While he was living in New Hampshire in the mid-1800s he knew a woman who thought all her illnesses, indeed all her troubles in life, were caused by witches. When she was sick the woman would threaten the witches, who although invisible undoubtedly were the source of her suffering.
To keep the witches at bay the woman wore a bead necklace made from mountain-ash (that's pyrus americana or sorbus americana to you botanists out there):
They were cut about three eighths of an inch in length, the bark being left on, and strung on string running through the pith. She was careful to keep them concealed, but sometimes they would work up above her collar and be conspicuous. This species of tree was once quite popular among New England witch-believers as a charm against witches...
Folklorist Fanny Bergen also reports that people in New England carried a piece of mountain-ash, or witch-wood, in their pockets to repeal witches.
The American mountain-ash is of the same family as the rowan tree, which is well-documented in European folklore. The Norse god Thor was saved from drowning by grasping a rowan tree, while in Scotland householders planted them at their front doors to repel evil. According to Wikipedia, it could also be used to prevent storms at sea, keep the unhappy dead in their graves, and prevent lightning from striking a house. That's a pretty potent tree! It seems likely that the English settlers in New England brought their beliefs about the rowan tree to the New World and transferred them to its American cousin.
Oddly, I've been to Scotland and seen a rowan tree but don't know if I have seen an American mountain-ash. I'll have to take a trip to the Aboretum and track one down!
This week's info is from John McNab Currier, “Contributions to New England Folk-Lore”, The Journal of American Folklore, Vol. 4, No. 14 (Jul – Sep. 1891) and Fanny Bergen, “Some Bits of Plant-Lore”, The Journal of American Folklore, Vol 5, No. 16 (Jan – Mar, 1892).