Downy mildew of lettuce

The causative agent of this disease is the oomycete Bremia lactucae, which leads to downy mildew infection. This obligate parasite targets the living tissues of the host plant, entering via stomata. It is widely considered one of the most economically damaging diseases for lettuce production globally due to its rapid spread and ability to evolve.

Lettuce is the primary host for Bremia lactucae, although the pathogen can also infect various other species within the Asteraceae family. These alternate hosts often serve as a continuous source of inoculum for commercial fields. The pathogen is highly adaptable, producing many physiological races that can overcome host resistance genes.

Symptoms typically appear as yellowish, angular spots on the upper leaf surfaces, confined by leaf veins. Under humid conditions, a characteristic white to greyish downy fungal growth appears on the underside of these spots. As the infection progresses, these areas necroticize and turn brown, rendering the lettuce leaves unsellable and reducing yields significantly.

The development of the disease is strictly dependent on high humidity and free water on the leaf surface. Cool temperatures, ranging between +10°C and +20°C, are optimal for spore production and germination. Because spores are easily dispersed by wind and splashing water, epidemics can occur very rapidly during periods of prolonged morning dew or overcast weather.

Effective management requires an integrated approach. The most successful strategy is the utilization of resistant lettuce varieties, although these must be updated regularly to counter new strains. Cultural practices include ensuring proper spacing to increase airflow, using subsurface or drip irrigation to keep foliage dry, and meticulous sanitation of debris. Chemical fungicides may be used as a supplementary measure under high disease pressure.