Primers for specific detection and identification of Pseudomonas syringae pv. maculicola and P. cannabina pv. alisalensis

In vitro, in planta, and comparative genomic analyses of Pseudomonas syringae pv. syringae strains of pepper (Capsicum annuum var. annuum)

Pseudomonas syringae pv. syringae (Pss) is an emerging phytopathogen that causes Pseudomonas leaf spot (PLS) disease in pepper plants. Pss can cause serious economic damage to pepper production, yet very little is known about the virulence factors carried by Pss that cause disease in pepper seedlings. In this study, Pss strains isolated from pepper plants showing PLS symptoms in Ohio between 2013 and 2021 (n = 16) showed varying degrees of virulence (Pss populations and disease symptoms on leaves) on 6-week-old pepper seedlings. In vitro studies assessing growth in nutrient-limited conditions, biofilm production, and motility also showed varying degrees of virulence, but in vitro and in planta variation in virulence between Pss strains did not correlate. Comparative whole-genome sequencing studies identified notable virulence genes including 30 biofilm genes, 87 motility genes, and 106 secretion system genes. Additionally, a total of 27 antimicrobial resistance genes were found. A multivariate correlation analysis and Scoary analysis based on variation in gene content (n = 812 variable genes) and single nucleotide polymorphisms within virulence genes identified no significant correlations with disease severity, likely due to our limited sample size. In summary, our study explored the virulence and antimicrobial gene content of Pss in pepper seedlings as a first step toward understanding the virulence and pathogenicity of Pss in pepper seedlings. Further studies with additional pepper Pss strains will facilitate defining genes in Pss that correlate with its virulence in pepper seedlings, which can facilitate the development of effective measures to control Pss in pepper and other related P. syringae pathovars.

IMPORTANCE

Pseudomonas leaf spot (PLS) caused by Pseudomonas syringae pv. syringae (Pss) causes significant losses to the pepper industry. Highly virulent Pss strains under optimal environmental conditions (cool-moderate temperatures, high moisture) can cause severe necrotic lesions on pepper leaves that consequently can decrease pepper yield if the disease persists. Hence, it is important to understand the virulence mechanisms of Pss to be able to effectively control PLS in peppers. In our study, in vitro, in planta, and whole-genome sequence analyses were conducted to better understand the virulence and pathogenicity characteristics of Pss strains in peppers. Our findings fill a knowledge gap regarding potential virulence and pathogenicity characteristics of Pss in peppers, including virulence and antimicrobial gene content. Our study helps pave a path to further identify the role of specific virulence genes in causing disease in peppers, which can have implications in developing strategies to effectively control PLS in peppers.

Three semi-selective media for Pseudomonas syringae pv. maculicola and P. cannabina pv. alisalensis

Three semi-selective media, DTarTA, SPbc, and SPamt, were developed and tested to isolate Pseudomonas syringae pv. maculicola (Psm) and P. cannabina pv. alisalensis (Pca) from Raphanus sativus seeds. DTarTA contained D-tartaric acid as a carbon source and potassium tellurite, ampicillin sodium, and methyl violet as antibiotics. DTarTA suppressed growth in 19 of the 24 pathovars from the P. syringae complex, whereas Psm and Pca grew and formed gray to black colonies. SPamt contained sucrose and peptone as nutrient sources and was supplemented with bromothymol blue and the same antibiotics present in DTarTA and Psm and Pca formed yellowish to dark brown colonies on the SPamt medium. SPbc contained sucrose and peptone and was supplemented with cephalexin and boric acid as antibiotics and Psm and Pca formed semi-translucent to white colonies on the SPbc medium. SPamt and SPbc suppressed the growth of several plant-associated bacteria (except the P. syringae complex). The growth of saprophytic bacteria in seeds on the different media was compared with that on King's B medium, using five types of commercially available Raphanus sativus seeds. The suppression rate of DTarTA was 85-99% and was lower for seeds with more saprophytic bacteria. The suppression rates of SPamt and SPbc were 90-99%. In detection tests using 10,000 seed samples mixed with Pca or Psm-contaminated seeds, it was possible to selectively isolate Psm and Pca using SPamt and SPbc, even when the colony numbers of the target bacterium constituted less than 10% of the total colonies. KEY POINTS: • Bacterial leaf spot and blight pathogens were selectively isolated from seeds. • DTarTA medium distinguishes these pathogens from P. syringae complex pathovars. • SPamp and SPbc media have different selectivity for plant-associated bacteria.

Complete and Draft Genome Sequences of the Cruciferous Pathogens Pseudomonas cannabina pv. alisalensis and Pseudomonas syringae pv. maculicola

Pseudomonas cannabina pv. alisalensis and P. syringae pv. maculicola cause bacterial leaf blight and bacterial leaf spot of crucifers (Brassicaceae). Both pathogens are threats to the cultivation of cruciferous crops. Here, we sequenced two strains of each pathogen, which will contribute to the development of countermeasures for the above diseases.

Pseudomonas cannabina pv. alisalensis and Pseudomonas syringae pv. maculicola cause bacterial leaf blight and bacterial leaf spot of crucifers (Brassicaceae). Both pathogens are threats to the cultivation of cruciferous crops. Here, we sequenced two strains of each pathogen, which will contribute to the development of countermeasures for the above diseases.