Quantitative and Health Evolution of National Yam Genetic Resources under Ex Situ Conservation in Côte d’Ivoire ()
1. Introduction
Yam of the genus Dioscorea, is an important crop cultivated in many countries around the world, particularly in West Africa in tropical zones [1]. Yam is the most important food crop in Côte d’Ivoire and contributes significantly to the food security of Ivorians, with an annual production of about 8 million tonnes [2]. Optimum yields range up to 60 - 70 t/ha, but the national average yield is 8 - 10 t/ha for the yam species Dioscorea rotundata and 10 - 15 t/ha for the yam species Dioscorea alata [3].
Yam is consumed in boiled, pounded, fried or braised form [4]. With a production of 4.2 million tonnes, i.e., 60% of national production, the D. alata yam species remains the most important yam species in Côte d’Ivoire [5]. However, 75% of the yam trade is dominated by the D. rotundata species, whereas D. alata is mainly self-consumed by producers [6].
Despite its importance, this foodstuff faces several difficulties, particularly in collecting genetic resources with a view to increasing both genetic variability and yields, as well as combating the various diseases and pests affecting this crop. To limit the extent of the harmful effects of climate change, it is essential to conserve genetic resources derived from studies, collections, introductions and creations. The conservation of genetic heritage forms the basis of all work in genetic improvement, agronomy and crop protection.
With a view to preserving the genetic variability of the national yam collection of the Centre National de Recherche Agronomique (CNRA), several research and collection projects have been carried out with funding from the CNRA, West Africa Agricultural Productivity Program (WAAPP), RTB Breeding-Yam (formerly AfricaYam) and RTB Breeding-Quality (formerly RTBfoods) projects. These projects have enriched yam collection through new collections of traditional varieties, the introduction of IITA hybrids and, above all, the ongoing creation of hybrids (715 cloned D. rotundata and 177 D. alata hybrids in G1; 762 cloned D. rotundata and 183 D. alata hybrids in G2, and 34 cloned D. alata and 30 D. rotundata hybrids in G3) since 2011.
The aim of this study is to take stock of the collection by analysing the composition, origin and quantitative and sanitary trends of yam genetic resources from 2011 to 2022, and to assess the incidence and severity of diseases (anthracnose, Yam Mosaic Virus) of yam accessions in the national collection of the Centre National de Recherche Agronomique (CNRA) from 2016 to 2022.
2. Materials and Methods
2.1. Materials
2.1.1. Study Site
The study was carried out in Bouake, in central Côte d’Ivoire, at the Food Crops Research Station (FCRS), located at 7˚4'N and 5˚2'W. The region has a typical Baouleen climate. The rainfall pattern consists of two rainy seasons, from April to July and September to October, and two dry seasons, from August and November to March [7]. The average annual rainfall is between 826.1 and 1396.8 mm (Figure 1). The soils at the Research Station for Food Crops are ferralitic, gravelly, branched, shallow and derived from granitic material [8].
Figure 1. Annual pluviometry of the study site from 2010 to 2022.
2.1.2. Plant Material
The material used in this study consists of yam accessions of the genus Dioscorea, comprising the species Dioscorea alata, Dioscorea rotundata, Dioscorea esculenta, Dioscorea bulbifera and Dioscorea dumetorum from the national yam collection. This collection consists of local or traditional varieties and improved varieties (hybrids) of various origins and varietal creations through intraspecific crosses.
2.2. Method
2.2.1. Experimental Design
The accessions were arranged in 7 blocks of 38 lines. Each accession consisted of 5 mounds. The dimensions were as follows:
The accessions are replanted each year on a plot at the CNRA’s Food Crops Research Station in Bouake, Côte d’Ivoire.
2.2.2. Observations and Measurements
The yam accessions in the collection were counted each year after the harvests. This was done from 2011 to 2022. It is possible to determine the loss or increase in accessions of each type of yam from one year to the next by using this count. This was achieved by subtracting the number of accessions planted from the number harvested.
Observations and measurements were made 3, 5 and 7 months after planting.
The average incidence and severity indices for each disease were calculated for anthracnose and mosaic virus.
They focused on the incidence and severity of anthracnose and Yam Mosaic Virus. Incidence of diseases (I) is defined as the proportion of diseased plants in each population. It is used to assess the development or spread of the disease. The incidence of each disease is determined by each microplot using the following formula:
Disease severity is the overall degree of damage (disease symptoms) caused to the plant or part of the plant (leaf, stem or root). Disease severity levels are assigned to each plant based on the observation of the characteristic symptoms of each disease under investigation. This is done on a scale from 1 to 5 [9]. Healthy plants have a score of 1 and diseased plants have a score from 2 to 5, i.e., from the least severe to the most severe (Table 1).
The severity index (or Symptom Severity Index, SSI) estimates by summing severity scores > 1 in a plot divided by total number of symptomatic plants. It is estimated by the following formula:
Table 1. Disease symptom severity rating scale [9].
Rating score |
Proportion of leaf area and stems showing symptoms |
1 |
Not present (healthy) |
2 |
<25% (mild) |
3 |
25% - 50% (moderate) |
4 |
50% - 75% (severe) |
5 |
>75% (very severe) |
2.2.3. Data Analysis
The various data (from 2011 to 2022) were analysed using Excel software. This included the number of accessions per year and health data on anthracnose and Yam Mosaic Virus. Averages were calculated and curves and histograms were plotted.
3. Results
3.1. Origin of Yam Accessions in the Collection in 2022
Of the 404 accessions in the collection in 2022, 303 accessions were of local origin, representing 69%. 108 accessions came from International Institute of Tropical Agriculture (IITA) 24%, 8 accessions from Puerto Rico (2%), 8 accessions from Cameroon (2%), 7 accessions from India (2%) and 2 accessions from Brazil (0.4%). The remaining 5 accessions came from New Caledonia, the Philippines, Martinique, Trinidad and Haiti (1%), i.e., 1 accession per origin (Figure 2).
Figure 2. Origin of yam accessions in the collection in 2022.
3.2. Evolution of the Number of Accessions in the Yam Collection
from 2011 to 2022
The number of yam accessions in the national collection varied from year to year and from species to species. Between 2011 and 2022, it decreased from 513 to 441 accessions. Losses and gains were also observed during the period of this study. Between 2011 and 2013, the number of D. alata yam accessions decreased from 284 to 200, a loss of 84 accessions, while the number of D. rotundata accessions decreased from 220 to 158, a loss of 62 accessions. This loss was very high for Dioscorea alata and Dioscorea rotundata yams, with 148 accessions lost each. An increase in the number of accessions was observed in 2014, with 67 accessions for D. alata and 42 accessions for D. rotundata, before a decrease in 2015 (36 accessions). From 2015 to 2017, a slight alternation of increase and decrease was observed, until the number reached 240 accessions for D. alata and 186 for D. rotundata in 2017. In 2018, the number increased to 281 accessions before oscillating between 281 and 283 until 2020, an increase of 43 accessions compared to 2017. A loss of 23 accessions was observed in 2021.
For D. rotundata, the number increased by 54 accessions in 2019 and 2020-2022, with a slight alternation between an increase and a loss of accessions. For the other 2 species, there was an increase in the number of accessions from 2011 to 2022. For D. esculenta, the number of accessions increased from 7 to 11 and for D. bulbifera from 2 to 3. The number of accessions for D. dumetorum remained constant at 2 from 2016-2022 (Table 2).
Table 2. Evolution of the number of accessions per type of yam from 2011 to 2022.
Year |
Different types of yams |
D. rotundata |
D. alata |
D. esculenta |
D. bulbifera |
D. dumetorum |
2011 |
220 |
284 |
7 |
2 |
0 |
2012 |
180 |
210 |
7 |
1 |
0 |
2013 |
158 |
200 |
7 |
1 |
0 |
2014 |
200 |
267 |
7 |
1 |
0 |
2015 |
182 |
243 |
7 |
1 |
0 |
2016 |
208 |
247 |
10 |
1 |
2 |
2017 |
186 |
240 |
11 |
2 |
2 |
2018 |
161 |
281 |
10 |
2 |
2 |
2019 |
215 |
283 |
11 |
3 |
2 |
2020 |
183 |
283 |
11 |
3 |
2 |
2021 |
208 |
251 |
11 |
3 |
2 |
2022 |
184 |
241 |
11 |
3 |
2 |
3.3. Health Status of Dioscorea alata and Dioscorea rotundata
Accessions
3.3.1. Incidence of Yam Mosaic Virus and Anthracnose Dioscorea
rotundata
Figure 3 shows the average incidence of anthracnose and Yam Mosaic Virus. For Yam Mosaic Virus, the incidence varied between 46.8% and 100% from 2016 to 2022, with the highest incidence (100%) before a resurgence in 2021, when 46.8% of the accessions in the collection were infected. For anthracnose, the infection rate varied between 28% and 100%, with the highest rate of 100% observed for 2 consecutive years from 2016 to 2018, and a decrease from 2018 to reach the lowest rate (28%) in 2020 (Figure 3).
Figure 3. Average incidence of anthracnose and Yam Mosaic Virus in D. rotundata accessions in the collection from 2016 to 2022.
3.3.2. Incidence of Yam Mosaic Virus and Anthracnose Dioscorea alata
The histograms below show the average incidence of Yam Mosaic Virus and anthracnose on Dioscorea alata accessions in the collection. In all years, the rate of Yam Mosaic Virus decreased over time, with the highest value of 100% in 2016 and the lowest value of 38.2% in 2021. As for anthracnose, a slight variation in the infection rate of the accessions was observed, ranging from 90.51% to 100% between 2016 and 2022 (Figure 4).
Figure 4. Average incidence of anthracnose and Yam Mosaic Virus in D. alata accessions in the collection from 2016 to 2022.
3.4. Degree of Symptoms of Yam Mosaic Virus and Anthracnose in
the Accessions of D. alata and D. rotundata in the Collection
3.4.1. Dioscorea rotundata
From 2016 to 2022, the average severity varied between 2 and 4. However, the average severity of Yam Mosaic Virus was constant from 2016 to 2017, before decreasing from 4 to 2 from 2017 to 2018. A slight variation in average severity was observed from 2018 to 2022, with an increase followed by a constant phase and a decrease (Figure 5). The average severity of anthracnose also fluctuated. From 2016 to 2017 there was a slight increase from 2 to 3, followed by a decrease in 2018, with the value 2 remaining constant until 2022 (Figure 5).
Figure 5. Average severity of anthracnose and Yam Mosaic Virus in D. rotundata accessions in the collection from 2016 to 2022.
3.4.2. Dioscorea alata
Variations in the average severity of Yam Mosaic Virus and anthracnose in the D. alata collection from 2016-2021 are shown in Figure 6. The average severity of Yam Mosaic Virus developed from 3.5% in 2016 to a peak of 4.3% in 2017. From 2018 to 2021 a decrease in severity was observed, from an average of 4.3% to the lowest severity of 1.6% in 2021. For anthracnose, the average severity varied between 4.5 and 2.3 for all D. alata yam accessions in the collection. Apart from 2017, when the accessions had a high anthracnose severity of 4.5, it was less severe in 2016 (3.8), 2018 (3.6), 2019 (3.4) and 2020 (3.3). The lowest anthracnose severity was observed in 2021 with a score of 2.3 (Figure 6).
Figure 6. Average severity of anthracnose and Yam Mosaic Virus in D. alata accessions in the collection from 2016 to 2022.
4. Discussion
The national genetic resources of yam of the genus Dioscorea have diverse origins. It is essentially composed of 5 species, namely D. alata, D. rotundata, D. esculenta, D. bulbifera and D. dumetorum enriched over the years by the various introductions of yam cultivars and and species of yam over the years from institutes such as: Overseas Office of Scientific and Technical Research (ORSTOM) and Institute of Savannahs (IDESSA), which has enabled the genetic resources of yam conserved ex situ at the RSFC of the CNRA of Bouake to have several provenances. Yam accessions and to increase their number. These results are in with those obtained by Kouakou in 2010 [6]. They indicated several parallel collections and introductions from countries in the sub-from countries in the sub-region, South America and the West Indies.
In 2011, following the post-election crisis, the genetic resources in Abidjan were transferred to Bouake to continue research. The samples collected in 2008 and 2009 were used to enrich the different yam species, bringing the total number of accessions to 502. The increase in the number of accessions is also due to the surveys and collections carried out in 2013. In addition, D. alata and D. rotundata clones from IITA were introduced in 2018 and evaluated at the Food Crops Research Station Bouake. As shown by the work of Dumont et al. (2010), important collections have been made in Côte d’Ivoire [10].
The decline in the number of accessions during this study was observed mainly in two species, Dioscorea alata and Dioscorea rotundata. This decline is thought to be due to post-harvest losses caused by poor harvesting and transport techniques, resulting in wounds and bruises, which are the main entry points for diseases and insects in the tubers. According to Nteranya and Adiel (2005), the high rates of tuber loss are due to poor agricultural practices at harvest, during transport from farms and during storage, which cause tubers to rot [11]. The susceptibility of certain cultivars to storage pests, as pointed out by Foua-bi in1982, means that certain cultivars, such as those of the D. alata species, are more susceptible to pest attack than those of the D. rotundata species, depending on the thickness and hardness of the tuber periderm, which has led to enormous losses in the D. alata species [12].
The ex-situ conservation method for genetic resources is limited by the repeated replanting of infected seeds in the same or neighboring fields. This leads to an accumulation of pathogens in the soil, creating increasing disease pressure over time [13], resulting in losses of accessions, as observed in our study. The same applies to annual changes in rainfall and temperature, which can favor or hinder the growth of specific pathogens. For example, humid conditions can promote the development of fungal rot, while dry seasons can stress plants and make them more susceptible to nematode attacks [14].
The prevalence of yam anthracnose in the D. alata collection was 100% for four consecutive years and varied from 100% to 20% in the D. rotundata collection. The severity varied from year to year for the D alata species and remained constant for the D. rotundata species from 2018 onwards. These results can be explained by the cultivation practices and the contaminated plant material used to establish the collections. Most of the cultivars used have not been improved and are often susceptible to diseases [15]. In 2005, George et al. showed that anthracnose is present in all regions of the world where yams are intensively grown, with D. alata species being more susceptible than other species [16]. The disease was more severe in 2017 due to heavy rainfall during the six months after planting, from June to September. Contrary to the results of the 2017 study by Yao et al., which showed that anthracnose severity varied by agroecological zone, it varied over the years in the same zone [17].
The prevalence of Yam Mosaic Virus was most pronounced in D. rotundata, with an incidence of 100% for four consecutive years, before decreasing to 46% in 2021, with a severity ranging from 4 to 2.13. The prevalence of Yam Mosaic Virus decreased over the years, reaching a low of 3.9% in 2022 in D. alata, with a severity ranging from 1.1 to 3.5. In fact, many cultivars, especially new improved cultivars, have shown some tolerance to viruses. As a result, improved varieties have a capacity for disease resistance that limits the effect of the virus on accessions [18]. Our results showed that all collections were 100% infected, which may be justified using plant material affected by Yam Mosaic Virus for the conservation of these genetic resources. In 2009, research by Séka et al. highlighted the strong presence of yam Yam Mosaic Virus in Bouake, with an incidence rate of over 80% and very high severity scores [19].
However, the environment can affect the virulence of mosaic and anthracnose, as well as the effectiveness with which they are transmitted. Extreme weather conditions, such as heatwaves and heavy rainfall, can create conditions conducive to disease outbreaks, even in resistant varieties, as observed in our study [14].
Therefore, we must continue to record yam accessions in the collection. This will lay the groundwork for future research focused on selecting and developing promising yam species, creating chemical composition tables, developing preservation techniques to produce enriched flour for children, processing compounds derived from these species to ensure their sustainability, and effectively managing natural resources to reduce pressure on wild species while increasing agricultural diversity [20].
The conservation method employed in our study is more restrictive than in vitro conservation, which is less space- and time-intensive. However, our goal is to distribute virus-free in vitro plants to farmers, researchers, and other interested parties both nationally and internationally [21].
5. Conclusion
Yam genetic resources under ex situ conservation in Côte d’Ivoire are composed of 5 species of the genus Dioscorea, of which the alata comprises 241 accessions, the rotundata 181 accessions, the esculenta 11 accessions, the bulbifera 3 accessions and the dumetrium 2 accessions to date. These genetic resources have undergone a discontinuous evolution, with loss of 72 accessions between 2011 and 2022. However, these losses have been minimised thanks to local and external collections and introductions during these years. In terms of health, two diseases have affected genetic resources: anthracnose and Yam Mosaic Virus. These diseases have had a detrimental effect on the various collections, especially anthracnose, which is highly prevalent in D. alata, and Yam Mosaic Virus in D. rotundata. This increase in the prevalence of anthracnose and Yam Mosaic Virus, particularly in these two species, is of concern to the CNRA, which could use existing biological control program to clean up yam genetic resources.
Acknowledgements
The authors would like to express their gratitude to the Centre National de Recherche Agronomique (CNRA).