Johnsongrass Mosaic Virus Revealed as Widespread and Genetically Diverse in Ethiopian Forage Grasses

A large-scale survey of forage grass collections in Ethiopia has uncovered a strikingly widespread and genetically distinctive population of johnsongrass mosaic virus, a member of the genus Potyvirus that ranks among the most economically important groups of plant viruses affecting grasses and cereal crops worldwide. Researchers screening nearly a thousand forage grass accessions found the virus associated with hundreds of plants spanning dozens of species, and in the process documented fifteen grass species that had never before been recorded as natural hosts of the pathogen. The study, published in BMC Plant Biology, also reveals that the Ethiopian isolates are so divergent from their counterparts elsewhere in the world that some fall below the internationally accepted threshold used to distinguish one potyvirus species from another, raising intriguing questions about the evolutionary trajectory of this virus in East Africa.

The research was carried out by Alok Kumar, Jean Hanson, Chris S. Jones, Fikerte Mulatu and Yilikal Assefa, all affiliated with the International Livestock Research Institute in Addis Ababa. Their motivation stemmed from a conspicuous gap in knowledge: although johnsongrass mosaic virus, commonly abbreviated JGMV, is recognized as an economically significant pathogen of grasses and cereals across the globe, its host range, genetic diversity and population structure in Ethiopian forage grasses had remained poorly understood. Forage grasses underpin livestock production in much of the region, so an uncharted viral landscape within these plants carried both scientific and agricultural implications. The team set out to identify which forage grass species in Ethiopia harbor the virus and to characterize the genetic diversity, population structure and evolutionary dynamics of the isolates circulating there.

The scale of the screening effort was considerable. The researchers examined a total of 949 forage grass accessions representing 33 genera, using a two-stage diagnostic strategy designed to balance throughput with specificity. The first stage employed a dot-blot assay, a technique in which plant tissue extracts are spotted onto a membrane and probed with virus-specific antibodies to reveal which samples carry viral proteins. Samples that tested positive in this serological screen were then subjected to reverse transcription polymerase chain reaction, or RT-PCR, using JGMV-specific primers designated GBV125F and GBV106R. These primers target the region of the viral genome encoding the coat protein, the structural protein that encapsidates the viral RNA and serves as a workhorse for potyvirus identification and classification.

The molecular confirmation was unambiguous. RT-PCR amplified the expected product of approximately 1.5 kilobases from samples corresponding to 249 accessions, which together represented 22 species across 10 genera. In other words, roughly a quarter of the screened accessions carried the virus, a prevalence that underscores how thoroughly JGMV has permeated the forage grass collections in Ethiopia. The researchers then selected representative isolates from this confirmed set for deeper genetic analysis, focusing on isolates drawn from 15 grass species belonging to six genera. Sequence analysis of these isolates confirmed JGMV infection in every case, and all fifteen species are now reported for the first time as natural hosts of the virus, a substantial expansion of the known host range of this pathogen.

The most consequential findings emerged when the team compared the amino acid sequences of the coat proteins of the Ethiopian isolates with those of JGMV isolates reported from around the world. The identity between Ethiopian isolates and international ones ranged from 70.4 percent to 81.1 percent, a range that sits uncomfortably close to, and in some cases below, the species demarcation threshold applied by the International Committee on Taxonomy of Viruses. The ICTV considers potyvirus isolates with coat protein amino acid identity above 80 percent to belong to the same species; values below that line suggest that the isolates may represent distinct species. The fact that some comparisons between Ethiopian and global isolates fell below this threshold means the Ethiopian virus population may be more than simply a geographically isolated branch of JGMV; it could represent distinct evolutionary lineages within the genus Potyvirus, a possibility the authors flag explicitly in their conclusions.

Phylogenetic analysis, which reconstructs the evolutionary relationships among virus isolates based on their sequences, grouped the JGMV isolates into three major clades. Within this framework, all of the Ethiopian isolates clustered together with isolates from Brazil in a distinct clade, separate from isolates collected elsewhere. This pattern is noteworthy because it links virus populations from two continents separated by the Atlantic Ocean, hinting at either shared ancestry, historical movement of virus or host material between the regions, or convergent evolutionary paths. The study does not resolve the mechanism behind this relationship, but the coherent clustering of Ethiopian isolates with Brazilian ones provides a clear phylogeographic signal that future work on virus dispersal will need to explain.

Population genetic analyses added further texture to the picture. The Ethiopian isolates exhibited high genetic diversity and significant genetic differentiation, indicating that the virus population in Ethiopia is both internally varied and substantially separated from populations elsewhere. Neutrality tests, statistical tools that compare the observed distribution of genetic variation with what would be expected under a neutral evolutionary model, provided evidence of historical population expansion, suggesting that the Ethiopian JGMV population has grown substantially at some point in its past. Such expansion is consistent with a virus that has successfully colonized a broad and abundant host landscape, and the fifteen newly identified host species offer ample substrate for continued diversification.

Recombination analysis revealed a further layer of complexity in the evolutionary history of the virus. Recombination, the exchange of genetic material between viral lineages, is a major driver of potyvirus evolution and can generate novel genotypes with altered host ranges or pathogenic properties. The analysis indicated that isolates from Ethiopia, Argentina, Brazil and Kenya were involved in recombination events, whereas isolates from the United States and Australia appeared relatively stable, showing no evidence of such exchanges. This geographic asymmetry suggests that recombination is not a uniform feature of the global JGMV population but rather concentrated in particular regions, with East Africa and South America emerging as hotspots of genetic exchange. For plant health authorities, recombination-prone populations are of particular interest because they can produce unexpected variants with the potential to overcome existing resistance in crop varieties.

The practical implications of the study extend beyond virology into the management of forage resources. Ethiopia’s forage grasses are central to livestock agriculture, and germplasm collections maintained for breeding and conservation purposes can inadvertently serve as reservoirs of viral pathogens if infection goes undetected. The finding that 249 of 949 accessions were positive for JGMV, spanning 22 species and 10 genera, illustrates how extensively the virus has infiltrated the forage gene pool in the country. The identification of fifteen new natural host species broadens the range of plants through which the virus can persist and move, complicating efforts to manage its spread. Because the study was funded by the CGIAR Research Program for Managing and Sustaining Crop Collections, and drew on antiserum for detecting Brachiaria-infecting potyviruses provided by the International Center for Tropical Agriculture, it reflects the kind of collaborative infrastructure that international agricultural research centers maintain to safeguard plant genetic resources.

The authors are careful to frame their conclusions as a foundation rather than a final word. The substantial divergence observed among the Ethiopian isolates suggests the possible emergence of distinct evolutionary lineages within the genus Potyvirus, but confirming whether these isolates warrant recognition as separate species will require complete genome sequencing rather than analysis of the coat protein region alone. The team also highlights the need for vector transmission studies to clarify how the virus moves among its hosts in the field, knowledge that is essential for assessing its epidemiological significance and designing control strategies. Until such work is done, the study stands as a vivid demonstration of how much viral diversity can remain hidden within a region’s plant collections, and of the value of systematic screening paired with molecular characterization. For Ethiopia’s forage grasses, and for the broader community studying potyvirus evolution, the message is clear: the virus population in East Africa is widespread, genetically rich, and evolutionarily distinctive enough to demand closer attention.

Subject of Research: Detection and genetic diversity of johnsongrass mosaic virus in Ethiopian forage grasses

Article Title: Detection and genetic diversity of johnsongrass mosaic virus isolates infecting fifteen new forage grass hosts in Ethiopia

Article References: Kumar, A., Hanson, J., Jones, C. S., Mulatu, F., & Assefa, Y. (2026). Detection and genetic diversity of johnsongrass mosaic virus isolates infecting fifteen new forage grass hosts in Ethiopia. BMC Plant Biology. https://doi.org/10.1186/s12870-026-10002-0

Image Credits: AI Generated

DOI: 10.1186/s12870-026-10002-0

Keywords: johnsongrass mosaic virus, Potyvirus, Ethiopia, forage grasses, RT-PCR, coat protein, genetic diversity, phylogenetics, recombination, ICTV species demarcation, plant virus epidemiology, germplasm screening