Search Results (183)

This retrospective review traces personal encounters along the complex path of plant yellowing diseases—graft-transmissible disorders historically attributed to elusive viruses, but later linked to phloem-invading, wall-less bacteria known as Mollicutes. These include two plant-infecting genera: the cultivable Spiroplasma and the non-cultivable ‘Candidatus Phytoplasma’. A third group—the walled, psyllid-transmitted Candidatus Liberibacter—was later implicated in closely similar syndromes. This shift in understanding marked a major turning point in plant pathology, offering new insights into yellowing diseases characterized by stunting, decline, and poor or deformed growth. The review focuses on key syndromes: citrus little leaf disease (LLD), or citrus stubborn disease (CSD), caused by Spiroplasma citri; and several Mollicute -related disorders, including safflower phyllody, Bermuda grass yellowing, and papaya dieback (PDD) (Nivun Haamir), the latter linked to ‘Candidatus Phytoplasma australiense’. Despite differing causes and vectors, citrus LLD-CSD and PPD share an erratic, unpredictable pattern of natural outbreaks—sometimes a decade apart—hindering grower engagement and sustained control efforts. While scientific understanding has deepened, practical management remains limited. The recent global spread of Huanglongbing (HLB), caused by Candidatus Liberibacter species, underscores the urgent need for improved strategies to manage this resilient group of phloem-limited bacterial pathogens. Full article

The global citrus industry faces unprecedented challenges due to Huanglongbing (HLB), which is associated with the bacterial pathogen Candidatus Liberibacter asiaticus (CLas). This study evaluates the field performance of 11 rootstocks, grafted with ‘Valencia’ orange (Citrus sinensis), under Florida’s HLB-endemic production conditions, while also examining the impact of systemic applications of oxytetracycline (OTC) via trunk injection. Mature trees received annual OTC injections and were assessed over two production seasons. In year 1, OTC-treated trees exhibited significant improvements regardless of the rootstock, including a 36% increase in yield, an 11% increase in juice TSS, and reduced leaf bacterial titers. During year 2, the positive effects of OTC were sustained, or even enhanced. CLas titers were reduced in both leaves and roots; yield increased by 70%; and fruit weight, juice color, and TSS also improved significantly. Moreover, OTC-injected trees exhibited a larger percentage of finer roots compared to non-injected trees. US-1688 and US-1672, both hybrids of C. maxima ‘Hirado’ and C. reticulata ‘Cleopatra’, emerged as the most productive rootstocks. These results demonstrate the importance of rootstock selection for sustainable citrus cultivation while highlighting the benefits of integrating the systemic delivery of OTC to manage HLB and maximize the resilience of citrus. Full article

The Asian citrus psyllid (ACP), Diaphorina citri Kuwayama (Hemiptera: Liviidae), is a vector of Candidatus Liberibacter asiaticus (CLas), the causal agent of citrus Huanglongbing (HLB). We examine the effectiveness of spraying adhesive on new shoots of orange jasmine (Murraya paniculata) to trap ACP in laboratory and field conditions and for the monitoring of ACP population dynamics and directional preferences. After 36 h of observation, orange jasmine plants with new shoots, with and without adhesive, are significantly (p < 0.05) more attractive to ACP than plants without new shoots. In field trials, orange jasmine with new shoots attracted more ACP, particularly females, than plants without new shoots. A male-biased ACP sex ratio occurred in a near-natural population. Orange jasmine with new shoots coated with adhesive more effectively trapped ACP than yellow sticky traps, particularly during the winter and early spring, when ACP densities were low. ACP has a strong phototropic response, preferring to feed and rest in south- and east-facing positions. Adhesive trapping shows potential for attracting adult ACP, especially in citrus orchards during cooler seasons, when host trees lack new shoots, and it may be particularly effective in doing so in urban areas and unmanaged citrus refugia (the primary sources of ACP infestations for commercial groves). Full article

Psyllids are vectors of fastidious plant pathogenic ‘Candidatus Liberibacter’ species that infect both the psyllid vector and plant host. Understanding the molecular and cellular basis of ‘Ca. Liberibacter’ interactions with the psyllid host will aid in identification of effectors involved in invasion and multiplication and facilitate transmission to the host plant. The differential expression of previously identified genes/loci with predicted involvement in tomato host–plant– ‘Ca. L. solanacearum’–prophage–Wolbachia endosymbiont dynamics was quantified by RT-qPCR amplification. Fifteen ‘Ca. Liberibacter solanacearum genes and/or prophage loci and four predicted Wolbachia spp. loci were analyzed in potato psyllids in a 14-day time-course study, post-48-h acquisition-access period by potato psyllids on ‘Ca. L. solanacearum’-infected tomato plants. The ‘Ca. L. solanacearum’-infected tomato host plants were used as an infected host ‘calibrator’ species lacking involvement of psyllid effectors. ‘Ca. L. solanacearum’ genes with predicted functions in adhesion, motility, transport, and virulence that are associated with the prophage lysogenic lifestyle were differentially expressed. In contrast, the prophage-loci expression was synchronous with early or late phase of psyllid-‘Ca. L. solanacearum’ infection, respectively. The observations are consistent with the previously in silico-predicted ‘Ca. L. solanacearum’ gene and prophage/Wolbachia loci functions and time-course global expression patterns. Knockdown of ‘Ca. L. solanacearum’ genes involved in invasion, biofilm formation, and colonization would be expected to impair the vertical and horizontal transmission of ‘Ca. L. solanacearum’ to psyllid offspring and host plants, respectively. Full article

Grapefruit (Citrus × paradisi Macfad.) is susceptible to Huanglongbing (HLB) disease, which prominently affects tree health and leads to a substantial loss of productivity. HLB-affected trees exhibit a nutritional imbalance expressed in either deficiencies or toxicities of the essential minerals required for plant growth, as well as changes in the production of plant metabolites. Hence, understanding foliar nutritional and metabolite fluctuations as HLB-elicited symptoms progress can assist growers in improving tree health management strategies. This study evaluated changes in foliar nutrient and phloem sap amino acid concentrations of HLB-affected grapefruit trees showing a mixed canopy of HLB-induced blotchy mottle and asymptomatic mature leaves. The trees used in our experiment were fruit-bearing seven-year-old grapefruit trees (cv ‘Rio Red’ on sour orange rootstock) grown in South Texas. Two types of foliage from HLB-affected trees were studied, (a) HLB-symptomatic and confirmed Candidatus Liberibacter asiaticus (CLas)-positive (IS) and (b) CLas-negative and HLB-asymptomatic (IA) mature leaves, which were compared to asymptomatic and CLas-free mature foliage from healthy trees (HY) in terms of their leaf nutrient and phloem sap amino acid contents. Hierarchical clustering based on leaf nutrient contents showed that 70% of IA samples clustered with HY samples, thus indicating that the levels of some nutrients were statistically similar in these two types of samples. The concentrations of the macronutrients N, Ca, Mg, and S and the micronutrients Mn and B were significantly reduced in HLB-symptomatic (IS) leaves, as compared to their IA and HY counterparts, which did not show statistically significant differences. Conversely, leaf Na concentration was approximately two-fold higher in leaves from HLB-affected trees (IA and IS) independent of symptom expression as compared to leaves from healthy trees. Significantly higher concentrations of glutamine and the S-containing amino acids taurine and cystathionine were observed in the IS leaves relative to the phloem sap of IA leaves from HLB-affected trees. In contrast, the phloem sap of IA (14%) and IS (41%) leaves from HLB-affected trees exhibited lower levels of γ-amino butyric acid (GABA) as compared to HY leaves. The results of this study highlight the changes in leaf nutrient and phloem sap amino acid profiles following CLas infection and HLB symptom development in grapefruit, and we discuss these results considering the strategies that growers can implement to correct the nutritional deficiencies and/or toxicities induced by this disease. Full article

Huanglongbing (HLB), associated with Candidatus Liberibacter asiaticus (CLas), has severely impacted global citrus production, with no economically viable control measures currently available. This study explored microwave treatment at 2450 MHz as an innovative physical method for HLB control, combining pathogen elimination efficacy with metabolomic analysis. In controlled experiments, 36 HLB-infected citrus plants were treated with 500 W or 250 W microwave irradiation and underwent 10 cycles, achieving up to 99.83% reduction CLas titer. Non-targeted metabolomic analysis identified 15 significantly altered metabolites, including upregulated beta-caryophyllene and lysophosphatidylinositols, and downregulated 5′-S-methyl-5′-thioadenosine. The results indicate that microwave treatment effectively suppressed CLas while simultaneously triggering citrus physiological metabolic changes. These findings suggest that microwave treatment could serve as a sustainable alternative to chemical controls. However, further optimization of parameters, such as wavelengths, voltages, currents, and safety protocols, will be essential for practical field implementation. Full article

Diaphorina citri is the primary global vector of “Candidatus Liberibacter asiaticus”, the bacterium responsible for Huanglongbing. Syntaxin-1A (Syx1A), a member of the Qa-SNARE family, is essential for vesicle fusion and signal transduction, though its function in hemipteran insects remains poorly understood. This study presents the first comprehensive analysis of Syx1A expression in D. citri. Transcripts were detected across all life stages, with peak expression in the salivary glands. RNAi silencing of Syx1A reduced mRNA levels by 39.0% in nymphs and 58.0% in adults, resulting in 58.3% nmortality in nymphs within 5 days and 73.3% in adults within seven days, along with significant weight loss. Treated females showed marked declines in fecundity, ovarian degeneration, and deficient yolk deposition. RT-qPCR confirmed significant downregulation of Vg1, VgA, and VgR. These findings establish Syx1A as a regulator of growth and reproduction in citrus psyllids via modulation of yolk synthesis. RNAi targeting of Syx1A represents a promising strategy for ecologically sound pest control and may contribute to efforts in halting the transmission of the Huanglongbing pathogen CLas. Full article

The Asian citrus psyllid (Diaphorina citri) is an economically important pest of citrus as it is a vector of the bacterium (Candidatus Liberibacter asiaticus, CLas) that causes huanglongbing disease (HLB). Understanding the virome of D. citri is important for uncovering factors that influence vector competence, to support biosecurity surveillance, and to identify candidate agents for biological control. Previous studies have identified several D. citri-associated viruses from various geographical populations of this pest. To further investigate virus diversity in this pest, high-throughput sequencing was used to analyse D. citri populations from the Samoan islands of Upolu and Savai’i. Eleven novel viruses from the Yadokariviridae, Botourmiaviridae, Nodaviridae, Mymonaviridae, Partitiviridae, Totiviridae, and Polymycoviridae were identified as well as some that corresponded to unclassified groups. In addition, microbiome analysis revealed the presence of several endosymbiotic microorganisms, including Wolbachia, as well as some plant pathogenic fungi, including Botrytis cinerea. However, the causative agent of HLB disease (CLas) was not detected in the RNA-Seq data. These findings highlight the complex and diverse microbiota associated with D. citri and suggest potential interactions and dynamics between microorganisms and psyllid-associated viruses. Further research is needed to understand the ecological significance of these discoveries, and whether the novel viruses play a role in regulating field populations of the psyllid. Full article

Among the new strategies for managing diseases in agricultural crops is the application of metallic nanoparticles due to their ability to inhibit the development of phytopathogenic microorganisms and to induce plant defense responses. Therefore, this research evaluated the effects of silver (AgNPs), zinc oxide (ZnONPs), and silicon dioxide (SiO₂NPs) nanoparticles on symptom progression and physiological parameters in two pathosystems: Pseudomonas syringae pv. tomato (Psto) in tomato (pathosystem one, culturable pathogen) and Candidatus Liberibacter solanacearum (CaLso) in pepper plants (pathosystem two, non-culturable pathogen). For in vitro pathosystem one assays, SiO₂NPs did not inhibit Psto growth. The minimum inhibitory concentration (MIC) was 31.67 ppm for AgNPs and 194.3 ppm for ZnONPs. Furthermore, the minimum lethal concentration (MLC) for AgNPs was 100 ppm, while for ZnONPs, it was 1000 ppm. For in planta assays, ZnONPs, AgNPs, and SiO₂NPs reduced the number of lesions per leaf, but only ZnONPs significantly decreased the severity. Regarding pathosystem two, AgNPs, ZnONPs, and SiO₂NPs application delayed symptom progression. However, only AgNPs significantly reduced severity percentage. Moreover, treatments with AgNPs and SiO₂NPs increased the plant height and dry weight compared to the results for the control. Full article

Huanglongbing (HLB), caused by Candidatus Liberibacter asiaticus (CLas), is the most devastating disease threatening global citrus production. Although no commercial citrus varieties exhibit complete HLB resistance, genotype-specific tolerance variations remain underexplored. This study conducted a comparative transcriptomic profiling of six commercially citrus cultivars in South China, four susceptible cultivars (C. reticulata cv. Tankan, Gongkan, Shatangju, and C. sinensis Osbeck cv. Newhall), and two tolerant cultivars (C. limon cv. Eureka; C. maxima cv Guanxi Yu) to dissect molecular mechanisms underlying HLB responses. Comparative transcriptomic analyses revealed extensive transcriptional reprogramming, with tolerant cultivars exhibiting fewer differentially expressed genes (DEGs) and targeted defense activation compared to susceptible genotypes. The key findings highlighted the genotype-specific regulation of starch metabolism, where β-amylase 3 (BAM3) was uniquely upregulated in tolerant varieties, potentially mitigating starch accumulation. Immune signaling diverged significantly: tolerant cultivars activated pattern-triggered immunity (PTI) via receptor-like kinases (FLS2) and suppressed ROS-associated RBOH genes, while susceptible genotypes showed the hyperactivation of ethylene signaling and oxidative stress pathways. Cell wall remodeling in susceptible cultivars involved upregulated xyloglucan endotransglucosylases (XTH), contrasting with pectin methylesterase induction in tolerant Eureka lemon for structural reinforcement. Phytohormonal dynamics revealed SA-mediated defense and NPR3/4 suppression in Eureka lemon, whereas susceptible cultivars prioritized ethylene/JA pathways. These findings delineate genotype-specific strategies in citrus–CLas interactions, identifying BAM3, FLS2, and cell wall modifiers as critical targets for breeding HLB-resistant cultivars through molecular-assisted selection. This study provides a foundational framework for understanding host–pathogen dynamics and advancing citrus immunity engineering. Full article

Phytoplasmas are wall-less, phloem-restricted bacteria responsible for numerous significant plant diseases worldwide. An increasing body of evidence indicates that phytoplasmas can coexist with other pathogens in mixed infections, including various 16Sr group phytoplasmas, ‘Candidatus Liberibacter’ species, viruses, spiroplasmas, fungi, and other difficult-to-culture phloem-limited bacteria. These interactions challenge established views regarding the causes, detection, and management of plant diseases. This review consolidates existing knowledge on the diversity and epidemiology of phytoplasma-related mixed infections, with a particular emphasis on documented co-infections across various host plants and regions, especially in tropical and subtropical areas. Mixed infections affect disease severity, symptom expression, vector behavior, and pathogen dissemination, highlighting the limitations of pathogen-specific diagnostic and control strategies. The necessity for tools to detect multiple pathogens, enhanced understanding of pathogen–pathogen and host–pathogen interactions, and comprehensive surveillance systems is emphasized. Ultimately, breeding for resistance must consider the complexities of natural co-infections to ensure effective protection of crops. Addressing the challenges presented by phytoplasma-related mixed infections is crucial for developing resilient and sustainable plant health strategies in the face of increasing ecological and agricultural pressures. Full article

Huanglongbing (HLB), caused mainly by Candidatus Liberibacter asiaticus (CLas), is a devastating disease threatening citrus production worldwide, leading to leaf mottling, fruit deformation, and significant yield losses. This study generated a comprehensive co-expression network analysis using RNA-seq data from 17 public datasets. Weighted gene co-expression network analysis (WGCNA) was applied to identify gene modules associated with citrus species, tissue types, and days post-infection (DPIs). These modules revealed significant enrichment in biological pathways related to stress responses, metabolic reprograming, ribosomal protein synthesis, chloroplast and plastid function, cellular architecture, and intracellular transport. The results offer a molecular framework for understanding HLB pathogenesis and host response. By elucidating module-specific functions and their correlation with species- and tissue-specific responses, this study provides a robust foundation for identifying key genetic targets. These insights facilitate breeding programs focused on developing HLB-tolerant citrus cultivars, contributing to the long-term sustainability and resilience of global citrus production. Full article

HLB is a severe and devastating disease affecting citrus plants, for which no cure is currently available. The pathogen CLas is an unculturable, phloem-limited α-proteobacterium associated with HLB. Phages, known for their ability to directly lyse pathogenic bacteria, have been widely recognized as effective control agents in both medical and agricultural fields. In this study, we identified 191 prophages across 48 Liberibacter genomes using comparative genomics and phylogenetic methods. These prophages were classified into 17 distinct types, with 13 novel types, named NF1 to NF13. Diversity analysis revealed that the number of prophages per genome ranged from one to eight, with an average of four. Annotation showed that the genome sizes of NF1–NF13 ranged from 10,160 bp to 72,736 bp, with an average size of 27,616 bp, containing between 7 and 66 genes (average: 27 genes). Further functional enrichment of the sequences within 5 kb upstream and downstream of the integration site identified six main aspects, involving four core biological processes: genome structure modification, gene expression regulation, stress response activation, and metabolic pathway modulation. It is speculated that the insertion of the prophage significantly impacts the host. Six virulence factors, including carB, clpC, flhA, algW, rffG, and IlpA, were identified in the prophages. Interestingly, prophages containing carB or clpC are predominantly found in CLas, which may contribute to the global colonization of CLas. Notably, among the 35 identified Type 4 prophages, 19 were found to be active. Type 4 is the only active prophage type, making it a promising candidate for developing phage therapy against CLas. This study is significant for a deeper understanding of the diversity of Liberibacter and the interactions between prophages and their hosts. Full article

Psyllids (Hemiptera: Psylloidea) are the main vectors of various phloem-limited plant pathogens, including ‘Candidatus Liberibacter’ species. ‘Candidatus Liberibacter solanacearum’ (CLso) has been associated with various plant disorders and economic losses in plants from the Solanaceae and Apiaceae families. Recently, it has been reported in Europe, primarily linked to carrots and celery. This situation presents a significant threat, prompting the need for a survey to assess the presence of the bacterium and its potential vectors. Plant and psyllid samples were collected from potato (Solanum tuberosum), carrot (Daucus carota) and other wild weed species in commercial fields and urban areas over two consecutive years (2022 and 2023). DNA was extracted from the samples, followed by conventional PCR and the sequencing of positive samples. The psyllid species Bactericera nigricornis was the dominant species in potato fields, while Bactericera trigonica was the most abundant in carrot fields, followed by Heterotrioza chenopodii, ranking as the second most abundant species in both cases. CLso-positive samples were found in D. carota, B. trigonica, H. chenopodii, B. nigricornis, and Trioza urticae. The sequencing results suggest the detected haplotypes are D and U. These findings raise concerns about the potential spread of CLso and the associated risk of significant economic losses. Full article

Huanglongbing disease (HLB), caused by Candidatus Liberibacter asiaticus (CLas), affects all commercial citrus species. Persian lime (Citrus latifolia Tanaka), a crop of global economic importance, has shown tolerance to this disease. Efforts are focused on extending the productive life of diseased trees through effective agronomic management. This study aimed to evaluate how different fertilization strategies influence the physiological and hormonal responses of Citrus latifolia on both healthy and HLB-affected plants. It compared the effects of low (Ma-1), medium (Ma-2), and high (Ma-3) doses of macronutrients, with and without the addition of micronutrients (Mi-1), using either soil (Mi-2) or foliar (Mi-3) applications. Treatments were applied every 18 days for one year. C. latifolia showed tolerance; however, HLB infection negatively affected growth parameters, photosynthetic activity, vascular bundle anatomy, reflectance at 550 and 790 nm, carbohydrate metabolism, and the concentration of salicylic acid and its biosynthetic precursors. The hormonal response showed higher levels of benzoic acid and lower levels of salicylic acid than those reported in susceptible citrus. Plants treated with low doses of macronutrients along with soil-applied micronutrients (Ma-1 + Mi-2) showed a 17.9% increase in growth, a 31.3% larger canopy volume, and an 83.3% reduction in starch accumulation compared to the treatment with high doses of macronutrients and both soil and foliar applied micronutrients (Ma-3 + Mi-3). These findings indicate that split soil fertilization with low-dose macronutrients and micronutrients might influence plant physiological responses, potentially improving disease management and decreasing fertilizer inputs. Full article