Sweet potato rot is a type of fungal disease caused by the fungus Phytophthora. It can affect the roots, stems, and tubers of sweet potato plants. Symptoms of sweet potato rot include dark brown spots on the surface of the tuber and a foul odor coming from the infected area. The infected areas may become soft and mushy as they rot away. In severe cases, entire sweet potatoes may become rotten and inedible. Sweet potato rot is most common in wet soil conditions, so it is important to ensure that your sweet potatoes are planted in well-draining soil and watered only when necessary.