The Witch's Curse is a major plot line in the film Into the Woods. The curse was cast by the witch's mother in the past.
The Witch who was once a beautiful woman was transformed into a hideous witch after her Magic Beans were stolen by The Baker's father. The Witch had inherited a garden containing these beans and was warned by her mother that if she were to loss any of the special beans that she would be punished. As punishment she was turned into a witch.
Being annoyed and angry at The Baker's father she placed a curse on them that their family tree would always be a barren one, this is the reason as to why The Baker and his wife were unable to have children. The Witch told them if they wanted the curse reversed, they had to bring her back four items; The Cow as White as Milk, The Cape as Red as Blood, The Hair as Yellow as Corn, and the Slipper as Pure as Gold. She asked them to deliver those items to her within three nights before the moon turns blue. The Witch also warned them that the things they are to acquire cannot be touched by her or the curse wouldn't be undone.
The Baker and his wife got the cow, Milky White, by trading Jack beans for the cow, the Baker got the cape from Red Riding Hood after he saved her and her grandmother from the wolf, and some hair from Cinderella's stepmother. The Baker's Wife also acquired hair, but from Rapunzel.
At the evening of the third night, the Witch revives Milky White who died one day prior and requested the baker, his wife, Jack, his mother, and Little Red Riding Hood to feed the cow with the cape, slipper, and hair. Jack was trying to milk the cow to the chalice that the witch handled to him, however it was discovered that the hair that was given to the cow was Rapunzel's whose hair the witch touched in the past. However the baker remembers about the hair he collected and feeds it to the cow. The milk is produced and the witch drinks it reverting to her former appearance.
After it is discovered during the argument between the baker, Cinderella, Jack, and Little Red Riding Hood that all the beans that the baker's father stole were used, the curse cast by the witch's mother fulfills and the witch is turned into a bog in the woods.