With a record production of 2.010 Million tons of marketable cocoa beans produced in 2016-2017 (ICCO, 2017), Ivory Coast cocoa farming covers an area of more than 2 176 000 hectares ( 6% of the national territory) and with more than one million farmers. In addition, cocoa generates more than 30% of export earnings and accounts for more than 15% of Ivorian GDP.
In Côte d’Ivoire, four main constraints render Ivorian cacao plantings extremely vulnerable to climate change:
i) ageing trees, where 19% of cacao plantings are older than 30 years;
ii) low levels of improved material, where 70% cacao plantings are ‘unimproved’ farmers’ selections;
iii) high pest pressure from mirids (30-40 %), Black Pod (20-45 %), and Cocoa Swollen Shoot Virus,
iv) increasingly longer periods of drought (3 to 4 consecutive months of drought in some cocoa producing regions).
All of these factors will constrain performance under drought. Apart from tree or leaf death (figure 1), drought causes pod and seed desiccation and rotting, and seeds sticking to the pod linings, thereby reducing productivity and quality (figures 2 and 3).
Four contrasting agro-climatic zones were selected for establishing test plots to collect primary data: Abengourou, Bouaflé, Divo and Soubré (see figure 4).
1) Abengourou area, located in eastern Côte d'Ivoire. Between 2004 and 2016 this zone was subject to an interannual evolution of rainfall marked by three (3) periods: two water deficit periods (2004 to 2006, and 2013 to 2016) characterized by negative pluviometric indices, between which, intervenes a surplus period (2007 to 2012) marked by positive rainfall indices. The linear trend curve with a positive slope of almost zero (0.0008) shows a slight upward trend of rainfall over the period 2004-2016.
2) Bouaflé area, located in central-western Côte d'Ivoire. In contrast to Abengourou, Bouaflé has been subject to an interannual change in rainfall marked by four (4) periods between 2004 and 2012: two periods of surplus (one in 2004 and the other from 2007 to 2012) characterized by positive indices and two deficit periods lasting two years each (2005 to 2006 and 2011 to 2012) characterized by positive indices. The linear trend line has a steep and negative slope (-0.0952) which indicates a downward trend of precipitation towards increased drought over the period 2004 to 2012.
3) Divo area, located in central-southern Côte d’Ivoire. The year-to-year change from 2004 to 2017 was marked by three periods: two surplus periods (2004 to 2007 and 2012 to 2017) separated by a deficit
period (2008 to 2011). The negative slope (-0.011) of the linear trend line reveals a general downward trend in rainfall over the period 2004 to 2017.
4) Soubré area, located in south-western Côte d'Ivoire. Between 2004 and 2017, Soubré's seasonal rainfall series was marked by 4 periods including two surpluses (2004 to 2006 and 2010 to 2013) and two deficit periods (2007 to 2009 and 2014 to 2017). The negative and steep slope (- 0.039) reflects a general downward trend over the entire period 2004-2017.
The cacao genotypes available for this study include a total of thirty-eight (38) CNRA hybrid families of which 15 (F1 to F15) were planted in the areas of Abengourou, Bouaflé, Divo and Soubré at the same time as 15 local farmer-selected cultivars, selected for their agronomic and quality performance.
Measurements and counts carried out tree by tree focused on the parameters below:
Production and technological quality parameters per tree:
- Number of healthy, gnawed and rotten pods
- Number of wilted chérelles and total chérelles (to calculate the rate of wilted chérelles)
- Pod weight
- Number of beans per pod (normal beans and flat beans)
- Weight of 100 ‘commercial’ cocoa beans
- Weight of 1 commercial cocoa bean.
Vigour parameters measured per tree:
- Collar diameter 30 cm above the ground
- Circumference of tree trunks 130 cm above the ground
- Height of the tree
Morphological and physiological parameters:
- Length and width of leaves to calculate leaf area (on 3 adult leaves / tree)
- Foliage density (on a scale of 1 to 4)
- Foliar density (on a scale of 1 to 4)
- Flush Intensity (on a rating scale of 1 to 4)
- Sensitivity to leaf loss following a long drought (range 0-5).
- Relative humidity of the soil at 20 cm depth
- Daily rainfall data at different times
- Pluviometry with direct reading on the different sites.
With the exception of field resistance to brown rot of hybrid family pods (assessed by the rate of rotten pods) a highly significant zone-family interactions (probability <0.0001) were identified for all other characters measured.
This suggests that for the characters involved, the agroecological zone impacts the behaviour of the families. In particular, the study demonstrated the negative impact of drought on bean production evaluated by the weight of 100 cocoa beans. Bean production was significantly lower in Abengourou (more intense drought) compared to Divo (low rainfall area). It is interesting to note, however, that for the majority of the parameters studied, six hybrid families were ranked among the top ten families in all study areas, thus highlighting the stability of these families for these traits regardless the study area.
The study also demonstrated the feasibility of selection using:
- three (3) criteria (vigour, leaf density and canopy density) that are potentially innovative for evaluating the resilience of cocoa trees to drought;
- four (4) farmer-selected families remarkable for their low rate of brown rot (4-9%) in Abengourou, a region where P. megakarya is the most damaging species in the field;
- nine (9) farmer-selected families remarkable for their average weight of 100 high cocoa beans (140.3 to 168.3 g), especially in Abengourou where the drought had a negative impact on the size of cocoa beans.
This study highlights the importance of evaluating farmer-selected material for a standard range of physiological traits in a follow-on phase. For the first phase of the 12-month project, the achievements presented are just as important as they are encouraging. Additional observations are nevertheless necessary to confirm the results thus obtained.
For further information, the focal point is Dr Tahi Gnion Mathias, Researcher, Cocoa Geneticist, Cocoa Program Manager at CNRA. Mail: email@example.com