Pseudomonas uvaldensis sp. nov., a bacterial pathogen causing onion bulb rot

Isolation and Characterization of Bacteria Associated with Onion and First Report of Onion Diseases Caused by Five Bacterial Pathogens in Texas, U.S.A

Bacterial diseases pose a severe challenge to growers and cause significant loss to the billion-dollar onion industry in the United States. Texas is the sixth largest onion producing state, yet the bacterial communities associated with short-day onion crops grown in Texas have not been studied. This study was conducted to identify, characterize, and understand the diversity of bacteria associated with onion production in Texas. In 2020, 190 foliar and 210 bulb samples were collected from onion crops in the Rio Grande Valley and Winter Garden regions of Texas. Sequencing of the 16s rRNA gene was used to identify each bacterial strains to a genus. The pathogenicity to onion of each bacterial strain was tested using three assays: a red onion scale assay, a yellow onion bulb assay, and a foliar assay. Whole genome sequencing was done to identify the onion-pathogenic strains to species. Collectively, isolates of 24 genera belonging to three phyla were detected, including 19 genera from foliar samples and nine genera from bulb samples. Isolates in the Phylum Proteobacteria, including 15 genera of Gram-negative bacteria, were the most abundant of the taxa, comprising 90.0% of the strains isolated. The diversity of foliar isolates was evenly distributed between Gram-positive and Gram-negative bacteria, while Gram-negative bacteria dominated the isolates from bulb samples. In total, 83.9% of the bacterial isolates were not pathogenic on onion, with only isolates of Pantoea, Pseudomonas, Burkholderia, Erwinia, Enterobacter, and Curtobacterium proving pathogenic. Strains of Burkholderia gladioli, Pseudomonas alliivorans, Pantoea agglomerans, P. ananatis, and P. allii are the first documented cases of these pathogens of onion in Texas. Identifying and characterizing the nature of onion microflora, including pathogens of onion, is vital to developing rapid disease detection techniques via pathogenomics and minimizing losses through the application of effective disease management measures.

Draft Genome Sequence and Annotation of Pseudomonas carnis Strain 20TX0167, Isolated from an Onion (Allium cepa)

The draft genome sequence of Pseudomonas carnis strain 20TX0167, isolated from a cold stored onion bulb, is described here. A comparative genomic study against the type strain of this species, B4-1^T, revealed differences in some genetic aspects.

Curtobacterium allii sp. nov., the actinobacterial pathogen causing onion bulb rot

A Gram-stain-positive, aerobic, and non-spore-forming bacterial strain, 20TX0166^T, was isolated from a diseased onion bulb in Texas, USA. Upon testing its pathogenicity on onion bulb, it produced pathogenic response which makes it first species of pathogen belonging to the phylum actinobacteria detected in onion. Phylogenetic analysis of the 16S rRNA gene sequence revealed that the strain belonged to the genus Curtobacterium and was most similar to Curtobacterium flaccumfaciens LMG 3645^T (100%), C. pusillum DSM 20527^T (99.5%), and C. oceanosedimentum ATCC 31317^T (99.5%). The estimated genome size of the novel species was 4.0 Mbp with a G + C content of 70.8%. The orthologous ANI (orthoANIu), ANI based on blast (ANIb), and dDDH values between the novel strain and the closest relative, C. flaccumfaciens LMG 3645^T, were 95.7%, 95.4%, and 63.3%, respectively. These values were below the recommended species cut-off threshold of 96% (ANI) and 70% (dDDH), suggesting the strain may be a novel species. Physiologic and phenotypic characters of this novel strain were also unique when compared with the closely related species. The major cellular fatty acids of this strain were anteiso-C_15:0 and anteiso-C_17:0. Using a polyphasic approach based on phenotypic and genotypic analyses, strain 20TX0166^T represents a novel species of the genus Curtobacterium, and the name Curtobacterium allii sp. nov. is proposed. The type strain is 20TX0166^T (= LMG 32517^T = CIP112023^T = NCIMB 15427^T).

Whole genome and 16S rRNA dataset of Pectobacterium carotovorum strain 21TX0081 isolated from a symptomatic onion foliage in Texas

Bacterial soft rot is an important disease in onion caused by Pectobacterium carotovorum, a Gram-negative bacterium belonging to the Pectobacteriaceae family. The soft rot disease may occur during the preharvest (mature bulbs) or postharvest (transit, storage) stage. We collected symptomatic onion plants from the field and isolated bacteria from the foliage and bulbs. We extracted genomic DNA from the bacterial strain 21TX0081 isolated from onion foliage, conducted a polymerase chain reaction (PCR) based on 16S ribosomal RNA (rRNA)-specific primers, and classified the strain to genus Pectobacterium. To ascertain the species, genome features, and novelty of the strain, we conducted whole genome sequencing and computed average nucleotide identity (ANI) and digital DNA-DNA hybridization (dDDH) values. We also annotated the genome of P. carotovorum strain 21TX0081 using Rapid Annotation using Subsystem Technology (RAST) server. The raw, untrimmed forward and reverse sequence reads of 16S rRNA, and whole genome annotation features have been deposited at Mendeley Data. The datasets generated from consensus 16S rRNA and whole genome sequence have been deposited in National Center for Biotechnology Information (NCBI) repository. They will be useful for future research on comparative genomics and developing tools for disease management in onion.