What to Do When Lettuce Gets Downy Mildew? Detailed Field Control Measures

Lettuce (scientific name: Lactuca sativa L. var. angustana Irish.), also known as celtuce, is a variety of the lettuce species in the genus Lactuca of the Asteraceae family that can form a fleshy tender stem, and is an annual or biennial herbaceous plant. It is also known by the aliases stem lettuce, asparagus lettuce, green lettuce, and wosun.

Morphological characteristics of lettuce

The stem looks like rings stacked one upon another, resembling a bamboo shoot. It has a taproot system, and after transplanting it develops many lateral roots that are shallow and dense, mainly distributed in the 20-30cm soil layer. The stem is shortened. The leaves are alternate, lanceolate or long ovate, with colors ranging from light green, green, dark green, or purplish red; the leaf surface is flat or wrinkled, with entire or notched margins. The shortened stem gradually elongates and thickens as the plant grows; after the stem tip differentiates flower buds, the stem thickens while the flower stalk elongates, forming a club-shaped fleshy tender stem. The flesh color is light green, emerald green, or yellow-green. The inflorescence is a conical capitulum with pale yellow flowers; each inflorescence has about 20 flowers and is self-pollinating, though cross-pollination sometimes occurs. The achene is dark brown or silvery white, with a pappus attached.

Dietary therapeutic effects of lettuce

Promoting digestion and relieving stagnation: Lettuce has a fresh taste with a slight bitterness that can stimulate digestive enzyme secretion and increase appetite. Its milky sap can enhance gastric juice, digestive gland secretion, and bile secretion, thereby promoting the function of various digestive organs, and is especially beneficial for patients with weakened digestive function, reduced acidity in the digestive tract, and constipation.

Diuresis and promoting lactation: The potassium content of lettuce is much higher than its sodium content, which helps maintain water and electrolyte balance in the body and promotes urination and milk secretion. It has certain dietary therapeutic effects on hypertension, edema, and heart disease patients.

Strengthening the body and preventing and fighting cancer: Lettuce contains a variety of vitamins and minerals and has the effect of regulating nervous system function. Among its organic compounds, it is rich in iron that the human body can absorb, making it very beneficial for patients with iron-deficiency anemia. The hot water extract of lettuce has a high inhibition rate against certain cancer cells, so it can also be used to prevent and fight cancer.

Laxative effect: Lettuce contains a large amount of plant fiber, which can promote intestinal wall peristalsis, facilitate the digestive tract, and help with bowel movements, and can be used to treat various types of constipation.

Field control measures for lettuce downy mildew

In recent years, with the expansion of lettuce planting area, lettuce downy mildew has become a relatively serious disease in spring and autumn cultivation, seriously affecting the quality and yield of lettuce and reducing economic benefits.

Field symptoms of lettuce downy mildew:  

The disease mainly damages leaves. It first appears on the leaves near the ground, then gradually spreads upward, developing from the outer leaves inward. Initially, pale yellow spots appear on the leaves; as the lesions expand, they are restricted by the leaf veins and become polygonal or irregular in shape. In severe cases, the lesions die and connect into patches, causing the entire leaf to turn yellow and dry up, seriously affecting plant growth. The typical symptom of this disease is the white downy mold layer that grows on the lesions on the underside of the leaves when it is humid.

Conditions for the onset of lettuce downy mildew  : The pathogen overwinters as mycelium inside seeds, or as oospores with diseased residues in the soil. The disease spreads through wind and rain, invading through the plant's stomata. The suitable temperature for infection is 15-17°C. It is more severe in rainy spring and autumn seasons. Overly dense planting, high field humidity, excessive watering, and poor drainage all make this disease more likely to occur.

Effective field control measures for lettuce downy mildew  :

Select disease-resistant varieties: Generally, pointed-leaf lettuce is more disease-resistant than round-leaf lettuce, such as white pointed-leaf lettuce, etc.  

Strict crop rotation and ridge cultivation.

Reasonable dense planting and maintaining field ventilation and light transmission.   Generally maintain a row spacing of 35-40cm and a plant spacing of 25cm, with about 6,000 plants planted per 667 sq m.  

Strengthen field management. Before the field plants close their rows, carry out timely intertillage, and increase the number of intertillage operations during the seedling stage. Water more in the early stage and less in the later stage. After the plants close their rows, reduce the frequency of watering, lower field humidity, prevent water accumulation in the field, promote plant growth, and inhibit pathogen development.  

Observe timely, detect the disease early, and clean the field promptly. Remove and burn diseased leaves in time, especially the lower old leaves, to increase air and light penetration.  

Use chemical control. Choose 58% metalaxyl-mancozeb wettable powder at 500 times dilution, 40% ethylicin wettable powder at 200 times dilution, or 72% Antaisheng wettable powder at 700 times dilution for spraying, alternating the chemicals, once every 7-10 days, spraying 2-3 times continuously, which can effectively control lettuce downy mildew and improve the yield and quality of lettuce.