White bulb rot (Sclerotium cepivorum)
The disease occurs mainly in the field and seldom causes injury in storage. The disease is called white rot because of characteristic basal bulb rot where the tissue is covered with white mat of fungal growth. Later numerous rounded black fungal bodies (sclerotia), each about the size of a pin's head, develop. The leaves of diseased plants decay at the base, turn yellow, wilt, fall over and die. The older leaves are the first to die. The roots of affected plants are usually rotted making the plants easy to pull.
Optimum temperature range for infection is from 15 to 18.3° C. The fungus survives in the soil as sclerotia and also in diseased onion sets and wild onion. It is most severe in light cool moist soils. Its host range includes Welsh onion, garlic, leek, shallot and some species of wild onion.
- Plant tolerant, resistant varieties, if available.
- Use healthy seeds.
- Practises long rotation (8-10 years) with cereals.
- Destroy wild onions and leeks.
- Manure from animals fed on diseased plant material should not be used on onion fields.