• Latent disease - although infection takes place in the field
• Softening of scales which take on a water soaked appearance
• Under moist conditions - greyish sporulating mycelial mat develops on the surface of the scales 

Mode of spread and survival:

• Fungus persists saprophytically on dead onion tissue, on humus in soil and as sclerotia near the surface of previous onion crop cells
• Sclerotia germinate in moist weather and produce conidia that are disseminated by air currents
• When conidia land on the onion necks that are cut or wounded they can geminate, penetrate and kill onion tissue

• Dusting the seed with benomyl at 1g/kg of seed
• Avoid excessive and late (after mid-July) applications of nitrogen
• Rapid and thorough drying directly after harvest
• Provide good ventilation for curing onions before storage