How to control the most stressful disease in onion farming?
Purple blotch is one of the most stressful disease in onion farming causing massive losses to farmers. As an onion farmer can you prevent it? How can one actually control it?
Kelvin Wairimu 
Purple Blotch
Purple blotch is one of the most stressful diseases in onion farming causing massive losses to farmers. As an onion farmer, do you know how you can prevent it? How can one actually control it? Purple blotch is a fungal disease caused by Alternaria porri fungus and is distributed in most parts of the world.
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What are the symptoms of purple blotch?
- Older leaves tend to be more susceptible than younger leaves.
- Water-soaked lesions forms which usually have a white centre.
- Edges of lesions become brown to purple and the leaf turns yellow above and below the lesions.
- With time, dark brown to black concentric rings form throughout the lesions. These are areas of sporulation of the fungus.
- As the disease progresses, lesions may girdle the leaf causing it to collapse and die.
- When bulb infection occurs, it is normally through the neck. If the fungus invades the bulb, the infected area is initially bright yellow; but eventually turns a characteristic red wine colour. This may cause a wet orange rot starting at the neck.
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What are the conditions that facilitate purple blotch development?
- Spores are formed during humid nights and leaf wetness periods greater than 12 hours.
- As the morning dew dries, spores become air-borne and are disseminated to susceptible onion tissue.
- 1-4 days are needed for symptoms to develop after infection.
- Disease development is greatest during prolonged periods of leaf wetness.
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How to control purple blotch?
- Use resistant varieties e.g. Red Passion F1 and Red Pinoy F1
- A fungicide spray program with broad-spectrum protective fungicides (such as Score+ Redomil gold) applied before infection can provide good protection.
- Planting on the recommended spacing and minimizing leaf wetness by using surface rather than sprinkler irrigation, good field drainage and correct plant spacing can reduce disease development.
- A rotation out of Allium to unrelated crops for several years can reduce disease as well.
- Maintaining a high level of field sanitation by removing all crop remains after harvest.
- Fields should always be free of weeds.
Reference: Seminis
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