Downy mildew of lettuce

Downy mildew, caused by the oomycete Bremia lactucae, is one of the most destructive diseases affecting lettuce crops worldwide. It is capable of attacking the plant at any growth stage, significantly impacting both the yield and the aesthetic marketability of harvested leaves.

The pathogen primarily targets lettuce varieties within the Lactuca genus. A major challenge for growers is the high variability of the fungus, which constantly evolves into new races. This capability allows the pathogen to overcome the resistance of cultivars that were previously considered resistant or tolerant to the infection.

Early symptoms manifest as pale green or yellow angular spots on the upper leaf surface, limited by the leaf veins. Under humid conditions, a whitish or greyish fluffy growth of sporangia appears on the underside of these spots. As the infection progresses, these areas turn brown, necrotic, and eventually merge, leading to leaf decay.

Disease development is favored by cool temperatures, typically between 15°C and 20°C, and high relative humidity. The presence of free moisture on the leaf surface is essential for the germination of sporangia. Dissemination occurs primarily via wind-borne spores, splashing water during irrigation, or movement of contaminated tools and equipment.

Effective management requires an integrated approach to minimize crop losses. This involves combining biological, cultural, and chemical practices to lower disease pressure. Key prevention and control measures include:

• Planting resistant or tolerant lettuce cultivars.
• Improving greenhouse ventilation to reduce leaf wetness.
• Ensuring proper crop rotation to break the pathogen's life cycle.
• Regular scouting and localized application of fungicides if needed.
• Maintaining proper spacing to enhance air circulation and reduce humidity.