation are unknown in citrus. To assess PLD response to water stress and gene expression/regulation in citrus, we subjected potted seedlings to increasing levels of soil drought. Evapotranspirational demand (ET) was estimated by measuring weight loss in pots, and water stress was further assessed by measuring ABA content. Three treatments were performed over a 3-week period: (a) Control treatment without drought stress (100% of daily ET); (b) mild water stress (50% ET); and (c) severe water stress (0% ET). ABA content increased during drought stress in both roots and leaves, being higher in leaves than in roots by the end of the experiment for any stress condition assayed. PLD enzymatic activity was monitored and expression of five PLD genes was studied. PLD activity increased linearly over time in response to increased soil drought and was around three times higher in roots than in leaves. PLD activation occurred initially in roots and then in leaves. PLD gene expression in response to stress soil drought differed between roots and leaves. These results show the potential of PLD as a suitable indicator of stress severity in citrus.

Accumulation of toxic copper in soil and development of copper-resistant pests are emerging challenges currently faced by the agricultural community worldwide. As an alternative, we have developed a ternary zinc chelate solution (TSOL) pesticide where zinc ions are the primary active ingredient. The material is composed of zinc, urea, and hydrogen peroxide. Urea was chosen as it is widely used as a plant fertilizer and can also bind to both zinc and hydrogen peroxide. No phytotoxicity was observed with TSOL on Meyer lemon (Citrus × meyeri) seedlings at a field spray rate of 800 μg/mL Zn metal concentration. Antimicrobial studies showed that TSOL exhibited improved killing efficacy against Escherichia coli and Xanthomonas alfalfae compared to Zn ions alone. Citrus canker field trials in a grapefruit (Chrysopelea paradisi) grove over three years showed that TSOL provided comparable disease protection to copper products at an equivalent or lower metal content.

Host disease resistance is the most desirable strategy for control of citrus canker, a disease caused by a gram-negative bacterium Xanthomonas citri subsp. citri. However, no resistant commercial citrus cultivar has been identified. Cybridization, a somatic hybridization approach that combines the organelle and nuclear genomes from different species, was used to create cybrids between citrus canker resistant ‘Meiwa’ kumquat (Fortunella crassifolia Swingle snym. Citrus japonica Thunb.) and susceptible grapefruit (Citrus paradisi Macfad) cultivars. From these fusions, cybrids with grapefruit nucleus, kumquat mitochondria and kumquat chloroplasts and cybrids with grapefruit nucleus, kumquat mitochondria and grapefruit chloroplasts were generated. These cybrids showed a range of citrus canker response, but all cybrids with kumquat chloroplasts had a significantly lower number of lesions and lower Xanthomonas citri subsp. citri populations than the grapefruit controls. Cybrids with grapefruit chloroplasts had a significantly higher number of lesions than those with kumquat chloroplasts. To understand the role of chloroplasts in the cybrid disease defense, quantitative PCR was performed on both cybrid types and their parents to examine changes in gene expression during Xanthomonas citri subsp. citri infection. The results revealed chloroplast influences on nuclear gene expression, since isonuclear cybrids and ‘Marsh’ grapefruit had different gene expression profiles. In addition, only genotypes with kumquat chloroplasts showed an early up-regulation of reactive oxygen species genes upon Xanthomonas citri subsp. citri infection. These cybrids have the potential to enhance citrus canker resistance in commercial grapefruit orchards. They also serve as models for understanding the contribution of chloroplasts to plant disease response and raise the question of whether other alien chloroplast genotypes would condition similar results.

Phytophthora species are important soil-borne, fungus-like pathogens that attack the root systems, trunk, and fruit of citrus trees at any age. The front of this identification sheet includes images of healthy and infested roots and descriptions of leaf and root symptoms. The back lists sampling procedures: soil collection, soil testing, and diagnosing phytophthora. Written by Jamie D. Burrow, Diane B. Bright, Tim D. Riley, and James H. Graham, and published by the UF Department of Soil and Water Science, July 2015. SL431/SS645: Phytophthora Identification and Sampling in Citrus Nurseries (ufl.edu)

Root weevils infest citrus groves throughout the citrus growing regions of Florida. Among the eight weevil species that have been identified in Florida citrus groves, five have some potential to cause economic problems for nurserymen and commercial growers. The most important weevil species are Diaprepes root weevil (Diaprepes abbreviatus), southern blue-green citrus root weevil (Pachnaeus litus), and the blue-green citrus root weevil (Pachnaeus opalus). The little leaf notcher (Artipus floridanus) and Fuller rose beetle (Asynonychus godmani) are of less concern, but may be locally important (Fig. 1). This paper will deal with Diaprepes and the blue-green root weevils because they are of major economic importance and frequently occur in citrus groves. This document is HS-1014, one of a series of the Horticultural Sciences Department, UF/IFAS Extension. Original publication date May 2005. HS-1014/HS260: Field Diagnosis of Citrus Root Weevil Damage (ufl.edu)

This 7-page fact sheet is part of the 2019–2020 Florida Citrus Production Guide. Written by Megan M. Dewdney, Evan G. Johnson, and James H. Graham, and published by the Plant Pathology Department, April 2019. PP-156/CG009: 2022–2023 Florida Citrus Production Guide: Phytophthora Foot Rot, Crown Rot, and Root Rot (ufl.edu)

Phytophthora species are important soil-borne fungal pathogens worldwide that attack the root systems, trunks, and fruit of citrus trees at any age. Historical reports from Florida indicated Phytophthora was a problem as early as 1876 on sweet orange seedlings. When roots or tree trunks are affected, the foliage turns yellow and exhibits poor growth, leaves drop, twigs die back, and fruit size and yield are reduced. As fungal infection of roots and bark progress, the above-ground symptoms increase in severity. Ultimately, trees may decline and die. These symptoms result from the inability of the tree's fibrous roots to take up nutrients and water from the soil, as well as blockage of movement to the tree's canopy via lateral roots and the trunk. The movement of photosynthates 'downward' is also impaired. The two main species of Phytophthora in Florida are P. nicotianae and P. palmivora. Populations of either Phytophthora spp. in the soil may increase by repeated infection of fibrous roots resulting in rapid reproduction under conditions of high moisture and warm temperatures (75-90°F). This document is HS-1015, one of a series of the Horticultural Sciences Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date May 2005. HS-1015/HS261: Field Diagnosis and Management of Phytophthora Diseases (ufl.edu)

We welcome the comments of Rillig et al. (2019) on Ryan & Graham (2018). A robust debate concerning the need to manage arbuscular mycorrhizal fungi (AMF) in agricultural systems has been long overdue. We agree with Rillig et al. (2019) that AMF can play an important role in many processes that may benefit the functioning and yield of agricultural systems. Thus, deliberate management can be warranted which favours AMF, but usually only under certain conditions and contexts. Although the role of AMF for crop yield was the main focus of Ryan & Graham (2018), Rillig et al. (2019) note other potential benefits, including biofortification, improved soil aggregation and reduced nutrient leaching. We contend that the literature pertaining to these processes expresses similar undue optimism as described for yield by Ryan & Graham (2018) because of a paucity of field-relevant experimentation, experimental rigour (in some instances) and agricultural systems context. In this Letter, we concentrate on the last issue, as Ryan & Graham (2018) provide specific examples of the first two. We note the broader context of the need for sustainable intensification of agriculture to provide food security for the growing world population without further land being converted to agriculture or degradation of the land resource, i.e. higher yields with improved resource use efficiency (see Fischer & Connor, 2018). Rillig et al. (2019) propose that AMF are such a critical component of yield and other services of agricultural systems that current systems should be transitioned to be AMF-adequate and that this should be done by redesigning systems to more closely resemble natural analogues in terms of inputs and biodiversity. We suggest that this AMF-centric approach will be relatively ineffective to promote change in current farming systems as it lacks an agricultural systems context. For instance, the approach appears to be advocating low inputs. We recognize the importance of efficiently using inputs and not over-supplying, but – clearly – the high-yielding agricultural systems for sustainable intensification and global food security must involve abundant nutrient inputs if crops are to achieve maximal yields, as these entail high nutrient outputs in produce. If nutrient inputs, including phosphorus (P), are less than outputs in produce, yields will inevitably decline: abundant AMF will not maintain yields (Ryan et al., 2000; Burkitt et al., 2007). In Africa, where the need for improved food security is greatest (Fischer & Connor, 2018), addressing this relationship through increased inputs is essential (Craswell & Vlek, 2013). We suggest that the soil and plant processes able to efficiently recycle nutrients for systems with low productivity are not necessarily the same as those required for systems with high inputs and outputs (in produce) of nutrients. Abundant and effective AMF may benefit yield or promote efficient use of inputs, or effective nutrient cycling, more effectively in the former than the latter. A clear understanding of the continuum of trade-offs between inputs and abundance of AMF and crop yield must be present before an increased focus on AMF is advocated. We consider that an AMF-centric approach to research will, alone, not contribute greatly to the achievement of sustainable intensification. Instead, we propose that research on AMF should be undertaken with a systems agronomy approach in which crop genotype (G), AMF and other soil biota, and environment (E) (including soil chemistry and physics, rainfall, temperature and so on) and farm management (M) are all considered initially with equal merit: see Section VI on G × E × M in Ryan & Graham (2018). The approach involves benchmarking credible physiological crop production limits in order to understand and remove the constraints to achieving them with the aim to develop regionally adapted, resource-efficient agronomic practices. As an example, Dimes et al. (2015) modelled maize yields in rain-fed systems in eastern and southern Africa, and found that low applications of nitrogen (N) fertilizer or legume residues, and improved weed control, could increase farmer yields by 40–150% and maintain stable yields across seasons. For the modern, relatively high-input and high-yielding, no-till cropping systems in southern Australia, Kirkegaard & Hunt (2010) reviewed the literature, modelled the data and proposed a way to combine agronomic practices and new varieties to improve water use efficiency. Follow-up field experiments largely confirmed model predictions, and new agronomic packages were then developed (Kirkegaard et al., 2014; Flohr et al., 2018). The eight high-priority research questions posed by Rillig et al. (2019) are all interesting and many, particularly the final one, are very similar to those of Ryan & Graham (2018). However, we suggest that many of them would benefit from a systems agronomy approach to ascertain whether the management of AMF is the most effective way to achieve goals. As shown by Ryan & Graham (2018), there is currently little evidence to suggest a need to manage AMF and/or that deleterious impacts accrue from farming systems low in AMF, except under some specific circumstances, such as sporadically following long bare fallows in subtropical northern Australia (Thompson, 1987; Ryan & Kirkegaard, 2012). If the management of AMF does appear to be warranted in a particular agricultural context, perhaps one with low-P soils and crops more dependent on AMF than are wheat and other cereals, strategies to utilize AMF can be investigated. However, we would expect that a well-managed system, with efficient use of nutrients, biocides and other inputs, as well as pragmatic implementation of best practices to ameliorate environmental problems and to maximize yield, will favour sufficiently symbiotic AMF by default. A necessary precondition for such a system to function optimally would be the application of P-efficient farming practices, as discussed by Ryan & Graham (2018), where soil P is maintained near the external critical P level of the target crop species (Simpson et al., 2015). Alternative means to achieve outcomes desired from the manipulation of AMF should also be considered. These include modified applications of P fertilizer to enhance the yield or reduce leaching of P, the adoption or development of crop genotypes with root systems better able to acquire P (e.g. Haling et al., 2016), and the adoption of no-tillage and stubble retention to improve soil aggregation or reduce erosion. In conclusion, the mechanisms and effects of symbiotic interactions between AMF and crop plants are extremely complex. This complexity increases with the broader awareness of diversity in interactions among AMF species and strain ecotypes, the other components of the soil biota, environmental variables, farm management and crop genotype (e.g. Gryndler et al., 2018; Ryan & Graham, 2018). The discovery of a group of arbuscule-forming fungal symbionts evolutionarily distinct from AMF, and possibly common in agricultural systems (Orchard et al., 2017), adds further complexity. Although an extensive AMF-centric literature concerning agricultural impacts has accumulated, we suggest that it has become increasingly removed from the agronomic context of its overarching goal of applying research outcomes to benefit agricultural systems. This goal is being lost either in the detail or in detachment from a useful agronomic systems context. As a consequence, conclusions are often overly optimistic, as exemplified by papers stating that AMF will play a major role in sustainable intensification and food security (Rodriguez & Sanders, 2014; Rillig et al., 2016; Thirkell et al., 2017). We contend that AMF will not be harnessed effectively for sustainable intensification without a strong systems agronomy approach overlaying all research. We consider sufficient evidence exists to adopt such an approach, as demonstrated by Ryan & Kirkegaard (2012) in their review on the role of AMF in the farming systems of southern Australia. This approach will encompass a transition towards P-efficient farming systems and promote the retention of abundant AMF and capture many of their purported benefits for yield and other processes, with minimal targeted management. For the management of AMF to be a part of intensifying agriculture to ensure global food security, and for research funding on AMF in agricultural systems to have real impact, communication and collaboration between agronomists and researchers focused on AMF is now essential. MHR is funded by ARC Future Fellowship FT140100103.

Management of brown rot, caused by Phytophthora nicotianae or P. palmivora, is needed on both processing and fresh-market fruit. While the disease affects all citrus types, it is usually most severe on Hamlin, Navel, and other early-maturing sweet orange cultivars. This 3-page fact sheet is part of the 2019–2020 Florida Citrus Production Guide. Written by M. M. Dewdney, E. G. Johnson, and J. H. Graham, and published by the Plant Pathology Department, March 2019. PP-148/CG022: 2022–2023 Florida Citrus Production Guide: Brown Rot of Fruit (ufl.edu)

The Candidatus Liberibacter asiaticus bacterium, associated with Huanglongbing (HLB) disease of citrus trees, moves downward in the phloem and infects the roots soon after transmission by the Asian Citrus Psyllid (Diaphorina citri) vector into shoots. Before canopy symptoms appear, 30–50% of the roots are damaged. Without aggressive management to reduce abiotic and biotic stress, root loss increases to 70–80%. An extensive survey of HLB-affected groves in central and south-central Florida indicated that a greater decline in fibrous root health as well as a greater expression of HLB symptoms is observed where irrigation water is high in bicarbonates (>100 mg L−1) and soil pH is >6.5. Over three seasons of survey, acidification of irrigation water in the central and south-central citrus growing regions of Florida reduced the decline in root density associated with HLB. Irrigation water treatment with sulfuric acid and soil amendment with elemental sulfur for 36 months to establish a soil pH range from 4.0 to 7.0 increased root growth, soil nutrient availability, and the uptake of Ca, Mg, Mn, and Zn in response to a gradual reduction in soil pH in young and mature Valencia orange groves on Swingle citrumelo rootstock. The reduction in soil pH increased yield and soluble solids in fruit and so would improve citrus production.