As pink root destroys onion roots, the plants cannot keep leaves supplied with moisture and nutrients, so they die. Bulbs may be small and immature. Bulb onions infected with pink root often rot in storage.
Always rotate onions with non-related crops. Grow onions in fertile, well-drained soil. As bulb onions gain size, taper off on watering so the soil stays somewhat dry.
Pull up affected plants and trim them for immediate consumption, and compost the trimmings in an active compost pile.