Purple Blotch Disease in Onion Crop – Symptoms, Causes, and Prevention

Purple blotch disease (Alternaria porri) poses a serious threat to onion crops, affecting leaves and stems, which can lead to significant yield losses. If not identified and controlled in time, this disease can severely impact both the quality and quantity of the harvest.

Symptoms of Purple Blotch Disease:

Initially, small, water-soaked yellowish spots appear on leaves. These spots gradually enlarge and turn brown or purple with a yellow halo around them. In severe cases, leaves start to dry out, and the stems may also get affected. Premature leaf drying hinders bulb development, leading to reduced yield.

How Does Purple Blotch Disease Spread?

Infected plant debris in the field serves as a primary source of the disease. The pathogen spreads rapidly through wind-borne spores. High humidity (80-90%) and moderate temperatures (18-25°C) accelerate its growth. Waterlogging in the field further increases the risk of infection.

Management of Purple Blotch Disease:

1. Agronomic Management: Practice crop rotation with non-host plants. Remove and destroy old infected plant debris. Avoid excessive nitrogen application, as it can promote disease development. Ensure proper drainage to prevent water stagnation.
2. Biological Management : Use of Trichoderma spp. – A bio-agent effective in controlling the fungal pathogen. Neem oil (5%) spray – A natural remedy to reduce disease severity. Cow dung slurry – Organic manure enhances the plant’s disease resistance.
3. Chemical Management : Mancozeb (75 WP) – Mix 2.5 g per liter of water and spray. Propiconazole (25 EC) – Mix 1 ml per liter of water and apply. Repeat the fungicide spray twice at 10-15 day intervals for effective control.
4. Irrigation Management: Prefer drip irrigation in the morning hours. Avoid sprinkler irrigation, as it increases moisture on leaves, promoting disease spread.

Preventive Measures:

1. Regular field monitoring – Take immediate action if early symptoms appear.
2. Seed treatment – Treat seeds with Thiram or Captan (2-3 g/kg seed) before sowing.
3. Timely fungicide application – Apply fungicides when the crop is 30-35 days old.
4. Weed management – Keep the field clean to eliminate disease-carrying agents.