Causal Agent: 

Fungus (Sclerotium rolfsii)

Characteristic Symptoms:
 
Affected onion bulb are usually covered with web of white mycelia with brown to black "sclerotial bodies".
   
Bulb size is reduced and eventually becomes rot.
   
Conditions for Disease Development:
   
The disease is favored by high soil moisture and high temperature.
   
The fungus survives in soil and crop residues for long periods of time in the form of sclerotia which be a new source of infection.
   
Management and Control:
 
Crop rotation (e.g. corn)  and soil solarization, deep plowing and flooding may help reduce the inoculum in the soil.
   
Restriction of movement of equipment and planting materials can help minimize spread and/or avoid introduction of this disease.
   
Spray with chemical fungicides like difenoconazole (e.g. ScoreⓇ, MontanaⓇ, PursueⓇ, BashⓇ), Pyraclostrobin (e.g. Cabrio 25 ECⓇ), metalaxyl+mancozeb (e.g. Ridomil Gold MZ 68 WGⓇ, Apron XL 350 ESⓇ) or carbendazim ( (Bavistin 50 DFⓇ, Goldazim 500 SSⓇ, Minx 500 SCⓇ, SuperdazimⓇ, Avert 50 WPⓇ, BiostinⓇ) to minimize infection.
   
Soak bulbs in fungicide after harvesting to prevent bulb rotting during storage.

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