Pests & Diseases
Lettuce is susceptible to several pests and diseases including aphids, caterpillars, slugs, armyworms, imported cabbageworms, cabbage loopers, leafhoppers, leafminers, lygus bugs, and thrips. Pests are more challenging to summer and fall crops than spring.
Lettuce aphids, identified by their multiple color schemes and various black markings, tend to burrow into the lettuce; they feed at or near the center of the plant, sucking out nutrients and causing the leaves to shrivel. While insecticides can help control aphid populations, field prep before planting can prevent insects from taking hold.
Caterpillars, particularly the saltmarsh caterpillar, tend to affect Southern California and other southerly areas of the United States. Larvae will often feed on leaves; insecticides are recommended, but physical barriers can be used to keep the insects away from crops.
Slugs tend to feed on leaves, creating holes, stripping stems, and leaving slime trails. Though generally cosmetic in nature, extensive damage can kill plants. Watering plants early in the morning or using drip irrigation will prevent excess moisture, which attracts slugs.
Armyworms feed on any part of the plant aboveground. Beet armyworms will typically feed down the leaves to crowns, killing seedlings and reducing crown formation.
Southern armyworms feed on nectar in groups before dispersing to other plants in the vicinity. Imported cabbageworms feed on the underside of leaves and cause holes. As they age, cabbageworms tend to move from outer to inner leaves, eventually reaching the head. Damage will prevent proper formation and can kill plants. Cabbage loopers feed on outer leaves, making large holes and/or damaging the head.
Leafhoppers typically cause minimal injury but spread various diseases. Weeds and infected plants should be removed to prevent the spread. Leafminers are rarely considered a major pest anywhere but Florida, having grown resistant to certain chemicals in the state. Once eggs are deposited in holes in leaves, newborns feed on both sides of leaves as they head to the base of the plant.
Lygus bugs, also known as tarnished plant bugs, are born inside the tissue of plants. Larvae create holes that expand into lesions and can eventually kill the plant. In large numbers, thrips will cause malformed leaves and spread disease.
Seedlings and young plants are susceptible to damping-off. Seedling stems will thin and leaves will wilt and turn brown. Proper soil temperature and sterilizing field tools can help control the disease.
Mature plants can fall victim to sclerotinia or lettuce drop, mildew, grey mold, and Rhizoctonia bottom rot. Mildew, whether downy mildew or powdery mildew, causes white lesions on leaves and stems that turn brown, shrivel, and die. Younger plants may not survive the infection.
Damp, humid areas serve as an ideal breeding ground. Butter and leaf varieties are more resistant than romaine or iceberg.
Tipburn in mature plants can occur from changes in temperature or soil moisture causing leaves to brown, affecting appearance. Too much heat or moisture can lead to rot.
Maintaining nutrient levels and proper irrigation will keep it at bay, along with resistant cultivars. While it is commonly believed tipburn is the result of calcium deficiency, studies have shown this is not the case.
Storage & Packaging
All types of lettuce are harvested and packed in the field; heads are placed into waxed cartons in counts of 20 to 24. To avoid crushing, pack cartons in two layers: a bottom layer stem down and a top layer stem up to keep the stem milk from damaging the leaves. Some varieties, including leaf, butter and romaine hearts, are put in bags and packed 24 to 36 per carton.
Depending on the crop, harvest mature heads about every 2 to 3 days. Good crops yield about 400 to 500 crates of iceberg or 800 to 1,000 crates of leaf lettuce per acre.
Harvested lettuce must be cooled prior to shipping. Vacuum cooling to quickly lower head temperatures to 34 to 38°F is recommended. Hydrocooling and forced-air cooling are also acceptable.
Cartons of iceberg stored at 32°F and 95% relative humidity will last 2 to 3 weeks, sometimes longer if the head is bagged and the leaves are dry. Leaf and butter lettuces will last 1 to 2 weeks in cold storage.
References: Clemson University Cooperative Extension, Illinois Integrated Pest Management Program, University of Arizona Cooperative Extension, University of California Agriculture & Natural Resources, University of Florida/IFAS Extension, USDA.