Evaluating Grafting and Dodder as Non-Vector Transmission Routes of ‘Candidatus Arsenophonus phytopathogenicus’ in Sugar Beet

Syndrome Basses Richesses (SBR) is a phloem-limited disease of sugar beet caused by ‘ Candidatus Arsenophonus phytopathogenicus’ (ARSEPH), a bacterium currently considered a facultative endosymbiont of its insect vector, Pentastiridius leporinus . Due to increasing disease incidence and the need for reliable vector-free resistance screening methods, this study evaluated alternative ARSEPH transmission routes using chip grafting and the parasitic plant dodder ( Cuscuta campestris ). Given that Arsenophonus species are unculturable and their insect vectors have complex life cycles so laboratory propagation is labor intensive, the development of vector-independent systems is desirable for controlled pathogen transmission and host–pathogen interaction studies. Chip grafting enabled successful ARSEPH transmission to sugar beet, though with low efficiency (~ 9.6%) and delayed pathogen detection. While C. campestris acquired and maintained high ARSEPH titers, it failed to transmit the bacterium to healthy plants, indicating limited systemic movement. In contrast, vector-mediated transmission via P. leporinus achieved 100% efficiency within 25 days, confirming a strong biological compatibility. These findings underscore the limitations of non-vector-based approaches for phloem-restricted pathogens and highlight the need for methodological optimization. The study provides key insights into ARSEPH’s complex transmission biology and supports a functionally specialized association with its insect vector.