Downy mildew

Downy mildew, caused by the pathogen Pseudoperonospora cubensis, is one of the most destructive diseases affecting cucurbit crops worldwide. This oomycete thrives in humid conditions and can cause severe defoliation, leading to significant yield losses in cucumbers, squash, and melons.

The pathogen primarily attacks the leaves of the host plants. Because it is highly contagious and can be wind-dispersed, an infection can spread rapidly across a field or greenhouse if left unmanaged, rendering affected plants unproductive.

Typical symptoms begin as small, pale green spots on the upper leaf surface, which eventually become angular and yellow due to the restriction by leaf veins. Under humid conditions, a dark grayish or purplish fuzzy growth appears on the undersides of these spots, representing the mass of sporangia.

The development of the disease is favored by temperatures between 15°C and 25°C and high humidity (above 80%). Prolonged leaf wetness, such as dew or rainfall, is essential for the zoospore infection process, making irrigation timing and ventilation critical management factors.

• Planting resistant or tolerant cultivars.
• Increasing plant spacing to improve air circulation.
• Avoiding overhead irrigation to reduce leaf wetness duration.
• Applying preventive and curative fungicides when conditions are favorable for disease.
• Removing and destroying crop residues immediately after harvest.