qPCR-Based Quantification As A Tool For Genetic Selection And Yield-Stable Breeding Against Restricted Phloem...

The emerging phloem‑restricted pathogens Candidatus Arsenophonus phytopathogenicus (ARSEPH) and Candidatus Phytoplasma solani (PHYPSO), transmitted by the planthopper Pentastiridius leporinus, threaten European potato (Solanum tuberosum L.) production. This study used quantitative real‑time PCR (qPCR) to quantify ARSEPH and PHYPSO in 40 varieties grown at two field locations in southwest Germany under high natural P. leporinus pressure. Tuber assays revealed near‑complete infection (95.7-100%), dominated by co‑infections, with ARSEPH more prevalent and generally reaching higher copy numbers than PHYPSO. ANOVA showed that ARSEPH load was primarily genotype‑dependent, with stable varietal rankings across locations, whereas PHYPSO titres were strongly affected by both location and genotype and exhibited a pronounced genotype × location interaction. Processing varieties carried higher ARSEPH loads than ware potatoes, while maturity class did not explain clear differences in pathogen titre. Spearman correlations indicated that increased pathogen load was statistically associated with earlier symptom expression, accelerated canopy senescence, and a shift from marketable (35-65 mm) to undersized tubers, resulting in moderate but consistent reductions in net yield and increased rubbery tuber incidence; P. leporinus abundance was tightly linked to canopy yellowing and yield loss. These findings support the use of qPCR‑derived pathogen copy number as a robust quantitative trait for assessing varietal susceptibility under high infection pressure and argue for its integration into potato breeding and integrated management strategies in regions facing rising ARSEPH and PHYPSO pressure.