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Nigeria Root Vegetables: Production, Utilization, Breeding, Biotechnology and Constraints
Written By
Emmanuel O. Ajayi, Pamela E. Akin-Idowu, Olaide R. Aderibigbe, Dorcas O. Ibitoye, Gloria Afolayan, Oluyemisi M. Adewale, Esther A. Adesegun and Benjamin E. Ubi
Submitted: 04 July 2022Reviewed: 29 July 2022Published: 17 October 2022
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This chapter describes the various facets, from agronomy to marketing, of Nigerian root vegetables including garlic, onion, turmeric, ginger and carrot being the world’s most significant and vital root vegetables which have high culinary and medicinal value. The chapter commences with their origin and history, universal spread, production figures, areas under cultivation and goes on to explain the botany, diversity, conservation, production practices, pests and diseases management, utilization, post-harvest technology and their uses as nutraceuticals. This chapter also presents the socio-economic, market analyses, export potential of these crops in Nigeria. It would be an important reference material for researchers, agricultural and food science students at both undergraduate and postgraduate level and policy makers; and be of great interest to experts and industries involved in root vegetables and spices trade. The in-depth information and knowledge about the genetic conservation, socio-economics, production, pests and diseases management and post-harvest technology of root vegetables in Nigeria provided in this chapter would greatly help in efforts towards improving their production and utilization for enhanced nutrition and healthy living.
National Horticutural Research Institute (NIHORT), Nigeria
Pamela E. Akin-Idowu
National Horticutural Research Institute (NIHORT), Nigeria
Olaide R. Aderibigbe
National Horticutural Research Institute (NIHORT), Nigeria
Dorcas O. Ibitoye
National Horticutural Research Institute (NIHORT), Nigeria
Gloria Afolayan
National Centre for Genetic Resources and Biotechnology, Nigeria
Oluyemisi M. Adewale
National Horticutural Research Institute (NIHORT), Nigeria
Esther A. Adesegun
National Horticutural Research Institute (NIHORT), Nigeria
Benjamin E. Ubi*
Deptartment of Biotechnology, Ebonyi State University, Nigeria
*Address all correspondence to: ben.ubi@ebsu.edu.ng
1. Introduction
Vegetables are plants cultivated for their leaves, succulent stems, young shoots, fruits or a combination of these parts. They form an integral part of everyday diet and include diverse plant species with considerable economic and medicinal properties. Vegetables are protective foods for maintenance of health and for prevention of diseases. Nigerian Root vegetables (NRVs) such as Ginger (Zingiber officinale), garlic (Allium sativum), turmeric (Curcuma longa), onion (Allium cepa) and carrot (Daucus carota) can be defined as vegetables cultivated for their edible underground parts. True roots such as taproots can be botanically differentiated from tuberous roots from non-roots such as bulbs, corms, rhizomes, and tubers, the word “root vegetable” being applied to all of them as it pertains to their agricultural and culinary usage. Root vegetables are rich in nutrients such as minerals, vitamins and fiber, and play important and valuable roles in nutritional, health, economic, social, cultural and ecological aspects of rural and urban communities in Nigeria and all over the world. Root vegetables constitute an important component of the Nigerian cultural heritage where they play vital roles in the tradition, food and income security of many households. Root vegetables (RVs) are able to make significant contributions to food security and nutrition, enhance livelihoods of marginal and smallholder farmers as well as improve the wellbeing of households. Advantages of planting NRVs include the ease of incorporating them into existing cropping systems, provide relatively higher earnings than most of the cash crops, they can be produced on small and barely productive lands, can be successfully cultivated under varying climatic conditions, and short production cycles. In addition, they require few purchased inputs, requires few resources and produces high yields with robust nutritional values. To realize higher return and the desired impact, it will be necessary to increase production of NRVs within the major areas of production as clusters of micro-enterprises. Pests and diseases can have devastating effects on vegetable crops if not well managed. According to [1], damage caused by pests on vegetable at various growth stages on the field to harvesting, storage and also during conveyance can lead to 5–40% crop loss annualy which poses a devastating effect on food and nutrition security for the rising Nigerian population. However, various approaches have been employed by researchers to tackle the menace of pests and diseases in root vegetable crops ranging from cultural practices, biological control, use of indigenous knowledge, plant extracts, pheromones, synthetic pesticides, use of improved planting materials and Integrated Pest Management (IPM) approach. This study reviews some of the efforts made so far in Nigeria on root vegetables to collect and conserve their germplasm, improve their production, utilization and trade, examine the major pests and diseases affecting them and the various approaches adopted by researchers in their management and finally post-harvest technology applied to them.
2. Collection and conservation of Nigeria root vegetable genetic resources
With high rate of population growth, demand for agricultural crops worldwide is expected to increase with a higher purchasing power per person which implies higher consumption and use of agricultural produce [2, 3, 4, 5]. Genetic materials of horticultural crops and their wild types are of great importance for food and nutrition security, and also serve as good sources of fodder, fuel, shelter, as well as sources of high-value industrial products to meet the high demand of an increasing global population. Genetic materials also provide useful sources of genetic variation required by plant breeders for crop improvement, and a broad genetic base within the gene pool is necessary to expand the scope of identifying and introgessing desirable genes underlying agronomically-important traits [6]. Genetic diversity has huge value for present and future generations, and more efforts must be made for its conservation and sustainable utilization [7]. Despite the usefulness of genetic resources, available genetic variability including landraces is getting eroded at an alarming rate, causing an enormous reduction of variability. This situation thus requires fast action to conserve germplasm [6]. The conservation, likewise sustainable use of germplasm is necessary in the promotion of food and nutrition security and gives room for securing diversity to respond to future challenges including the increasing climate change [8].
The Genebank of the National Centre for Genetic Resources and Biotechnology (NACGRAB) holds one of the most extensive collections in Nigeria, with a total record of over 10,500 accessions of 40 different crops, comprising of wild relatives, landraces, as well as old and more recent cultivars of crops, with germplasms of various vegetables being well represented. Vegetables include various genera and species; and are a vital element for a balanced diet which supplies vitamins, antioxidants, minerals, fiber, amino acids, as well as other compounds that improves health, and contribute to nutrition security [9]. Roughly a million accessions of crops that are entirely or partly used as vegetables are conserved ex-situ [9]. However, only 7% of the total global ex-situ conservation are fully used as vegetables which are mostly leafy or fruit vegetables. Root vegetables are not well represented in genebanks as they are mostly conserved in the fields; thus, are potentially exposed to pest and diseases and environmental variation. To avoid the risk of losing available germplasm, such crops are however amenable to in vitro conservation which is cost and labour intensive and requires consistent material transfer to fresh growth media. Root vegetables like ginger are conserved in vitro at the NACGRAB’s genebank in Nigeria.
Of the ca. 7.4 million accessions of Plant Genetic Resources for Food and Agriculture (PGRFA) that are ex-situ conserved globally, only 7% (i.e. ca. 518,000 accessions) are vegetables [10]. Of these global vegetable collections, only alliums are well-represented root vegetables in ex-situ collections [9]. As a result, the level of representation of root vegetables in both National and global collections, calls for the need to explore, collect and conserve more of the variabilities in root vegetables. Though we still have the benefit of a vast agrobiodiversity, there is the need to be conscious that two out of five plant species are endangered with losses occuring on a regular basis [11] due to the increasing climate change, extension of human settlements, and substitution of landraces with hybrid cultivars. Thus, the need for additional exploration for collection and conservation of root vegetable genetic materials in areas of vegetable diversity to maintain valuable germplasm for the present and future.
3. Biotechnology: tool for root vegetables production and improvement in Nigeria
3.1 Onions
Onion being a crop that is propagated by vegetative means with high heterozygosity, the cultivars are often low in reproductive fertility, making the breeding of this diploid species a challenging effort. Onion has a relatively long breeding cycle and genetic complexity, as well as sensitivity to inbreeding depression which determines the conventional methods adopted for onion improvement. Traditional breeding methods for onion involve mass selection for disease/pest resistance, improving or maintaining quality traits such as a bulb color, shape and increasing yields and shelf-life. This has given rise to the development of many good clones. According to [12], the yield potential of onion has remained relatively constant over centuries despite rigorous breeding efforts. Conventional breeding of diploids plants often involves screening and backcrossing of large number of plants in order to obtain the desired genotype. Selection of many desirable traits at the initial stage can be ineffective and/or time consuming. Hence, onion breeders have most times needed to screen a large number of seedlings (up to a million) to enable identification of a single clonal line that can pull through to the release of a successful cultivar. Biotechnology application has provided unparallel opportunities for plant production and quality improvement [13, 14, 15].
Possibilities for improving agronomically-important traits are limitless with the biotechnological applications like in vitro culture techniques, marker-assisted breeding technologies, genetic engineering, genome editing or a combination of all the novel gene technologies. To increase onion production and improvement through biotechnology, the following directions for research of this crop plant are suggested: (1) ploidy manipulation as an alternative technique to induce haploids in onions [16]. More haploid regenerants should be produced by ploidy manipulation to improve onion for breeding purpose; (2) embryo rescue techniques to enable successful intergeneric and interspecific hybridization as has been widely reported for other species [17]. Hybrid embryo developments have been developed through embryo rescue. These hybrids could have horticulturally- and agronomically- important traits; (3) protoplast regeneration and fusion have been used to improve a plant’s agronomic and horticultural characteristics such as pests and diseases resistance [18]. This technique should be used to produce more somatic hybrids that are pests and diseases resistant. Somatic variation can lead to creation of additional genetic variation in onion. Tolerance to herbicides, environmental or chemical stresses have been developed via this technique [19]. Somatic hybridization is an alternative technique to overcome both intraspecific and interspecific cross incompatibility to a large extent [20], and this technique could be used to introduce horticulturally- and agronomically-important genes in onions.
As genetic diversity is the basic input for breeding programmes [21] an understanding of genetic diversity among Nigeria onion germplasm collection is imperative for onion breeding.
Ijeomah et al. [22] studied and revealed the genetic diversity of 10 cultivars of spring onions in Nigeria using one SSR and three ISSR markers. The four markers yielded a total of 26 polymorphic alleles with polymorphic information content (PIC) values ranging from 0.6402 to 0.7569. The resulting UPGMA dendrogram showed that the 10 cultivars studied formed two main clusters with one subgroup showing no genetic distance among them. This study indicated the efficiency of SSR and ISSR markers to estimate the extent of genetic polymorphisms of spring onion cultivars with potential utility towards the conservation and management of Allium species.
3.2 Ginger
Most ginger improvement efforts have been restricted to evaluation and selection of naturally-occurring clonal variants. Conventional crossing efforts have been largely ineffective as a result of rare flowering and poor seed setting. Efforts at evolving high yielding clones through mutation and polyploid breeding indicated lack of success [23, 24]. Furthermore, the seed stock (rhizomes for vegetative propagation) seriously suffers from fungal and bacterial diseases such as Pseudomonas solanacearum (bacterial wilt), Fusarium oxysporum (yellow leaf), Pythium aphanidermatum (soft rot), Phyllosticta zingiberi (leaf spot), leading to heavy crop losses [25, 26]. Underground rhizomes are usually used as vegetative propagules for ginger which accounts for its very low multiplication rate [25, 26]. Different explant types used for micropropagation of ginger and other related species include meristem, axilliary buds, shoot tips and aerial pseudostems, although the commonly used explants are rhizome buds and shoot tips which have been reported as responsive explants for large-scale micropropagation to generate pathogen-free propagules [27].
An optimum fragment (explant) size is required for initiating successful tissue cultures. Sathyagowri and Seran [28] reported that rhizome buds of 0.5 cm in length were best for initiating ginger in vitro culture and shoot multiplication among the different tested explant sizes of 0.5, 1.0 and 2 cm long. The establishment of clean in vitro culture ginger from rhizome explants can be a daunting task as these underground explant is laden with pathogens resulting in contamination of cultures. As a general rule, absence of browning and freedom from contamination are criteria for the explants’ survival for subsequent shoot multiplication. Surface sterilization of explants is commonly carried out with disinfectants such as ethanol (C2H5OH), sodium hypochlorite (NaOCl) and mercuric chloride (HgCl2). Ginger aseptic cultures have been obtained by surface sterilization of the rhizome buds explants with 0.1% HgCl2 solution for 10–20 min [16], turmeric [29]. Rout et al. [30] established a surface sterilization protocol for ginger sprouting buds explants using 2% (v/v) Teepol for 15 min followed by 0.2% (w/v) HgCl2 solution for 25 min and several changes of sterile distilled water. Contamination-free in vitro culture of ginger can also be optimally achieved by sterilizing the rhizome buds with 20% Clorox for 20 min [27]. Although bacterial contamination can be a common challenge with the undgerground ginger rhizome bud explants, this problem can be overcomed by incorporating antibiotics to the initial culture medium.
Maintaining optimum environmental parameters such as light and temperature should also be of utmost consideration to ensure optimal growth of cultures; the cultures being generally incubated under photoperiod regime of 16 h light/day and 8 h dark/night with cool, white fluorescent light, 60–70% relative humidity and temperature of 25 ± 2°C in culture room [26, 30]. MS meadia [31] containing 0–5 mg/l cytokinin (BAP or kinetin) eiher used alone or in combination with auxin (NAA or IAA or IBA) are commonly used for multiple shoots induction and subsequent regeneration of ginger plantlets (Table 1) [27, 32]. Nkere and Mbanaso [33] investigated the optimal concentrations of phytohormones for in vitro ginger culture, and found a combination of 0.05 mg/l NAA and 4.0 mg/l BAP yielding the highest mean shoot multiplication rate of 4.25 (Table 1). Although shoot survival is a key factor in in vitro propagation, a treatment combination of 1.0 mg/l BAP and 0.05 mg/l NAA recorded a relatively high survival rate of 80% and resulted in approximately 4-fold mean shoot multiplication rate in ginger as shown in Table 1 [33]. Sharma and Singh [34] indicated that kinetin (Kn) outperformed BAP for vegetative bud multiplication. However, [25] reported that BAP at 17.76 μM (4.0 mg/l) yielded a 4-fold multiplication rate after the second subculture (Table 1). Balachandran et al. [35] also found using BAP alone resulted in better shoot multiplication than when BAP was combined with Kn in turmeric and ginger clonal propagation. Considering performance of the explants and the need to lower the cost associated with micropropagation, media containing the lowest concentrations of NAA (0.05 mg/l) and BAP (1.0 mg/l) which resulted in a very good survival rate of 80% and about 4-fold mean shoot multiplication rate [33] was recommended for in vitro ginger micropropagation. Zeatin was also reported as being more effective for microrhizome induction of ‘Bentong’ ginger, although its effectiveness on shoot multiplication was poor compared with BAP [32], with zeatin’s less effectiveness for shoot multiplication relative to BAP likely attributed to its high oxidative cleavage with prolonged incubation.
Studies on in vitro propagation of ginger directly from rhizome explants.
3.3 Garlic
Garlic research in Nigeria has been initiated with the breeding strategy focusing on collection, introduction, adaptation and selection of superior lines. However, there is a considerable morphological and physiological variation within and among cultivars [36]. Analysis of genetic diversity and relatedness between individuals is important for breeding purpose. Although garlic displays wide morphological differences [36], clonal propagation narrows garlic variation, given rise to a genetic bottleneck. This situation complicates garlic breeding programs geared towards improving preferred agronomic traits. In view of this, assessing the morphological and molecular polymorphisms in garlic genotypes originating from Nigeria is important for breeding programmes. Molecular characterization of 115 garlic accessions has been done in Ethiopia using 11 SSR markers [37]. There is paucity of information on genetic diversity studies of garlic germplasm in Nigeria using molecular markers.
Garlic is cultivated vegetatively due to its sexual sterility [38]. Vegetative propagation is achieved via division of the ground and aerial bulbs which results in lower multiplication rate. Many of the elite garlic cultivars succumb to diseases incited by viruses, nematodes and fungi, as well as insect pests [39]. Virus infection has been reported to reduce the bulb yield by 20–60%, and up to 80% with mixed infection, depending on cultivar and stage of infection [40]. The low propagation rate coupled with continuous accumulation of serious diseases such as viruses observed in the field warrants the development of in vitro propagation of garlic [41, 42]. There is a great influence of genotype on garlic in vitro cultures [43]. A combination of 1 mg/l 2,4-D + 5 mg/l BAP + 5 mg/l NAA resulted in 100% callus induction from root apices of garlic. The shoot has been reported to have higher callus induction frequency and higher callus fresh weight relative to root apices in some cultivars of garlic [44]. The use of root tips as explant greatly increases the regeneration potential than shoot tips as the number of regenerated shoots per explant was higher than that obtained from callus induced from shoot apices [42, 44].
3.4 Turmeric
Turmeric (C. longa L.), a cross-pollinated and triploid species (2n = 3x = 63), is propagated vegetatively through its yellow-fleshed rhizomes. Due to its numerous beneficial uses, the National Root Crops Research Institute (NRCRI) Umudike, Nigeria commenced research on turmeric in 1998 by pioneering collection of genetic resources and indigenous knowledge pertaining to its production and use. An active gene bank consisting of strong genetic base is a basic requirement for a sound crop improvement programme. Minor root and tuber crops nearly lacked a gene bank at the onset to enable take-off of meaningful crop breeding efforts. However, several germplasm exploration trips within the last decade resulted in the collection of a total of 76 accessions of Turmeric [45]. Ekiti State topped the list of collections (12) while Abia and Kwara were the least with one collection each. The passport data of the colections including the number collected and the local utilization at the source of collection are well documented to guide future breeding work. Following multi-locational evaluation at different locations in Nigeria—Jos, Otobi, Umudike and Igbariam, 10 genotypes were identified as promising and need to be further evaluated before their official registration and release to farmers.
Scarcity of planting materials resulting from low multiplication rate of turmeric (C. longa) limit their massive production. Micropropagation would help solve the challenge of limited planting materials. The phytohormone, 2,4-D, has been reported as the most effective auxin for callus induction in ginger and turmeric [46, 47]. MS media containing cytokinin (BAP or kinetin) either alone or in combination with auxin (NAA or IAA or IBA) are commonly utilized to induce multiple shoots and subsequent plantlet formation in turmeric (Table 2) [48, 49]. MS media containing 3.0 mg/l BAP yielded the best average vigor survival rating of [35]. This can be compared with results of other researchers as shown in Table 2 [29, 50]. This finding is in conformity with [51] who reported that the higher BAP concentration decreased shoot multiplication rate in turmeric among the different BAP levels (1–6 mg/l) tested. In turmeric, maximum rooting to multiple shoots was notable on half strength MS medium supplemented with 0.5 mg/l NAA [52]. Rooting on MS + 0.5 mg/l IBA of microrhizomes produced multiple roots/explants [53].
There is an enormous variation in Nigerian agricultural production landscape in terms of age groups, social class, gender, educational attainment and background. The motivations behind farmers’ enterprises, sustainability of systems are basically not the same. Therefore, the general assumption that farmers are solely motivated by the need to enhance food security does not always hold true. There are other intrinsic and extrinsic factors that propel farmers in their enterprise. Farmers have the desire for mastery of their enterprise and sense of achievement which arises when they have good harvests from their crops. As indicated in a Nigerian study, farmers of NRVs particularly onion, garlic, ginger in Kano state vary greatly in educational attainment (Table 3). Adewale and Oladeji [56] reported that onion, garlic, ginger farmers in Kano state are spread over all education levels. In Imo and Oyo states, farmers also have one form of education or the other [57]. This submission lends credence to the position of [58] that Nigerian farmers are not totally illiterate as they have one form of educational attainment or another. With this educational attainment, farmers are able to play a positive role in their societies as formal education provides an important foundation for knowledge, decision making and acceptance of vital innovation [59].
Variable
Characteristics
Producers (%)
Marketers (%)
Gender
Male
100.0
100.0
Female
0.0
0.0
Age
18–30
20.9
29.0
31–50
51.2
57.0
51 and above
27.9
14.0
Marital status
Single
2.3
14.0
Married
97.7
86.0
Educational attainment
Non formal
38.0
63.0
Primary
17.8
20.0
Secondary
24.8
17.0
Tertiary
3.9
0.0
Islamic
15.5
Not indicated
Experience in spices enterprise (years)
1–9
14.6
17.1
10–19
38.0
45.7
20–29
29.0
14.3
30–39
13.0
22.9
40–49
4.6
0.0
< 50
0.8
0.0
Table 3.
Comparative socio-economic characteristics of spices producers and marketers in Kano State, Nigeria.
The gender distribution of NRV farmers is also highly variable and location-specific. The different roles men and women play in the society and in agricultural value chains are caused by gender disparity which causes differences in the distribution of resources, activities, decision-making, wealth as well as enjoyment of entitlement and rights [60]. There appears to be a dominance of male farmers involved in NRVs in Kano state. This is however due to the socio-cultural and religious inclination that places restrictions on the women gender. However, in Oyo state both male and female are involved in production and marketing of root vegetable crops such as garlic, turmeric etc. As is common with production of other vegetables, there are more females involved in the production, processing and marketing [57]. Majority 51.3% of producers and 57.0% of marketers were within the age bracket of 31–50 years indicating that both producers and marketers are still strong, mentally alert and active. These enterprises thus provide windows of opportunity for younger generations to participate. Furthermore, people engaged in root vegetable production and marketing are mostly married as the marriage institution is held in high esteem among rural citizens whose occupation is mostly agrarian in nature. A large proportion (38.0%) of producers and 45.7% of marketers had more than 10 years of experience in production and marketing of root vegetables, respectively. This indicates that a large proportion of them are experienced in the production and marketing of root vegetables; they are therefore knowledgeable in the enterprise and are able to fully comprehend the intricacies and complexities of the enterprises.
Large household size usually characterizes farming households in Nigeria. According to [61], Nigeria has a national average of 5.9 persons per household. Large household is essential to traditional agriculture in Nigeria where the main source of labor is the family members. In some cases, members of the community are hired to work on the farms at pre-agreed sums of money, by barter (by giving crops or animals in return for work done on the farm) or by rendering services such as working on other farmers farms in return. In Kano state, a large proportion of these farmers have large household size which range between 10 and 20 persons [56]. This resonate the position of [62] who submitted that farmers in northern Nigeria maintain large households to meet the large labor requirements during farming seasons. This way, they make use of the children and dependents for farming activities rather than pay for hired labor. Majority of NRV are cultivated on farm sizes that ranged between <1 and 5 ha of farm land. This reveals that majority are small-holder farmers which confirm the characteristic peasant nature of agriculture in Nigeria. This is also as submitted by the National Bureau of Statistics [63] revealing the system of agriculture among farmers is on small holdings. Small holder production however is labor intensive and thus able to provide employment to a large share of rural population. Furthermore, when proper conditions are put in place to enable small farms grow and have access to markets, off-farm employment increases.
The annual income from root vegetables production is also shown in Table 4. It is indicative of a promising enterprise as majority (54.3%) earn an annual income between of
100,000–499,000 and 38.6% earn between
500,000 and 999,000. It reveals that root vegetables are capable of increasing the income of producing households which resound the view of [64] that root vegetables such as ginger, garlic, turmeric are crops of high value with the potentiality of enhancing household income and improving livelihoods.
Table 4.
Household size, landholding and income of root vegetable production in Kano state.
The estimates of marketing margin and marketing efficiency of garlic and ginger is shown in Table 5. The net marketing margin for garlic is
3, 449.0 per bag. For ginger, the net marketing margin is
2277.0. Ginger has a higher average marketing efficiency (1.37) which implies that for every
1.00 spent in purchasing ginger in Kano state, the marketer will accrue 37 k as returns to his/her investment. For garlic in Kano state, with an average marketing efficiency of 1.31, it implies that for every
1.00 spent in purchasing garlic, a return of 31 k will be gained. This further reveals root vegetable production and marketing as a promising enterprise capable of generating sufficient profit.
Variables
Garlic
Ginger
Total Marketing Cost
10,629
6156
Total Revenue
14,078
8433
Marketing margin
3449
2277
Marketing efficiency
1.31
1.37
Table 5.
Marketing margin and efficiency analysis for garlic and ginger in Kano state.
4.1 Economic values and contribution to livelihoods
Root vegetables such as ginger, garlic onion do not need expanse of land for profitable production, they can be cultivated without excessive investments as they are usually produced with little inputs (cash, labor and land). They are good crops to be included in small-scale farming systems and are also suitable for small garden production [64]. Farmers can derive a lot of benefits from root vegetables as crop with high value that requires little cash that can yield more than double the income accrued from staples and other horticultural crops [64]; to enhance household income and thus improve their livelihoods. As a production enterprise, they can be a source of additional employment opportunities for the farm family and money realized can be used as a ‘safety net’ during periods of need in order to improve livelihoods. Value-adding activities and sales of processed root vegetables can also be a good potential for small-scale processing (on-farm or off-farm) industries to generate higher income. Their production is especially an avenue to provide opportunities for women due to the fact that they can easily be grown in gardens in and around homes. In addition to providing opportunities for the womenfolk to start commercial enterprises and be able to participate in the local economy, root vegetables production enables women earn income for themselves, be involved in trading, create social networks, enhance their family status and their social status in the community and to provide added security to their household in case of abandonment or adversities.
5. Agronomy and production of root vegetables in Nigeria
5.1 Onion (A. cepa)
Onion is an important spice and one of the vegetables that is commonly consumed in Nigeria which produces underground bulb that are edible, with about two million metric tonnes of Onions produced annually in Nigeria, according to Mr. Muhammad Abubakar, the Minister of Agriculture and Rural Development, at the 4th Regional Onion annual conference held in Kano, an event organized by Regional Observatory on Onions for West and Central Africa, in conjunction with National Onion Producers, Processors and Marketers Association of Nigeria (NOPPMAN). Nigeria is part of the largest onion producers in the world with over 2,000,000 tonnes produced annually [65]; Nigeria being ranked sixth among the top 10 producers of green onion, and 11th position in dry onion production globally. Major growing ares of onion in Nigeria include Kano, Kaduna, Jigawa, Sokoto, Plateau, Bauchi and Kebbi states. Production figures in 2012 indicated about 240,000 tons of green onions and 1,350,000 tons of dry onions were produced in Nigeria (Figures 1 and 2), with dry onion production having an increasing trend. Nigeria had a world share of 5.5% of a total 4,339,925 tons of green onions produced in 2012 and 1.6% share of a total 82,815,927 tons of dry onions produced globally in 2012 [66].
Figure 1.
Production/yield quantities of onions, dry in Nigeria.
Figure 2.
Production/yield quantities of onions, shallots, green in Nigeria.
5.1.1 Climate and soil
Onions can grow in different climatic conditions. However, it requires moist soil during the early growth stages, likewise hot or dry weather during the maturity and harvesting periods. It produces flowers prematurely in very cold weather conditions (resulting in smaller bulb size). Onions require a loose soil with reasonable depth. Clay soil is not suitable as it gets waterlogged and hardens when dried up. The soil needs to be slightly acidic (pH between 6.0 and 7.0), must also be fertile with humus [67].
5.1.2 Land preparation
The land should be properly plowed, farm manure could be plowed into the soil. Ridges are constructed after plowing when planting is about to start.
5.1.3 Planting operations
Onions are planted either by seeds or by setts, the seeds need to be raised in the nursery for 6–8 weeks under a conducive condition. Planting is done at a spacing of 10 cm apart around November or December. It takes about 3–4 months for the crop to reach maturity.
5.1.4 Management
The farm is kept weed-free because onion can not withstand weed competition. Weed should be removed regularly so as to enhance high yield as weed competition reduces yield. First weeding is done 14–21 days after transplanting. The plants are watered regularly because onion plants can not absorb water from deep layers of the soil, that is why the top soil is always kept moist. Irrigation system (either drip or surface) can be used in as much as the soil is not flooded, because onions do not thrive in soils that are flooded and poorly drained. After irrigation, mulching is used to conserve water and inhibit weed growth. Onion does not require much fertilizer in the case where the soil is fertile. However, 20 tons/ha of poultry manure should be incorporated into the soil a week before planting. Also, 75 kgN/ha can be applied at 4 and 6 weeks after transplanting [67].
5.1.5 Harvest and storage
Maturity period of onions is 3–4 months, and once the leaves begin to dry off naturally, it can be harvested. Onions are harvested by uprooting the bulb and cutting off the roots and leaves. In Sokoto, it was reported that onion takes between 112 and 161 days from sowing to harvest or 15 and 8 weeks for the early and late sown onions, respectively, while the time of forming bulbs was 56 days [68]. Maximum plant height of 69 cm has been recorded and number of leaves of between 10 and 13 has also been observed. Yield of up to 48 t/ha has been recorded in the early crop but yield of less than 20 t/ha has been observed for those planted after December [68]. Thorough sun-drying after harvesting is very necessary before bagging for transportation to market or storage in a well ventilated and perfectly constructed silo. This is necessary to reduce the moisture content as much as possible thereby increasing its shelf life.
5.2 Carrot (Daucus carota)
Carrots are one of the most well-known, delicious, very effective, nutritious root vegetables. Carrot is majorly grown in the Northern part of Nigeria, especially in Zaria, Sokoto, Kano and Jos [69]; and carrot production trends in Nigeria are shown in Figure 3. Carrot is described as a root vegetable with diverse colors like orange (most common), purple, black, red, white, and yellow. Carrot comprises mostly the taproot which is the main edible part and the green leafy part which is crispy when fresh and also eaten but not common.
Figure 3.
Production/yield quantities of carrots and turnips in Nigeria.
5.2.1 Weather and soil
Carrot is a cold-weather crop, but can also thrive well in warm climates. Optimum temperature for good growth is 16–20°C, temperatures beyond 28°C will drastically reduce its growth. Carrot grows well in sandy soil, when planted in rows of at least 10 cm apart on a raised seedbed, it starts to germinate at about 1–3 weeks and can be harvested at any size, but better when it has turned bright orange for enhanced flavor. There are several varieties of carrot that are cultivated in Nigeria with a high rate of productivity, some of which include the Danvers variety which is larger in size and requires a balanced level of soil fertility to grow well. Other varieties include Chantanay, Nantes, Armsterdam, among others. Carrots are well adapted to a wide variety of soils, although deep, loose, well-drained soils rich in humus are suitable for commercial carrot farming. In Nigeria, the best soil for cultivation of carrots is loamy or sandy loam soils rich in humus [70].
5.2.2 Land preparation
Soil should be properly prepared by repeated deep plowing (at least 20–30 cm deep), harrowing, leveling, and cleaning will enhance desired yield. The soil must be loose, friable, deep, and well-drained in order to enhance effective germination of seeds. Because carrot seeds are very small and delicate, a fine seedbed of convenient size should be prepared before sowing. Carrots are taproots that penetrate and grow downwards so while cultivating the carrots, you should avoid rocky or stony areas to prevent stunted growth. It is advisable to make ridges for planting carrots, which should be higher than those for planting a crop like maize. Ridges are necessary at a considerable height level for the maximum penetration of the carrot.
5.2.3 Planting
Carrots are propagated using seeds, clean, disease-free and viable seeds from reliable sources will enhance good productivity. Complete seeds germination takes approximately 7–21 days. Seeds are lightly covered with soil after planting. Some farmers irrigate the field about 24 h prior to sowing to ensure that enough water is present in the soil at the time of sowing. The Fertilizer recommendations should be based on soil analysis.
5.2.4 Irrigation
Light irrigation should be done immediately after sowing while subsequent irrigations are applied as necessary. Excess water makes carrot to be light colored, short with large diameter.
5.2.5 Harvest
In case of special markets, carrots can be harvested early when they have not fully developed; or else, they should be allowed to reach full maturity stage in the soil. But they will become hard and unfit for consumption if they are retained in the full maturity stage. Carrots are harvested when the roots are ca. 1.8 cm or larger in diameter at the upper end. After harvesting, carrots are carefully washed, sorted by size then packaged for future endeavors.
5.3 Turmeric (Curcuma longa)
Turmeric (Curcuma Longa Linn) is a type of root vegetable belonging to the same family (Zingiberaceae) as ginger [71]. It is a tropical perennial plant, originated from India and Indonesia, which is widely cultivated throughout the tropical regions of the world. It is one of the most essential spices used as a culinary ingredient world wide for which it is referred to as the “golden spice of life” [72]. World production level of turmeric is 11–16 tonnes annually. Nigeria being the fourth largest world producer, produces about 3% of the world annual production [73]. About 76 cultivars of turmeric exist in the gene bank of the National Root Crop Research Institute (NRCRI), Umudike with some being evaluated in multilocational trials [74]. The prevailing favorable edaphic and climatic conditions in Nigeria place the country in a position to play a leading role in turmeric production. Turmeric is cultivated both under rain fed and irrigated conditions.
5.3.1 Soil requirement
Turmeric also thrives well in deep soil tilt with heavy manure for high yields. Fertile and friable, well-drained, loamy soil ranging from sandy loam to clay loam with high organic matter are required for turmeric cultivation [75]. Moreover, flat land with gentle or no slope is equally recommended.
5.3.2 Land preparation/planting
This commences with selection and clearing of the site followed by bed preparation because turmeric thrives well on beds, ridges or even on mounds at the onset of rains possibly around April or when the rains must have stabilized around May or June. Bed size of 3 × 2 m is advised so as to reduce human movement on the beds during farming operations. Seedbed can be prepared using tractor or manually using a spade or a hoe. Turmeric is propagated by rhizomes using mother rhizomes as planting material [76], planting distance of 30 x 50 cm [77] at a depth of 10 cm to give optimum yield [78] is recommended. The rhizomes are about 10–15 g with one or two buds [79] therefore, about 1 ton of setts are needed to plant 1 ha.
5.3.3 Mulching
Mulching is important in cultivation of turmeric, the first mulching should be done immediately after planting followed by a second mulching at 8 weeks after planting. Mulching aids moisture conservation, enhances germination, controls weeds, modifies soil temperature, adds nutrients to the soil and improves soil fertility for optimum yield. Mulching can be done with elephant grass (straw) at the rate of 12 t/ha [78].
5.3.4 Weeding
Pre-emergence herbicide can be used to control weeds in turmeric farm. Weed control is normally done within 4–6 weeks after planting depending on rate of weed emergence. According to [80], under Umudike condition, critical time of weed interference is between 8 and 12 weeks after planting. Yield loss in turmeric due to weed competition can range between 3 and 55%.
5.3.5 Fertilizer application
Fertilizer at the rate of 60 kg N, 13 kg P and 25 kg K/ha 2 weeks after planting [81] on a sandy loam Ultisol has been reported. According to [82], this recommendation translates to application rate of 200 kg/ha N:P:K 15:15:15 and augmenting with 30 kg N/ha. Animal dung or droppings can also be used to improve soil fertility in the absence of inorganic fertilizer as application of poultry manure has been reported to influence rhizome yield of turmeric [83]. Application of lime at the rate of 2 t/ha to soil with pH of 5.91 in combination with 200 kg/ha of NPK 15:15:15 fertilizer has been reported for turmeric production on an Ultisol in South eastern Nigeria [84, 85] whereas organic manure of 30–40 t/ha plowed into the soil and inorganic fertilizer of 60 kgN, 50 kg P2O2 and 120 kg K2O per hectare in split doses is recommended by National Horticultural Research Institute (NIHORT), Ibadan [67].
5.3.6 Harvesting and yield
Maturity period for turmeric is 7–9 months after planting when the leaves turn yellow and starts wilting. Harvesting is carried out by uprooting the whole rhizome with spade or hoe after which the mother and finger rhizomes are separated. Yields of turmeric ranges from 20 to 25 t/ha. Some cultivars under research-managed farms yielded 35,000 t/ha [67, 75].
5.4 Ginger (Zingiber officinale)
According to FAOSTAT (Figure 4), Nigeria is the third-largest producer of ginger in the world producing more than 300,000 tonnes between 2014 to 2018. The global ginger market share for Nigeria is about 11%, after only India (35%) and China (18%). Nigeria produces over 400,000 Metric tonnes of Ginger across the 36 states annually. Cultivation of ginger began in Nigeria in the year 1927 around Kwoi, Kubacha, Kafanchan and Kagarko areas of southern Kaduna State and some neighboring parts of Plateau State, but it is now cultivated in different parts of the country [86]. Kaduna, Bauchi, Benue, Gombe and Nasarawa are the top five producing states of ginger in Nigeria. Ginger is now cultivated in Sokoto, Osun, Anambra, Zamfara, Akwa Ibom, Oyo, Abia and Lagos states, but southern Kaduna remains the largest producer [87]. In Nigeria, two varieties (reddish and yellow) are commonly grown but different cultivars like UG1, UG2 and Maran are available in the country, UG1 produced higher yields than UG2 and it is reported to be more pungent [88]. Average yield per hectare is about 13–27 metric tonnes in Nigeria compared to the world average of about 35–40 metric tonnes.
Figure 4.
Production/yield quantities of ginger in Nigeria.
5.4.1 Seed selection
Selection of ginger seed for planting as reported by respondents in Jaba region is based on the size of the tuber irrespective of the variety, those with width of about 5–6 cm and thickness of about 2–3 cm are often selected for planting. Apart from seed size, number of budding tendencies which is about 5 and above and non-physical damage to the tuber are also part of criteria for seed selection which is usually carried out after harvest around September to October.
5.4.2 Planting
As reported by respondents in the Jaba region, this is normally done in the month of April to early May. This is carried out manually after adequate moisture is ensured following successive rains. Planting entails digging with hoe, majorly by a male, while children or females drop the bud seedling in the hole and cover it. Planting distance is usually 10–13 cm side wise, this helps to reduce weed competition. After planting, the entire field is covered with dry grass to enhance quick germination and protect the seedling from the effect of sun heat. The dry grass is normally left on the field for about 1 month when the ginger is expected to have fully germinated, after which it would be removed and packed along the furrow to decompose.
5.4.3 Fertilizer application
According to Alhaji Adamu Shekari while describing the importance of of applying fertilizer to ginger stated that “the profit of ginger farming is determined by fertilizer” [89]. The general view from farmers indicated that fertilizers are applied three times during the growing period of ginger. The first is during field preparation which is mainly organic manure. Respondents gave reasons for this practice that inorganic fertilizers may cause the young ginger plants to wilt or die because it may be too hot for them when applied. This agrees with the findings of [90] that organic manure like ash has high pH of about 10.2 that neutralizes soil acidity. The next fertilizer applications involved majorly inorganic fertilizers like NPK or Urea which are applied after second and third weeding.
5.4.4 Harvesting
Maturity of ginger is usually reached when leaves of ginger plant begin to turn yellowish brown in color indicating readiness for harvest. Harvesting is done by digging out the tuber majorly by male workers while female workers or children collect the tuber into containers.
5.5 Garlic (Allium sativum)
India produces average yield of about 5.23 t/ha making it one of the world’s largest garlic producers [91]. Spain, Egypt, Korean Republic, Argentina, Italy, China, and the United States are other growing countries. According to [92], garlic production is about 10 million tonnes per annum which represents only ca. 10% of bulb onions production. Garlic has been cultivated for decades in the northern states of Nigeria like Kano, Sokoto, Borno, Bauchi, Jigawa, Katsina, and Zamfara [93]; and garlic production figures in Nigeria are shown in Figure 5. Its wide distribution is denoted by its common “native” names in different societies. It is called “Tafarnuwa” in Hausa and “Ayu” in Yoruba [94].
Figure 5.
Production/yield quantities of garlic in Nigeria.
5.5.1 Climatic and soil requirements
Garlic can grow well in both tropical and sub-tropical environment [95] but it is a cold weather perennial crop that has a high requirement for nutrients and water [96]. It grows well in region with 600–1200 mm annual precipitation and temperatures ranging between 5–25°C and 25–40°C [97]. Fertile loamy soils free from stone and gravels and that is well-drained is suitable for garlic production. Heavy soils are unsuitable for growing garlic because the bulbs produced under such condition will be deformed and difficult to harvest [97].
5.5.2 Seed preparation and treatment
Garlic cloves that are about 8–12 g in size which are detached from the bulbs and soaked in clean water for about 6 h before planting and removal of the outer skin from the bulblets and dried are used as planting materials. These cloves are mixed with fungicide and insecticide to control fungicides and seed attacking insects before planting [97].
5.5.3 Planting
Garlic has fibrous root and the bulbs comprise small bulbils referred to as cloves, which are the vegetative planting materials for the crop [98]. Garlic can be planted either by dibbling, drilling or broadcasting [97]. Dibbling method is commonly practiced, it involves putting one clove in a hole 7 × 15 cm at a depth of 3–6 cm, placing the growing point upwards and covering lightly with soil. About 350–600 kg cloves are needed to plant one hectare corresponding to 400,000–500,000 plants/ha [97]. Garlic produces well when cultivated on fertile well-drained sandy or silt-loam soils with good moisture retention capacity [99]. Ahmed et al. [100] recommended large clove size and irrigation at 3-day interval for good performance under semi-arid conditions like Sokoto in Nigeria.
5.5.4 Fertilizer and its application
Nitrogen is a major nutrient required for growing garlic. Bulb growth was significantly affected by applied nitrogen [101]. Longer leaves and higher number of leaves were recorded when nitrogen rate was increased to 100 kgN/ha [102]. In another experiment [96], increasing rates of nitrogen application to 150 kgN/ha increased growth and yield components of garlic but higher rates of nitrogen above 150 kg significantly reduced growth and yield. Farooqui et al. [103] reported that 200 kgN/ha significantly increased yield parameters such as neck thickness, bulb diameter, number of cloves/bulb and fresh weight of 20 cloves. However, at Samaru, Nigeria [104] growth and yield of garlic was significantly increased with application of nitrogen with 15 t/ha recorded as maximum yield when 90 kgN/ha was applied. Meanwhile, [105] reported significant increase in bulb yield with application of 75 kgN/ha but clove weight increased only at 150 kgN/ha after which a significant decrease was observed. Magaji et al. [99] recommended application of 50 kgN/ha for improved plant height (cm), number of leaves/plant, leaf area, number of bulbs, and the total yield of the garlic.
5.5.5 Harvesting
At 18 weeks after planting when leaves of garlic are partially dry and bend to the ground, it is assumed that garlic has reached optimum maturity period and the bulbs are harvested [97].
6. Pest and disease management in turmeric (Curcuma longa)
In Nigeria many pests attack turmeric in the farm. According to [106] such pests include leaf roller, shoot borer, scale insects to mention but a few. The diseases includes leaf blotch, leaf spot and rhizome rot. The symptoms appear as small oval, rectangular or irregular brown spots on any side of leaves and soon turning to dirty brown, they decrease yield and are controlled by spraying chemicals. The incidence of rhizome rot, leaf spot, leaf blotch, thrips, leaf folder and cutworm are the major cause of the yield loss in turmeric [107]. Nirmal et al. [108] reported that the rhizome rot results in 50–80% loss during storage. Jagtap et al. [109] recorded 34–57% yield loss due to the incidence of leaf spot Colletotrichum capsici ((Syd.) Butler and Bisbyl). The survey conducted in various turmeric growing states of South India revealed that the rhizome rot is caused predominately by Pythium aphanidermatum (Edson.) Fitz. [110, 111]. Fungicides like metalaxyl, mancozeb, carbendazim, alliete, propiconozle, hexoconozole etc. are widely used by the farmers for the management of disease complex in turmeric [107].
Sarathi et al. [111] has evaluated the efficacy of C. longa (Turmeric) as possible botanical pesticide for managing insect pests of okra on the field and they observed differences in efficacy and yield between lambda-cyhalothrin and C. longa which was used as the extraction solvent. Their study revealed that C. longa compete well with lambda-cyhalothrin in controlling pest infestation and produced yields that were significantly higher than plots without treatments. Therefore, they recommended that farmers should consider using it as a botanical pesticide. Sarathi et al. [111] also reported the antifungal activity of two varieties of turmeric (white and red) rhizome extracts on fusarium wilt pathogen (Fusarium oxysporum f. sp. lycopersici). Their findings revealed that both varieties had comparable antifungal activity on the mycelial growth of the test pathogen at 15% extract concentration and that the highest mycelial growth was 17.7 and 25.2%, respectively.
6.1 Pest and disease management in ginger (Zingiber officinale)
Ginger cultivation is affected by both biotic and abiotic factors. Biotic factors are viruses, bacteria, fungi and nematodes [112, 113] with bacteria being the most important causing wilt and soft rot. The next major pathogen after bacteria is fungi which causes rhizome rot, soft rot, sclerotium rot and yellows disease. Nematode causes root-knot disease while viruses causes mosaic and chlorotic fleck in ginger plants leading to reduction in rhizome yield of the plants. In River State of Nigeria, a survey was conducted by [114] and it was reported that the most abundant arthropods in stored ginger was mites then Lasioderma serricorne. Reports have it that Aspergilus flavus, Aspergilus niger, Fusarium oxysporium and Rhizopus sp. were fungi pathogens isolated from dried ginger samples with high occurrences of A. flavus. According to a survey in Rivers State, Nigeria, stored ginger has no immunity against arthropod pest infestation and fungal infection. It was also observed that population dynamics of arthropod pests of stored ginger was affected by seasonal variation. The four main diseaseS and pestS of ginger as reported by THE National Agricultural Advisory services (NAADS) Uganda are rhizome rot, soft rot, root-knot nematode and rhizome scale.
6.1.1 Rhizome rot
It is a partly/completely decomposition of rhizome tissue; root may dry, wet, soft or slimy and color turns black. This rot is caused by the fungus (Fusanum Oxysporum) both in the field and post-harvest when ginger is washed in dirty (recycled) water, poor aeration and storage in contaminated sheds where rotten piles of reject ginger is allowed to accumulate.
6.1.2 Soft rot
Soft rot is caused by fungus Pythium gracile, either alone or in combination with a bacterium Erwinia species. The fungus Pythium is a water-mold and develops maximally in moist conditions when temperatures are favorable. Prolonged wet weather, therefore, creates ideal conditionS for development of soft-rot disease in ginger.
6.1.3 Root knot nematode
It is caused by the nematode (Meloidogyne incognita) which attacks both the root and ginger rhizome, resulting in warty overgrowth on them. The nematode normally burrows themselves into the soil and form knots on the roots and rhizome. In this way, they attack the plant by feeding on the food and nutrients which should be used by the plant for growth and production. Therefore, because of starvation with nutrients and food, the plant dies.
6.1.4 Rhizome scale
The size and shape of this scale insect is about 0.5–2 mm in diameter and usually circular in shape. They normally have a shell color brown on top which act as coverage and protection. It attacks the rhizome of the plant by sucking out the juice from it thus resulting in wilting and death of the plant. It is hard or difficult to treat this pest (using insecticides or even hot-water treatment) because of the shell forming on top which protects the insect from dying.
Apart from these major pest and diseases, others include Fungi and bacteria such as Thread blight Ceratobasidium sp.; Corticium sp., Stem rot (Athelia rolfsii), Leaf spot (Magnaporthe Grisea).
6.1.5 Nematodes
Burrowing Nematode—Radopholus similis (associated with rhizome rot); Reniform nematode- Rotylenchulus reniformis (yellowing leaf drying and stunting); Pin nematode Criconemoides onoensis
6.1.6 Yellow leafspot of ginger
Over the years, ginger yellow leaf spot disease has posed a serious challenge to increased ginger production in Nigeria [115]. The most susceptible stage which is three- to four-leaf stage coupled with high humidity has been observed to be conducive for the disease spread. Ginger plants of up to 6–7 months old are also susceptible to this disease. Grasses have been reported as reservoir hosts while agents of dispersal are rainwater and wind [116]. This disease spread widely in ginger-growing areas with resistance to benomyl (Benlate 50wp), mancozeb (Diathane m-45), and Kocide 101 (copper hydroxide) treatments. In the rainforest agro-ecology of Nigeria, though there may be appearance of early signs of the disease but severity is usually noticeable towards the end of vegetative growth.
6.1.7 Management of diseases
The most recommended method used to control these pest and diseases is the use of hot water treatment (51°C). Spraying indoxacarb at 10 ml in 15 l of water, or novaluron at 10 ml in 15 l of water at 15 days interval has been reported to be very effective in controlling shoot borer and leaf roller. Dipping the seed rhizome in quinalphos prepared by dissolving 1 ml in 1 l of water can be used to control rhizome scale insects that destroys rhizomes. Good drainage coupled with treatment of seed rhizome by dissolving 3 g of carbendazim and mancozeb in 1 l of water for almost 30 min before storage should be done to prevent rhizome rot. Moreover, treating seed rhizome by dissolving 2 g of streptomycin in 1 l of water for 30 min canbe used to effectively control bacterial wilt that causes milky ooze when rhizomes are gently pressed.
6.2 Pests and disease management in onion (Allium cepa) and garlic (Allium sativum)
Nutrient-deficient and poorly irrigated soils, low technology adoption rates coupled with pests and diseases have curtailed onion and garlic cultivation in Nigeria. Ewuziem and Alleluyanatha [116] highlighted purple blotch (Alternaria porri), black mold diseases (Aspergillus niger), neck and bulb rot (Botrytis allii), Onion twister (Colletotrichum cingulata), downy mildew (Peronospora destructor) pink rot (Pyrenochaeta terrestris) and bulb rot incited by Fusarium oxysporium as fungal diseases of economic importance affecting onion and garlic cultivation in northern Nigeria.
The biggest threats to making profit on onion or garlic cultivation in northern Nigeria is insect pests, majorly thrips. Thrips tabaci (Thysanoptera: Thripidae) constitute a major threat to onion which is capable of reducing bulb yield if onion is planted late. Other minor insect pests are Zonocerus variegatus and Spodoptera exigua. Late onions can harbor about 600 thrips in a plant, with the largest population being harbored by the third youngest leaf, notwithstanding the age of the crop [117]. Thrips are also controlled with the use of Lambda-cyhalothrin, neem formulation (2–3 ml/l). Malathion (2 teaspoon/l), Diazinoon 50 WP (1 table spoon/4 l), Bayfidan (triadimenol) 20 EC (2 teaspoon/4) or Dimethoate (0.05%).
Report by Kebbi Agricultural and Rural Development Authority (KARDA) in Kebbi State of Nigeria stated that a major limitation to profitable cultivation of onions is the incidence of a disease locally called ‘Danzazzalau’. The devastating effect of this disease on onion productivity necessitated the study reported and concluded that Fusarium equiseti was responsible for the disease. Fusarium equiseti living in the soil affect the roots, stem plate and fleshy leaf bases of the onion plant.
6.2.1 Purple blotch of onion
Pattern of variation for resistance to purple blotch (Alternaria porri) of onions (A. cepa L.) in North western Nigeria was also reported by [118]. According to the report, five cultivars of onion: Ori local, Kaharda, Sokoto, Red Creole and Koumassa were selected based on genetic backgrounds diversity in respect of resistance to Alternaria porri (Ellis.) Cif. In a complete diallel cross of some cultivars, 25 F1s generated and their parents evaluated in a yield trial at Sokoto and Talata Mafara in Zamfara State both of Nigeria, with 31.20%, 30.58% and 5.42% disease incidence observed as phenotypic, genotypic and environmental coefficients of variability, respectively [119].
6.3 Garlic (A. sativum)
Emechebe et al. [118] evaluated the insecticidal properties of garlic aqueous extract on beans (Phaseolus valgaris) and maize (Zea mays) pests at different concentrations and concluded that there was relationship between extracts and mortality of Sitophilus zea mays and Callosobruchs maculatus. The report of the study carried out by [120] revealed that garlic (A. sativum) at 5% aqueous extract concentration exhibited antifungal potential when tested against the mycelial growth of southern blight pathogen (Sclerotium rolfsii) of tomato. Percentage inhibition was reported to be 77% at 5% concentration in vitro and disease severity was as low as 2.7 in vivo.
6.4 Pests and disease management in carrot
Carrot is affected by a variety of pests and diseases [121]. Reducing of plant vigor and growth are part of the effects of nematodes on crops. In nematode affected fields, some plants will be observed to be heavily infested while others will not, resulting in uneven crop maturity or reduction in the quality of the produce [122, 123]. Root knot nematodes (Meloidogyne species) causes general reduction in plant vigor likewise severe distortions and root swelling thereby reducing the marketability of root crops like carrots [124]. Flea beetle, white flies (Daccus sp.), aphids, cutworms, and horn worms are among insects that destroy carrot plants [125]. The insects can be controlled by planting resistant varieties and judicious use of short acting pesticides like malathion and carbaryl (Sevin). Calcium deficiency causes blossom end rot which can be treated by addition of lime to soil or spraying the leaves with calcium solution. This disorder can also be contolled by maintaining adequate soil moisture [122, 123]. Hill et al. [122, 123] concluded that in most locations, diseases severely restrict carrot cultivation. In Nigeria and other African countries, small holder carrot cultivation has experienced a great increase both as food and cash crops in recent years, and effective management of pests and diseases is essential for sustained carrot production. Wittwer [126] highlighted major fungal diseases of carrot as Alternaria and Cercospora leaf spots or blights, leaf mold, Fusarium wilt, target spot or early blight some of which are soil-borne. Plants are affected at any developmental stage by Septoria leaf spot [127]. Powdery mildew caused by Oidium lycopersicum is another important fungal disease. According to [128] scouting for disease and rogueing infected plants once they are observed is very important. Soil-borne diseases of carrot according to [129] are bacterial soft rot, cavity spot, cottony rot, crown rot, phytophthora, root die-back, root knot nematode and southern blight [126]. Carrot is considered as being capable of producing higher yields and returns to vegetable farmers in some part of the country after the rainy season [130], which calls for effective crop protection to maximize farmers’profit.
7. Postharvest technology of Nigeria root vegetables
Root vegetables are mainly functioning roots. Bulbs, corms, rhizomes, and tubers; they have high moisture content making them highly perishable. A moisture content of up to 52 and 72% have been reported for ginger and garlic, respectively [131], which make these root vegetables highly vulnerable to huge postharvest losses. For instance, up to 50% of onions harvested in the northern states of Nigeria were lost during storage [132]. The postharvest losses of these commodities require that they are taken through a number of postharvest operations in order to keep them wholesome for consumption and increase their utilization. There are different low-cost technologies that have been applied in Nigeria, the primary processing involves sorting, blanching, curing, splitting or peeling and drying to a moisture level of 7–12%. In some cases, pretreatments are applied to improve the quality of the final product. This step varies depending on the crop being processed. These postharvest operations will be discussed in this chapter.
7.1 Postharvest operations
7.1.1 Curing
This helps to release the unique aroma of root vegetables such as ginger, garlic and tumeric that are used as spices, curing is usually done before storage by spreading the bulbs to a thickness of about 5 cm in a shaded and well-ventilated place for 3–4 days. This allows the bulbs to receive dry air which helps to maintain the quality in storage. It is important to remove the tops before curing. Garlic can also be cured by packing in jute sacks in a well-ventilated shady place [97]. The processing of ginger in Nigeria has however not been standardized. Ginger rhizomes are sorted, washed and splitted or peeled. The traditional methods vary considerably resulting in mold growth and loss of important volatile oils [133]. Curing enhances the valued yellow color and aroma of tumeric which is attributed to its curcumin content. The rhizomes must however be separated from the fingers before curing
7.1.2 Peeling
The skin of whole ginger must be peeled to allow for removal of the water content as it constitutes a barrier for evaporation during drying. Peeling is usually done with knives by scraping off the skin. The use of mechanical peeling machine is gaining popularity among ginger processors; however, the abrasion of machine is similar to hand-scrapping. The time spent during peeling is very critical as it contributes to the loss of volatile oils. In Nigeria, peeling is majorly carried out when the ginger is meant for culinary purposes [134]. Peeled ginger does not attract high prices compared to the splitted ginger and farmers consider it time consuming.
7.1.3 Splitting
Ginger splitting is the most common and widely acceptable processing operation that is carried out before drying. Flavor components are concentrated under the peel, thus splitting helps to retain as much as 20% of the flavor that could be lost due to peeling.
7.1.4 Blanching
Blanching is not desirable for ginger and garlic. Although blanching is generally helpful in inactivating spoilage causing organisms and enzymes, ginger, onions and garlic take exceptions as the specific flavors they are valued for are reduced by blanching. Tumeric’s quality, on the other hand, is enhanced by blanching in the presence of an alikali [135]. In a study conducted by [136] blanching at 100°C for 10 min before drying was shown to yield tumeric powder with higher color value and curcumin content compared to those that were not blanched or those that were boiled before drying.
7.1.5 Sun drying
Sun drying still remain the predominant dehydration methods for ginger, garlic and onions in Nigeria. The processing of the Nigerian ginger has not been standardized resulting in low rating in international market and loss of foreign exchange earnings. The microbial load, organoleptic properties and chemical composition usually fall short of specifications. There have been reports of the presence of salmonella, aflatoxin and molds in dried split ginger and ground ginger from Nigeria which have been attributed to poor drying and occurrence of fungal infection. This limits Nigeria’s access to the international market thereby creating the need for serious efforts in improving food safety and quality of dried root vegetables.
7.1.6 Storage
Cool environment with temperature of between 10 and 15°C is suitable for storage of dried rhizomes, slices and splits. Higher temperatures above 23°C can cause losses of up to 20% of its oleoresin. These postharvest operations are usually carried out on ginger from November to January in the northern states of Nigeria, but middlemen can do further sorting and cleaning after getting them from the farmers. The price of these root spices is determined by the extent of sorting, drying and packing done at individual farmers’ level. Ginger that have been thoroughly washed and dried hygienically to produce a white-light cream slices is referred to as “American standard”. The lower grade is usually called Mozo and the lowest grade of ginger are the unsorted dried ginger rhizomes.
7.2 Culinary uses
Garlic, onions and ginger are consumed fresh or dried in form of spice and also as ingredient to flavor various dishes. Tasty spice powders are produced from garlic cloves, ginger and tumeric rhizomes and onion bulbs by drying mainly in the sun. Garlic is also used to cover the odor and flavor of salted meat and fish. Dry root spices in powdered form are used in many homes and restaurants in Nigeria. It is a common seasoning applied to pasta, pizza, and grilled chicken. They are common components of spice mix which are usually seasoned with salt.
7.3 Traditional/medicinal uses
In addition to their nutritional benefits, root vegetables also have medicinal value, and their consumption has been used traditionally to treat digestive tract problems, morning sickness, arthritis, high level of cholesterol, etc. They are also good for healthy eyes and good-looking skin. Garlic posseses digestive, carminative and anti-rheumatic properties. Since acient times, garlic is used in formulation of Ayurveda curing muscular pain, giddiness, lungs, heating intestinal ulcer, etc. Different phyto-remedies including ginger, turmeric, onion, and garlic have achieved foremost importance in prevention of metabolic syndrome. Ginger is well known for its medicinal properties, ginger can help with alleviating cough, flu, rheumatoid arthritis and travel sickness based on reports. It aids digestion and said to be an inflammation fighter. It is also said to reduce cholesterol and blood pressure level and provides better blood circulation. Ginger is recognized as one of the most important constituents in herbal medicine. Herbal medicine practitioners use ginger as medicine for treating many diseases.
7.4 Value added products
Despite the volume of ginger production in Nigeria, the country is yet to tap into the global market of value-added products; the level of value addition to ginger, garlic, tumeric and onions in Nigeria is very low. Some of the processed products include dried slices, powder, paste, ginger beer, spice mixes, teas, ginger honey, pickle, oil, oleoresin, candy, soft drinks and juice. Increased storage life, reduced transportation cost and foreign exchange earnings are the main advantages of preparing value-added products from these root vegetables.
7.4.1 Dehydrated products
Garlic and ginger are exported either as dried flakes or powder. These spice powders are gaining considerable recognition globally and locally. They are popular in household, restaurants, eateries, caterers, and clubs. Ease of handling and usage has made the demand for these processed products increase everyday.
7.4.2 Enhanced Ogi paste
Ogi is a cheap and readily available health-sustaining fermented porridge commonly consumed in West Africa. It has been documented as the most popular fermented health food in many countries in West Africa. It is economical, easy to process and the raw materials are readily available, and a choice food for weaning children [137, 138]. The inclusion of ginger and garlic to ogi has been explored as a means of enhancing its taste and acceptability. Inclusion of garlic has been shown to improve the color and texture, while the use of ginger improves the aroma. Inclusion of both contributes to improved nutritional qualities of ogi.
7.4.3 Spiced jam
Fruit jams are important in the diet and eaten by people of all ages. In order to improve the nutritional content and the health functionality of jams, produced watermelon, apple and pineapple jams spiced with garlic, ginger and/or turmeric. A good jam which has a soft even consistency and exhibits a high nutritional and antioxidant quality was produced. The total soluble solid and total titratable acidity were found to be within recommended range by Codex Alimentarius [139].
7.4.4 Oleoresin
Oleoresins are commonly used in many industries because of their strong aroma and flavor. They possess some advantages over raw or dried spices as flavoring agents [140]; their flavors are 5–20 times stronger than that of the corresponding spices and they are commonly sold as spice drop. Oleoresins are essential oils, their microbiological advantages, uniformity in flavor, pungency, and ease of storage and conveyance made them to be preferred. For a good yield of oleoresin, ginger should be harvested at 7–8 months after planting [141]. Through the collaboration of the Raw Materials Research and Development Council (RMRDC) and Belphins Nigeria Ltd., Kaduna State ginger oleoresin was produced in Nigeria which showed favorable physico-chemical and microbial properties [142]. The traditional method is usually a manual process which entails preliminary processing and hand pressing while the improved method comprise of chemical extraction and mechanical expression. Locally, the demand for ginger oleoresin is on the increase from both food and pharmaceutical industries but local production has not been able to meet the demand; there is still a dependence on importation. Therefore, the country has huge potentials for expanding oleoresin production to take advantage of the ever expanding local and global oleoresin market.
7.5 Nutraceuticals
The awareness and concerns for the physical wellbeing of humans has been a major driving force for the nutraceutical market in Africa. The inclusion of nutraceuticals in the diet is gaining popularity because they are considered to be natural, safe with less side effects compared to products that are chemically derived. The processing of nutraceuticals has become a means of livelihood especially in the South-west and South-east of Nigeria. Based on industry reports, nutraceuticals market value of Africa is projected to be over USD 430 billion by 2025 and it’s expected to register a good Compound Annual Growth Rate (CAGR) of 6.05% during the projected period of 2020–2025 [143]. Nigeria, Namibia, Morocco and Egypt are markets said to drive the sales of pet nutraceuticals. Neutraceuticals in Nigeria are mainly herbs, medicinal and aromatics. The industry is active but small in value involving traditional productions.
7.6 Chemical and nutritional composition of root vegetables
The place of cultivation and postharvest treatments influence variation in the chemical and nutrient components of root vegetables. Tables 6 and 7 shows the nutritional composition and chemical components of some root vegetables in Nigeira; with the highlights of nutrients, antioxidant and antimicrobial properties presented below.
Garlic (DW)
Ginger (DW)
Turmeric (DW)
White onions (WW)
Red onions (WW)
Moisture (%)
4.55
6.37
8.92
89.62
88.48
Ash (%)
4.08
6.30
2.85
3.33
3.17
Crude protein (%)
15.33
8.58
9.40
3.22
3.02
Fat (%)
8.58
5.35
6.85
2.17
6.50
Crude fiber (%)
2.10
3.25
4.60
3.83
2.83
Carbohydrate (%)
73.22
68.15
67.38
87.44
84.48
Sodium (mg/100 g)
4.10
4.10
NA
0.40
0.37
Calcium (mg/100 g)
26.30
25.76
21.00
1.36
1.32
Iron (mg/100 g)
5.29
3.46
4.50
0.09
0.12
Phosphorous (mg/100 g)
10.19
12.56
63.00
3.09
3.68
Potassium (mg/100 g)
54.00
215.00
46.00
14.29
16.02
Zinc (mg/100 g)
0.34
0.04
NA
0.026
0.03
Manganese (mg/100 g)
0.001
0.002
NA
0.01
0.01
Magnesium (mg/100 g)
4.10
5.00
0.92 (ppm)
0.86
0.97
Vitamin C (mg/100 g)
8.00
4.80
8.40
14.67
18.00
Table 6.
Nutritional composition of selected root vegetables in Nigeria.
Root vegetables like ginger, onions, garlic and turmeric are known for their rich mineral and very low fat content. Many Root vegetables have absolutely no calorie content thereby making it a weight loss diet. Root vegetables contain significant quantities of minerals such as magnesium, iron, calcium, and potassium.
7.6.2 Antioxidants
Root vegetables which are also used as spices are excellent sources of natural antioxidants; they contain antioxidant enzymes like catalase, glutathione peroxidase and superoxide and non-enzymatic antioxidants such as ascorbic acid, polyphenols, carotenoids and chelating agents. Ginger contains polyphenol compounds like gingerol and its derivatives like zingiberone, bisabolene, camphene, geranial, linalool, borneol and oleoresin. These bioactive compounds are responsible for the sensory attributes and capacity to delay food spoilage or inhibit disease causing reactions or agents that have been reported. Diverse bioactive compounds have been reported in garlic [152]. Flavonoids (flavones and quercetins) and sulfur-containing compounds (allyl-cysteine, diallyl sulfide, and allyl trisulfide are found in garlic, and [153] reported the antioxidant properties of the garlic compounds allyl cysteine, alliin, allicin, and ally disulfide. The S- allyl-L-cysteine sulfoxide is converted into allicin (which produces the unique odor associated with garlic). Consumption of garlic preparations have been shown to reduce blood lipid peroxidation while increasing the vitamin E concentration [154]. The curcumin in turmeric is a natural phenolic compound. Apart from being a coloring pigment, it has the capacity to neutralize free radicals [155].
7.6.3 Antimicrobial properties
The biological and antimicrobial properties of spices are also linked to the presence of bioactive compounds. Allicin present in garlic exhibits a broad spectrum of effects on a variety of fungal species. Phenolic compounds in ginger have high antimicrobial activities and effectiveness for the control of certain viral, bacterial and fungal diseases. These plants are used in many countries for food preservation. Gingerols and gingerdiol are the main anti-fungal components of ginger. Some antiviral properties have been reported for ginger. Ginger is reported to be effective in management of hepatitis C virus infection where viral clearance is affected [156, 157].
Vegetables plays pivotal role in the Nigerian economy by providing food, nutritional and economic security to households and higher returns to producers. In addition, vegetable crops have high productivity and short maturity cycle leading to higher returns per unit area and time. Globally, without significant increase in yield of vegetables, the strategy to reduce poverty is impossible. In Nigeria with diverse agro-ecological conditions that favor the cultivation of several types of fruit, vegetables and other classes of crops, diversification into NRV cultivation provides interesting and profitable opportunities. A very promising premise in the production of NRVs is the ease of cultivation, they do not need large area of land for profitable production. Successful cultivation could be achieved without excessive investments as they could be cultivated with little inputs like cash and land. Their production, though more labour intensive can provide twice the amount of employment in one hectare of land used for production compared to staple food crop production. Their value chain is longer with more complex stages than staple crops as a result of available job opportunities [158]. This is beneficial for generating additional employment opportunities in rural areas where labour abounds for attaining widespread and equitable growth.
NRVs are particularly viable enterprise for women, the landless and youths because they can be suitably produced in gardens, in and around homesteads, likewise provide opportunities for profit-oriented enterprises and opportunity to contribute to the local economy [64]. There are good potentials for entrepreneurship development by small-scale on-farm and off-farm processing that provides higher income from value-adding activities and sales of processed spices. The growing demand for high-value crops world-wide is an opportunity for rural households to diversify towards spices enterprises considering the strong potential for higher returns to land, labor and capital.
There is the need for breeding programmes to develop improved cultivars of existing cultivated NRVs. This will encourage farmers into more cultivation and commercialization with the attendant benefits inherent in cultivation and potential impacts of NRVs. In spite of the diverse opportunities inherent in production of NRVs, farmers still operate under high cost of production such as high transportation costs which reduce their productivity to a very large extent. In Nigeria, other factors that affect high productivity in small scale farming include input availability and use. As opined by [159], a continuous cycle of low productivity, income, input availability and use is prevalent among farmers in Nigeria as yields involve combination of education by extension services, access to appropriate and timely inputs along with ability to access finance to purchase inputs. It also further buttress the findings of [160] who posited that poor road network affect the quality of life of producers, their ability to transfer produce to markets which increases spoilage of harvest stalling enterprise sustainability, productivity and income.
As shown in Table 8, there are diverse constraints militating against optimum production of root vegetables. These constraints include challenges occasioned by infrastructural factors like high cost of inputs such as seeds, seedlings and other requirements for a successful production enterprise. This ranks as a prime constraint in the production of root vegetables in Kano state, Nigeria. Other infrastructural constraints are high transportation costs, high labor costs, non- availability of inputs and labor for farming activities as well as pest and diseases. Institutional constraints that challenge optimum production of root vegetables include inadequate technical advice, extension support, knowledge of production of root vegetables and poor market information.
Constraints
Mean
Rank
High cost of inputs (seeds, seedlings, fertilizers)
Production of root vegetables should be popularized especially among rural, smallholder farmers who have limited land resources and agronomic inputs. Their short production cycles compared to other field crops and their high-value, low-volume, high-yields should be promoted as added advantages. The general public needs to be well-informed about accessible and affordable sources of micronutrients and antioxidants which impacts highly on human health and wellbeing. More researches on integrated conservation techniques, promotional campaigns to enhance consumption and utilization as well as promotion and development of value chains are required for a better understanding of benefits inherent in nutrient-dense NRVs. Reviews such as this that provide insights into the great benefits of root vegetables grown in Nigeria including perspectives on agronomy, genetics and breeding, biotechnology, nutraceutics and socio-economics, would ultimately enhance their production and utilization for healthy lives.
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Emmanuel O. Ajayi, Pamela E. Akin-Idowu, Olaide R. Aderibigbe, Dorcas O. Ibitoye, Gloria Afolayan, Oluyemisi M. Adewale, Esther A. Adesegun and Benjamin E. Ubi
Submitted: 04 July 2022Reviewed: 29 July 2022Published: 17 October 2022
Open access peer-reviewed chapter
