Onion white rot

Onion white rot or mouldy nose is caused by the pathogen sclerotium cepivorum that destroys the roots, killing the plant. It then produces resting bodies that can last for many years in the soil.

Identifying Features
Leaves wilt and turn yellow. In wet conditions the plant may not wilt, but can become loose in the soil. When plants are lifted; white fluffy growth can be found on the base. In advanced cases, you will see black, pinhead-sized, regular spheres. These are the sclerotia, or resting bodies, that will carry the disease from one crop to the next.

Onion white rot appears similar to onion fly or bean-seed fly, but also shows a white mould. These pests tend to go for onions rather than other alliums.

Treatment
Remove and destroy affected plants and a few inches of surrounding soil. Do not grow other alliums on the same site for another 8 years. This fungus survives in the soil for a very long time. There is currently no chemical control.

Garlic spray
It is currently being investigated whether spraying the intended growing site with garlic spray two to three weeks before sowing alliums will reduce a white rot infection. The theory is that the garlic releases sclerotium spores into action, but as there are no plants on which to feed, the spores die.

Prevention
Practising crop rotation will help leave sufficient time between allium crops to reduce sclerotia in lightly infected soil, but will do little to prevent infection in a heavily infected site.

Hosts
Potential pathogen hosts include:


 * Garlic
 * Leek
 * Onion
 * Shallot
 * Spring onion