Pseudomonas syringae is a Gram-negative bacterium that infects a wide range of plants, causing significant economic losses in agricultural production. The pathogen exhibits a high degree of genetic and phenotypic diversity, which has led to the classification of P. syringae strains into different pathovars based on their host range and disease symptoms. Copper-based products have traditionally been used to manage infections in agriculture, but the emergence of copper-resistant strains has become a significant concern. Biological control is a promising strategy to manage P. syringae, as it offers an environmentally friendly and sustainable approach to disease management. The review includes an overview of the biology and epidemiology of P. syringae, and of the mechanisms of action of various biological control agents, mainly microorganisms (antagonistic bacteria, and fungi) and bacteriophages. Specifically, this review highlights the renewed interest in bacteriophages (bacteria-infecting viruses) due to their advantages over other eco-friendly management methods, thanks to their bactericidal properties and potential to target specific pathogenic bacteria. The potential benefits and limitations of biological control are also examined, along with research directions to optimize the use of this approach for the management of P. syringae.

Pistachio crops have a great economic potential, as their global production has increased dramatically over the past few decades. Therefore, it is important to maintain the healthy phytosanitary status of pistachio crops. In a Chilean pistachio orchard, a dieback of the trees was observed, with blighting of twigs and severe necrosis in the trunk and twigs. Bacterial isolation, pathogenicity tests and molecular characterization were conducted to determine the causal agent of the observed disease. The bacterial isolation and analysis of 16S rRNA gene led to the identification of Pantoea genus bacteria. Pathogenicity tests carried out on fruits inoculated with Pantoea isolates induced large necrosis seven days post-inoculation. Further inoculations were carried out on pruning cuttings and on the trunk of 18-month-old pistachio plants. Thirty-one weeks after inoculation, necrotic lesions were observed in the wood of pistachio plants. Sequence analysis of housekeeping genes enabled the isolated bacterium to be identified as Pantoa agglomerans, and to verify its role as the causal agent of the pistachio dieback with necrotic lesions. This is the first report of an occurrence of P. agglomerans inducing dieback in pistachio.

Tomatoes are susceptible to bacterial diseases, mainly related to some Pseudomonas syringae pathovars. Many Pseudomonas species are considered innocuous, but some have shown the ability to opportunistically infect tomato plants. Antimicrobial compounds have been used to control pathogenic organisms, and this can lead to environmental selection of phenotypically resistant bacteria. We assessed the diversity of Pseudomonas species associated with tomato plants from Chilean orchards and analyzed antimicrobial resistance among the isolated strains. A total of 64 Pseudomonas isolates (P. syringae, P. viridiflava, P. fluorescens, P. koreensis, P. gessardii, and P. azotoformans) were evaluated for their phenotypic resistance to seven antimicrobial compounds, including copper, streptomycin, and five other antibiotics typically not used in agriculture. The results showed that 95%, 86%, 70%, 53%, 45%, and 1.6% of the isolates were resistant to rifampin, ampicillin, copper, chloramphenicol, streptomycin, and tetracycline, respectively, with no isolates being resistant to gentamicin. A total of 96.9% of Pseudomonas isolates exhibited a multiresistant phenotype to at least two of the antimicrobials tested. The most frequent multiresistance phenotype was Cu-Str-Amp-Cm-Rif (23.4%). The presence of Pseudomonas strains tolerant to conventional bactericides, metals, and other antimicrobials makes these bacteria an emerging threat to the agriculture industry and to human health.

In Chile, edible herbs are mainly grown by small farmers. This type of horticultural crop typically requires intensive management because it is highly susceptible to insects, some of which transmit viruses that severely affect crop yield and quality. In 2019, in coriander plants tested negative for all previously reported viruses, RNA-Seq analysis of one symptomatic plant revealed a plethora of viruses, including one virus known to infect coriander, five viruses never reported in coriander, and a new cytorhabdovirus with a 14,180 nucleotide RNA genome for which the species name Cytorhabdovirus coriandrum was proposed. Since all the detected viruses were aphid-borne, aphids and weeds commonly growing around the coriander field were screened for viruses. The results showed the occurrence of the same seven viruses and the alfalfa mosaic virus, another aphid-borne virus, in aphids and weeds. Together, our findings document the presence of multiple viruses in coriander and the potential role of weeds as virus reservoirs for aphid acquisition.

Pseudomonas syringae pv. syringae and Xanthomonas arboricola pv. juglandis are the causal agents of bacterial canker in cherry and walnut blight, respectively, which cause significant production losses worldwide. These diseases have traditionally been controlled by copper-based agrochemicals and, more recently, antibiotics. However, the prolonged use of these compounds has led to the emergence of resistant bacterial strains. The search for new, efficient, and environmentally friendly biocontrol alternatives has intensified. Phages are promising candidates due to their ability to specifically infect and lyse bacterial pathogens. Endolysin enzymes are responsible for bacterial cell wall degradation, and although they have been extensively studied in medical and veterinary contexts, their application in agriculture remains limited. In this study, 17 putative endolysins were identified from bacteriophages infecting X. arboricola pv. juglandis and P. syringae pv. syringae. Based on conserved domain analyses, 12 were classified as glycosidases, four as amidases, and one as an endopeptidase. From these, a recombinant amidase (LysP) and a recombinant glycosidase (LysX) were expressed in E. coli, purified, and evaluated as pure enzymes. Both endolysins exhibited significant antimicrobial activity, reducing P. syringae pv. syringae viability by 62–78.3% and X. arboricola pv. juglandis viability by 51.5–53.1%, respectively. These findings highlight these recombinant endolysins as promising candidates for the development of biocontrol strategies against bacterial plant pathogens.

In recent years, the cultivated area of hazelnuts in Chile has increased significantly. Along with this rapid expansion, biotic constraints that affect the optimal development of the crop have been identified. Among these, bacterial blight disease caused by Xanthomonas arboricola pv. corylina has been particularly relevant. This pathogen has a global distribution and is present in all hazelnut-producing countries. In the spring of 2023, hazelnut orchards were sampled from the Maule to Biobío Regions of Chile. The Chilean isolates recovered from hazelnut tissues showing symptoms of bacterial blight were characterized by their ability to grow on different semi-selective media, their carbohydrate utilization profiles, hypersensitivity response in tobacco plants, and biochemical tests. Additionally, the isolates were identified based on the 16S rRNA gene and multilocus sequence analysis (MLSA) on the rpoD, gyrB, and atpD genes. The results showed that the X. arboricola pv. corylina Chilean isolates differed from previously reported isolates in other geographic areas as they are capable of metabolizing sorbitol and mannitol. Using MLSA and average nucleotide identity (ANI) comparison, these isolates were grouped into four and five phylogenetic clades, respectively, representing a significant difference from what has been reported in similar international studies.