📈 Diseases of Onion and Garlic

Onion (Allium cepa) and garlic (Allium sativum) are among the most important bulb crops grown in India. Both crops are susceptible to several fungal, bacterial, and viral diseases that cause major pre-harvest and post-harvest losses.

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Purple Blotch

Causal Organism: Alternaria porri (Ellis) Cif.

This is the most common and widespread foliar disease of onion and garlic in India.

Symptoms

• Small, water-soaked lesions on leaves that enlarge into elliptical, purplish-brown spots
• Spots have concentric zonation and are surrounded by a yellow halo
• Severely affected leaves dry up from the tip downward
• On seed stalks, lesions cause stalk bending and poor seed set

Favourable Conditions

Factor	Detail
Temperature	25-30 degrees C
Humidity	Above 80%
Rainfall	Frequent rains during crop growth
Thrips injury	Wounds from thrips feeding facilitate fungal entry

Management

• Spray Mancozeb (0.25%) or Iprodione (0.2%) at 10-15 day intervals starting from 30 days after transplanting
• Control thrips with Fipronil or Lambda-cyhalothrin to reduce wound-mediated infections
• Maintain adequate plant spacing for good air circulation
• Use resistant varieties such as Arka Kalyan and Arka Niketan

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Stemphylium Blight

Causal Organism: Stemphylium vesicarium (Wallr.) Simmons

Symptoms

• Small, light yellow to orange, oval spots on leaves that rapidly enlarge
• Lesions become dark brown to black and merge, causing extensive leaf blight
• Leaves die from the tip downward; young seedlings may be killed
• Often occurs in association with purple blotch

Management

• Spray Mancozeb (0.25%) or Chlorothalonil (0.2%) beginning at the first sign of disease
• Remove and destroy infected crop debris
• Avoid excessive irrigation that maintains prolonged leaf wetness

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Basal Rot

Causal Organism: Fusarium oxysporum f. sp. cepae

Symptoms

• Yellowing and wilting of leaves from the tip, beginning with the outer leaves
• White mycelial growth at the base plate of the bulb
• Basal plate becomes soft and brown; roots decay
• Bulbs become watery and rot during storage

Management

• Use disease-free sets and transplants
• Seed treatment with Carbendazim (2 g/kg) or Trichoderma viride (4 g/kg)
• Practice crop rotation for 4-5 years with non-allium crops
• Avoid waterlogging and ensure good drainage

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Onion Smut

Causal Organism: Urocystis cepulae Frost

Symptoms

• Dark, raised, elongated blisters (sori) on cotyledons and first leaves of seedlings
• Blisters rupture to release black, powdery spore masses (teliospores)
• Infected seedlings are stunted and may die; older plants are rarely affected

Management

• Treat seed with Thiram (3 g/kg) before sowing
• Soil application of Thiram or Carbendazim in the nursery bed furrow
• Transplanting is safer than direct seeding in endemic areas

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White Rot

Causal Organism: Sclerotium cepivorum Berk.

Symptoms

• Yellowing and wilting of foliage; plants are easily pulled from the soil
• Dense, white, fluffy mycelium covers the base plate and roots
• Small, black sclerotia (0.5-1 mm) develop on the decaying tissue
• Sclerotia persist in the soil for over 20 years

Management

• Long crop rotation (at least 8-10 years) without allium crops
• Soil fumigation with Metham sodium in heavily infested fields
• Apply Iprodione or Tebuconazole as a seed-furrow drench
• Avoid moving infested soil or planting material between fields

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Summary Cheat Sheet

Disease	Causal Organism	Hallmark Symptom	Core Management
Purple blotch	Alternaria porri	Purplish concentric leaf lesions	Fungicidal spray + thrips control
Stemphylium blight	Stemphylium vesicarium	Rapid leaf blight from tip	Early fungicide and sanitation
Basal rot	Fusarium oxysporum f. sp. cepae	Basal plate rot and root decay	Healthy sets + rotation + drainage
Onion smut	Urocystis cepulae	Black blisters on seedlings	Seed treatment + nursery hygiene
White rot	Sclerotium cepivorum	White mycelium and black sclerotia at base	Long rotation + sanitation + drench

References

2 sources • [1] [2]

[1]

Indian Council of Agricultural Research (ICAR)

Official

[2]

CABI Crop Protection Compendium

Website