First detection and biological characterization of African eggplant mosaic virus, an aphid, transmissible potyvirus infecting tomato in Switzerland

In 2025, tomato (Solanum lycopersicum) plants exhibiting irregular fruit marbling and spotting were observed at three production sites in Ticino, Switzerland. Transmission electron microscopy, generic potyvirus RT-PCR followed by Sanger sequencing, and subsequent High-throughput sequencing confirmation identified the causal agent as African eggplant mosaic virus (AEMV; Potyvirus melongenae), marking its first detection outside of Africa and its first natural occurrence on tomato. The Swiss isolates represent a highly divergent AEMV strain, sharing only 77.5% full-genome nucleotide identity with the Tanzanian reference isolate. While the P1 region showed extreme hypervariability (51% nt identity), the coat protein (83% nt and 92% aa identities) firmly supported its taxonomic assignment to AEMV. This high sequence divergence suggests toward a long-standing, previously unsampled cryptic reservoir. Biological assays demonstrated efficient transmission by the aphid Myzus persicae and confirmed that while mature plants exhibit only fruit symptoms, early infection of young plantlets leads to severe growth retardation. RT-qPCR analysis showed a significant correlation between viral load and fruit symptom expression. Epidemiological tracing linked all three Swiss outbreaks to a single nursery, suggesting a point-source introduction through infected plantlets. A targeted survey of 81 tomato samples collected in 2024, across 12 Swiss cantons yielded no additional positives, indicating the outbreak is currently localized. To support surveillance, species-specific RT-qPCR diagnostic assays were developed. This study provides a blueprint for the rapid characterization, risk assessment, and containment of emerging plant viruses introduced through international trade, and supplies the diagnostic tools and epidemiological framework needed to manage this virus in the European tomato sector.