Publications

3992

Huanglongbing (HLB, or citrus greening disease) affects all citrus varieties worldwide. In the United States, Asia, and South America, the causal agent is ‘ Candidatus Liberibacter asiaticus’ ( CLas), a phloem-limited, uncultured alphaproteobacterium. The hemipteran insect vector Diaphorina citri (Asian citrus psyllid) acquires and transmits CLas in a circulative, propagative manner. In addition to CLas, D. citri hosts multiple symbiotic bacterial species including Wolbachia (wDi). In D. citri, wDi has been sequenced and studied, but specific roles in D. citri biology are unknown. Using well-established quantitative PCR methods, we measured CLas titer in D. citri collected from four groves in central Florida with distinct HLB management strategies and tested whether CLas and wDi titer were correlated in a subset of these insects. Grove site had the largest effect on CLas titer. Sex had no effect on CLas titer, whereas a higher wDi titer was correlated with noninfected insects. Our results suggest that more directed follow-up research is necessary and important to clarify whether field management tactics influence CLas titer in D. citri and to better understand gene-by-environment interactions among D. citri, wDi, and CLas. Now that millions of trees in Florida have been treated with injectable formulations of oxytetracycline, which is likely to decrease bacterial populations in D. citri, this study may represent the last biologically meaningful snapshot of grove-level vector-pathogen ecology in the state during the HLB epidemic.

The thymidylate kinase (tmk) gene is indispensable for the proliferation and survival of phytoplasma. To reveal the molecular variation and phylogeny of the tmk genes of Candidatus phytoplasma ziziphi, in this study, the tmk genes of 50 phytoplasma strains infecting different resistant and susceptible jujube cultivars from different regions in China were amplified and analyzed. Two sequence types, tmk-x and tmk-y, were identified using clone-based sequencing. The JWB phytoplasma strains were classified into three types, type-X, type-Y, and type-XY, based on the sequencing chromatograms of the tmk genes. The type-X and type-Y strains contained only tmk-x and tmk-y genes, respectively. The type-XY strain contained both tmk-x and tmk-y genes. The type-X, type-Y, and type-XY strains comprised 42%, 12%, and 46% of all the strains, respectively. The type-X and type-XY strains were identified in both susceptible and resistant jujube cultivars, while type-Y strain was only identified in susceptible cultivars. Phylogenetic analysis indicated that the tmk genes of the phytoplasmas were divided into two categories: phylo-S and phylo-M. The phylo-S tmk gene was single-copied in the genome, with an evolutionary pattern similar to the 16S rRNA gene; the phylo-M tmk gene was multi-copied, related to PMU-mediated within-genome transposition and between-genome transfer. Furthermore, the phylogenetic tree suggested that the tmk genes shuttled between the genomes of the Paulownia witches’ broom phytoplasma and JWB phytoplasma. These findings provide insights into the evolutionary and adaptive mechanisms of phytoplasmas.

The Asian citrus psyllid (ACP) is the vector of Candidatus Liberibacter asiaticus (CLas), the causal agent of citrus greening or Huanglongbing (HLB), one of the most devastating citrus diseases worldwide. The citrus industry in Georgia (U.S.A.) is in the process of a rapid expansion, and based on experiences with HLB in Florida, there is great concern about the potential impacts of HLB on this emerging industry. Prior to 2023, ACP had been identified in residential citrus trees in isolated Georgia counties but little to no testing of psyllids for CLas had occurred. However, in 2023, one individual psyllid collected from Chatham County was confirmed positive for CLas by PCR and sequencing. Furthermore, during 2023, ACP adults and nymphs were identified for the first time in a Georgia commercial citrus grove. The finding of ACP in a commercial planting represents a significant risk for CLas dissemination, and thereby has the potential to stall the rapid expansion of Georgia’s citrus industry. In the coming years, surveillance and testing of ACP from commercial groves will be essential for the early detection and management of HLB and its vector to reduce HLB spread within Georgia’s commercial groves.

El tomate es una de las hortalizas de mayor consumo a nivel mundial, y México ocupa los primeros lugares en exportación. Plagas y enfermedades que atacan el cultivo de tomate obligan a los productores a usar plaguicidas, causando contaminación ambiental. En la actualidad se buscan nuevos productos que sean amigables con el medio ambiente, pero que a su  vez tengan efectividad para el control de plagas y enfermedades; una opción viable es la menadiona bisulfito de sodio (MBS). El objetivo de la investigación fue determinar el efecto bioestimulante de la MBS en plantas de tomate infectadas con Candidatus Liberibacter solanacearum (CaLso), así como evaluar la incidencia y severidad de la enfermedad en las plantas. El experimento se desarrolló bajo condiciones de invernadero en tomate de la variedad Floradade, cuyas plantas se trataron con las concentraciones de MBS antes de la inoculación. Se establecieron ocho tratamientos, cuatro inoculados con CaLso y tratados con MBS (55, 80, 100 y 500 ppm), otro con tratamiento químico e inoculado, un tratamiento testigo inoculado y dos tratamientos sin inocular, uno como testigo absoluto y otro solo tratado con 500 ppm MBS. Enseguida se inoculó con CaLso y las aplicaciones de MBS fueron realizadas a intervalos de 15 días posteriores a la inoculación. Se evaluaron variables agronómicas, incidencia y severidad de la enfermedad. La MBS logró reducir la severidad de 23.5 a 41.7 % en plantas inoculadas con la bacteria CaLso, e incrementó las variables agronómicas de altura deplanta, peso fresco y peso seco de la planta y raíz, firmeza y grados Brix. Los resultados mostraron que la MBS puede ser integrada  para el manejo de la enfermedad causada por CaLso en el cultivo del tomate.