Downy mildew of Lettuce

Bremia lactucae

Bremia lactucae is a pathogenic oomycete responsible for causing downy mildew in lettuce crops. Downy mildew of lettuce, caused by Bremia lactucae, is a common and economically significant disease that affects various lettuce varieties. Here's an informative description of Bremia lactucae, covering its appearance, identification, and management:

1. Symptoms:

  • Leaf Lesions: Initial symptoms typically appear as small, pale green to yellowish lesions on the upper surface of lettuce leaves.
  • Downy Growth: On the undersides of infected leaves, a characteristic downy growth (sporulation) develops. This downy growth is often white and can be observed as fluffy patches.
  • Lesion Expansion: Lesions may enlarge rapidly, and severe infections can lead to necrosis and distortion of the affected leaves.

2. Disease Cycle:

  • Infection: The pathogen initiates infection through sporangia, which are produced on infected plant tissues.
  • Spore Dispersal: Sporangia are dispersed by wind, water, or other means, leading to the infection of nearby lettuce plants.
  • Germination: Upon reaching a susceptible host, sporangia germinate to release zoospores, which then infect new plant tissues, completing the cycle.
  • Survival: The pathogen can survive between growing seasons in infected plant debris and is favored by cool, humid conditions.

3. Identification:

  • Leaf Symptoms: Identification often begins with the observation of small lesions on the upper leaf surface and the characteristic downy growth on the undersides of leaves.
  • Fluffy Patches: The white, fluffy appearance of the downy growth is a key identifier.
  • Lesion Patterns: Lesions may exhibit angular or irregular patterns on the leaves.

4. Management:

  • Resistant Varieties: Planting lettuce varieties that are resistant or less susceptible to downy mildew is an effective strategy.
  • Fungicide Applications: Fungicides, especially those effective against oomycetes, can be applied preventively or curatively. Application timing is crucial, and a rotation of different fungicide classes is recommended to reduce the risk of resistance development.
  • Cultural Practices: Practices such as spacing plants to enhance air circulation, avoiding overhead irrigation, and managing nitrogen levels can help create less favorable conditions for disease development.
  • Sanitation: Removal and destruction of infected plant debris contribute to reducing the source of inoculum.

5. Importance:

  • Economic Impact: Downy mildew caused by Bremia lactucae can lead to significant yield losses and reduce the quality of lettuce crops.
  • Global Distribution: The pathogen is widespread and can affect lettuce crops in various regions.