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Lettuce    

Lactuca sativa. Compositae. Lettuce is an annual salad vegetable. A cultigen possibly derived from the wild lettuce Lactuca serriola, it originated east of the Mediterranean, in the region encompassing Asia Minor, Transcaucasia, Iran and Turkestan. First used as a medicinal plant it was treated as a food plant as early as 4500BC. By the 1st century AD it was in general use by the Greeks and Romans. Original forms of the plant were non-headforming loose-leaf types and the headed types did not appear until the 16th century. It is now widely cultivated in both temperate and tropical regions and is also important as a glasshouse crop in cooler regions.

Lettuce is usually grown where mean temperatures are in the range 10-20ºC. Higher temperatures prevent head formation and cause bolting and also produce a bitter flavour in the leaves. Cool temperatures and adequate soil moisture are required during heading. The range of cultivars enables lettuce to be produced in the open throughout most of the year. Some are hardy enough to overwinter in milder areas but cloches or cool glass are needed in regions experiencing frost. The ideal site for lettuce is an open area where the soil is light, well-drained and fertile, with an optimum pH around 6.0. Well-rotted organic manure should be incorporated to increase the moisture retention of the soil. The crop is fairly tolerant of high salinity.

Lettuce may be either sown direct, or in seed trays or peat blocks for subsequent transplanting. They do not transplant well in hot weather, so later sowings are best direct-sown or established by using peat blocks or modular-raised plants. Seeds should be sown in drills 2cm deep and at intervals of about two weeks for continuity of supply. Mosaic-tested seed is preferable as it reduces the risk of virus infection. Sowing can be made from early spring until autumn but germination, particularly of the butter-head types, is often erratic when soil temperatures are greater than 25ºC. Watering will help to reduce soil temperature and improve the germination of sowings made during the summer. Except during mid-summer, when lettuce does readily recover from transplanting, thinnings from direct sowings can be used to provide a successive crop maturing approximately ten days later. Plants should be kept well watered, particularly during the later stages of growth.

The earliest sowings for outdoor production should be made in trays or blocks under protection or direct-seeded in frames or under cloches, where they can be harvested in situ or provide transplants for establishing later in open ground.

The main period of outdoor sowings should commence as soon as soil temperatures begin to rise in the spring. A succession of sowings with appropriate cultivars will provide harvests through the summer months until autumn. The later crops will benefit from protection from cloches during the autumn. Autumn sowings for a protected winter crop can either be sown in the open or under glass for transplanting to frames or cool glass during the later autumn.

Cold-hardy cultivars can be overwintered outdoors to provide an early crop during late summer and thinned to approximately 8cm apart during the autumn. Final thinning should be carried out during the following spring when growth recommences to allow 30cm between plants. Overwintered crops benefit from a topdressing of a nitrogen fertiliser during early spring.

The choice of cultivar will to a large extent determine the final plant spacing. However, it is possible to modify the growth habit of lettuce by planting at high density to encourage the formation of leaves and inhibit head formation. The leaves can then be cut off 2-3cm above soil level and the remaining stumps will produce a second crop that will be ready to harvest 4-7 weeks later. For maximum leaf growth, plants should be sown in a fertile weed-free soil in rows 12cm apart and a final within-row spacing of approximately 2.5cm between plants. Cos lettuce are more suited for leaf production than other types.

For heading lettuce, the time from planting to maturity varies from 60-80 days during the summer to 90-145 days during the cooler period of the year. The harvested crop is highly perishable, particularly at high temperatures and low humidity. Shelf life can be extended to two weeks at 1ºC and 95% relative humidity.

The large number of lettuce cultivars available can be broadly divided into two groups – the cabbage and cos types. Cabbage lettuce are further divided into the soft-leaved butterheads (loose-heads) in the US) and the crisp-leaved ‘crispheads’. The butterheads are earlier maturing than the crispheads but have a greater tendency to bolt. The cos lettuce are upright in shape with thicker, long leaves. They normally take longer to mature than the cabbage type. In addition to the heading lettuces there are a number of non-heading or ‘salad bowl’ types which are less prone to bolting and from which leaves can be harvested as required throughout the summer months. Leaf shape can range from entire to deeply indented and colours extend from green to red. Forms with a thick main stem that can be cooked are called celtuce.

The following cultivars cover a range of types and maturity periods. For early sowing under protection: ‘Little Gem’, ‘Tom Thumb’, ‘Winter Destiny’, and ‘Salad Bowl’ types. Main summer sowings in the open: (Butterheads): ‘All the year round’, ‘Avondefiance’, ‘Buttercrunch’, ‘Continuity’, ‘Red Sails’, ‘Reskia’, ‘Royal Oak Leaf’, ‘Ruby’, ‘Sabine’, ‘Unrivalled’. (Cos): ‘Little Gem’, ‘Valmaire’, ‘Winter Density’; (Crispheads): ‘Avoncrisp’, ‘Great Lakes’, ‘Iceberg’, ‘Malika’, ‘Minetto’, ‘Musette’. ‘Salad Bowl’ types, ‘Black Seeded Simpson’: Early Curled Simpson’, ‘Grand Rapids’, ‘Lollo Biondo’, ‘Lollo Rosso’, ‘Oakleaf’, ‘Red Oakleaf’, ‘Red Salad Bowl’, ‘Saladini’, ‘Slobolt’, ‘Wallopo’. Protected winter crop: (Butterheads): ‘Columbus’, ‘Cynthia’, ‘Dandie’, ‘Kwiek’, ‘Magnet’, ‘Pascal’, ‘Ravel’; (Crisphead): ‘Kelly’s’. Outdoor overwintering crop: (Butterhead): ‘Imperial Winter’; (Cos): ‘Little Gem’, ‘Lobjoits Green’.

Lettuce ring spot is caused by the fungus Microdochium panattonianum. Small, round, brown-yellow spots occur on the underneath surfaces of the leaves and elongated, sunken areas resembling slug damage develop on the midribs. Other, usually less important, leaf spots are caused by the fungi Pleospora herbarum and Septoria lactucae. These diseases can be controlled by fungicide sprays and infected crop debris should be destroyed. Marginal leaf spot, in which the leaf edges shrivel, is caused by infection with the soil borne bacterium Pseudomonas marginalis. The bacteria enter the leaves through the stomata and measures which reduce humidity and prevent the leaves being wetted will help control the disease. Lettuce can also be affected by bacterial soft rot (Erwinia carotovora), damping-off and foot rot (Rhizoctonia solani), downy mildew (Bremia lactucae), grey mould (Botrytis cinerea) and Sclerotiia rot (Sclerotinia sclerotiorum). The aecidial stage of the rust Puccinia opizii occasionally occurs on lettuce; urediospores and teliospores are produced on Carex species, the alternate hosts.

The aphid-transmitted beet western yellows virus causes intense inter-veinal yellowing of the outer, maturing leaves. Good weed control should reduce the incidence of this disease, which is present in weeds such as cleavers (Galium aparine), groundsel (Senecio vulgaris), hairy bittercress (Cardamine hirsutum), shepherd’s purse (Capsella bursa-pastoris) and wild radish (Raphanus raphanistrum); in North America the virus infects beetroot and sugar beet. Lettuce big-vein virus causes pale yellow vein-banding, especially near the bases of the outer leaves. It is transmitted by the zoospores of a soil-borne, phycomycete fungus, Olpidium brassicae, which enter the plant roots. Lettuce should be grown in uncontaminated soil otherwise soil sterilisation will be necessary. Plants affected by the seed-borne lettuce mosaic virus are likely to be stunted, yellowed, and have crinkly leaves or poorly developed hearts. Certified seed, with less than 0.1% or 0.01% infection, can be obtained. Cucumber mosaic virus sometimes produces symptoms similar to those of lettuce mosaic.

Lettuces may become infested with several species of polyphagous foliar aphids and also by the lettuce root aphid (Pemphigus bursarius). This root-feeding species which migrates from poplars is difficult to control unless a soil drench of suitable contact or systemic insecticide is applied early at the first sign of attack; in areas where attacks persist resistant cultivars should be planted instead of susceptible ones. The foliage may also be attacked by slugs and snails, caterpillars of cabbage moth (Mamestra brassicae), the stem eelworm (Ditylenchus dipsaci) springtails and the chrysanthemum leaf miner (Phytomyza syngenesiae). In North America caterpillars of the omnivorous leaf roller (Platynota stultana) may also infest lettuces. Cutworms attack plants at soil level and can cause extensive damage especially on light soils during dry summers. The roots of lettuces may be damaged by the caterpillars of swift moths (Hepialus species), the grubs of chafer beetles (various species), wireworms (Agriotes species), symphlids (Scutigerella species) and the root knot eelworm (Meloidogyne species).

Indian Lettuce   

Lactuca indica (Compositae). An erect perennial up to 1.3m; both leaves and stems contain latex. The centre of origin is China, but the vegetable is now grown in India, Malaysia, Indonesia, the Philippines and Japan. In warm climates, seeds are grown in nursery beds or containers and transplanted to well prepared beds at a spacing of approximately 30 x 30cm. Propagation by root cuttings is also possible and axillary buds from the base may develop to produce a ratoon crop. In temperate areas, seeds may be sown early in the year under greenhouse conditions, in temperatures ranging from 20-25ºC and seedlings transferred to 25-30cm pots when 10-15cm high. The first leaves may be harvested about 60 days from sowing or planting.

Stem Lettuce   

(Asparagus lettuce, Chinese lettuce, celtuce). Lactuca sativa variety asparagina (Compositae). The centre of origin is China, where it is widely grown, although it is also cultivated in many parts of tropical Southeast Asia. This is a non-heading form of lettuce: the mature leaves are large, coarse and inedible but the basal leaves are narrow and lanceolate. The thickened stems may grow to 1m; both immature stems and young leaves and stems are both used as a cooked vegetable. Propagation and planting are similar to the method described for Indian lettuce (Latuca indica). In temperate climates, this crop can be grown under greenhouse conditions, with a temperature range of 20-25ºC. Seeds may be sown early in the year and seedlings transferred to 25-30cm pots when about 15cm high.

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