A Quantitative Real-Time PCR Assay for Detection and Quantification of the Ginseng Alternaria Leaf and Stem Blight Pathogen Alternaria panax

Ginseng Alternaria leaf and stem blight, caused by Alternaria panax, imposes substantial yield and economic losses to the ginseng cultivation industry. Current diagnostic methods for ginseng diseases primarily rely on pathogen isolation from infected tissues, a procedure that is laborious, time-consuming, and inherently low in sensitivity. This study has therefore developed a rapid, specific and sensitive SYBR Green-based quantitative real-time PCR (qPCR) assay for detecting A. panax in plants, seeds, and soil. The developed qPCR assay exhibited high sensitivity and repeatability, with a detection limit of 0.074 fg/μL of target amplicon DNA (0.619 ng/μL of genomic DNA) and a coefficient of variation below 2%. In artificially inoculated tissues (leaves, stems and seeds), Ct values decreased progressively with increasing incubation time, reflecting pathogen proliferation. Analysis of field-collected leaves and stems showed a strong overall correlation between Ct values and visual disease grades. Surveying of ginseng-growing areas revealed that A. panax was detected in asymptomatic leaves and stems at rates of 12.12% and 14.29%, respectively, and in 14.46% of soil samples and 23.73% of seed samples. This qPCR assay presented here provides a robust tool for forecasting early disease, tracking the primary inoculum of the pathogen and its transmission chains, and screening of both ginseng seed lots and candidate soils for ginseng Alternaria leaf and stem blight prior to planting.