Onion is one of the most widely grown vegetable and spice crops in India. Farmers cultivate it in almost every state because of its high demand throughout the year. However, onion is also a delicate crop that can be easily affected by several diseases caused by fungi, bacteria, and viruses. These diseases not only reduce yield but also affect bulb size, quality, and storability. Managing onion diseases early and using preventive methods is the key to getting a healthy and profitable crop.
1. Major diseases of onion
Several diseases commonly attack onion crops, especially under humid or poorly managed conditions. The most common ones are:
a) Purple Blotch (Alternaria porri)
Symptoms: Small water-soaked spots on leaves that turn purple or brown with yellow margins. Leaves dry up from the tips downward.
Favourable conditions: Warm and humid weather promotes rapid spread.
Management: Remove infected leaves and avoid overhead irrigation. Spray recommended fungicides like mancozeb or propiconazole at the first sign of infection.
b) Stemphylium Blight
Symptoms: Yellowish-brown spots appear on older leaves, later turning dark and causing leaf tip drying.
Management: Avoid overcrowding, maintain good air circulation, and spray protective fungicides when the disease appears.
c) Downy Mildew (Peronospora destructor)
Symptoms: Pale yellow patches on leaves with a greyish-purple fungal growth on the lower surface. Leaves bend, collapse, and die prematurely.
Favourable conditions: Cool, moist, or foggy weather.
Management: Ensure good drainage, avoid late evening irrigation, and spray fungicides such as metalaxyl or copper oxychloride to prevent spread.
d) Basal Rot (Fusarium oxysporum)
Symptoms: Yellowing and wilting of leaves, followed by rotting at the base of the bulb. When pulled out, the roots appear black or decayed.
Management: Use disease-free bulbs and practice crop rotation for at least 3 years. Avoid waterlogging and apply biofungicides like Trichoderma harzianum during soil preparation.
e) Bacterial Soft Rot
Symptoms: Soft, watery decay in bulbs with a foul smell. The rot often starts at the neck or wounds.
Management: Avoid mechanical injury during harvesting and drying. Cure bulbs properly before storage and avoid excessive moisture.
f) Onion Smut and White Rot
These are soil-borne diseases that affect seedlings and roots. The best control is through crop rotation, solarization, and soil treatment before sowing.
2. Integrated disease management practices
Seed treatment: Treat seeds with fungicides or biocontrol agents like Trichoderma before sowing to prevent seed-borne diseases.
Crop rotation: Do not grow onion or related crops like garlic continuously in the same field. Rotate with cereals or pulses to break the disease cycle.
Field sanitation: Remove and destroy diseased plants and debris after harvest. This prevents pathogens from surviving in the soil.
Proper irrigation: Overhead or frequent watering increases humidity and disease spread. Use furrow or drip irrigation instead.
Balanced fertilization: Avoid excessive nitrogen, as it encourages soft tissue that is more prone to infection.
Resistant varieties: Use locally recommended disease-tolerant or resistant varieties wherever available.
Storage management: After harvest, cure bulbs properly in shade for 10–15 days before storing. Keep storage rooms well-ventilated and dry.
3. Biological and eco-friendly options
Farmers can use Trichoderma viride or Pseudomonas fluorescens to treat soil and roots at sowing or transplanting. Neem-based products can also help reduce fungal infection while being safe for the environment.
Conclusion
Diseases in onion can spread quickly and cause heavy losses if not managed on time. The best approach is to focus on prevention — healthy seeds, crop rotation, proper irrigation, and good field hygiene. Combine biological control and safe fungicide use as needed. With regular field monitoring and timely action, farmers can protect their onion crop, improve yield, and ensure better quality bulbs that fetch higher market prices.
