Purple Blotch is a fungal disease that can cause significant damage to onion crops. The disease manifests as purple or brownish-purple lesions on the leaf stem and bulbs and is caused by the fungus Alternaria porri. The fungus spores can survive in soil, plant debris, and infected onion bulbs, making it a challenging disease to manage. Crop rotation and disease-resistant onion varieties can also be effective management strategies. In severe cases, fungicides may be necessary but should be combined with other management strategies for best results.
Purple Blotch Management in Onion
What is Purple Blotch in Onion?
- Purple Blotch is a fungal disease that affects onions, causing purple or brownish-purple lesions on the leaves, stems, and bulbs.
- The disease is caused by the fungus Alternaria porri, which thrives in warm and humid conditions.
- The fungus spores can survive in soil, plant debris, and infected onion bulbs, infecting new crops.
- Purple Blotch can cause significant yield losses in onions, as infected plants may produce smaller bulbs.
Onion Plant Details
- Onion (Allium cepa) is a biennial plant that is grown as an annual crop.
- The plant produces a bulb that is used as a culinary ingredient and also has medicinal properties.
- Onions are a cool-season crop typically grown in the fall or winter in temperate regions or in the spring in colder regions.
Causes/Pathogen of Purple Blotch in Onion
- Purple Blotch in onions is primarily caused by the fungus Alternaria porri. This disease is prevalent in warm and humid regions where onions are grown.
- The fungus can also affect other plants in the Allium family, such as garlic and leeks. Long-day onion cultivars, especially sweet Spanish onions, are more susceptible to the disease than common yellow cultivars.
- All short-day cultivars are also susceptible. The fungus can survive in soil and plant debris, infecting new onion crops, and can also be transmitted through infected seedlings or bulbs used for planting.
The Disease Cycle of Purple Blotch in Onion
- The disease is soil-borne, and the fungus can survive in soil, infected bulbs, plant debris, and even on the roots of weeds.
- The pathogen overwinters in crop residue or the soil surface, and spores are produced during warm and humid weather.
- Symptoms of Purple Blotch appear 1-4 days after infection, and new lesions produce black spores within five days.
- Spores are produced at night and released in the morning when humidity decreases, and they are spread by wind, splashing rain, or irrigation.
- Lesions typically occur when the leaf wetness lasts 16 hours or more, and flecking may occur at 12 hours.
- Purple Blotch and Stemphylium leaf blight can sometimes occur on the same plant, but microscopy is necessary to distinguish between the fungi.
Conditions Favorable for Purple Blotch Spread in the Field
- Purple Blotch disease thrives in hot and humid weather, with temperatures ranging from 21-30°C.
- High relative humidity (between 80-90%) also creates favorable conditions for the disease to develop and spread.
- The fungus that causes Purple Blotch can survive in the soil for several years, making crop rotation an important management strategy.
- Overcrowding onion plants in the field can also increase the Purple Blotch infection. Poor air circulation in the field can also contribute to the development and spread of the disease.
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