Downy mildew of Brassicas

Peronospora parasitica

Peronospora parasitica is a water mold (oomycete) that causes downy mildew in plants, particularly in the Brassicaceae family, which includes cruciferous vegetables such as cabbage, broccoli, and mustard. The disease is commonly known as downy mildew of crucifers or crucifer downy mildew. Here is an informative description of Peronospora parasitica, including its appearance, identification, and management:

1. Symptoms:

  • Leaf Lesions: The most common symptoms are the appearance of yellow-green lesions on the upper surface of leaves.
  • Underside Growth: On the undersides of leaves, a downy, grayish-white mass may develop, consisting of the reproductive structures (sporangiophores and sporangia) of the pathogen.
  • Leaf Curling: Infected leaves may exhibit curling or distortion, and severe infections can lead to defoliation.

2. Disease Cycle:

  • Infection: Peronospora parasitica thrives in cool, humid conditions. The pathogen initiates infection through sporangia, which are produced on infected plant tissues.
  • Spore Dispersal: Sporangia are dispersed by wind, rain, or other means, and they can infect nearby plants under favorable conditions.
  • Germination: Upon reaching a susceptible host, sporangia germinate to release zoospores, which then infect plant tissues, perpetuating the cycle.
  • Survival: The pathogen can survive between growing seasons in infected plant debris.

3. Identification:

  • Yellow Lesions: Identification often begins with the observation of yellow lesions on the upper surface of leaves.
  • Downy Growth: The characteristic downy growth on the undersides of leaves is a key identifier.
  • Laboratory Analysis: Microscopic examination of infected tissues may reveal the presence of sporangia, aiding in definitive identification.

4. Management:

  • Resistant Varieties: Planting resistant cultivars can be an effective strategy in managing downy mildew.
  • Crop Rotation: Crop rotation helps reduce the build-up of inoculum in the soil.
  • Fungicide Applications: Fungicides, especially those effective against oomycetes, can be applied preventively or curatively. Application timing is critical.
  • Sanitation: Removal and destruction of infected plant debris contribute to reducing the source of inoculum.
  • Avoiding Overhead Irrigation: Minimizing prolonged leaf wetness by avoiding overhead irrigation can help create an unfavorable environment for the pathogen.

5. Importance:

  • Economic Impact: Downy mildew caused by Peronospora parasitica can lead to significant yield losses and reduced quality of cruciferous crops.
  • Global Distribution: The pathogen has a wide geographical distribution, affecting cruciferous crops in various regions.