Lettuce (Lactuca sativa) is one of the most popular and widely consumed vegetables today, as well as historically. The Greeks, Persians, Romans, Egyptians, and Chinese considered lettuce a leafy delicacy and grew seeds for oil. Columbus brought lettuce and seeds on his voyage to the New World, as did most European explorers in their colonization efforts, rooting lettuce in America and most countries around the world. In the United States, the average person eats more than 25 pounds of lettuce each year.
Derived from wild, stemmy varieties, lettuce was cultivated over the centuries into what today are firmer heads with more leaves. Cultivation and breeding over the years has honed in on best features, including leaf shape, flavor, and resistance to disease and bolting. Newer hybrid strains have been bred to resist brown blight and mildew.
Iceberg lettuce, also called roundhead or crisphead, got its name in the early 1900s as it was shipped and stored in ice-filled containers. Romaine is so named because of its popularity in early Rome. Mesclun is not a variety itself, but a salad mix of baby lettuce and other greens including radicchio, arugula, mustard greens, and others.
References: Texas A&M AgriLife Extension Service, University of Arizona Cooperative Extension, University of Illinois Extension.
TYPES, VARIETIES & CUTS
Lettuce comes in four main varieties: leaf lettuce, including red and green varieties, where the thinner leaves and loose heads can range in taste from delicate to slightly bitter; butter lettuce including both Bibb and Boston, which is a sweeter variety when grown in cooler weather; romaine or cos, popular in Caesar salads, has dark leaves and a firm and crunchy leaf rib; and crisphead or iceberg has tightly packed leaves with a higher water content.
Top producers Arizona and California account for most U.S. production with New Jersey as a contributor as well. In Arizona, butter lettuce or butterhead, including Bibb and Boston, tend to be easier to grow than crisphead, though iceberg is the state’s leading lettuce. Romaine is second in production.
In California, butterhead, leaf, and romaine lead the charge. Within the butterhead type, a popular variety is Margarita. Common leaf varieties in the state include both green and red; greenleaf varieties consist of Big Star, Burgam’s Green, North Star, and Tropicana, while redhead varieties include Red Fox and Red Tide. Romaine varieties include Darkland, Green Towers, Progeny, and Sun Belt.
New Jersey’s Bibb and Boston classifications varieties include Esmeralda, Ermosa, and Optima. Common leaf types include Black Seeded Simpson, Grand Rapids, Salad Bowl, and Two Star (all green), and red leaf varieties consist of New Redfire, Red Express, and Red Sails. Under the romaine umbrella, popular varieties include Green Forest, Green Towers, Ideal Cos, and Pyramid Cos.
References: Agricultural Marketing Resource Center, California Dept. of Agriculture, Rutgers University, University of Arizona Cooperative Extension, University of California Division of Agriculture and Natural Resources, University of Maryland Extension.
PESTS & DISEASE
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 crops.
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 southern areas of the United States. Yellow-brown with long hairs and stripes as adults, the larvae will often feed on the leaves. Insecticides are recommended, but physical barriers can be used to keep the insects away from crops. Bacillus thuringiensis is a nonchemical alternative to control caterpillars. 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. Larvae tend to be dark green or grey and striped. 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. Introducing natural enemies, removing weeds, and applying insecticides can be used to control armyworms. Imported cabbageworms are green in their larval stage, feeding on the underside of lettuce leaves and causing holes. As they age, cabbageworms tend to move from the outer leaves to the inner ones, eventually reaching the center of the head. Damage will prevent proper head formation and can kill the plant. Removing weeds and insecticides can help combat cabbageworms (early afternoon applications are most effective).
Cabbage loopers are identified by their striped, white bodies in larval stage and dark colors and silver markings as adults. They commonly feed on a plant’s outer leaves, leaving large holes or reaching and damaging the head. Various insecticides, especially newer ones, are considered effective though introducing natural enemies is an option as well.
Leafhoppers, with white spots on their heads and thorax or black spots on hind wings, typically cause minimal injury, but spread various diseases. Weeds and infected plants should be removed to prevent additional the spread of disease.
Identifiable by black or grey coloring and yellow markings, leafminers are rarely considered a major pest anywhere but Florida, having grown resistant to certain chemicals in that particular state. Once eggs are deposited in holes in leaves, newborns will feed on both sides of the leaves as they head to the base of the plant. While the insects have developed resistance to many common pesticides, certain types are still effective in controlling populations.
Lygus bugs, also known as tarnished plant bugs, can range in color from yellow-green to bronze, with a triangle on the back. Born inside the tissue of the plant, larvae create holes that expand into lesions and can eventually kill the plant. Weed removal helps prevent infestations; insecticides and the introduction of natural predators such as spiders can reduce numbers.
In large numbers, thrips can easily cause malformed leaves and spread disease. Pale yellow, brown, or black in color, thrips easily migrate through crops. While various insecticides exist to reduce incidence, weeding and avoiding mowing can help.
Seedlings and young plants are susceptible to damping-off, which can occur when a number of pathogens infect seeds and cause them to suddenly rot and die. 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 be at risk for sclerotinia, mildew, grey mold, and Rhizoctonia bottom rot. Sclerotinia, also known as lettuce drop, causes leaves to wilt and grow fungus that can destroy the crown. Fungicides can help deter the disease, as well as plowing. 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 for the disease. Butter and leaf varieties are more resistant than romaine or crisphead.
Grey mold causes discoloration of leaves and can cause rot throughout the plant. Young or weak tissues are most susceptible; proper pruning and spacing between plants can help with prevention. Rhizoctonia bottom rot is a soil fungus that can cause damping-off for seedlings, along with dark, sunken lesions on heads that will often lead to collapse. Clearing fields can help prevent the disease, as well as consistent weeding and the use of fungicides.
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 can keep tipburn 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.
References: Clemson University Cooperative Extension, Cornell University Cooperative Extension, Illinois Integrated Pest Management Program, NC State University Cooperative Extension, Ohio State University Extension, University of Arizona Cooperative Extension, University of California Agriculture & Natural Resources, University of Florida/IFAS Extension.