“The folklore of the plant is meagre, considering its wide distribution, but there are a number of curious superstitions connected with it. In some parts there is a belief that it thrives best if planted on Maundy Thursday; in others, that if planted under certain stars it will become watery. In Devonshire the people believe that the potato is a certain cure for the toothache – not taken internally, but carried about in the pocket. It is by several writers mentioned as a reputed cure for rheumatism in the same way; only it is prescribed that, in order to be an effective cure in such cases, the potato should be stolen. Mr. Andrew Lang mentions an instance of faith in the practice of this cure, which he came across in a London drawing-room. He regards this belief as a survival of the old superstitions about mandrake, and as analogous to the habit of African tribes who wear roots round the neck as protection against wild animals.”

Benjamin Taylor, Storyology : Essays In Folk-Lore, Sea-Lore, And Plant-Lore (1900)

ADDENDUM : It may be worth noting here that, if you were suffering from rheumatism in Aveyron, and, in attempting a cure, stole a potato from the Wild Boy of that place, he would begin to scream. Not unlike a mandrake root pulled from the soil.