Onion smut

Onion smut (Urocystis magica) is a tough disease of the onion because it can last for years in the soil as spores. When the onion crop is planted the spores germinate and enter the fresh seeds.

Symptoms
When the plant grows lesions form on the leaf and any bulbs that form will be covered in black spots. Most of the time infected seeds will die within 3–5 weeks of planting.

Treatment
None. Lift and burn diseased plants.

Prevention
Leeks and onions should not be grown in the same site for 8 years.