DNA-based detection of Aphanomyces cochlioides in soil and sugar beet plants

Aphanomyces cochlioides, the causal agent of seedling damping-off and Aphanomyces root rot (ARR) of sugar beet, causes yield losses in major sugar beet growing regions. Currently, a 4-week soil bioassay and a 2-day culture-based assay are used to diagnose presence of A. cochlioides. However, these assays can be time-consuming and lack sensitivity. In this study we developed a sensitive, specific, and rapid assay to detect and quantify DNA of A. cochlioides. We developed a TaqMan qPCR assay targeting a region of the mitochondrial genome of A. cochlioides representing a unique gene order for Aphanomyces with genus-specific primers and a species-specific probe. The qPCR assay detected A. cochlioides in 12 naturally infested field soil samples with disease severity index (DSI) values of 48-100, in sugar beet seedlings 5-7 days after planting, and with as little as 1 fg of pure A. cochlioides DNA. Adult sugar beet roots with ARR symptoms were sampled to further validate this qPCR assay. Aphanomyces cochlioides was detected in 95% of these samples using this qPCR assay, while only 23% of the same samples were positive using a culture-based assay. This shows the improved sensitivity of this qPCR assay for disease diagnosis and could provide growers with ARR risk of a field, which would help them make informed disease management decisions. However, further research is required to translate the results of this study to growers fields to quantify A. cochlioides with a high degree of accuracy.