<span>It's a metaphor, darling. A nursery (whether it's a plant nursery or a nursery school) is a place where small plants or human beings are carefully nutured so that they can grow. "He" (whoever this mysterious "he" is that you're referring to) is asserting that factories are places where bad things like "disease and vice" can similarly grow and flourish.</span>
The primary purpose of the Farmers Alliance was to put an end to increasingly lower payments to farmers by creating price floors at which all crops would be grown, to increase revenue for all farmers in general.
