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 to 20°C. 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°C 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 tip
burn.
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 15mm. 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.
Fertilisation
Fertilisation applications should be
based on soil analysis. Over fertilisation may result in increased
susceptibility of the crop to various diseases or disorders. Apply 600kg/ha of vegetable
fertiliser or Compound C (5:15:12) at planting. Alternate with Flower, Fruit
and Shrub (Compound J) Fertiliser 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
fertiliser.
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 300mm. 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 to 2 l/ha.
Rotation
To avoid a build-up of pests and
diseases follow a legume to leaf crop, root crop to 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.
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.
Spray with Carbaryl 85 WP, or Decis to
control bollworms.
Disease Control
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 to 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.
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