Profitable Lettuce Production Guide article first appeared on Agribusiness Magazine. Like their Facebook Page for more tips
Lettuce is a high value crop which in Zimbabwe can be farmed in many regions as the temperature range requirements are compatible with most places. In an unfortunate scenario, when one considers market gardening projects to undertake locally, lettuce is hardly top of the list, whilst it is relatively easy to farm and is not financially intensive. This is simply because it requires no major infrastructure development and can be done at a small scale. The following are basic guidelines which should provide a knowledge base for one to undertake lettuce production.
Climatic Conditions
Lettuce is a cool season crop that grows best within temperatures of 12 oC to 20 oC. It does not suffer from light frosts and winter cold except near maturity. Severe frost before harvest can scorch leaves and heads. Temperatures above 27 oC affect head development and plant edible quality and also promote premature seed stalk development. High temperatures also inhibit germination and can cause a high incidence of tipburn.
Water Requirements
The crop has high moisture requirements and not more than 50% of the available water in the root zone should be depleted before irrigation.
Soil Requirements
The plant grows well on a wide variety of soils ranging from light sand to heavy clay. However, best results are obtained on fertile loams that are rich in organic matter. A pH between 5.5 and 7.0 is optimum. Lettuce should be grown on soils with high water holding capacity and proper drainage for good root growth and plant performance.
Propagation
Lettuce is propagated from seed.
Soil preparation
Lettuce seed is small and as such requires soils that are not prone to crusting. The soil should be worked to a fine tilth, without clods and it should be as level as possible in order to ensure uniform emergence.
Planting
Raised beds are ideal for lettuce production, and they help prevent damage from soil compaction and flooding. They also improve air flow around the plants, resulting in reduced disease incidence. Plant populations generally range from 60 000 to 100 000 plants per hectare.
Lettuce is generally sown directly in the field to a depth of 10 to 15 mm. The seedlings are later thinned out to the desired spacing, and they are sometimes used for transplanting. Seedlings for transplanting may also be raised in seed trays or seedbeds and transplanted about five weeks after sowing.
Fertilization
Fertilization applications should be based on soil analysis. Over fertilization may result in increased susceptibility of the crop to various diseases or disorders. Apply 600 kg/ha of Vegetable Fertilizer or Compound C (5:15:12) at planting. Alternate with Flower, Fruit and Shrub (Compound J) Fertilizer at the same rate. All plants will benefit by additional application of Nutrifol No. 1 foliar spray to overcome nutrient deficiencies and increase yield. Lettuce does NOT require top dressing fertilizer.
Irrigation
Lettuce has a shallow root system and as such requires frequent but lighter irrigations. The root penetrates the soil to a depth of only 300 mm. Water should be applied throughout the growing period and reduce it when the heads become full. A water shortage tends to promote bolting.
Weed control
Weeds are controlled mechanically, manually or chemically. Mechanical weed control can only be practised before planting because of close spacing. Weeds are removed by hand or hoeing or pulling between plants in the rows. Chemical control can be achieved through the application of Ronstar 25 EC pre-emergent at 3 to 4 l/ha. Transplant 24 hours later and follow with a light irrigation. Grasses including Shamva grass can be controlled using Agil 100 EC post- emergent at 1-2 l/ha.
Rotation
To avoid a build-up of pests and diseases follow a legume – leaf crop – root crop – fruit crop rotation.
Pest control
Cutworms (Agrotis sp)
Cutworm may be problematic during the seedling stage. They are usually found two to five cm below the soil surface in the vicinity of plants that would have been cut off. Cut worms are active during the night and they cut stems just above or below the soil surface.
Chemical sprays recommended by Windmill are Carbaryl 85 WP, Pyrinex/ Chlorpyrifos, Lambda, and Fenvalerate.
Diamond Back Moth (DBM)
This an important insect pest that must be controlled. Windmill has the following products that can be applied. Malathion 25 WP, Cartap hydrochloride/ Suntap, DDVP 100 EC, Match/Lufenuiron, Malathion 25 WP, or Tamaron 600 SL.
Aphids
Heavy populations of Aphids can result in stunted growth of young plants. Their presence may also contaminate the heads, thereby, affecting the appearance of the heads. Aphids act as vectors of small viral diseases. Full cover sprays with Dimethoate 40 EC, or Malathion 25 WP, or Diazinon.
American bollworm
Larvae penetrate at the bottom of the leaves and eat their way into the heads.
Disease control
Spray with Carbaryl 85 WP, or Decis to control bollworms.
Septoria leaf spot (Septoria lactucae)
The disease is widespread and damaging in hot weather. Infected plants have small, yellowish spots on the outer leaves. These spots grow and become large, irregular and brown. The centre of the spot is pale creamy brown with many black dots.
The disease symptoms are characterised by spots or patches of white to greyish, talcum powder like growth. Tiny pinhead sized, spherical structures that are first white, later yellow-brown then brown and finally black, may be present singly or in groups.
The disease can be prevented by spraying Copper Oxychloride 85 WP.
Downy mildew (Bremia lactucae)
Downy mildew occurs frequently during cold, moist weather in spring or early autumn, and in cool, humid areas in summer.
Disease symptoms consist of light green or yellow lesions on the upper surface of the leaves which are first noticed on older leaves. The lesions later become necrotic, limited by veins and angular. A white downy mould is also noticed on the lower leaf surface.
Control the disease by planting resistant cultivars. Use disease-free seeds. Always practise crop rotation. The disease can be prevented by spraying Copper Oxychloride 85 WP or Dithane/Mancozeb at 10-14 day intervals. Rotate these products to avoid chemical resistance.
Powdery mildew ( Erysiphe cichoracearum)
The disease occurs frequently and it reduces the quality of the crop.
The disease symptoms are characterised by spots or patches of white to greyish, talcum powder like growth. Tiny pinhead sized, spherical structures that are first white, later yellow brown and finally black, may be present singly or in groups.
The disease can be controlled using crop rotations, especially with non-host crops. Use disease-free seeds and always plant disease-resistant cultivars.
Sclerotinia rot
Infected plants initially wilt on hot days. A severe wet rot is seen on the stem near the soil. This rot spreads to the roots and into the head, which becomes wet and slimy. A white, cottony mould develops on rotten tissue. Hard, irregular, black sclerotia can be seen in the white mould on rotten tissue. The outer leaves whither and drop flat, followed by the inner ones, if the base of the leaf has rotted.
The disease can be controlled through rotations with non-host plants such as onions. Always plant disease-resistant cultivars. Use only registered chemicals.
Bacterial rot complex
The disease is caused by a combination of bacteria that occur on the leaf surfaces without causing damage. The bacteria get into the leaves and cause rotting when the plant is stressed or damaged.
Disease symptoms consist of leaf spots which start off under water-soaked conditions and later turn brown, before developing soft leaf rot and death of affected tissue.
Control the disease by planting disease-free seeds and always plant disease resistant- cultivars. Discard infested plants and remove them from the field. Always keep your field free of weeds.
Lettuce mosaic virus
The disease is seed borne and is transmitted by the green peach aphid.
Early infections result in vein clearing followed by mottling, recurving of the leaves, and increased marginal frillness. Infected and mature plants are yellow and stunted and cannot be harvested.
Control the disease by planting disease-free seeds, and always plant disease-resistant cultivars. Always keep your field free of weeds.
Harvesting
The heading types are harvested when the heads are fully grown and firm while the loose leaf types are picked when the leaves have reached the required size.
Lettuce is harvested by hand by cutting off the plant just above the soil surface to keep most of the outer leaves around the head. Harvesting should be done very early in the morning because lettuce wilts rapidly.
