Onions are consumed in almost every household in Zambia. Not only are onions grown for domestic consumption but also for commercial purposes. Onions are one of the few crops in Zambia that will always have a ready market. This simple guide will equip onion growers with recommended practices in the cultivation of onions.
Climatic
Requirements
Onions are a cool season crop.
Germination of onions is slow at 6 to 7°C. The optimum germination temperature
range is 10 to 35°C, and the maximum temperature is 40°C. Onion is adapted to a
growing season with air temperatures at 13 to 24°C. Low temperatures early in
the season are desirable with higher temperatures after bulb formation. Onion
is tolerant to frost but seedlings are generally only tolerant down to -1°C.
Onions are sensitive to
photoperiod. Long days are favourable to onion production as this enhances leaf
development and which, in turn, is directly related to bulb size. Early
varieties require 13 hours for bulb initiation while late varieties require 16
hours for bulb initiation. Onions begin to form bulbs when day length reaches
the appropriate duration for the cultivar, provided temperatures are high
enough. Early seeding or transplanting is, therefore, essential. Cool weather
during early growth of the plant promotes formation of seed stalks (bolting).
Onion bulbs grow quickly in warm than cool temperatures. At 40°C, bulb
formation is retarded. In onion, there is an interaction between day-length and
temperature in the bulbing process of an individual cultivar.
Soil
Requirements
Soils should be well-drained and well-supplied with organic matter. Sandy loams without stones are optimal. Heavy clay soils should be avoided. Coarse sand soils may be used for specialty onion production under irrigation.
Planting
Direct seeding will generally require 5 to 8kg/ha. Use high quality seed that has been suitably treated with fungicide dressing. For lands, when transplanting from seedbed, use a seed rate of 3kg/ha. Plant the seedbed at 10g/sq.m and often 300sq.m is adequate to plant out 1 hectare. The recommended planting period is from February to March for transplanting 6 to 8 weeks later when plants are 5 to 7mm in diameter (pencil thickness).
Irrigation
Onions have a shallow and
limited root system which explores mainly the upper 30cm of the soil. This crop
should be irrigated frequently throughout the growing season. Soil moisture
should not be allowed to fall below 50% of water holding capacity. Most soils
should receive 2.5cm of water per week from the combination of rainfall and irrigation. Soil moisture is important in the growth
of new roots; the soil moisture must reach the base of the bulb periodically if
the newly formed roots from the stem are to grow into the soil. New roots will
not grow into dry soil.
Crop
Nutrition
Onions require a fertile but well-balanced
soil. Manure is not recommended as weeds are a serious
problem for this crop and also due to variable nitrogen content. Soils must
have a pH of 6.5 to 6.8 for satisfactory crops. On peat soils, pH of 5.5 is
sufficient. The soil must contain adequate calcium for crop growth. This means
that calcium must be evenly distributed and incorporated into the field. Crop
failure is common on fields with inadequate liming. Also, some cultivars may be more sensitive
than others to low pH.
1.
Nitrogen - Apply most of the nitrogen pre-plant incorporated (at
least 2/3 of the required amount). Side-dress the remainder in mid-to-late June
after the seeded onions are about 15cm tall. Excessive nitrogen especially in
July can cause delayed maturity (thick necks) and soft bulbs.
2.
Phosphorus - Should be banded if possible at the time of seeding.
Otherwise relatively heavy applications of phosphorus must be broadcast and
pre-plant incorporated.
3.
Potash - Potash should be broadcast and pre-plant incorporated.
Application rates depend on the level in the soil.
Micronutrients
a.
Copper - deficiency occurs on acid or peat soils. Copper may be
mixed with the fertilizer and applied. On peat soils (as an initial
application), 50kg of copper sulphate per hectare is recommended. Copper
Sulphate can be applied by spraying it onto the soil surface and incorporating
it into the soil (this material is extremely corrosive to metal).
b.
Manganese - At high soil pHs a deficiency may show up. Soil
application of this element is not suggested due to the large amounts required.
Foliar applications of manganese sulphate are recommended, starting when the
plants are about 15cm tall with 1.5 to 2.75kg manganese per hectare in 300 litres
of water and repeated in 4 to 5 sprays 10 days apart. Use the low rate on small
plants increasing the rate as the season progresses.
Fertilizer
Application
For seedbeds, apply basal
fertiliser Compound S (7:21:7 9S 0.04B) at the rate of 45g/m2. After
2 to 3 weeks of emergence, apply Ammonium Nitrate (34.5% N) as top dressing at
the rate of 35g/m2.
For lands, apply basal
fertiliser of Compound C (5:15:12 11S.01B) fertiliser at the rate of 600kg/hectare.
Apply Ammonium Nitrate (34.5% N) at the rate of 100kg/hectare after 4 to 6
weeks after transplanting.
Weed
Control
Onions do not compete well with weeds. Good weed control requires integration of cultural and chemical techniques. Herbicides will provide pre-emergence and post-emergence control of annual weeds but repeat applications may be necessary. Cultivation and hand weeding are usually required to supplement chemical control. Onions should be planted in soil where the annual weed seed population has been reduced by cultural procedures such as crop rotation, fallowing or stale-seedbed technique. Specialty onions can be successfully grown by transplanting through black plastic mulch. This method provides excellent weed control and crop growth. Care must be taken to avoid fields where residual herbicides from previous years persist in the soil as crop injury may occur. For pre-emergent control of annual weeds and broad leaf weeds, apply Ronstar 25 EC or Linurex 50 WP. Use Agil 40 EC for post-emergent weed control.
Pest
Control
1.
Nematodes
Onions are often prone to
nematode attack. It is recommended that one apply Basamid GR against them in
the seedbed.
2.
Onion Maggot
Onion maggots overwinter in
the pupal stage with adult flies emerging in the summer. Adults resemble the
common housefly but are slightly smaller (6mm) and pale grey. The elongate
white eggs are laid in the soil at the base of onion plants. The creamy-white
larvae emerge within one week, and reach a length of 7mm when fully grown. If
onion maggots have been a problem it would be possible to use onion setts along
the margins of the field as a trap crop. When injury is seen, or if onion
maggots are known to be a problem in your area, apply a fallow treatment. After
harvest remove and dispose of any onions that are left in the field. Diazinon
30 EC is a good remedy.
3.
Onion Thrips
Onion thrips are minute
insects that puncture the leaves or stems and suck up the exuding sap. This
causes the appearance of whitish blotches on the leaves. The insects may be
found in large numbers between the leaf sheaths. Thrips are slender, yellow,
active insects, at most 1mm long. They usually enter the field border areas
first and become problems especially under hot, dry weather conditions.
The threshold for thrips can
be determined by counting their numbers. This is done by generally looking at
the newest leaves on the plant - the greatest number of thrips will be found
between the new leaves. Count the number of thrips per plant on 30 to 50 plants
throughout the field to calculate the average number of thrips per plant. You
then divide by the leaf number to give the average number of thrips per leaf.
Apply insecticide treatments when the number of thrips observed exceeds the
threshold of three thrips per leaf for cooking, onions, or one thrips per leaf
for Spanish and green bunching onions. After the crop is harvested, the tops
should be raked together and burned. A chemical recommendation will be
Malathion 25 WP.
4.
Cutworms
Upon detection of cutworms,
apply Dursban/ Pyrinex 48 EC, Fenveralate 20 EC, or Lamda-Cychlothrin 5 EC.
Diseases
Control
1.
Damping-off and Root rots
Damping-off occurs in seedlings,
which may topple over and die because of decay at the soil surface. Surviving
seedlings may be stunted because of a brownish rot on the roots and shoot. Pink
root is a type of root rot, which results in reduced bulb size. Affected plants
turn yellow and wither and roots have a pink colour, which eventually turns
brown to black.
Control:
Treat seed with Thiram 80 WP for damping-off. To control pink root-rot,
practise a rotation of several years.
2.
Botrytis Leaf Blight
Botrytis leaf blight develops
as white spots 1 to 5mm in length with light green to silvery halos. The
centres of the spots become sunken and straw coloured. When spots become
numerous, the leaf tips die back down the entire length of the leaf.
Control: A
regular spray of Dithane M45 schedule throughout the summer is necessary to
control Botrytis leaf blight.
3.
Downey Mildew
Downy mildew first appears on
leaves as elongated patches varying in size and slightly paler than the rest of
the foliage. Under moist conditions, these areas turn violet grey, which may
spread to surrounding tissue. Leaves fold over at the affected areas and the
leaf tips wither. Onions that are severely infected do not cure properly and are
susceptible to storage rots.
Control: A
regular spray of Dithane M45 schedule throughout the summer is necessary to
control Downey mildew.
4.
Purple Blotch
Purple blotch appears on
leaves as brown spots 1 to 3cm in length with red-purple margins. The brown
areas may have alternating dark and light zones giving a target board effect.
Leaves weakened by purple blotch may fall over. Purple blotch frequently
develops after Botrytis leaf blight or downy mildew has appeared.
Control: A
regular spray of Dithane M45 schedule throughout the summer is necessary to
control Purple blotch. Practice a 2- year rotation and destroy infected crop
debris after harvest and destroy refuse heaps of onions culled from storage.
5.
White Rot
This is a very destructive
disease of the onion family. The characteristic symptoms are a white fluffy
fungal growth and soft rot around the base of the bulbs. Masses of tiny black
sclerotia form in the fungal growth and in bulb tissues. These sclerotia allow
the fungus to survive in the soil for 4 to 5 years or longer.
Control: The
use of infected onion setts or transplants can introduce the disease to new
areas. Do not introduce the disease from infected areas, equipment, pallet
boxes, etc. Follow a 4 to 5 year rotation. For chemical control, use Quintozine
75 WP or Allisan 50 WP.
6.
Bacterial Diseases
There are at least three different diseases caused by bacteria; soft rot, slippery skin, and sour skin. The bacteria causing slippery skin and sour skin enter the onion through wounds on leaves and when heavy irrigation or rainfall results in water standing in leaf axils and the neck. These bacteria can enter the bulb prior to harvest at windrowing or through injuries at harvest. Soft rot often occurs when bulbs are damaged by onion maggot, bulb diseases, or mechanical injury.
Harvesting
and Handling
You can always tell when
onions have stopped growing. The leaves will lose their colour, weaken at the
top of the bulb and flop over. It's not good to leave the onions in the ground
for longer than two weeks after the tops die because they become open to
organisms that can cause rots in storage, or they might even start growing
again. Pull your onions up on a sunny day if you can, then let them sit in the
sun for another day or so to dry (in hot climates this usually takes just a few
hours). This drying kills the root system at the bottom of each bulb. The roots
will be like little brittle wires when they are dry. Picking the right day to
pull the onions can determine how well the onions will keep. If you harvest
them after some rainy weather, they will have a lot more moisture in them and will
not dry out well.
Storage and Conditioning
a.
Curing
Undercut or pull and windrow onions when at least 60% of tops have fallen down, and leave in windrow until inside neck tissues are dry before topping and storing. If the season is too damp for field curing, dry in storage at temperatures between 27 to 35°C with relative humidity at 70 to 80% for 2 weeks or cure with forced ventilation at 27 to 35°C. Curing with forced air at 35°C can be complete in as little as 48 hours. Best skin colour develops when onions are cured at 24 to 32°C with 80% relative humidity. Properly cured onions will have a tight neck and dry outer scales that rustle. Onions for sets are pulled and cured in late summer or early fall while the tops are still green, and when the bulbs reach 2cm in diameter. Larger setts produce a high proportion of "bolters.''
b.
Storage
Following the curing period, onions should be cooled and held at 0°C and 65 to 70% relative humidity. With forced air circulation, it may be possible to allow humidity as high as 85%. Root growth and decay are stimulated at high humidity, and sprouting occurs at high temperatures. Thick-necked onions will not keep and should be removed before storing if possible. Onions are frequently stored in bulk storage, two or four meters deep, constructed so that the bulbs can be cured by forcing air up through them. This method is considered more efficient and economical than curing and storing in crates. Properly cooled onions can be successfully stored at 0°C for 6 to 8 months.
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