Avoid growing onions with white bulbs as these are more susceptible than yellow or red ones. Onion white rot A dense, fluffy white fungal growth around the roots and base of the bulb indicates white rot infection caused by the fungus Sclerotium cepivorum . Infected plants should be removed promptly. Avoid growing onions, or other related crops, on the same site for at least eight years. There is no chemical control or resistant variety available.
Infected leaves develop off-white to grey, fluffy fungal growth and start to discolour and die back.
Infections that occur early in the season may result in undersized bulbs. The disease is caused by the fungus Peronospora destructor and encouraged by dampness or humidity. Remove infected leaves promptly and spray with a fungicide containing mancozeb. Onion fly Onions are particularly prone to attack by the larvae of the pest Delia antiqua, but leeks, shallots and garlic may also be attacked. The larvae eat the roots of the bulbs and, if the infestation develops during late summer, they may also burrow into the bulbs.