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Phytophthora porri

Phytophthora porri:

Phytophthora porri is a water-mold pathogen responsible for causing diseases in various Allium species, including onions and leeks. The diseases it causes are commonly known as downy mildew or purple blotch. Here's an informative description:

Appearance and Symptoms:

  1. Purple-Brown Lesions: Initial symptoms include the development of small, water-soaked lesions on leaves. As the lesions progress, they often turn purple-brown and can have a characteristic rectangular shape.

  2. Lesion Expansion: Lesions may expand, coalesce, and cover significant portions of the leaf surface.

  3. White Fungal Growth: Under humid conditions, a white, downy, fungal growth may appear on the lower surface of infected leaves.

  4. Leaf Death: Severe infections can lead to the death of entire leaves, affecting the overall health and yield of the plant.

Identification:

  1. Lesion Appearance: Identify the characteristic purple-brown lesions on the leaves of Allium crops.

  2. Fungal Growth: Look for the white downy fungal growth on the lower leaf surface, especially in humid conditions.

Disease Cycle:

  1. Survival: The pathogen can survive in infected plant debris and soil.

  2. Spore Production: Sporangia are produced on infected leaves, and these can be dispersed by wind and water.

  3. Infection: Infection occurs through the stomata of leaves during wet and humid conditions.

Management:

Cultural Practices:

  1. Crop Rotation: Avoid planting Allium crops in the same location consecutively.

  2. Sanitation: Remove and destroy infected plant debris to reduce the inoculum in the field.

  3. Avoid Overhead Irrigation: Minimize leaf wetness by using drip irrigation or other methods that keep foliage dry.

Resistant Varieties:

  1. Plant Resistant Varieties: Use onion or leek varieties that are resistant to Phytophthora porri when available.

Fungicide Application:

  1. Foliar Fungicides: Application of fungicides, especially during periods conducive to disease development, can help manage downy mildew.

Regular Monitoring:

  1. Field Scouting: Regularly monitor Allium fields for early symptoms of downy mildew, especially during periods of high humidity.