Causal Agent: 

Fungus (Alternaria porri)


Characteristic Symptoms:
Affected leaves and stem initially have small water-soaked spots.
The spots will eventually enlarge and  turn into purple or brown lesions covered with brown or dark gray spores.
The fungus usually infects dead or dying leaf tissue.
Onion leaves become more susceptible as they age.
Severe infection can cause blighting of leaves.
Conditions for Disease Development:
The pathogen is soil-borne and survives on plant debris for long periods of time.
The disease is severe with prolonged humid conditions.
Spores are disseminated by wind and rain splashes.
Diseased plants are source of secondary inoculum, which spreads from plant to plant in the field.
Management and Control:
Use pathogen-free seed.
Remove and destroy infected plants as soon as symptoms are observed to minimize spread of disease.
Avoid prolonged moisture to minimize disease severity.
Avoid wounding/injuries as this may predispose plants to infection.
Crop rotation for 2-3 years may help reduce inoculum level in the soil.
Drench nursery beds with fungicides like  promocarb hydrochloride (e.g. ProplantⓇ, PrevicureⓇ) or fosetyl aluminum (e.g. AlietteⓇ), metalaxyl+mancozeb (e.g. Ridomil Gold MZ 68 WGⓇ, Apron XL 350 ESⓇ), mancozeb (e.g. Attain M-80Ⓡ, Achem Mancozeb 80 WPⓇ, Micron 80 WOⓇ, VanzebⓇ) to minimize incidence and prevent introduction of disease to the field. 

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