Identification, genetic diversity, and pathogenicity of Ralstonia pseudosolanacearum causing cigar tobacco bacterial wilt in China

Development of a lytic Ralstonia phage cocktail and evaluation of its control efficacy against tobacco bacterial wilt

Introduction

Bacterial wilt (BW) caused by Ralstonia pseudosolanacearum is a devastating soil-borne disease. Bacteriophages are important biocontrol resources that rapidly and specifically lyse host bacteria, showing good application potential in agricultural production.

Methods

This study isolated nine phages (YL1-YL9) and, using host range and pot experiments, identified two broader host range phages (YL1 and YL4) and two higher control efficacy phages (YL2 and YL3), which were combined to obtain five cocktails (BPC-1-BPC-5).

Results

Pot experiments showed that BPC-1 (YL3 and YL4) had the highest control efficacy (99.25%). Biological characterization revealed that these four phages had substantial thermal stability and pH tolerance. Whole genome sequencing and analysis showed that YL1, YL2, YL3, and YL4 belonged to the genus Gervaisevirus. AlphaFold 3 predictions of tail fiber protein II structures showed that YL1 differed significantly from the other phages. Amino acid sequence alignment revealed that the ORF66 (YL1) "tip domain" of contained a higher proportion of aromatic and positively charged amino acids. However, the surface of the ORF69 (YL4) "tip domain" exhibited more positively charged residues than ORF66 (YL2) and ORF70 (YL3). These characteristics are hypothesized to confer a broader host range to YL1 and YL4.

Discussion

This study demonstrates that phages assembling a broad host range and high control efficacy have better biocontrol potential, providing high-quality resources for the biological control of BW.

Whole-Genome Sequence and Characterization of Ralstonia solanacearum MLY102 Isolated from Infected Tobacco Stalks

BACKGROUND/OBJECTIVES

Bacterial wilt disease is a soil-borne disease caused by Ralstonia solanacearum that causes huge losses to crop economies worldwide.

METHODS

In this work, strain MLY102 was isolated and further identified as R. solanacearum from a diseased tobacco stalk. The genomic properties of MLY102 were explored by performing biochemical characterization, genome sequencing, compositional analysis, functional annotation and comparative genomic analysis.

RESULTS

MLY102 had a pinkish-red color in the center of the colony surrounded by a milky-white liquid with fluidity on TTC medium. The biochemical results revealed that MLY102 can utilize carbon sources, including D-glucose (dGLU), cane sugar (SAC) and D-trehalose dihydrate (dTRE). Genome sequencing through the DNBSEQ and PacBio platforms revealed a genome size of 5.72 Mb with a G+C content of 67.59%. The genome consists of a circular chromosome and a circular giant plasmid with 5283 protein-coding genes. A comparison of the genomes revealed that MLY102 is closely related to GMI1000 and CMR15 but has 498 special genes and 13 homologous genes in the species-specific gene family, indicating a high degree of genomic uniqueness.

CONCLUSIONS

The unique characteristics and genomic data of MLY102 can provide important reference values for the prevention and control of bacterial wilt disease.

Genetic Diversity of Ralstonia solanacearum Causing Tobacco Bacterial Wilt in Fujian Province and Identification of Biocontrol Streptomyces sp

Tobacco bacterial wilt is a highly destructive soilborne disease caused by the Ralstonia solanacearum species complex, exhibiting a significant risk to global flue-cured tobacco cultivation and resulting in substantial economic loss. In this study, 77 isolates were collected from three prominent flue-cured tobacco cultivation areas in Fujian, China (Nanping, Sanming, and Longyan), in 2021 and 2022. The isolated strains were classified through phylotype-specific multiplex polymerase chain reaction (Pmx-PCR) and physiological tests. The analysis showed that all the strains were associated with phylotype I, race 1, and biovar III. Subsequent phylogenetic analysis using partial egl gene sequences classified the 77 isolates into 5 distinct sequevars: 13, 15, 16, 17, and 34. Notably, a remarkable predominance of sequevar 15 was observed in Fujian Province, while sequevar 16 was first reported on tobacco in China, which was identified in other plants, expanding the understanding of its host range and distribution in the country. In addition, a Streptomyces strain extracted from the rhizosphere soil of tobacco was found to inhibit the growth of multiple sequevars of tobacco R. solanacearum, indicating its broad-spectrum antagonistic properties. Furthermore, pot experiments showed that the strain St35 effectively controlled tobacco bacterial wilt. The isolate St35 was conclusively identified as Streptomyces gancidicus according to the morphological and genetic features. In summary, the present study demonstrated the genetic diversity and distribution of tobacco R. solanacearum strains in the Fujian province of China, as well as the identification of a candidate biological control agent for the management of tobacco bacterial wilt.

An Interspecies Recombinant Sida Yellow Mosaic China Virus Isolate and Betasatellite Cause a Leaf Curl Disease in Tobacco in Hainan, China

Tobacco (Nicotiana tabacum) is an herbaceous crop. Cigar tobacco, a group of tobacco cultivars, has recently been planted in a few provinces in China. Since its introduction, symptoms such as leaf curling and vein thickening have appeared. Here we report a begomovirus, Sida yellow mosaic China virus-Hainan isolate (designated SiYMCNV-HN), associated with the betasatellite (designated SiYMCNB-HN) as the causal agent of a leaf curl disease in cigar tobacco (N. tabacum cv. Haiyan101) in Hainan Province, China. Phylogenetic and recombination analyses indicate that SiYMCNV-HN is an interspecies recombinant with a SiYMCNV isolate as the major parent and a Sida yellow vein Vietnam virus isolate as the minor parent. Full-length infectious clones of SiYMCNV-HN and SiYMCNB-HN were generated, which were highly infectious and induced high pathogenicity through agroinfiltration in Nicotiana benthamiana and N. tabacum. This newly reported recombinant begomovirus poses potential threats to tobacco plantations in the region.