Phytopathogenic bacteria are economically important because they affect crop yields and threaten the livelihoods of farmers worldwide. The genus Xanthomonas is particularly significant because it is associated with some plant diseases that cause tremendous loss in yields of globally essential crops. Current management practices are ineffective, unsustainable and harmful to natural ecosystems. Bacteriophage (phage) biocontrol for plant disease management has been of particular interest from the early nineteenth century to date. Xanthomonas phage research for plant disease management continues to demonstrate promising results under laboratory and field conditions. AgriPhage has developed phage products for the control of Xanthomonas campestris pv. vesicatoria and Xanthomonas citri subsp. citri. These are causative agents for tomato, pepper spot and speck disease as well as citrus canker disease.Phage-mediated biocontrol is becoming a viable option because phages occur naturally and are safe for disease control and management. Thorough knowledge of biological characteristics of Xanthomonas phages is vital for developing effective biocontrol products. This review covers Xanthomonas phage research highlighting aspects of their ecology, biology and biocontrol applications.

Providence virus (PrV) is a member of the family Tetraviridae, a family of small, positive-sense, ssRNA viruses that exclusively infect lepidopteran insects. PrV is the only known tetravirus that replicates in tissue culture. We have analysed the genome and characterized the viral translation products, showing that PrV has a monopartite genome encoding three ORFs: (i) p130, unique to PrV and of unknown function; (ii) p104, which contains a read-through stop signal, producing an N-terminal product of 40 kDa (p40) and (iii) the capsid protein precursor (p81). There are three 2A-like processing sequences: one at the N terminus of p130 (PrV-2A1) and two more (PrV-2A2 and PrV-2A3) at the N terminus of p81. Metabolic radiolabelling identified viral translation products corresponding to all three ORFs in persistently infected cells and showed that the read-through stop in p104 and PrV-2A3 in p81 are functional in vivo and these results were confirmed by in vitro translation experiments. The RNA-dependent RNA polymerase domain of the PrV replicase is phylogenetically most closely related to members of the families Tombusviridae and Umbraviridae rather than to members of the family Tetraviridae. The unique genome organization, translational control systems and phylogenetic relationship with the replicases of (+ve) plant viruses lead us to propose that PrV represents a novel family of small insect RNA viruses, distinct from current members of the family Tetraviridae.

Providence virus is the only member of the family Carmotetraviridae and carries a positive single stranded RNA genome that encodes three open reading frames. The smallest open reading frame encodes the structural proteins. The largest open reading frame encodes a large putative protein, p130. The second overlapping open reading frame encodes two non-structural proteins; p40, a proposed accessory protein and p104, the replicase, containing the RdRp domain. Till date, p130 and p40 are not associated with any related open reading frames in the databases. The purpose of this study is to identify sequences within these non-structural proteins with potential roles in replication and evolution using computational tools. Our results revealed that p130 has a putative arginine-rich sequence which lies in the disordered region also found in the Umbravirus, Groundnut rosette virus p27. Analysis of the p40 revealed a sequence with a coiled-coil conformation and surface-exposed characteristics comparable to the interaction domain of Tombusvirus, Tomato bushy stunt virus p33 accessory protein. The hypothetical two transmembrane helix topology of PrV p104 oriented the putative localization signal at the N-terminus, the same way the localization signal of Tomato bushy stunt virus p92 is oriented. This study concluded that Providence virus non-structural proteins are structurally related to Tombusvirus and Umbravirus accessory proteins and contain sequences with predicted functions in replication. Findings from this study have led us to propose a co-evolutionary event between an insect and plant virus resulting in a hybrid virus with the potential to infect and replicate in both host plant and animal systems. Key words: Providence virus, non-structural proteins, p40, p130, sequence comparison, replication, evolution.

Xanthomonas vasicola pv. musacearum is responsible for the widespread Banana Xanthomonas Wilt in banana cultivation regions across the globe. Biocontrol measures for disease management remain limited amidst increasing antimicrobial resistance and unsustainable conventional agricultural practices. The purpose of this study is to explore a viable alternative or adjunct strategy through the use of bacteriophages for disease management. Kintu was isolated from sewage and displayed clear and circular plaques measuring 3 mm. Based on transmission electron microscopy, Kintu displays siphovirus characteristics, including an icosahedral head and a non-contractile tail. Kintu infects 78% (22 out of 28) Ugandan Xvm strains, has an optimal multiplicity of infection of 1, a 10 min adsorption and latent period, a 35 min burst period, and a burst size of 15 particles per bacterium. Phage titers remain stable for two and half months (75 days) in SM buffer at -20 oC and − 40 oC but decrease significantly (p ≤ 0.0001) at 4 oC. Kintu is active at pH 3 and 11, maintains viability at temperatures between 25 oC and 120 oC and tolerates UV irradiation for up to 2 min and 20 s. Kintu inhibits Xvm growth at MOI ratios of 0.1, 1 and 10. The genome is a double stranded DNA molecule that consists of 48,985 base pairs and a G + C content of 51.71%. Antibiotic resistance genes or genes associated with a lysogenic life cycle are absent. There is limited sequence similarity of Kintu with other phages, making it a novel phage belonging to an unclassified genus of the class Caudoviricetes. Kintu is a novel bacteriophage that infects and lyses Xanthomonas vasicola pv. musacearum, the causative agent for Banana Xanthomonas Wilt. Its stability across diverse temperatures and pH conditions highlights its potential as a biocontrol agent for managing the disease.

Background: Xanthomonas vasicola pv. musacearum is a gram-negative bacterium that causes Banana Xanthomonas Wilt (BXW), a disease that severely affects banana crops. This results in reduced banana production, affects food security and the livelihoods of smallholder farmers. Despite the use of conventional management practices, the disease remains widespread and continues to affect banana plantations. The study aims to determine the host range of bacteriophage filtrates isolated from soils surrounding BXW-infected plants. Methods: Xanthomonas vasicola pv. musacearum was isolated from exudates of infected BXW-banana plant pseudostems and cultured on agar containing yeast extract, peptone, glucose, cephalexin and cycloheximide (YPGA-CC). The bacterial strains were confirmed using polymerase chain reaction. Soil samples were collected from three banana gardens affected byBXW. The soil-derived bacteriophage filtrates were enriched using double strength YPG broth while the host range was determined using the spot assay technique. Results: Bacterial isolates obtained from the exudates of the infected banana plants formed yellowish mucoid colonies on YGPA-CC plates. Xanthomonas vasicola pv. musacearum strains were confirmed by PCR, which yielded a band of approximately 398 base pairs on agarose gel. The host range was between 81% and 86% when tested on 21 Xanthomonas vasicola pv. musacearum strains. No plaque formation was detected on non-Xanthomonas strains. The clear plaques observed on Xanthomonas vasicola pv. musacearum lawns indicate successful lytic infection of the bacteriophage filtrates against the bacteria. Conclusions: The bacteriophage filtrates isolated from soil surrounding BXW-infected banana plants exhibited a broad host range against Xanthomonas vasicola pv. musacearum strains. These findings highlight their potential as biocontrol agents for managingBXW. To advance their application, further studies are required to isolate individual phage types from the filtrates and characterize them for potential use as biocontrol agents.