Rebecca and the Horror Harvest — Margaret Pearce

Three wizards have moved into Rebecca's district with their nasty hobgoblins. Why are they so helpful, giving free bags of potatoes to everyone? Rebecca finds out that the tainted potatoes keep all whom eat them under the spell of the wizards.
Rebecca and the house goblins struggle to destroy the potatoes but the wizards are planting more of the horror harvest. They are getting nearly all of the district under their control. Rebecca is helped by the three old sisters, the Misses Fritons. She makes friends with the bridge troll which also helps her in her fight against the dangerous wizards.