Hello there,
The answer to your question is Wilt of Potato. This disease in plants is caused by the soil borne bacteria called Pseudomonas solanacearum. It is called ring disease because of the brown ring formed on the xylem. Potato wilt bacterium mainly inhabits the roots, and enters the root system at points of injury caused by farm tools or equipment and soil pests.
Hope this information helps you.
Good luck!!