On Crops: Bulb onions, garlic, shallots, leeks, chives, and ornamental varieties of Allium
The foliage yellows and wilts. This may coincide with the plants being nearly ready to harvest, so the disease can go unnoticed until the crop is harvested. When pulled, the plant will lift free from the soil easily. On inspection of the basal plate (where the roots sprout from) a fluffy white mould can be found, which may be peppered with tiny black dots like poppy seeds. The black dots are 'sclerotia', which will drop off in the soil and wait until conditions are right for them to germinate and spread the disease to the next onion family crop. The optimum germination temperature for the sclerotia is 15-18C (59-64F), and they may remain viable in the soil for 7-20 years.
Bulbs are softened and parts may be inedible. They will not store, and may transfer the disease to unaffected bulbs in storage, so must be used immediately. In severe cases, the bulb will have turned black and be totally rotten.
Rotate onions to a fresh site each year to prevent disease build-up. Only buy certified disease-free sets and seedlings and inspect all plants carefully before planting, or grow only from seed.
Gather up and burn onion debris. Do not compost it. Wash boots and tools well to avoid transferring the disease to other parts of the garden. Set aside bulbs from affected plants for short-term consumption, because they will rot in storage. Avoid growing onions in the same spot again for at least eight years.