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Lettuce Pests and Pest Control
Weeds
Herbicides recommended for use on lettuce will not provide total, season long weed control. Good weed control requires integration of cultural and chemical methods. Lettuce should be planted to land free of perennial weeds, where the annual weed seed population has been reduced by cultural practices such as crop rotation, fallowing or stale seedbed. Herbicides should be ordered early to avoid problems of short supply at planting. Care must be taken to avoid fields where residual herbicides from previous years persist in the soil as crop injury may occur.
Diseases
Gray Mold - (Botrytis) fungus
Characteristics: Gray mold can appear on plants at all stages although initial infection is often on seedlings in the greenhouse. Seedlings look like they have damping off; while older plants rot at the stem or on lower leaves in contact with soil. A slimy rot spread upwards into the head. A dense fuzzy gray mold appears on infected areas and dark, hard sclerotia (fruiting bodies) may also develop. Disease spreads mostly under moist conditions.
Control: Seed should be dressed with suitable fungicide. Use sterile seedbeds and flats and avoid thick stands of seedlings and transplants. Spray greenhouse seedlings at weekly intervals and once or twice later in the field at 10 day intervals. Use a 3 or 4 year crop rotation. Plow down crop refuse promptly after harvest. Orient rows in direction of prevailing winds.
Lettuce drop - (Sclerotinia) fungus
Characteristics: Symptoms begin on the stem near the soil surface. A severe wet rot develops rapidly and spreads downward to roots and upward through the head. Once the base of the leaf is rotted, the leaf wilts, withers and dies. Symptoms successively develop from outer to inner leaves. The head becomes a wet, slimy mass. During wet conditions, a white cottony mold develops on rotted plant parts; hard irregular black sclerotia (pea sized bodies) may occur in the mold.
Control: Sclerotia can persist for many years in the soil. Wet conditions favour disease. Avoid, if possible, land where clover, soybeans, snap or dry beans, lettuce, peas, tomatoes, carrots, cucurbits or cabbage have been grown recently. Practice a 3 year rotation with non-susceptible crops (grasses, corn, cereals, onions or beets). After harvest, disc the field promptly to destroy production of sclerotinia. Use row and plant spacings to encourage good air movement or plant on raised beds.
Dampening Off, Stunt (pythium spp. and other fungi)
Characteristics: This is a frequent problem in the greenhouse and the field. Lettuce is extremely susceptible. Seedling emergence may be poor. There may be sudden collapse and death of seedlings. Cool, damp conditions favour this disease. Soilless mixes can contain high levels of pythium unless properly treated before use.
Control: Use raised beds or well drained soils for early seeding or transplanting. Properly condition greenhouse soils before seeding and maintain favourable conditions for transplant growth. Use the best available fungicides for seed treatment or soil drenches in the greenhouse.
Downy Mildew - (fungus)
Characteristics: Usually only a problem in coastal areas and on late field lettuce. Downy mildew can affect seedlings or mature plants. Symptoms appear first on oldest leaves. Yellowish or light-green blotchy areas appear on the upper sides of leaves. A white, downy mold then appears on the undersides of the leaf spots; finally, the affected areas die. The fungus overwinters in crop residue. Spores are spread by wind. Spore production is greatest at temperatures below 19ºC and at relative humidities approaching 100%.
Control: At first sign of disease, spray with a fungicide at 7 to 10 day intervals, using a spreader-sticker. Carry out a 2 to 3 year rotation. Use cultivars with resistance to this disease.
Rhizoctonia Bottom Rot (fungus)
Characteristics: Infection occurs on lower leaves touching the ground. The disease progresses up into the head causing a dark brown, slimy decay. Later the head may dry out leaving a dry, mummified plant. The pathogen lives indefinitely in the soil.
Control: Avoid rotating potatoes and other susceptible crops with lettuce. Grow lettuce on 8 to 15 cm high ridges. Plow down crop residue immediately after harvest.
Aster yellows - (mycoplasma)
Characteristics: Yellowing and curling occur on the youngest leaves. At heading, head leaves are dwarfed and curled and heads remain soft. The mycoplasma can overwinter in many perennial weeds and is spread to lettuce by leafhoppers during their feeding.
Control: Eliminate weed hosts in field headlands and ditch banks. Do not plant lettuce adjacent to earlier lettuce plantings that contain infected plants. Grain, grass, carrot or celery crops should not be planted close to lettuce. Apply controls for the leafhopper vector.
Slime
Characteristics: A physiological disease of hot, humid weather, often aided by bacteria. The greatest single danger to mature lettuce, it produces a wet, slimy decay on lettuce in the field, in transit or in the market. Usually the large, internal leaves are affected first.
Control: Aim for a sequence of harvests at optimal maturity by successive sowings. Do not overcrowd plants and avoid overwatering. Harvest as soon as mature, pre-cool heads to 1ºC and keep them cool.
Pink Rib (non pathogenic disorder)
Characteristics: It first appears as a pink discoloration at the base of the mid veins of the lettuce leaves. This discoloration extends throughout the veins of the outer leaves and then extends into the younger leaves. The cause is unknown but high temperatures may be involved since it shows up most during the summer and much less in June, September and October harvests. This problem can cause a high loss of marketable yield in the Atlantic area.
Control: Crisp head lettuce is most affected during hot weather. Avoid July to mid August harvest schedules. Romaine lettuce and leaf lettuce seem to be much less prone to this disorder.
Insects
Cutworms
Characteristics: Cutworms are common pests in the Atlantic Provinces. Most of the damage is to newly set plants in the field. Plants are commonly cut off at ground level but some species can climb and feed on the lower leaves of the plants.
Control: See Atlantic Provinces 'Guide to Pest Management' for vegetable crops.
Aster (six-spotted) leafhopper
Characteristics: Leafhoppers can carry aster yellows from plant to plant as they feed. They are small (4 mm long), slender, wedge-shaped insects. They are greenish-yellow in color. Once a leafhopper feeds on an infected plant, it can spread it to all the other plants it feeds on. They feed on many other crops such as potatoes, tomatoes, celery, spinach, lettuce, onion and squash.
Control: Planting time treatments include granular or soil-injected insecticides. Alternatively, spraying regularly throughout the season can be done. Spray crop and field boundaries at weekly or twice-weekly intervals from time of transplanting or emergence of seedlings. Twice-weekly applications are only necessary during periods of heavy infestation, usually during late July and early August.
After harvest disk down crop residue immediately. Pest resistance can develop to insecticides so alternate pesticide groups. Monitor leafhopper numbers. Thresholds have been established.
Cabbage Looper
Characteristics: The cabbage looper gets its name from the way it forms a loop as it walks. It is a smooth green larvae with two white stripes along the back and two along the sides. The cabbage looper is capable of causing significant damage to lettuce. Cabbage loopers do not winter over in this region. Adult moths migrate into the region during the summer. Cabbage looper tends to be more problematic during late summer.
Control: Monitor moth numbers with traps and apply preventative insecticide sprays as necessary.
Aphids
Characteristics: Aphids are small, soft-bodied, slow- moving insects. They are often found in large colonies on the undersurface of leaves. A colony consists of winged and wingless adults and various sizes of nymphs. Aphids may be black, yellow or pink, but mostly are various shades of green. Lettuce aphids are small, green or pinkish and feed on the inner leaves and within the heads. Other aphids feed on the undersides of leaves and curl or stunt them.
Lettuce root aphids will also feed on the roots thereby stunting the plants especially under dry conditions.
Control: Use granular insecticides and/or foliar insecticides as necessary. Do not second crop lettuce unless insecticide is banded. Also, root aphid can be partly controlled by irrigation.
Tarnished Plant Bug
Characteristics: Tarnished plant bugs are small 6 mm long insects, somewhat oval in shape. They are mottled white and yellow with some black marking on the wings. They fly in from nearby hay fields or weedy areas along the margins of fields. They feed along the main veins of the leaf and cause browning along the lesions they make while feeding.
Control: Spray with an appropriate insecticide as needed.
Slugs
Characteristics: Slugs eat the leaves of the crop and cover it with unsightly slime tracks.
Control: Baits containing metaldehyde, a specific poison for slugs, can be used but are considered too costly in commercial plantings. Slugs prefer areas which are cool, moist and high in organic matter. Sod crops, weedy fence lines and hedgerows fulfil these conditions. Since slugs can over-winter fairly easily, cultural practices aimed at controlling them should begin at least one year before the susceptible crop is put in.
Wireworm
Characteristics: Early in the spring, adult wireworms (click beetles) lay their eggs around grass roots. The larvae hatch in about a week and, depending on the species, will live for 1 to 5 years in the ground feeding on roots and seeds. Wireworms require 3 or more years to complete their life cycle. Wireworms of all sizes and ages are present in the soil throughout the year as there is always an overlapping of generations. The wireworms, or larvae, are yellow, white or darker shades of brown. Fully developed larvae may be 1.2 to 4 cm long and have a hard, smooth surface. When a larva is mature, it pupates in the fall. It then becomes an adult beetle and waits until spring to emerge. Wireworms are often numerous in land that has been in sod for several years. They are also more abundant in heavy poorly drained soil.
Wireworms are sometimes confused with millipedes. Millipedes have numerous pairs of legs and coil up when disturbed, while wireworms have three pairs of legs near the front of the body and do not coil up.
Control: Avoid planting crops highly susceptible to wireworms in a field that has been recently in sod.
This information has been taken from www.gov.pe.ca