Downy mildew
Bremia lactucae
What is Downy mildew (Bremia lactucae)?
Bremia lactucae, a complex organism with multiple races, infects lettuce and other plants such as artichoke, cornflower, and strawflower. It exhibits host-specific forms, generally infecting only their original hosts. Symptoms of downy mildew include light green to yellow angular spots on upper leaf surfaces with white fluffy growth on the lower sides. Lesions turn brown and dry up over time, primarily affecting older leaves. In severe cases, leaves may die, and systemic infection can cause stem discoloration. Cotyledon infection can be fatal for young seedlings, and greenhouse-grown lettuce transplants are also susceptible.
How does Downy mildew (Bremia lactucae) occur?
Bremia lactucae reproduces and spreads through the production of spores. The spores are dispersed by wind, water, or human activity. Infection occurs when the spores land on susceptible plant surfaces, particularly on lettuce leaves. Under favorable conditions of high humidity and leaf wetness, the spores germinate and penetrate the plant tissues, establishing the infection. The pathogen reproduces within the plant, producing new spores that are released and spread to other plants, continuing the cycle of infection and reproduction.
Symptoms
1 - Impacts on Plants
Mycosphaerella species cause reduced vigor in plants, leading to stunted growth, decreased yield, and low-quality produce. They also inflict leaf damage and defoliation resulting in leaf death and potential defoliation.
2 - Impacts on Environment
Mycosphaerella species cause ecological disruption by disrupting plant populations, thereby impacting biodiversity and ecological interactions within the affected ecosystem.
Solutions
1 - Irrigation controls and Greenhouse protection
• Use irrigation systems like drip irrigation to minimize leaf wetness and humidity, which can help reduce disease severity. • Implement measures to protect greenhouse-grown transplants from downy mildew to prevent its introduction into the field.
2 - Resistant cultivars
• Optimal control: Growing resistant cultivars is the most effective method for managing downy mildew. • Availability limitations: Resistant cultivars may not be available for all areas or seasons. • Adaptability concerns: Pathogen variability can lead to new virulent strains that can overcome resistance over time.
3 - Fungicide application
• Several fungicides are available for controlling downy mildew caused by Bremia lactucae, including systemic options like metalaxyl/mefenoxam and protective options like cymoxanil. These fungicides inhibit the growth and spread of the pathogen, providing effective control. Follow recommended guidelines for fungicide application to ensure effective control.
