Purple Blotch: A Fungal Infection in Allium Crops

Purple blotch is a type of fungal infection caused by a fungus known as Alternaria porri. This infection can affect various parts of the crops above the ground, including the bulb. Allium crops, such as onion and garlic, are particularly vulnerable to this disease, leading to significant losses in yield.

Favorable Environmental Conditions

The growth of the pathogen is facilitated by warm and moist environmental conditions. These conditions provide an ideal habitat for the fungus to thrive and spread.

Disease Cycle

Overwintering and Spore Dispersal

During winter, the pathogen survives in crop residue, either on or near the soil surface. As the humidity decreases during the night, the fungus produces spores that are released in the morning. These spores can be carried by wind or splashing rain to healthy plants, initiating new infections.

Infection and Symptoms

Early Infection

Upon infection, small water-soaked lesions develop on the older leaves. These lesions have whitish centers and gradually enlarge as the infection progresses. The margins of the lesions become purplish and display light yellow concentric rings.

Advanced Infection

If left uncontrolled, the disease can cause yellowish-brown discoloration of leaves, loss of erectness, and wilting. Younger leaves become more susceptible to infection as the bulb matures. Additionally, during harvesting, if the pathogen enters neck wounds, it can infect the bulbs, making them papery over time.

Management and Control

Given the significant losses associated with purple blotch infection, it is crucial to implement disease control measures. There are several methods available for management and control:

Chemical Control

Several fungicides have proven to be effective against purple blotch due to their preventive and curative properties. Recommended fungicides include:

  • TRINITY GOLD 425WP: Mix 50g in 20 liters of water
  • ABSOLUTE 375SC: Mix 10ml in 20 liters of water
  • COLONIZER 440WP: Mix 50g in 20 liters of water
  • TOWER 720WP: Mix 50g in 20 liters of water
  • MILLIONAIRE 690WDG: Mix 40g in 20 liters of water
  • GEARLOCK TURBO 250WP: Mix 25g in 20 liters of water
  • PYRAMID 700WP: Mix 50g in 20 liters of water
  • DOMAIN 250EC: Mix 10ml in 20 liters of water

It is advisable to add INTEGRA at a rate of 3ml per 20 liters of the fungicide mixture. INTEGRA is a sticker, penetrant, and spreader that enhances the effectiveness of the fungicide. To prevent the pathogen from developing resistance, it is recommended to alternate between different fungicides instead of relying on a single chemical throughout the crop season. Timely control and management of the pathogen are crucial.

Other Methods

Apart from chemical control, other methods can help prevent and manage purple blotch:

  • Use certified or disease-free seeds for planting.
  • Practice crop rotation with non-host crops.
  • Maintain field sanitation by removing crop residues and debris.
  • Implement proper weed control measures.
  • Consider using resistant varieties that are less susceptible to purple blotch.


Purple blotch, caused by the fungus Alternaria porri, is a fungal infection that affects allium crops, leading to significant losses. By implementing effective management strategies, such as chemical control and other preventive measures, farmers can reduce the impact of this disease on their crops and improve overall yield.

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