Diseases of Onion

Purple Blotch

Causal Organism:

• Alternaria porri
• Sub-division: Deuteromycotina

Etiology:

• Fungus produces air-borne conidia that spread through wind or splashing water.
• Survives on plant debris and seed bulbs, serving as primary sources of inoculum.

Symptoms:

• On Leaves:
  • Circular to oblong lesions with concentric velvety rings.
  • Yellow halo may appear around lesions.
  • Infected leaves break at the point of infection and hang down.
  • Severely infected plants dry up completely.

Disease Cycle:

• Primary Infection: Pathogen survives in plant debris and seed bulbs.
• Secondary Infection: Spread through air-borne conidia by wind or rain splash.

Favorable Conditions: Warm humid weather (20-30°C) with rains or heavy dew. High relative humidity (>80%).

Management:

• Cultural Practices:
  • Use disease-free bulbs for planting.
  • Remove and destroy plant debris after harvest.
  • Practice crop rotation with non-host crops.
• Seed Treatment: Treat seeds with Carboxin + Thiram @ 2g/kg seed.
• Chemical Control: Spray Mancozeb @ 0.25% at regular intervals during favorable conditions. Repeat applications every 10-15 days.

Stemphylium Leaf Blight

Causal Organism:

• Stemphylium vesicarium
• Teleomorph: Pleospora allii
• Sub-division: Deuteromycotina

Etiology:

• Produces air-borne conidia that are dispersed by wind.
• Survives on plant debris and seed bulbs.
• Perithecia (fruiting bodies) may form in infected tissues under favorable conditions.

Symptoms:

• On Leaves and Leaf Sheaths:
  • Initial lesions are small, light yellow to brown, and water-soaked.
  • Lesions coalesce, leading to extensive blighting.
  • Higher lesion density on the side facing the prevailing wind.
  • Centers of lesions change color from brown to tan, then dark olive brown, and finally black.
  • Perithecia may appear as small, black, pinhead-like raised bodies.

Disease Cycle:

• Primary Infection: Pathogen survives in plant debris and seed bulbs.
• Secondary Infection: Spread by air-borne conidia through wind or rain splash.

Favorable Conditions:

• Warm humid weather with rains or heavy dew.
• Optimal temperature range: 20-25°C.
• High relative humidity (>80%).

Management:

• Cultural Practices: Crop rotation to minimize pathogen buildup. Destroy infected plant debris after harvest.
• Chemical Control:
  • Spray Carbendazim + Mancozeb @ 0.25%.
  • Apply at the first sign of disease and repeat every 10-14 days under favorable conditions.