Downy mildew of corn salad

The causative agent of the disease is the oomycete Peronospora valerianellae, which is responsible for downy mildew. This pathogen is highly host-specific, primarily targeting plants within the Valerianaceae family, particularly Valerianella locusta, commonly known as corn salad or lamb's lettuce.

The disease primarily affects corn salad crops grown in both greenhouse conditions and open fields. It is a significant threat during periods of cool, moist weather which are typical for the growing season of this leafy green, often compromising the entire yield of the crop.

Symptoms typically include chlorotic, yellowing patches on the upper surface of leaves, often bounded by the leaf veins. In conditions of high humidity, a dense, grayish-purple mold develops on the underside of the leaves, which corresponds to the conidiophores of the pathogen releasing spores.

Development and spread are favored by temperatures between 12°C and 18°C and high relative humidity. The spores are easily dispersed by wind, rain splashes, and irrigation water, facilitating rapid transmission between plants and neighboring fields within a short timeframe.

The economic impact is severe as the disease ruins the marketable quality of the leaves. Infected plants show accelerated wilting and necrosis, leading to complete crop failure in favorable conditions for the pathogen, rendering the produce unsuitable for sale or consumption.

• Implement strict crop rotation patterns to break the pathogen's life cycle.
• Select resistant varieties of corn salad whenever they are commercially available.
• Maintain low planting density to promote airflow and decrease leaf moisture duration.
• Ensure efficient greenhouse ventilation to keep relative humidity below critical levels.
• Utilize appropriate fungicide applications at the first sign of symptoms, adhering to pre-harvest intervals.