ONION

Onions are one of the mineral and vitamin-rich vegetables in Nigeria.  They are rich in vitamin C and they are good sources of dietary fiber and folic acid. Onions also contain calcium, iron, and high protein quality. They are low in sodium and has no fat. They also contain quercetin, an anti-oxidant. Despite their nutritional value, onions are one of the farms produce plagued by post-harvest wastage due to spoilage and rot. This article is about harvesting and post-harvest handling and storage of onions to ensure effective storage of onions.

In Nigeria, onion is grown mostly in Kano, Kaduna, Jigawa, Sokoto, Plateau, Bauchi and Kebbi States. In 2012 alone, an estimate of about 240,000 tons of green onions and 1,350,000 tons of dry onions were produced in Nigeria.

 

Maturity of onions

Onion thrives well in the hot and dry season, producing smaller bulbs but very good quality onion bulbs. This is probably why the vegetable thrives very well in Northern Nigeria. Cool and wet growing seasons result in higher yields and bigger bulbs that tend to be of lesser quality due to the bulb rot problem. Moderate season favors onion growth, but ample rainfall and cooler temperature tend to favor leaf production and bacterial diseases.

Signs of Maturity

• The neck of the onions can no longer support the weight of the foliage, thus it falls over.
• Plants don’t put on new leaves
• Leaves start drying and the plant is resistant to pull out of the ground
• Bulb matures within 100 to 140 days from the date of sowing.

Harvesting

Only top onions when the neck is dry and no green tissue remain (roll the neck of the onion bulb between your fingers if the tissue doesn’t slide, then it’s ready for topping).

Stop irrigation at maturity about one to two weeks before harvest to prevent bulbs from being waterlogged. Harvest bulbs during the cooler part of the day (early morning and late evening) but not on a wet day. This will prevent the occurrence of brown stains and black mold.  You can use a fork to loosen the bulbs before pulling the top by hand. Be careful when pulling out but avoid damaging the bulbs.

Onion are usually harvested within three months, especially the bulb onions. Be on the look out for when the plants begins to yellow because that is the right time to harvest it. You can also induce the process by breaking the tops of the onions as that makes it ripen before bringing out flowering stalks. Do not wait until it starts producing flower stalks.

For the green onions, that is usually due for harvesting when it gets to 5 inches tall as it has been discovered to have more stronger flavor if it,s allowed to grow taller.

It is usually best to plant onions by the mid-October, though it can be planted at other times but just that it needs uniform nutrients and moisture through out the growing season. Furrow irrigation is encouraged for the plants to produce very well.

Onions farming is not for mediocre but is meant for those who want to make millions because it has the capacity to do just that. Go to various major markets in Nigeria and see that onions business is so huge as a bag of onions currently sell between N25,000, N30,000 and N45,000 as the case may be.

Other Business Opportunity In Onion Farming

You can buy a bag of onions at the rate of N4,500 and N5,000 from the Northern states of the country like Kebbi, Kano, Abuja or from any other states that grow it  and have it brought down to any place of your choice while you source for the ready markets locally as it is consumed everywhere in the country.

Pulling and windrowing

Onions shouldn’t be pulled until at least 50% of the plants have lodged. Lodging occurs when the onion plant has fallen over. During lodging, nutrients translocate from the leaves to the bulbs. After lodging, bulb size increases by about 25-33%. In order to ensure that the bulb isn’t harvested until it is fully bulbed, it is recommended to carry out a pulling test. The pulling test involves gently pulling the individual plant from the ground. If the plant comes out of the ground easily then it is ready for harvest but if not it is an indication that the root is still very active and bulbing isn’t complete.

Avoid pulling onions and leaving them in the sun when the temperature is very high (80%-90%) because they can get sunscald. Clip off the top of the foliage approximately 1 inch above the bulb and trim the roots. This will prevent the bulb from decaying and sprouting during storage. Also, if wet onions are placed in boxes, it takes longer for them to cure properly and the high moisture encourages disease development, rooting, and sprouting.

Harvesting Tip

Avoid bruising during harvesting because bruises provide direct entry points for diseases.

 

Field drying and Curing

Layer the onion bulbs in such a way that the leaves lay overtop the bulbs to protect them from direct sun impact. The bulbs can also be dried in a greenhouse or high tunnel. The temperature should be below 85⁰F.

Curing is the drying process intended to dry off the neck and outer scale leaves of the bulbs to prevent the loss of moisture and the attack by decay during storage. Heat, good ventilation, and low humidity are essential for curing. Usually, the outer layer of the bulb that is contaminated by soil falls away easily when the bulbs are cured. This exposes the more attractive under-layer. A cured onion bulb would lose 3-5% of its weight, the neck is tight and outer scales are dried until they rustle. Onions can either be dried in the field or collected in trays and stacked in a warm, covered area with good ventilation. Onions can also be cured by tying the tops of the bulbs bunches and hanging them on a horizontal pole in well-ventilated shades. Curing in shade is advisable because it improves bulb color and reduces significant losses during storage.

 

Grading and Packaging of Onions

After curing, onions are graded before they are stored or transported to the market. The thick-necked, bolted, doubles, injured, and decayed bulbs are picked out. The onions are graded into extra-large (˃ 6cm diameter), medium (4-6cm diameter), and small (2-4cm diameter).

Packaging should be small for easy handling during transportation. Onions can be packed in open mesh jute bags or wooden baskets. The use of a nylon net bag for packaging onions is recommended because it results in less storage loss due to the good ventilation it provides.

Storage of Onions

Onions shouldn’t be stored unless adequately dried either on the field or by artificial means. It is important to dry the neck tissue and outer scales until they rustle when handled otherwise the bulb will rot in storage. Onions may be stored in bulk in special houses made up of thatched roof and walls made up of bamboo and wire mesh for good air circulation. Onions are spread on the dry and damp proof floor or racks.

 

Major Onion storage problems

• Sprouting and rooting are major challenges of onion storage. Sprouting is controlled by temperature while rooting is controlled by relative humidity. A temperature below 10-25⁰C increases sprouting and the higher the relative humidity the higher rooting occurrence. Sprouting can be preserved if Maleic hydrazide at 2500ppm is sprayed at 75-90 days after transplanting to the field.
• Weight loss is higher when the temperature is above 35⁰C. Onions are stored at a temperature of 30-35⁰C with a relative humidity of 65-70% but for cold storage, onions are kept under temperature between 0-2⁰C at a relative humidity of 60-70%.
• Storage rot can be prevented through proper cleanliness in the store and crops can be sprayed with 0.1% carbendazim after 90 days if transplanting and just before harvest.

Onion Storage tips

• The periodic turning of onion bulbs and the removal of the rotten, damaged, and sprouting bulbs is essential while storing onions.
• Avoid direct sunlight on onion bulbs to reduce sunscald, fading color, and quality deterioration.

Storage Diseases

• Botrytis neck rot

Onions are susceptible to Botrytis neck rot during storage. The diseases are characterized by grey fungi growth, often watery in nature and it affects the neck area and the outer scales. The fungal infection usually spreads quickly through the whole onion. Bruising of onion bulbs during harvest, storing under humid conditions, and exposing inner tissues due to breakage of outer scales increases the incidence of Botrytis neck rot. Proper curing of onions prior to storage will reduce the incidence of this disease.

• Black mold

This is caused by Aspergillus niger and it is characterized by black discoloration at the necks of the onions. The black discoloration can sometimes be found on the outer scales. Bruised onion are susceptible to this fungus. Black mold causes the tissue to become water-soaked which often induces bacterial soft rot. Storage at low temperature delays the growth of the fungus. Cleanliness during harvesting and postharvest handling prevent occurrence. Also avoiding the bruising of the bulb during harvesting and post-harvest handling reduces the occurrence of this disease.

• Blue mold

This is caused by Penicillium. Blue mold induces watery soft rot of onion tissues and/or blue-green discoloration at the neck or other tissues. Reducing mechanical damage and proper curing often reduces the incidence of this fungus.

• Bacterial soft rot

This disease is caused by Erwinia and Pseudomonas. Bacteria rot caused by Erwinia often appear healthy on the outside but when cut open some of the inner scales are brown, water-soaked and have a cooked appearance with a characteristic foul smell. Bacterial rot caused by Pseudomonas infect outer scales and is characterized by yellow slime which produces a sour odor.

Tips to control fungal and bacterial diseases in stored onions

• Pre-harvest application of Rovral, a registered fungicide.
• Harvest the bulbs at proper maturity.
• Minimise bruising of bulbs during harvesting and post-harvest handling.
• Cure onion bulbs properly.
• Store as quickly as possible.

References

Bringing it home: Harvest tips for high quality onions. Christy Hoepting CCE Vegetable program. Empire state producer expo small scale onions, Syracue, NY. January 19, 2017.

Growing onions by the editors. Retrieved from https://www.almanac.com/plant/onions  on 7 November, 2019

Harvest and Post-harvest tips for best onion bulb quality by Christy Hoepting, CCE Cornell vegetable program.

https://uaccville.wordpress.com/2014/07/10/thousands -of-onions-harvested-and-curing/amp/

https://www.alibaba.com

https://www.alliumnet.com

https://www.theatlantic.com

Onion post-harvest handling and storage. Shika Agblor and Waterer Doug. Departement of Plant Science University of Saskatchewan (June 2001).

Onion: Post harvest technology.

The secret of big home grown onion by Bruce Fraser. Fine gardening from www.finegardening.com/article/the-secret-to-big-homegrown-onions retrieved on

June 29, 2019.

Training of trainers on post-harvest management of onion. Kukom Edoh Ognakossan. World Vegetable center, West and central Africa, Dry regions.

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