As onion fusarium fungi destroy onion roots, the plants cannot make new growth. Bulbs may be small and immature. Bulb onions infected with fusarium quickly rot in storage.
Grow onions in fertile, well-drained soil. As bulb onions gain size, taper off on watering so the soil stays somewhat dry. Once you harvest your onions, handle them gently to avoid bruising, which can invite problems with fusarium in the stored onions.
Pull up affected plants and trim them for immediate consumption. Compost the trimmings in an active compost pile.