A novel in vitro root inoculation assay to screen potato genotypes for resistance to Common Scab

Common scab in potato is caused by multiple Streptomyces species that harbour various virulence factors. Varietal resistance is commonly evaluated with multi-year – multi-location field trials with known high infection potential or using the phytotoxin thaxtomin applied to in vitro mini tubers or potato tissue culture. In this study, we aimed to develop an efficient root inoculation assay to assess the resistance levels of potato genotypes and to evaluate whether the assay could identify resistant and susceptible genotypes, thus facilitating selection of scab resistant clones. We isolated 24 potential Streptomyces strains from fields in Ireland, of which 11 were identified as S. europaescabiei . All S. europaescabiei strains tested positive for txtAB gene but lacked n ec1 and t omA genes. The root inoculation assay resulted in plants exhibiting necrotic symptoms on roots and stunted root growth. Image analysis software was used to collect quantitative data from our assay. We observed a Spearman’s rank correlation of 0.61 between field data and our assay using a panel comprising 50 clones from the bi-parental cross Electra × Désirée and five control varieties. The root inoculation assay is rapid, as symptoms are observed within 6 to 10 days post-inoculation, and requires minimal manipulation since a bacterial suspension is applied instead of purified thaxtomin. Notably, this assay identifies resistant and susceptible progeny reliably, with some disparities between the resistance pattern in the field and the assay. This tool has potential to be useful for screening large numbers of genotypes and discarding the susceptible ones in a breeding program.