A characteristic symptom of TSWV infecting potato is leaf spots with a target appearance similar to early blight, but the spots are smaller and tend to be clustered. Clustering of spots can result in affected leaves having large areas that are completely dark brown to black, more often at the leaf base. Stems and petioles often have dark brown areas. Growing points can be killed.
Similar to the situation with other virus diseases, and unlike fungal diseases such as early blight, typically there are plants with no symptoms next to affected plants.
These pictures were taken in July 2014 of affected leaves from potato crops on Long Island. Prior to development of symptoms, Western flower thrips, a vector of TSWV, were observed in these crops at a higher density than considered normal in this region.