Causal Agent:
Fungus (Alternaria porri)
| Characteristic Symptoms: | |
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Affected leaves and stem initially have small water-soaked spots. |
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The spots will eventually enlarge and turn into purple or brown lesions covered with brown or dark gray spores. |
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The fungus usually infects dead or dying leaf tissue. |
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Onion leaves become more susceptible as they age. |
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Severe infection can cause blighting of leaves. |
| Conditions for Disease Development: | |
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The pathogen is soil-borne and survives on plant debris for long periods of time.
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The disease is severe with prolonged humid conditions.
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Spores are disseminated by wind and rain splashes.
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Diseased plants are source of secondary inoculum, which spreads from plant to plant in the field.
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| Management and Control: | |
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Use pathogen-free seed.
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Remove and destroy infected plants as soon as symptoms are observed to minimize spread of disease.
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Avoid prolonged moisture to minimize disease severity.
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Avoid wounding/injuries as this may predispose plants to infection.
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Crop rotation for 2-3 years may help reduce inoculum level in the soil.
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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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