Liberibacters Associated with Citrus Huanglongbing in Brazil: 'Candidatus Liberibacter asiaticus' Is Heat Tolerant, 'Ca. L. americanus' Is Heat Sensitive

Identification and Distribution of the 'Candidatus Liberibacter asiaticus'-Asian Citrus Psyllid Pathosystem in Saudi Arabia

Citrus greening disease was first reported in Saudi Arabia during the 1970s when characteristic foliar and fruit symptoms were observed in commercial citrus groves. However, 'Candidatus Liberibacter asiaticus' (CLas) was not detected in symptomatic trees until 1981 to 1984 when CLas-like cells were observed by transmission electron microscopy in leaves collected from symptomatic citrus groves in southwestern Saudi Arabia. Despite the anticipated establishment of the CLas-Asian citrus psyllid (ACP) (Diaphorina citri Kuwayama) pathosystem, CLas presence has not been verified in suspect trees nor have ACP infestations been documented. Given the recent expansion of citrus production in Saudi Arabia, a systematic country-wide survey was carried out to determine the potential CLas distribution in the 13 citrus-growing regions of the country. Citrus trees were surveyed for the presence of CLas-psyllid vector(s) and characteristic disease symptoms in commercial and urban citrus trees. Adult psyllids collected from infested citrus trees were identified as ACP based on morphological characteristics. Real-time quantitative PCR amplification (qPCR) of the CLas β-subunit of the ribonucleotide reductase (RNR) gene from citrus leaf and fruit samples and/or ACP adults revealed that trees were positive for CLas detection in 10 of the 13 survey regions; however, CLas was undetectable in ACP adults. Phylogenetic and single nucleotide polymorphism (SNP) analyses of a PCR-amplified, cloned fragment of the CLas 16S rRNA gene (∼1.1 kbp) indicated Saudi Arabian isolates were most closely related to Florida, U.S.A., isolates. Analysis of climate variables indicated that the distribution of the ACP-CLas pathosystem observed in Saudi Arabia was consistent with published predictions of terrains most likely to support establishment.

Sub-optimal temperatures lead to altered expression of stress-related genes and increased 'Candidatus Liberibacter solanacearum' accumulation in potato psyllid

Introduction

The potato psyllid Bactericera cockerelli is the insect vector of the fastidious bacterium 'Candidatus Liberibacter solanacearum'. The bacterium infects both B. cockerelli and plant species, causing zebra chip (ZC) disease of potato and vein-greening disease of tomato. Temperatures are known to influence the initiation and progression of disease symptom in the host plant, and seasonal transitions from moderate to high temperatures trigger psyllid dispersal migration to facilitate survival.

Methods

'Ca. L. solanacearum' -infected and uninfected psyllids were reared at previously established 'permissible', optimal, and 'non-permissible' and temperatures of 18°C, 24°C, and 30°C, respectively. Gene expression profiles for 'Ca. L. solanacearum'-infected and -uninfected adult psyllids reared at different temperatures were characterized by Illumina RNA-Seq analysis. Bacterial genome copy number was quantified by real-time quantitative-PCR (qPCR) amplification.

Results

Relative gene expression profiles varied in psyllids reared at the three experimental temperatures. Psyllids reared at 18°C and 30°C exhibited greater fold-change increased expression of stress- and 'Ca. L. solanacearum' invasion-related proteins. Quantification by qPCR of bacterial genome copy number revealed that 'Ca. L. solanacearum' accumulation was significantly lower in psyllids reared at 18°C and 30°C, compared to 24°C.

Discussion

Temperature is a key factor in the life history of potato psyllid and multiplication/accumulation of 'Ca. L. solanacearum' in both the plant and psyllid host, influences the expression of genes associated with thermal stress tolerance, among others, and may have been instrumental in driving the co-evolution of the pathosystem.

Current Epidemic Situation and Control Status of Citrus Huanglongbing in Guangdong China: The Space–Time Pattern Analysis of Specific Orchards

Huanglongbing (HLB) is the most harmful bacterial disease in citrus production in the world, and has been seriously ravaging the citrus groves of South China since the 1930s. The surveillance of the epidemiological characteristics of HLB is of utmost priority for citrus production in this region. In order to explore the effects of disease control measures, analyses on the space–time statistical features of the HLB epidemic, from 2019 to 2021, within six orchards in the Guangdong province are presented. Overall, the number of citrus plants in the orchards usually slightly decreased year by year. The reduction was mainly related to the level of plant susceptibility, which is correlated with citrus varieties. The maximum disease severity (incidence and race increment) was correlated with the awareness of this disease and the management intensity applied by the manager. A higher disease index was found in the conventional management orchards than in the comprehensive prevention and control orchards. Proper insect-protective screen houses can effectively prevent the epidemic of HLB, without affecting the fruit quality, and can also aid with higher yields. A high correlation was found between the geometry and topography of orchards and the HLB epidemic due to the wind direction from May to September and the Asia citrus psyllid activity characteristics. For flat orchards, the incidence of HLB in the north and entrance areas was higher than that in the southwest. In the mountain area, the incidence of the windward side in the south was higher than that of the leeward side in the north. Diseased trees tended to have an edge effect in the grove, whereas the trees of the same disease scale were found clustered in their distribution. These results allow a better understanding of HLB epidemiology and provide guidance for the early warning of HLB in new groves in areas that are severely affected by this disease. Furthermore, they also provide a scientific basis for the comprehensive prevention and control of HLB in old groves.

A new and accurate qPCR protocol to detect plant pathogenic bacteria of the genus 'Candidatus Liberibacter' in plants and insects

Four pathogenic bacterial species of the genus 'Candidatus Liberibacter', transmitted by psyllid vectors, have been associated with serious diseases affecting economically important crops of Rutaceae, Apiaceae and Solanaceae families. The most severe disease of citrus plants, huanglongbing (HLB), is associated with 'Ca. Liberibacter asiaticus' (CaLas), 'Ca. Liberibacter americanus' (CaLam) and 'Ca. Liberibacter africanus' (CaLaf), while 'Ca. Liberibacter solanacearum' (CaLsol) is associated with zebra chip disease in potatoes and vegetative disorders in apiaceous plants. Since these bacteria remain non-culturable and their symptoms are non-specific, their detection and identification are done by molecular methods, mainly based on PCR protocols. In this study, a new quantitative real-time PCR protocol based on TaqMan probe, which can also be performed in a conventional PCR version, has been developed to detect the four known phytopathogenic species of the genus Liberibacter. The new protocol has been validated according to European Plant Protection Organization (EPPO) guidelines and is able to detect CaLas, CaLam, CaLaf and CaLsol in both plants and vectors, not only using purified DNA but also using crude extracts of potato and citrus or psyllids. A comparative analysis with other previously described qPCR protocols revealed that this new one developed in this study is more specific and equally or more sensitive. Thus, other genus-specific qPCR protocols have important drawbacks regarding the lack of specificity, while with the new protocol there was no cross-reactions in 250 samples from 24 different plant and insect species from eight different geographical origins. Therefore, it can be used as a rapid and time-saving screening test, as it allows simultaneous detection of all plant pathogenic species of 'Ca. Liberibacter' in a one-step assay.

Insight into resistance to 'Candidatus Liberibacter asiaticus,' associated with Huanglongbing, in Oceanian citrus genotypes

Huanglongbing (HLB), the most destructive citrus disease, is associated with unculturable, phloem-limited Candidatus Liberibacter species, mainly Ca. L. asiaticus (Las). Las is transmitted naturally by the insect Diaphorina citri. In a previous study, we determined that the Oceanian citrus relatives Eremocitrus glauca, Microcitrus warburgiana, Microcitrus papuana, and Microcitrus australis and three hybrids among them and Citrus were full-resistant to Las. After 2 years of evaluations, leaves of those seven genotypes remained Las-free even with their susceptible rootstock being infected. However, Las was detected in their stem bark above the scion-rootstock graft union. Aiming to gain an understanding of the full-resistance phenotype, new experiments were carried out with the challenge-inoculated Oceanian citrus genotypes through which we evaluated: (1) Las acquisition by D. citri fed onto them; (2) Las infection in sweet orange plants grafted with bark or budwood from them; (3) Las infection in sweet orange plants top-grafted onto them; (4) Las infection in new shoots from rooted plants of them; and (5) Las infection in new shoots of them after drastic back-pruning. Overall, results showed that insects that fed on plants from the Oceanian citrus genotypes, their canopies, new flushes, and leaves from rooted cuttings evaluated remained quantitative real-time polymerase chain reaction (qPCR)-negative. Moreover, their budwood pieces were unable to infect sweet orange through grafting. Furthermore, sweet orange control leaves resulted infected when insects fed onto them and graft-receptor susceptible plants. Genomic and morphological analysis of the Oceanian genotypes corroborated that E. glauca and M. warburgiana are pure species while our M. australis accession is an M. australis × M. inodora hybrid and M. papuana is probably a M. papuana × M. warburgiana hybrid. E. glauca × C. sinensis hybrid was found coming from a cross between E. glauca and mandarin or tangor. Eremocitrus × Microcitrus hybrid is a complex admixture of M. australasica, M. australis, and E. glauca while the last hybrid is an M. australasica × M. australis admixture. Confirmation of consistent full resistance in these genotypes with proper validation of their genomic parentages is essential to map properly genomic regions for breeding programs aimed to generate new Citrus-like cultivars yielding immunity to HLB.

An Overview of the Mechanisms Against "Candidatus Liberibacter asiaticus": Virulence Targets, Citrus Defenses, and Microbiome

Citrus Huanglongbing (HLB) or citrus greening, is the most destructive disease for citrus worldwide. It is caused by the psyllid-transmitted, phloem-limited bacteria "Candidatus Liberibacter asiaticus" (CLas). To date, there are still no effective practical strategies for curing citrus HLB. Understanding the mechanisms against CLas can contribute to the development of effective approaches for combatting HLB. However, the unculturable nature of CLas has hindered elucidating mechanisms against CLas. In this review, we summarize the main aspects that contribute to the understanding about the mechanisms against CLas, including (1) CLas virulence targets, focusing on inhibition of virulence genes; (2) activation of citrus host defense genes and metabolites of HLB-tolerant citrus triggered by CLas, and by agents; and (3) we also review the role of citrus microbiome in combatting CLas. Finally, we discuss novel strategies to continue studying mechanisms against CLas and the relationship of above aspects.

Probing the Application of OmpA-Derived Peptides to Disrupt the Acquisition of 'Candidatus Liberibacter asiaticus' by Diaphorina citri

Huanglongbing (HLB) is currently the most devastating disease of citrus worldwide. Both bacteria 'Candidatus Liberibacter asiaticus' (CLas) and 'Candidatus Liberibacter americanus' (CLam) are associated with HLB in Brazil but with a strong prevalence of CLas over CLam. Conventionally, HLB management focuses on controlling the insect vector population (Diaphorina citri; also known as Asian citrus psyllid [ACP]) by spraying insecticides, an approach demonstrated to be mostly ineffective. Thus, development of novel, more efficient HLB control strategies is required. The multifunctional bacterial outer membrane protein OmpA is involved in several molecular processes between bacteria and their hosts and has been suggested as a target for bacterial control. Curiously, OmpA is absent in CLam in comparison with CLas, suggesting a possible role in host interaction. Therefore, in the current study, we have treated ACPs with different OmpA-derived peptides, aiming to evaluate acquisition of CLas by the insect vector. Treatment of psyllids with 5 µM of Pep1, Pep3, Pep5, and Pep6 in artificial diet significantly reduced the acquisition of CLas, whereas increasing the concentration of Pep5 and Pep6 to 50 µM abolished this process. In addition, in planta treatment with 50 µM of Pep6 also significantly decreased the acquisition of CLas, and sweet orange plants stably absorbed and maintained this peptide for as long as 3 months post the final application. Together, our results demonstrate the promising use of OmpA-derived peptides as a novel biotechnological tool to control CLas.

Growth of 'Candidatus Liberibacter asiaticus' in Commercial Grapefruit Juice-Based Media Formulations Reveals Common Cell Density-Dependent Transient Behaviors

The phloem-restricted, insect-transmitted bacterium 'Candidatus Liberibacter asiaticus' (CLas) is associated with huanglongbing (HLB), the most devastating disease of citrus worldwide. The inability to culture CLas impairs the understanding of its virulence mechanisms and the development of effective management strategies to control this incurable disease. Previously, our research group used commercial grapefruit juice (GJ) to prolong the viability of CLas in vitro. In the present study, GJ was amended with a wide range of compounds and incubated under different conditions to optimize CLas growth. Remarkably, results showed that CLas growth ratios were inversely proportional to the initial inoculum concentration. This correlation is probably regulated by a cell density-dependent mechanism, because diluting samples between subcultures allowed CLas to resume growth. Moreover, strategies to reduce the cell density of CLas, such as subculturing at short intervals and incubating samples under flow conditions, allowed this bacterium to multiply and reach maximum growth as early as 3 days after inoculation, although no sustained exponential growth was observed under any tested condition. Unfortunately, cultures were only transient, because CLas lost viability over time; nevertheless, we obtained populations of about 10⁵ genome equivalents/ml repeatedly. Finally, we established an ex vivo system to grow CLas within periwinkle calli that could be used to propagate bacterial inoculum in the lab. In this study we determined the influence of a comprehensive set of conditions and compounds on CLas growth in culture. We hope our results will help guide future efforts toward the long-sought goal of culturing CLas axenically.

Prevalent Transmission of 'Candidatus Liberibacter asiaticus' over 'Ca. Liberibacter americanus' in a Long-Term Controlled Environment

In Brazil, citrus huanglongbing (HLB) is associated with 'Candidatus Liberibacter americanus' (CLam) and 'Ca. Liberibacter asiaticus' (CLas). However, there are few studies about HLB epidemiology when both Liberibacter spp. and its insect vector, the Asian citrus psyllid (ACP, Diaphorina citri), are present. The objective of this work was to compare the transmission of HLB by ACP when both CLam and CLas are present as primary inoculum. Two experiments were performed under screenhouse conditions from April 2008 to January 2012 (experiment 1) and from February 2011 to December 2015 (experiment 2). The experiments were carried out with sweet orange plants infected with CLam or CLas as inoculum source surrounded by sweet orange healthy plants. One hundred Liberibacter-free adult psyllids were monthly confined to the source of inoculum plants for 7 days with subsequent free movement inside the screenhouse. Fortnightly, nymphs and adults of psyllids were monitored. Psyllid and leaf samples were collected periodically for Liberibacter detection by PCR or quantitative PCR. CLas was detected more frequently than CLam in both psyllid and leaf samples. No mixed infections were detected in the psyllids. A clear prevalence of CLas over CLam was observed in both experiments. The final HLB incidences were 16.7 and 14.5% of Liberibacter-positive test plants, and CLas was detected in 92.3 and 93.1% of these infected plants. Mixed infection was observed only in 3.8% of infected test plants in experiment 1. These results endorse the shift in the prevalence of CLam to CLas observed in citrus orchards of São Paulo, Brazil.

A Review of the 'Candidatus Liberibacter africanus' Citrus Pathosystem in Africa

It has been nearly 100 years since citrus growers in two distinct regions in the northern provinces of South Africa noticed unusual symptoms in their citrus trees, causing significant crop losses. They had no idea that these symptoms would later become part of an almost global pandemic of a disease called greening or huanglongbing (HLB). The rapid spread of the disease indicated that it might be caused by a transmissible pathogen, but it took >50 years to identify the causative agent as 'Candidatus Liberibacter africanus'. Recently, the disease appeared in more African countries, spreading by both infected planting material and Trioza erytreae. To date, five 'Ca. L. africanus' subspecies have been identified in various rutaceous species, with 'Ca. L. africanus subsp. clausenae' the only subspecies for which a biovar was detected in citrus. Efforts to detect and differentiate HLB-causing Liberibacter species are ongoing, and recent developments are discussed here. This review focuses on aspects of the African form of HLB, including its specific bacterial species and subspecies, its main insect vector, its geographic distribution, and current management strategies.

Effect of Plant Age, Temperature, and Vector Load on 'Candidatus Liberibacter solanacearum' in Planta Titer and Shoot Proliferation Symptoms in Carrot

A decade ago, shoot proliferation symptoms (i.e., witches' broom) in carrots were believed to be the cause of 'Candidatus Phytoplasma' and Spiroplasma infection, yet in recent years this association appeared to have weakened, and a closer association was found with the yet-unculturable, psyllid-transmitted Gram-negative bacterium 'Candidatus Liberibacter solanacearum'. In Israel, carrots are grown throughout the year, yet shoot proliferation symptoms tend to appear only in mature plants and mostly in late spring to early summer. We hypothesized that factors such as plant age, temperature, and vector load, which vary during the year, have a critical effect on symptom development and examined these factors under controlled conditions. Here we show that young carrot seedlings are as prone as older plants to develop shoot proliferation symptoms after 'Ca. L. solanacearum' inoculation. Surprisingly, we found that the local 'Ca. L. solanacearum' haplotype was extremely sensitive to constant temperature of 30°C, which led to a significant reduction in bacterial growth and symptom development compared with 18°C, which was very conducive to symptom development. We have also found that inoculations with 10 or 20 psyllids per plant results in faster symptom development compared with inoculations with two psyllids per plant; however, the difference between vector loads in disease progress rate was not significant. These data provide important insights to the effects of plant age, growth temperature, and vector load on 'Ca. L. solanacearum' and its associated symptoms and further strengthen the notion that 'Ca. L. solanacearum' is the main responsible agent for carrot witches' broom in Israel.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Early detection of Huanglongbing with EESI-MS indicates a role of phenylpropanoid pathway in citrus

Huanglongbing (HLB), a devastating disease for citrus worldwide, is caused by Candidatus Liberibacter asiaticus (CLas). In this study, we employed a novel extractive electrospray ionization-mass spectrometry (EESI-MS) method to analyze the metabolites in leaves of uninfected and HLB-infected Newhall navel orange. The results showed that uninfected and HLB-infected leaves could be readily distinguished based on EESI-MS combined by multivariable analysis. Nine phenolic compounds involved in phenylpropanoid pathway, such as p-coumaric acid, naringin, and apigenin, were principal components to distinguish the leaves of uninfected and HLB-infected Newhall navel orange. Gene expression was also conducted to further explore the molecular mechanism of phenylpropanoid branch pathway in HLB. The expression of genes (4CL, HCT, CHI, CHS, CYP, and C12R) involved in phenylpropanoid branch pathway was increased in asymptomatic and early period of HLB-infected leaves, while decreased in later period of HLB-infected leaves. This study provides a novel method for early detection of citrus HLB and suggests the regulation mechanism of phenylpropanoid pathway in the interaction between citrus and CLas.

Protocol for Successful Transmission of 'Candidatus Liberibacter asiaticus' from Citrus to Citrus Using Diaphorina citri

A protocol to successfully transmit the huanglongbing (HLB) pathogen, 'Candidatus Liberibacter asiaticus', between citrus plants by using the Asian citrus psyllid (ACP) and an alternative way to help growers control ACP are proposed. Best results were obtained when pathogen acquisition by adults reared on fully symptomatic 'Ca. Liberibacter asiaticus'-positive plants, latency, and inoculation occurred at ambient air temperatures ranging from 24 to 28°C and when a single infective adult ACP was confined for 7 days on soft, newly developing vegetative shoots (stages v2 to v4). No infection resulted from confinement of infective ACP adults on mature leaves (stage v6). Under the described conditions, single ACP adults could successfully transmit 'Ca. Liberibacter asiaticus' to an average of 56.5% (35 to 83%) of plantlets with v2 to v4 shoots growing in 0.3-liter tubes and to 80.5% (76 to 86%) of plants with v2 to v4 shoots growing in 4.7-liter pots. The use of single insects and plantlets reduces labor, space, and other resources necessary to undertake transmission tests. It also reduces time needed for transmission studies and should help accelerate research on HLB. The results were used to develop an index for favorability to infection (IFI) to determine orchard vulnerabilities to 'Ca. Liberibacter asiaticus'. The IFI is based on the heterogeneous population of new shoots that occurs on tree canopies and may offer alternative or complementary alternatives to the laborious and costly insect surveys currently used in most instances to determine threshold levels for insecticide applications.

Modeling 'Candidatus Liberibacter asiaticus' Movement Within Citrus Plants

The phloem-limited 'Candidatus Liberibacter asiaticus' (Las) causes huanglongbing, a destructive citrus disease. Graft-inoculated potted plants were used to assess Las speed of movement in phloem in the greenhouse, and the impacts of temperature on plant colonization in growth-chamber experiments. For assessment of Las speed, plants were inoculated at the main stem and assessed over time by quantitative PCR (qPCR) or symptoms at various distances from the inoculum. For colonization, the plants were inoculated in one of two opposite top branches, maintained at from 8 to 20°C, from 18 to 30°C, or from 24 to 38°C daily range, and assessed by qPCR of samples taken from noninoculated shoots. For all experiments, frequencies of Las-positive sites were submitted to analysis of variance and binomial generalized linear model and logistic regression analyses. Probabilities of detecting Las in greenhouse plants were functions of time and distance from the inoculation site, which resulted in 2.9 and 3.8 cm day^-1 average speed of movement. In growth chambers, the temperature impacted plant colonization by Las, new shoot emission, and symptom expression. After a 7-month exposure time, Las was absent in all new shoots in the cooler environment (average three per plant), and present in 70% at the milder environment (six shoots, severe symptoms) and 25% in the warmer environment (eight shoots, no visible symptoms). Temperature of 25.7°C was the optimum condition for plant colonization. This explains the higher impact and incidence of huanglongbing disease during the winter months or regions of milder climates in Brazil.

Early Population Dynamics of "Candidatus Liberibacter asiaticus" in Susceptible and Resistant Genotypes After Inoculation With Infected Diaphorina citri Feeding on Young Shoots

Huanglongbing is a highly destructive citrus disease associated with “Candidatus Liberibacter asiaticus” (Las), a phloem−limited and non-culturable bacterium, naturally transmitted by the psyllid Diaphorina citri. Although diverse approaches have been used to understand the molecular mechanisms involved in the pathogen–host interaction, such approaches have focused on already infected and/or symptomatic plants, missing early events in the initial days post-inoculation. This study aimed to identify the time course of Las multiplication and whole-plant colonization immediately following inoculation by infected psyllids feeding for 2 days. Thus, the experimental approach was to track Las titers after psyllid inoculation in new shoots (NS) of Citrus × sinensis (susceptible), Murraya paniculata (partially resistant), and Bergera koenigii (fully resistant). Soon after psyllid removal, Las titers dropped until the 10–12th days in all three species. Following this, Las titers increased exponentially only in C. × sinensis and M. paniculata, indicating active bacterial multiplication. In C. × sinensis, Las reached a stationary phase at ∼5 log Las cells/g of tissue from the 40th day onward, while in M. paniculata, Las increased at a lower rate of up to ∼3 log Las cells/g of tissue between the 40th and 60th days, decreasing gradually thereafter and becoming undetectable from the 160th day onward. In B. koenigii, Las titers decreased from the start and remained undetectable. In C. × sinensis, an average of 2.6 log of Las cells/g of tissue was necessary for Las to move out of 50% of the NS in 23.6 days and to colonize the rest of the plant, causing a successful infection. Conversely, the probability of Las moving out of the NS remained below 50% in M. paniculata and zero in B. koenigii. To our knowledge, this is the first study on Las dynamics and whole-plant colonization during the earliest stages of infection. Identification of critical time-points for either successful multiplication or Las resistance may help to elucidate initial events of Las–host interactions that may be missed due to longer sampling intervals and at later stages of infection.

Genetic diversity and population structure of 'Candidatus Liberibacter asiaticus' associated with citrus Huanglongbing in India based on the prophage types

Huanglongbing (HLB), also known as 'citrus greening', is an extremely destructive disease of citrus worldwide. HLB is associated with three species of the fastidious proteobacterium, Candidatus Liberibacter asiaticus (CaLas), Ca. L. africanus and Ca. L. americanus with CaLas being the most widely distributed around the world and the only species detected and described so far in India, one of the major global citrus fruit producers. Prophages are highly dynamic components in the bacterial genome and play an important role in intraspecies variations. Three types of prophages, Type 1, Type 2 and Type 3 have been identified and described in CaLas so far. In the present study, 441 CaLas isolates sampled across 18 Indian states were used for prophage typing. Based on detection of three prophage types by PCR, all the eight probable combinations of CaLas prophages were identified, including single Type 1 (26.5%), single Type 2 (18.8%), single Type 3 (1.4%), Type 1 + Type 2 (20.4%), Type 1 + Type 3 (12.5%), Type 2 + Type 3 (4.8%), Type 1 + Type 2 + Type 3 (11.3%) and None type (4.3%). Prophage types were confirmed by PCR amplicon sequencing and subsequent phylogenetic analysis. By discovery of all 3 prophages and based on genetic identity and genetic distance, CaLas populations from eighteen citrus growing states were separated into two major Prophage Typing Groups (PTGs): PTG1 and PTG2. The PTG1 comprised of CaLas from North-West India and PTG2 from rest of the country (North-East, Central and South India), and both major groups were further divided into two (PTG1-A, PTG1-B) and three (PTG2-A, PTG2-B and PTG2-C) subgroups respectively. The findings of CaLas population patterns provide evidence for independent origins of HLB-associated CaLas. CRISPR (clustered regularly interspaced short palindromic repeats) array was also detected in CaLas isolates. This is the first report evaluating the genetic variation of a large population of CaLas bacterium in India using the PCR markers from the prophage regions which would certainly assist the ongoing HLB management efforts in India.

Evidence That 'Candidatus Liberibacter asiaticus' Moves Predominantly Toward New Tissue Growth in Citrus Plants

'Candidatus Liberibacter asiaticus' (Las) is an unculturable, phloem-limited, insect-transmitted bacterium associated with the Asiatic form of huanglongbing (HLB), the most destructive citrus disease. In Asia and the Americas, it is transmitted by the Asian citrus psyllid (Diaphorina citri Kuwavama). Despite considerable research, little is known about the processes involved in plant infection and colonization by Las. This study was conducted to determine whether the basal portion (below girdling) of the plant is an important route for Las to move laterally from a point of inoculation on a branch to pathogen-free branches elsewhere in the canopy, and to quantify the influence of actively growing tissues on vertical upward (acropetally) or downward (basipetally) movement of Las. Nongirdled and fully or partially girdled stems of potted plants of 'Pera' sweet orange, graft-inoculated above or below girdling, were sampled in distinct regions and assessed by qPCR, 6 months postinoculation. Las invaded all regions of partially and nongirdled plants but remained restricted to the inoculated regions of fully girdled plants, evidence that in planta bacterium movement is limited to the phloem. In fully girdled plants, starch accumulated above the girdling site, probably because of changes in flow of phloem sap. To study the influence of actively growing tissues, inoculated 'Valencia' sweet orange plants were kept intact or were top- or root-pruned to force production of new tissues, and sampled at 15-day intervals. Las migrated rapidly and most predominantly toward newly developing root and leaf tissues. The rapid and predominant movement of Las to newly developed shoots and roots would explain failures of canopy heat treatments and pruning to cure HLB-affected trees, and reinforces the need to protect rapidly growing new shoots from feeding by D. citri in order to minimize transmission and spread of the pathogen by the vector within and between orchards.

Root samples provide early and improved detection of Candidatus Liberibacter asiaticus in Citrus

Huanglongbing (HLB), or Citrus Greening, is one of the most devastating diseases affecting agriculture today. Widespread throughout Citrus growing regions of the world, it has had severe economic consequences in all areas it has invaded. With no treatment available, management strategies focus on suppression and containment. Effective use of these costly control strategies relies on rapid and accurate identification of infected plants. Unfortunately, symptoms of the disease are slow to develop and indistinct from symptoms of other biotic/abiotic stressors. As a result, diagnosticians have focused on detecting the pathogen, Candidatus Liberibacter asiaticus, by DNA-based detection strategies utilizing leaf midribs for sampling. Recent work has shown that fibrous root decline occurs in HLB-affected trees before symptom development among leaves. Moreover, the pathogen, Ca. Liberibacter asiaticus, has been shown to be more evenly distributed within roots than within the canopy. Motivated by these observations, a longitudinal study of young asymptomatic trees was established to observe the spread of disease through time and test the relative effectiveness of leaf- and root-based detection strategies. Detection of the pathogen occurred earlier, more consistently, and more often in root samples than in leaf samples. Moreover, little influence of geography or host variety was found on the probability of detection.

Drench Application of Systemic Insecticides Disrupts Probing Behavior of Diaphorina citri (Hemiptera: Liviidae) and Inoculation of Candidatus Liberibacter asiaticus

Candidatus Liberibacter asiaticus (CLas) is a phloem-limited bacterium that is associated with the Huanglongbing (HLB) disease of citrus and transmitted by the psyllid, Diaphorina citri. There are no curative methods to control HLB and the prevention of new infections is essential for HLB management. Therefore, the objective of our study was to determine the effects of systemic insecticides, such as the neonicotinoids imidacloprid, thiamethoxam, and a mixture of thiamethoxam and chlorantraniliprole (diamide) on the probing behavior of CLas-infected D. citri and their effect on CLas transmission. The electrical penetration graph (EPG-DC) technique was used to monitor the stylet penetration activities of CLas-infected D. citri on sweet orange [Citrus sinensis (L.) Osbeck] ‘Valencia’ treated with systemic insecticides. Systemic insecticides disrupted the probing behavior of CLas-infected D. citri, in a way that affected CLas transmission efficiency, particularly by negatively affecting the stylet activities related to the phloem phase. All insecticides reduced (by 57–73%) the proportion of psyllids that exhibited sustainable phloem ingestion (waveform E2 > 10 min), with significant differences observed on plants treated with thiamethoxam and thiamethoxam + chlorantraniliprole. The transmission rate of CLas with high inoculum pressure (five CLas-infected D. citri per plant and a seven-day inoculation access period) to untreated control plants was 93%. In contrast, CLas transmission was reduced to 38.8% when test plants were protected by systemic insecticides. Our results indicated that all insecticides tested presented a potential to reduce CLas inoculation by an average of 59%; therefore, these insecticides can be used to reduce the spread of HLB.

Genome-wide analyses of Liberibacter species provides insights into evolution, phylogenetic relationships, and virulence factors

'Candidatus Liberibacter' species are insect-transmitted, phloem-limited α-Proteobacteria in the order of Rhizobiales. The citrus industry is facing significant challenges due to huanglongbing, associated with infection from 'Candidatus Liberibacter asiaticus' (Las). In order to gain greater insight into 'Ca. Liberibacter' biology and genetic diversity, we have performed genome sequencing and comparative analyses of diverse 'Ca. Liberibacter' species, including those that can infect citrus. Our phylogenetic analysis differentiates 'Ca. Liberibacter' species and Rhizobiales in separate clades and suggests stepwise evolution from a common ancestor splitting first into nonpathogenic Liberibacter crescens followed by diversification of pathogenic 'Ca. Liberibacter' species. Further analysis of Las genomes from different geographical locations revealed diversity among isolates from the United States. Our phylogenetic study also indicates multiple Las introduction events in California and spread of the pathogen from Florida to Texas. Texan Las isolates were closely related, while Florida and Asian isolates exhibited the most genetic variation. We have identified conserved Sec translocon (SEC)-dependent effectors likely involved in bacterial survival and virulence of Las and analysed their expression in their plant host (citrus) and insect vector (Diaphorina citri). Individual SEC-dependent effectors exhibited differential expression patterns between host and vector, indicating that Las uses its effector repertoire to differentially modulate diverse organisms. Collectively, this work provides insights into the evolution of 'Ca. Liberibacter' species, the introduction of Las in the United States and identifies promising Las targets for disease management.

Resistance to 'Candidatus Liberibacter asiaticus,' the Huanglongbing Associated Bacterium, in Sexually and/or Graft-Compatible Citrus Relatives

Huanglongbing (HLB) is the most destructive, yet incurable disease of citrus. Finding sources of genetic resistance to HLB-associated 'Candidatus Liberibacter asiaticus' (Las) becomes strategic to warrant crop sustainability, but no resistant Citrus genotypes exist. Some Citrus relatives of the family Rutaceae, subfamily Aurantioideae, were described as full-resistant to Las, but they are phylogenetically far, thus incompatible with Citrus. Partial resistance was indicated for certain cross-compatible types. Moreover, other genotypes from subtribe Citrinae, sexually incompatible but graft-compatible with Citrus, may provide new rootstocks able to restrict bacterial titer in the canopy. Use of seedlings from monoembryonic species and inconsistencies in previous reports likely due to Las recalcitrance encouraged us to evaluate more accurately these Citrus relatives. We tested for Las resistance a diverse collection of graft-compatible Citrinae species using an aggressive and consistent challenge-inoculation and evaluation procedure. Most Citrinae species examined were either susceptible or partially resistant to Las. However, Eremocitrus glauca and Papua/New Guinea Microcitrus species as well as their hybrids and those with Citrus arose here for the first time as full-resistant, opening the way for using these underutilized genotypes as Las resistance sources in breeding programs or attempting using them directly as possible new Las-resistant Citrus rootstocks or interstocks.

Murraya paniculata and Swinglea glutinosa as Short-Term Transient Hosts of 'Candidatus Liberibacter asiaticus' and Implications for the Spread of Huanglongbing

Murraya paniculata and Swinglea glutinosa are aurantioid hosts of the Asian citrus psyllid (ACP) Diaphorina citri, the principal vector of 'Candidatus Liberibacter asiaticus' (Las). Las is the pathogen associated with huanglongbing (HLB), the Asian form of which is the most devastating disease of Citrus species and cultivars (Rutaceae: Aurantioideae). M. paniculata is a common ornamental and S. glutinosa is grown as an ornamental, a citrus rootstock, and a hedgerow fence plant. Because of the uncertain status of these plants as reservoirs of Las, a series of cross-inoculation bioassays were carried out in different environments, using infected Valencia sweet orange (Citrus × aurantium) infected shoot tops as a source of inoculum and D. citri nymphs and adults reared on M. paniculata and S. glutinosa to inoculate pathogen-free Valencia orange plantlets. In contrast to sweet orange, Las was more unevenly distributed and reached much lower titers in M. paniculata and S. glutinosa. Infections in M. paniculata and S. glutinosa were also transient. Very few insects that successfully acquired Las from M. paniculata and S. glutinosa were able to transmit the pathogen to healthy citrus. Transmission rates were low from M. paniculata (1.0%) and S. glutinosa (2.0%) and occurred only in a controlled environment highly favorable to Las and ACP using 10-day-old adults that completed their life cycle on Las-positive plants. Our study showed that in HLB-endemic areas, M. paniculata and S. glutinosa can be deemed as epidemiologically dead-end hosts for Las and are not important alternative hosts of the pathogen for transmission to citrus. However, under a combination of conditions highly favorable to Las infection and transmission and in the absence of effective quarantine procedures, these plants could eventually serve as carriers of Las to regions currently free from HLB.

Lessons from One Fastidious Bacterium to Another: What Can We Learn about Liberibacter Species from Xylella fastidiosa

Huanglongbing is causing economic devastation to the citrus industry in Florida, and threatens the industry everywhere the bacterial pathogens in the Candidatus Liberibacter genus and their insect vectors are found. Bacteria in the genus cannot be cultured and no durable strategy is available for growers to control plant infection or pathogen transmission. However, scientists and grape growers were once in a comparable situation after the emergence of Pierce’s disease, which is caused by Xylella fastidiosa and spread by its hemipteran insect vector. Proactive quarantine and vector control measures coupled with interdisciplinary data-driven science established control of this devastating disease and pushed the frontiers of knowledge in the plant pathology and vector biology fields. Our review highlights the successful strategies used to understand and control X. fastidiosa and their potential applicability to the liberibacters associated with citrus greening, with a focus on the interactions between bacterial pathogen and insect vector. By placing the study of Candidatus Liberibacter spp. within the current and historical context of another fastidious emergent plant pathogen, future basic and applied research to develop control strategies can be prioritized.

Unsuitability of indigenous South American Rutaceae as potential hosts of Diaphorina citri

BACKGROUND

'Candidatus Liberibacter asiaticus' is transmitted by Diaphorina citri, an insect with a wide range of hosts in Rutaceae. Species related to Citrus occur in Brazilian forests where they may serve as hosts for psyllids and infested citrus orchards.

RESULTS

The suitability of plants as hosts of D. citri was classified into four groups. Group I (high suitability): Citrus × aurantium 'Valencia', 'Citrus limonia', Murraya paniculata (syn. Murraya exotica L.) (Aurantioideae: Aurantieae) and Bergera koenigii (Aurantioideae: Clauseneae). Group II (intermediate to low suitability): Citrus (Poncirus) trifoliata 'Pomeroy', Citrus wintersii, Swinglea glutinosa (Aurantieae) and Clausena lansium (Clauseneae). Group III (not suitable): Aegle marmelos, Atalantia buxifolia, Citrus ('Microcitrus') sp. (Aurantieae) and Helietta apiculata (Amyridoideae). Group IV (non-hosts): Glycosmis pentaphylla (Clauseneae), Balfourodendron riedelianum, Casimiroa edulis, Esenbeckia febrifuga, Esenbeckia leiocarpa, Metrodorea stipularis, Zanthoxylum rhoifolium (Amyridoideae) and Dictyoloma vandellianum (Cneoroideae). Insects survived longer on newly differentiated leaves compared with fully expanded soft leaves. Psyllids either did not develop or did not survive for long on most Group IV species, all of which, with the exception of G. pentaphylla, occur naturally in Brazilian forests.

CONCLUSION

Citrus relatives occurring in forests near citrus orchards are not suitable hosts of D. citri and, therefore, do not contribute to huanglongbing spread. © 2018 Society of Chemical Industry.

Classical biological control of the African citrus psyllid Trioza erytreae, a major threat to the European citrus industry

Citrus greening or huanglongbing (HLB) is the main threat to the European citrus industry since one of its vectors, the African citrus psyllid, Trioza erytreae, has recently become established in mainland Europe. In this context, classical biological control programmes should be implemented to reduce the spread of the psyllid. The aims of this study were to: i) disentangle the parasitoid complex of T. erytreae combining morphological and molecular characterization; and ii) to study the biology of its main parasitoids in its area of origin in South Africa for their future importation into Europe. The main citrus producing areas of South Africa were surveyed during 2017. In contrast to previous studies, the parasitoid complex of T. erytreae included three species of primary parasitoids: Tamarixia dryi, Psyllaephagus pulvinatus and another parasitoid of the genus Tamarixia. Molecular analysis showed that it is a new species closely related to T. dryi. Tamarixia dryi was the most abundant parasitoid but its relative abundance varied among sampling sites. The sex ratio (males/females) of T. dryi and Tamarixia sp. decreased with T. erytreae size and became female biased when psyllid nymphs were larger than 0.6 and 1.2 mm², respectively. These parasitoids were attacked by three species of hyperparasitoids, Aphidencyrtus cassatus, Marietta javensis and a species of the genus Aphanogmus. Aphidencyrtus cassatus, the most abundant hyperparasitoid, tended to emerge from large nymphs, and adult females lived as long as those of T. dryi. The implications of these results are discussed within the framework of the introduction of T. dryi into Europe.

Host and environmental factors influencing 'Candidatus Liberibacter asiaticus' acquisition in Diaphorina citri

BACKGROUND

Diaphorina citri is a vector of 'Candidatus Liberibacter asiaticus' (CLas) associated with citrus Huanglongbing. In this study, the infection and titers of CLas in the psyllid, were monitored for life cycle stage, sex, host-plant CLas titer, host-plant genotype, and ambient temperature.

RESULTS

Acquisition efficiency of CLas by D. citri was highest in nymphs reared at 25 °C on a host plant with high CLas titers but was independent of the host genotypes assessed and of vector sex. We further observed that D. citri nymphs acquired CLas more rapidly than adults based on acquisition access periods (AAPs). CLas did not multiply in the alimentary canal, hemolymph, and salivary glands of adults for 18 days after a 3-day AAP as adult. However, CLas multiplication was detected in hemolymph and salivary gland of adults after the bacterium was acquired by nymphs. Eighty percent of salivary glands of adults contained CLas 18 days after a 3-day AAP as nymph compared to 10% 18 days after a 3-day AAP as adults.

CONCLUSION

Different factors tested herein influenced CLas acquisition efficiency of D. citri, CLas multiplication and spread inside the psyllid. These observations serve to better understand mechanisms of CLas infection in D. citri. © 2018 Society of Chemical Industry.

The Asian Citrus Psyllid Host Murraya koenigii Is Immune to Citrus Huanglongbing Pathogen 'Candidatus Liberibacter asiaticus'

The Asian citrus psyllid (ACP) Diaphorina citri, vector of 'Candidatus Liberibacter asiaticus' (CLas), the putative causal agent of citrus Huanglongbing (HLB), is controlled by application of insecticides, which, although effective, has resulted in serious biological imbalances. New management tools are needed, and the technique known as "trap crop" has been attracting attention. A potential plant for use as a trap crop in the management of the ACP is Murraya koenigii (curry leaf). However, for this plant to be used in the field, it needs to be attractive for the vector and must not harbor CLas. To verify the potential of curry leaf as trap crop for the management of HLB, we investigated the ability of D. citri to transmit CLas to M. koenigii, and to other test plants, including M. paniculata (orange jasmine) and cultivar Valencia sweet-orange seedlings. For the tests, the insects were reared on a symptomatic CLas-infected plant and allowed to feed on the three test plant species. The overall maximum transmission rate for the citrus seedlings was 83.3%, and for orange jasmine was 33.3%. Successful transmission of CLas by ACP to the curry-leaf seedlings was not observed, and it was treated as immune to CLas. Supported by the previous results that M. koenigii is attractive for ACP, these results indicate that curry leaf is an excellent candidate for use as a trap crop, to improve the management of the insect vector and consequently of HLB.

Huanglongbing Control: Perhaps the End of the Beginning

Huanglongbing (HLB) is one of the most destructive citrus plant diseases worldwide. It is associated with the fastidious phloem-limited α-proteobacteria 'Candidatus Liberibacter asiaticus', 'Ca. Liberibacter africanus' and 'Ca. Liberibacter americanus'. In recent years, HLB-associated Liberibacters have extended to North and South America. The causal agents of HLB have been putatively identified, and their transmission pathways and worldwide population structure have been extensively studied. However, very little is known about the epidemiologic relationships of Ca. L. asiaticus, which has limited the scope of HLB research and especially the development of control strategies. HLB-affected plants produce damaged fruits and die within several years. To control the disease, scientists have developed new compounds and screened existing compounds for their antibiotic and antimicrobial activities against the disease. These compounds, however, have very little or even no effect on the disease. The aim of the present review was to compile and compare different methods of HLB disease control with newly developed integrative strategies. In light of recent studies, we also describe how to control the vectors of this disease and the biological control of other citrus plant pathogens. This work could steer the attention of scientists towards integrative control strategies.

Molecular mechanisms underlying heat or tetracycline treatments for citrus HLB control

Huanglongbing (HLB), a destructive plant bacterial disease, severely impedes worldwide citrus production. In our previous reports, we revealed the molecular mechanisms of host plant responses that underlie thermotherapy against HLB. In this study, we investigated the molecular mechanism underlying heat or tetracycline treatments on the HLB bacterium, 'Candidatus Liberibacter asiaticus' (Las) by focusing on Las prophage/phage conversion under stress conditions. By comparing the prophage FP1 and FP2 copy number to the copy number of 16S rDNA in HLB-affected plants, we found that the relative copy number of both FP1 and FP2 increased significantly, ranging from 3.4- to 6.7-fold change when Las-infected samples underwent a temperature shift from 23 to 37, 42 or 45 °C. When treated with tetracycline at 50-150 and 200-250 µg/ml, respectively, the relative copy number of both FP1 and FP2 increased by 3.4- to 6.0-fold. In addition, analyses of Las prophage structural gene and antirepressor gene copy numbers showed similar trends for all treatments. Furthermore, transmission electron microscopy provided direct evidence of lysogenic to lytic conversion upon temperature increase. These results not only provide new insight into the molecular mechanisms underlying heat or tetracycline treatment but also suggest a novel HLB control strategy by enhancing the endogenous conversion from Las prophages to phages.

Postharvest Development of 'Candidatus Liberibacter solanacearum' in Late-Season Infected Potato Tubers under Commercial Storage Conditions

Zebra chip (ZC) disease of potato is associated with the putative pathogen 'Candidatus Liberibacter solanacearum', which is transmitted by the potato psyllid Bactericera cockerelli (Hem., Triozidae). The present study was initiated to investigate 'Ca. L. solanacearum' development during and following typical commercial storage practices. Using bacteriliferous psyllids, Russet Norkotah potato tubers were infested in field cages 14, 10, and 4 days before harvest. Changes in 'Ca. L. solanacearum' detection rate, 'Ca. L. solanacearum' titer, and concentrations of phenolic compounds were documented throughout storage. 'Ca. L. solanacearum' titer continued to increase during storage. Although significant increases in the frequency of 'Ca. L. solanacearum' detection were observed in all infestation treatments, the impact of 'Ca. L. solanacearum' infection on tuber quality remained comparatively low in plants infected 4 days before harvest, because the majority of the tubers remained asymptomatic. Minimizing storage and retail chain movement durations would help to limit 'Ca. L. solanacearum' impact on tuber quality in tubers infected 14 and 10 days before harvest. This study also demonstrated that 'Ca. L. solanacearum' can relocate from a newly infected leaf to a tuber in as little as 4 days. Psyllid management is recommended until at least 4 days before green harvest, when psyllid pressure is high in fields in which tubers are destined for commercial storage.

Dynamics of Huanglongbing-associated Bacterium Candidatus Liberibacter asiaticus in Citrus aurantifolia Swingle (Mexican Lime)

BACKGROUND

The bacterial disease citrus huanglongbing (HLB), associated with "Candidatus Liberibacter asiaticus" (C.Las) has severely impacted the citrus industry, causing a significant reduction in production and fruit quality. In the present study, it was monitored the C.Las population dynamics in symptomatic, HLB-positive Mexican lime trees (Citrus aurantifolia Swingle) in a tropical, citrus-producing area of Mexico. The objective of this study was to identify the dynamics of the population of huanglongbing-associated bacterium Candidatus Liberibacter asiaticus and its insect vector in Citrus aurantifolia Swingle (Mexican lime).

MATERIALS AND METHODS

Leaf samples were collected every 2 months over a period of 26 months for quantification of bacterial titers and young and mature leaves were collected in each season to determine preferential sites of bacterial accumulation. The proportion of living and dead bacterial cells could be determined through the use of quantitative real-time PCR in the presence of ethidium monoazide (EMA-qPCR).

RESULTS

It was observed a lower bacterial titer at high temperatures in the infected trees relative to titers in mild weather, despite a higher accumulation of the insect vector Diaphorina citri in these conditions. This study also revealed seasonal fluctuations in the titers of bacteria in mature leaves when compared to young leaves. No statistically significant correlation between any meteorological variable, C.Las concentration and D. citri population could be drawn.

CONCLUSION

Although, HLB management strategies have focused on vector control, host tree phenology may be important. The evaluation of citrus phenology, C.Las concentration, ACP population and environmental conditions provides insights into the cyclical, seasonal variations of both the HLB pathogen and its vector. These findings should help in the design of integrative HLB control strategies that take into account the accumulation of the pathogen and the presence of its vector.

Characterization of host plant resistance to zebra chip disease from species-derived potato genotypes and the identification of new sources of zebra chip resistance

'Candidatus Liberibacter solanacearum' (Lso), an uncultivable phloem-limited phytopathogenic bacteria, is known to be associated with Zebra Chip disease (ZC), which represents a major threat to potato production in the US and elsewhere. This pathogen is transmitted by the phloem-feeding potato psyllid, Bactericera cockerelli Sulc (Hem. Triozidae). Currently, there are no reports of resistance to ZC in cultivated potatoes. This greenhouse study was conducted to evaluate Lso transmission success and the susceptibility of 11 potato breeding clones, representing diverge genetic background, to ZC, in relation to a susceptible commercial cultivar, Russet Burbank. Individual plants were exposed to two Lso-positive potato psyllids for 48 hours. The percentage of successful Lso transmission varied across the evaluated genotypes ranging between 7 and 57%. Freshly-cut and fried tubers showed significant variation in ZC symptom severity among the breeding clones, with several genotypes expressing relative tolerance when compared to Russet Burbank. None of the evaluated clones showed statistically lower Lso titers than Russet Burbank with the exception of one genotype in the second year of the study. However, the presence of a non-significant relationship between average symptom severity and Lso titer indicated variations in phenotypic responses (i.e. tolerance) to Lso existed among evaluated breeding lines. Breeding clones A07781-3LB, A07781-4LB and A07781-10LB had relatively lower Lso titer (low susceptibility) and tuber symptom expression (high tolerance) among the tested genotypes. As these three clones represent full siblings, the observed effects could be indicative of the presence of a genetic basis for resistance/tolerance to ZC. Findings provide a better understanding of resistance/tolerance to ZC, and contribute to continued efforts in breeding for resistance to this disease.

Seasonal Variation of 'Candidatus Liberibacter asiaticus' Titers in New Shoots of Citrus in Distinct Environments

The major citrus area of Brazil occupies near 450,000 ha between the Triângulo Mineiro (TM) region of Minas Gerais State and the south of São Paulo State (SPS). Significant climatic variation occurs between regions which could affect huanglongbing (HLB) progress, which is lower in TM. To investigate this possibility, young sweet orange shoots were sampled periodically over 2 years to determine 'Candidatus Liberibacter asiaticus' titers in naturally infected trees in orchards in Analândia, central SPS, and Frutal and Comendador Gomes, within TM. Data-loggers recorded local temperature and relative humidity hourly. In the laboratory, five 'Ca. L. asiaticus'-free Diaphorina citri adults were placed on each sampled shoot for 48 h to feed and acquire the pathogen. Shoots and insects were individually analyzed by quantitative polymerase chain reaction to determine 'Ca. L. asiaticus' titers. The incidence of 'Ca. L. asiaticus'-positive shoots, 'Ca. L. asiaticus' titers, and acquisition rates were lower for shoots from Comendador Gomes than those from Frutal or Analândia. Stronger association was observed between 'Ca. L. asiaticus' titers and the number of hours below 15°C (h < 15°C) or above 30°C (h > 30°C), and cumulative rainfall registered during the 15 days prior to sampling of shoots on each occasion. 'Ca. L. asiaticus' titers associated positively with h < 15°C and rainfall and negatively with h > 30°C. The slower spread and lower incidence of HLB in TM may be related to lower incidences of 'Ca. L. asiaticus'-positive young shoots and lower titers of 'Ca. L. asiaticus' in the same shoots as a consequence of the warmer and drier conditions.

Unusual Five Copies and Dual Forms of nrdB in "Candidatus Liberibacter asiaticus": Biological Implications and PCR Detection Application

"Candidatus Liberibacter asiaticus" (CLas), a non-culturable α-proteobacterium, is associated with citrus Huanglongbing (HLB, yellow shoot disease) currently threatening citrus production worldwide. Here, the whole genome sequence of CLas strain A4 from Guangdong of China was analyzed. Five copies of nrdB, encoding β-subunit of ribonucleotide reductase (RNR), a critical enzyme involving bacterial proliferation, were found. Three nrdB copies were in long form (nrdBL, 1,059 bp) and two were in short form (nrdBS, 378 bp). nrdBS shared >99% identity to 3' end of nrdBL and had no active site. Sequences of CLas nrdB genes formed a distinct monophyletic lineage among eubacteria. To make use of the high copy number feature, a nrdB-based primer set RNRf/RNRr was designed and evaluated using real-time PCR with 262 HLB samples collected from China and USA. Compared to the current standard primer set HLBas/HLBr derived from the 16S rRNA gene, RNRf/RNRr had Ct value reductions of 1.68 (SYBR Green PCR) and 1.77 (TaqMan PCR), thus increasing the detection sensitivity three-fold. Meanwhile, RNRf/RNRr was more than twice the stability of primer set LJ900f/LJ900r derived from multi-copy prophage. The nrdB-based PCR thereby provides a sensitive and reliable CLas detection with broad application, especially for the early diagnosis of HLB.

Biolistic transformation of Carrizo citrange (Citrus sinensis Osb. × Poncirus trifoliata L. Raf.)

The development of transgenic citrus plants by the biolistic method. A protocol for the biolistic transformation of epicotyl explants and transgenic shoot regeneration of immature citrange rootstock, cv. Carrizo (Citrus sinensis Osb. × Poncirus trifoliata L. Raf.) and plant regeneration is described. Immature epicotyl explants were bombarded with a vector containing the nptII selectable marker and the gfp reporter. The number of independent, stably transformed tissues/total number of explants, recorded by monitoring GFP fluorescence 4 weeks after bombardment was substantial at 18.4 %, and some fluorescing tissues regenerated into shoots. Fluorescing GFP, putative transgenic shoots were micro-grafted onto immature Carrizo rootstocks in vitro, confirmed by PCR amplification of nptII and gfp coding regions, followed by secondary grafting onto older rootstocks grown in soil. Southern blot analysis indicated that all the fluorescing shoots were transgenic. Multiple and single copies of nptII integrations were confirmed in five regenerated transgenic lines. There is potential to develop a higher throughput biolistics transformation system by optimizing the tissue culture medium to improve shoot regeneration and narrowing the window for plant sampling. This system will be appropriate for transformation with minimal cassettes.

Variations in Zebra Chip Disease Expression and Tuber Biochemistry in Response to Vector Density

Zebra chip (ZC) disease, caused by 'Candidatus Liberibacter solanacearum', which is transmitted by the potato psyllid, has negatively affected potato production in the United States for over a decade. The present study was conducted to evaluate the affect of the number of bacteriliferous psyllids on 'Ca. L. solanacearum' titer, levels of amino acids, carbohydrates, phenolics, and, subsequently, symptom severity in potato tubers. 'Red La Soda' and 'Russet Norkotah' potato were planted in the field and later inoculated with 'Ca. L. solanacearum' using 5, 10, and 30 bacteriliferous potato psyllids. In both cultivars, the increase in the number of psyllids resulted in elevated 'Ca. L. solanacearum' titer and symptom severity. In the cases of amino acids and reducing sugars, responses to vector density appeared to be cultivar specific. Overall, phenolic compounds showed a consistent increase following infection, a response that, in most cases, was positively correlated with the number of infective psyllids. Results quantified the impact of the number of infective psyllids on 'Ca. L. solanacearum' titer and biochemical content of infected tubers previously shown to be correlated with the degree of symptom expression. Managing vector numbers is currently the only effective approach for minimizing losses to ZC in grower's fields. However, our findings on physiological responses to vector density suggest that, in combination with chemical control, development of more tolerant or resistant potato genotypes, with emphasis on interrupting pathways that are associated with increased phenolic activity levels, may lead to more sustainable management of ZC in the future.

Deciphering the Bacterial Microbiome in Huanglongbing-Affected Citrus Treated with Thermotherapy and Sulfonamide Antibiotics

Huanglongbing (HLB) is a serious citrus disease that threatens the citrus industry. In previous studies, sulfonamide antibiotics and heat treatment suppressed ‘Candidatus Liberibacter asiaticus’ (Las), but did not completely eliminate the Las. Furthermore, there are few reports studying the bacterial microbiome of HLB-affected citrus treated by heat and sulfonamide antibiotics. In this study, combinations of heat (45°C or 40°C) and sulfonamide treatment (sulfathiazole sodium–STZ, or sulfadimethoxine sodium—SDX) were applied to HLB-affected citrus. The bacterial microbiome of HLB-affected citrus following thermotherapy and/or chemotherapy was characterized by PhyloChip^TMG3-based metagenomics. Our results showed that the combination of thermotherapy at 45°C and chemotherapy with STZ and SDX was more effective against HLB than thermotherapy alone, chemotherapy alone, or a combination of thermotherapy at 40°C and chemotherapy. The PhyloChip^TMG3-based results indicated that 311 empirical Operational Taxonomic Units (eOTUs) were detected in 26 phyla. Cyanobacteria (18.01%) were dominant after thermo-chemotherapy. Thermotherapy at 45°C decreased eOTUs (64.43%) in leaf samples, compared with thermotherapy at 40°C (73.96%) or without thermotherapy (90.68%) and it also reduced bacterial family biodiversity. The eOTU in phylum Proteobacteria was reduced significantly and eOTU_28, representing “Candidatus Liberibacter,” was not detected following thermotherapy at 45°C. Following antibiotic treatment with SDX and STZ, there was enhanced abundance of specific eOTUs belonging to the families Streptomycetaceae, Desulfobacteraceae, Chitinophagaceae, and Xanthomonadaceae, which may be implicated in increased resistance to plant pathogens. Our study further develops an integrated strategy for combating HLB, and also provides new insight into the bacterial microbiome of HLB-affected citrus treated by heat and sulfonamide antibiotics.

Transmission and Propagation of 'Candidatus Liberibacter asiaticus' by Grafting with Individual Citrus Leaves

Huanglongbing (HLB) is a chronic, progressive decline disease in citrus associated with a systemic infection by the bacterium 'Candidatus Liberibacter asiaticus'. Transmission of the bacterium in the field is by the Asian citrus psyllid, Diaphorina citri Kuwayama. Experimental propagation of 'Ca. L. asiaticus' is done primarily by grafting pieces of bud wood from an infected plant. To produce a small-scale model system for investigation of pathogen biology, we investigated grafting single leaves from infected citrus plants as sources of inoculum for propagation of the bacterium. In total, 162 plants ranging in age from 3 to 18 months were grafted. Grafting with intact asymptomatic and HLB-symptomatic leaves resulted in 61 of 78 (78%) and 35 of 41 (85%) of the plants infected with 'Ca. L. asiaticus', respectively. Inoculum consisting of the leaf petiole only or only an inoculum tissue remnant under the bark of the receptor tree resulted in 6 of 12 (50%) and 7 of 31 (23%) infected trees, respectively. Real-time polymerase chain reaction (qPCR) assays verified the infection in plants, a majority of which developed the foliar blotchy mottle symptom considered diagnostic for HLB, while some plants also displayed the stunted, chlorotic shoots for which the disease is named. The qPCR data together with the symptoms displayed demonstrated that individual leaves from infected trees can serve as effective inoculum sources for transmission and propagation of 'Ca. L. asiaticus' via grafting.

Characterization of a Recombinant Cathepsin B-Like Cysteine Peptidase from Diaphorina citri Kuwayama (Hemiptera: Liviidae): A Putative Target for Control of Citrus Huanglongbing

Huanglonbing (HLB) is one of the most destructive disease affecting citrus plants. The causal agent is associated with the phloem-limited bacterium Candidatus Liberibacter asiaticus (CLas) and the psyllid Diaphorina citri, vector of disease, that transmits the bacterium associated with HLB. The control of disease can be achieved by suppressing either the bacterium or the vector. Among the control strategies for HLB disease, one of the widely used consists in controlling the enzymes of the disease vector, Diaphorina citri. The insect Diaphorina citri belongs to the order Hemiptera, which frequently have cysteine peptidases in the gut. The importance of this class of enzymes led us to search for enzymes in the D. citri transcriptome for the establishment of alternatives strategies for HLB control. In this study, we reported the identification and characterization of a cathepsin B-like cysteine peptidase from D. citri (DCcathB). DCcathB was recombinantly expressed in Pichia pastoris, presenting a molecular mass of approximately 50 kDa. The enzyme hydrolyzed the fluorogenic substrate Z-F-R-AMC (Km = 23.5 μM) and the selective substrate for cathepsin B, Z-R-R-AMC (Km = 6.13 μM). The recombinant enzyme was inhibited by the cysteine protease inhibitors E64 (IC50 = 0.014 μM) and CaneCPI-4 (Ki = 0.05 nM) and by the selective cathepsin B inhibitor CA-074 (IC50 = 0.095 nM). RT-qPCR analysis revealed that the expression of the DCcathB in nymph and adult was approximately 9-fold greater than in egg. Moreover, the expression of this enzyme in the gut was 175-fold and 3333-fold higher than in the remaining tissues and in the head, respectively, suggesting that DCcathB can be a target for HLB control.

Seasonal Variation in Populations of 'Candidatus Liberibacter asiaticus' in Citrus Trees in Paraná State, Brazil

Huanglongbing (HLB) is considered one of the most destructive diseases of citrus because the plants rapidly become unproductive, enter a decline, and eventually die. HLB is caused by the phloem-limited bacterium 'Candidatus Liberibacter' spp. The objective of this study was to evaluate seasonal variation of the in planta population of 'Ca. Liberibacter asiaticus' in the foliage of citrus trees in Brazil using real-time polymerase chain reaction (qPCR). Eleven plants (naturally infected, then screened) in the field with very mild and localized symptoms of HLB were confirmed to be 'Ca. L. asiaticus' infected by conventional PCR, and the canopies were divided into four quadrants. The bacterial population in the trees was tested on a monthly basis for up to 20 months by quantifying 'Ca. L. asiaticus' DNA using qPCR 'Ca. L. asiaticus'-specific primers (As84F/As180R). The average cycle threshold (C_t) values, which relate to 'Ca. L. asiaticus' titer, were analyzed using a mixed model. Significant differences were observed in C_t values between seasons (F = 8.77, P = 0.0004), and abrupt changes were observed in C_t values in different quadrants of the trees. Autumn had the lowest C_t values, indicating the highest 'Ca. L. asiaticus' titer, and, thus, is considered the best period to detect 'Ca. L. asiaticus' infection in foliage of citrus trees in southern Brazil. In addition to the seasonal changes in C_t values, there was an initial decline in the C_t value in the months following initial detection, the rate of decline slowing with time. Concomitant with the increase of the bacterial population in the host, there was an increase in severity of HLB symptoms in the trees over time (Spearman's rank correlation, r = -0.4083, P < 0.0001). The results identify the optimal season to sample foliage for 'Ca. L. asiaticus' in southern Brazil (autumn) and confirm the importance of sample timing to maximize detection of 'Ca. L. asiaticus' and, thus, contribute to the search for effective measures to manage HLB.

Infection Density Dynamics of the Citrus Greening Bacterium "Candidatus Liberibacter asiaticus" in Field Populations of the Psyllid Diaphorina citri and Its Relevance to the Efficiency of Pathogen Transmission to Citrus Plants

Huanglongbing, or citrus greening, is a devastating disease of citrus plants recently spreading worldwide, which is caused by an uncultivable bacterial pathogen, "Candidatus Liberibacter asiaticus," and vectored by a phloem-sucking insect, Diaphorina citri. We investigated the infection density dynamics of "Ca. Liberibacter asiaticus" in field populations of D. citri with experiments using field-collected insects to address how "Ca. Liberibacter asiaticus" infection density in the vector insect is relevant to pathogen transmission to citrus plants. Of 500 insects continuously collected from "Ca. Liberibacter asiaticus"-infected citrus trees with pathological symptoms in the spring and autumn of 2009, 497 (99.4%) were "Ca. Liberibacter asiaticus" positive. The infections were systemic across head-thorax and abdomen, ranging from 10(3) to 10(7) bacteria per insect. In spring, the infection densities were low in March, at ∼ 10(3) bacteria per insect, increasing up to 10(6) to 10(7) bacteria per insect in April and May, and decreasing to 10(5) to 10(6) bacteria per insect in late May, whereas the infection densities were constantly ∼ 10(6) to 10(7) bacteria per insect in autumn. Statistical analysis suggested that several factors, such as insect sex, host trees, and collection dates, may be correlated with "Ca. Liberibacter asiaticus" infection densities in field D. citri populations. Inoculation experiments with citrus seedlings using field-collected "Ca. Liberibacter asiaticus"-infected insects suggested that (i) "Ca. Liberibacter asiaticus"-transmitting insects tend to exhibit higher infection densities than do nontransmitting insects, (ii) a threshold level (∼ 10(6) bacteria per insect) of "Ca. Liberibacter asiaticus" density in D. citri is required for successful transmission to citrus plants, and (iii) D. citri attaining the threshold infection level transmits "Ca. Liberibacter asiaticus" to citrus plants in a stochastic manner. These findings provide valuable insights into understanding, predicting, and controlling this notorious citrus pathogen.

Localization and Distribution of 'Candidatus Liberibacter asiaticus' in Citrus and Periwinkle by Direct Tissue Blot Immuno Assay with an Anti-OmpA Polyclonal Antibody

'Candidatus Liberibacter asiaticus' (CaLas), a non-cultured member of the α-proteobacteria, is the causal agent of citrus Huanglongbing (HLB). Due to the difficulties of in vitro culture, antibodies against CaLas have not been widely used in studies of this pathogen. We have used an anti-OmpA polyclonal antibody based direct tissue blot immunoassay to localize CaLas in different citrus tissues and in periwinkle leaves. In citrus petioles, CaLas was unevenly distributed in the phloem sieve tubes, and tended to colonize in phloem sieve tubes on the underside of petioles in preference to the upper side of petioles. Both the leaf abscission zone and the junction of the petiole and leaf midrib had fewer CaLas bacteria compared to the main portions of the petiole and the midribs. Colonies of CaLas in phloem sieve tubes were more frequently found in stems with symptomatic leaves than in stems with asymptomatic leaves with an uneven distribution pattern. In serial sections taken from the receptacle to the peduncle, more CaLas were observed in the peduncle sections adjacent to the stem. In seed, CaLas was located in the seed coat. Many fewer CaLas were found in the roots, as compared to the seeds and petioles when samples were collected from trees with obvious foliar symptoms. The direct tissue blot immuno assay was adapted to whole periwinkle leaves infected by CaLas. The pathogen was distributed throughout the lateral veins and the results were correlated with results of qPCR. Our data provide direct spatial and anatomical information for CaLas in planta. This simple and scalable method may facilitate the future research on the interaction of CaLas and host plant.

Ultrastructural Changes and Putative Phage Particles Observed in Sweet Orange Leaves Infected with 'Candidatus Liberibacter asiaticus'

Huanglongbing (HLB), also known as citrus greening, is currently the most destructive citrus disease. Anatomical analyses of HLB-affected sweet orange were carried out by light and electron microscopy. As compared with healthy citrus, the phloem plasmodesmata were plugged with callose, and in some samples the phloem was collapsed. Chloroplast structures were deformed. Prophage sequences occupy a significant portion of the genome of 'Candidatus Liberibacter asiaticus' and have been used to distinguish strains from Yunnan and Guangdong provinces in China and Florida. Interestingly, a large number of possible putative phage particles were observed attached on the surface of 'Ca. L. asiaticus' cells in plants inoculated with strain FJ3 from Fujian Province, China. Phage particles have been observed previously only in periwinkle plants artificially inoculated in Florida with 'Ca. L. asiaticus' that carried the SC1-type prophage. PCR assays verified the presence of the SC1-type prophage sequences previously described from this bacterium in Florida in the FJ3 isolate. This is the first time that suspected phage particles have been observed in sweet orange trees infected with 'Ca. L. asiaticus.'

Control of virus diseases of citrus

Citrus is thought to have originated in Southeast Asia and horticulturally desirable clonal selections have been clonally cultivated for hundreds of years. While some citrus species have nucellar embryony, most cultivation of citrus has been by clonal propagation to ensure that propagated plants have the same traits as the parent selection. Clonal propagation also avoids juvenility, and the propagated plants produce fruit sooner. Because of the clonal propagation of citrus, citrus has accumulated a large number of viruses; many of these viruses are asymptomatic until a susceptible rootstock and/or scion is encountered. The viruses reported to occur in citrus will be summarized in this review. Methods of therapy to clean selected clones from viruses will be reviewed; the use of quarantine, clean stock, and certification programs for control of citrus viruses and other strategies to control insect spread citrus viruses, such as mild strain cross-protection and the use of pest management areas will be discussed.

Association of 'Candidatus Liberibacter solanacearum' with a vegetative disorder of celery in Spain and development of a real-time PCR method for its detection

A new symptomatology was observed in celery (Apium graveolens) in Villena, Spain in 2008. Symptomatology included an abnormal amount of shoots per plant and curled stems. These vegetative disorders were associated with 'Candidatus Liberibacter solanacearum' and not with phytoplasmas. Samples from plant sap were immobilized on membranes based on the spot procedure and tested using a newly developed real-time polymerase chain reaction assay to detect 'Ca. L. solanacearum'. Then, a test kit was developed and validated by intralaboratory assays with an accuracy of 100%. Bacterial-like cells with typical morphology of 'Ca. Liberibacter' were observed using electron microscopy in celery plant tissues. A fifth haplotype of 'Ca. L. solanacearum', named E, was identified in celery and in carrot after analyzing partial sequences of 16S and 50S ribosomal RNA genes. From our results, celery (family Apiaceae) can be listed as a new natural host of this emerging bacterium.

Metabolite signature of Candidatus Liberibacter asiaticus infection in two citrus varieties

Huanglongbing (HLB), also known as Citrus Greening Disease, is caused by the bacterium 'Candidatus Liberibacter asiaticus' (CLas) and is a serious threat to the citrus industry. To understand the effect of CLas infection on the citrus metabolome, juice from healthy (n = 18), HLB-asymptomatic (n = 18), and HLB-symptomatic Hamlin (n = 18), as well as from healthy (n = 18) and HLB-symptomatic (n = 18) Valencia sweet oranges (from southern and eastern Florida) were evaluated using (1)H NMR-based metabolomics. Differences in the concentration of several metabolites including phenylalanine, histidine, limonin, and synephrine between control or asymptomatic fruit and symptomatic fruit were observed regardless of the citrus variety or location. There were no clear differences between the metabolite profiles of Hamlin fruits classified by PCR as asymptomatic and control, suggesting that some of the control fruit may have been infected. Taken together, these data indicate that infection due to CLas presents a strong metabolic response that is observed across different cultivars and regions, suggesting the potential for generation of metabolite-based biomarkers of CLas infection.

Detection of citrus huanglongbing-associated 'Candidatus Liberibacter asiaticus' in citrus and Diaphorina citri in Pakistan, seasonal variability, and implications for disease management

We report the detection of the huanglongbing (HLB)-associated bacterium 'Candidatus Liberibacter asiaticus' from both plants and insects in Pakistan and the seasonal variability in the numbers of 'Ca. L. asiaticus'-positive psyllid vector, Diaphorina citri. Our studies showed that 'Ca. L. asiaticus' was detectable from trees in areas with maximum temperatures reaching nearly 50°C (average maximum of 42°C). However, the bacterium was present at very low levels in psyllids both in summer (June to August) and autumn (September to November) in contrast to reports from Florida, where the bacterium was detectable at very high levels during October to November. We hypothesize that hot summer temperatures in Pakistan may interfere with acquisition and replication of 'Ca. L. asiaticus' in psyllids and may lead to dead or non transmissible 'Ca. L. asiaticus' in plants. Psyllid counts were very low in both summer and winter, showed a population peak ('Ca. L. asiaticus'-positive vectors) in spring, and showed a larger peak ('Ca. L. asiaticus'-free psyllids) in autumn. Natural thermotherapy during hot summers and a low vector population during environmental extremes may have played a major role in long-term survival of the citrus industry in Pakistan. These results may be useful in developing management strategies for U.S. citrus industries in Texas and California.

Repertoire of novel sequence signatures for the detection of Candidatus Liberibacter asiaticus by quantitative real-time PCR

BACKGROUND

Huanglongbing (HLB) or citrus greening is a devastating disease of citrus. The gram-negative bacterium Candidatus Liberibacter asiaticus (Las) belonging to the α-proteobacteria is responsible for HLB in North America as well as in Asia. Currently, there is no cure for this disease. Early detection and quarantine of Las-infected trees are important management strategies used to prevent HLB from invading HLB-free citrus producing regions. Quantitative real-time PCR (qRT-PCR) based molecular diagnostic assays have been routinely used in the detection and diagnosis of Las. The oligonucleotide primer pairs based on conserved genes or regions, which include 16S rDNA and the β-operon, have been widely employed in the detection of Las by qRT-PCR. The availability of whole genome sequence of Las now allows the design of primers beyond the conserved regions for the detection of Las explicitly.

RESULTS

We took a complimentary approach by systematically screening the genes in a genome-wide fashion, to identify the unique signatures that are only present in Las by an exhaustive sequence based similarity search against the nucleotide sequence database. Our search resulted in 34 probable unique signatures. Furthermore, by designing the primer pair specific to the identified signatures, we showed that most of our primer sets are able to detect Las from the infected plant and psyllid materials collected from the USA and China by qRT-PCR. Overall, 18 primer pairs of the 34 are found to be highly specific to Las with no cross reactivity to the closely related species Ca. L. americanus (Lam) and Ca. L. africanus (Laf).

CONCLUSIONS

We have designed qRT-PCR primers based on Las specific genes. Among them, 18 are suitable for the detection of Las from Las-infected plant and psyllid samples. The repertoire of primers that we have developed and characterized in this study enhanced the qRT-PCR based molecular diagnosis of HLB.

Detection of Huanglongbing disease using differential mobility spectrometry

The viability of the multibillion dollar global citrus industry is threatened by the "green menace", citrus greening disease (Huanglongbing, HLB), caused by the bacterial pathogen Candidatus Liberibacter. The long asymptomatic stage of HLB makes it challenging to detect emerging regional infections early to limit disease spread. We have established a novel method of disease detection based on chemical analysis of released volatile organic compounds (VOCs) that emanate from infected trees. We found that the biomarkers "fingerprint" is specific to the causal pathogen and could be interpreted using analytical methods such as gas chromatography/mass spectrometry (GC/MS) and gas chromatography/differential mobility spectrometry (GC/DMS). This VOC-based disease detection method has a high accuracy of ∼90% throughout the year, approaching 100% under optimal testing conditions, even at very early stages of infection where other methods are not adequate. Detecting early infection based on VOCs precedes visual symptoms and DNA-based detection techniques (real-time polymerase chain reaction, RT-PCR) and can be performed at a substantially lower cost and with rapid field deployment.