The role of soil abundance of TxtAB in potato common scab disease severity

Common scab is an economically costly, soil-borne disease of potato endemic in many potato growing regions. The disease is caused by species of Streptomyces bacteria that produce the phytotoxin Thaxtomin A. The primary disease management tool available to growers is planting resistant cultivars, but no cultivar is fully resistant to common scab and partially resistant cultivars are often not the preferred choice of growers because of agronomic or market considerations. Therefore, growers would benefit from knowledge of the presence and severity of common scab infestations in field soils to make informed planting decisions. We implemented a qPCR diagnostic assay to enable field detection and quantification of all strains of Streptomyces that cause common scab in the United States through amplification of the Thaxtomin A biosynthetic genes. Greenhouse trials confirmed that pathogen abundance was highly correlated with disease severity for five distinct phytopathogenic Streptomyces species, though the degree of disease severity was dependent on the pathogen species. Correlations between the abundance of the Thaxtomin biosynthetic genes from field soil with disease on tubers at field sites across four U.S. states and across two years were not as strong as correlations observed in greenhouse assays. We also developed an effective ddPCR diagnostic assay that also has potential for field quantification of Thaxtomin biosynthetic genes. Further improvement of the PCR assays and added modeling of other environmental factors that impact disease outcome, such as soil composition, can aid growers in making informed planting decisions.