ASSESSMENT OF THREE CASSAVA VARIETIES RESPONSES TO CASSAVA BACTERIAL BLIGHT (CBB) IN THE SEVEN AGRO-ECOLOGICAL ZONES OF CÔTE D’IVOIRE DURING A SURVEY IN 2017.

Several cassava varieties are grown in Côte d’Ivoire. Some of them are more widespread than others. Their dissemination depends on characteristics such as yield, taste and dry matter content. Cassava bacterial blight pressure constitutes a threat in all cassava agroecological zones. This study aims to survey the widely used varieties Conclusion:-This study showed the geographical repartition of the three cassava varieties assessed and also the most widespread in Côte d’Ivoire. It also showed that the local ones (Akama and Yace) were still very much accepted than the improved one (Yavo). The presence of Akama and Yavo considered as sweet varieties in the AEZ6 help to understand that the geographical distribution of the varieties is changing. The varieties are in a process of adaptation in a new environment previously defined as unfavourable for their growth. Except Yace which didn’t show a susceptibility to CBB in the AEZ4 and AEZ5, the others were susceptible in all the AEZ where they were found at different rates. Yavo was the most susceptible in all AEZ excluding in the AEZ6 where it was less susceptible than Akama and Yace; however, the varieties susceptibility differed from one AEZ to another. The AEZ6 was characterized by the high level of Akama and Yace susceptibility while the AEZ5 was characterized by those of Yavo susceptibility. These zones were followed by the AEZ4 for Akama and Yavo and the AEZ1 for Yace. This behaviour in the AEZ pointed out an interaction between the varieties, the pathogen and the environment. Since these factors seems to affect the relationship between the disease and the varieties, it would be important to test these varieties for other control strategies in order to prevent yield losses due to the strong pressure of CBB. Ethics approval and consent to participate'Not applicable' for this section.

Cassava is naturally drought tolerant, has a greater adaptability to climate and soil, thrives in different texture of soil and can even grow on poor and acid soils, which are often detrimental to other crops such as maize, millet and sorghum. It is also useful for the prevention of hunger through the gradual harvesting of tuberous roots and leaving the surplus in the soil. It is also available throughout the year for households and in times of agricultural and social instability (Burns et al, 2010; PACIR, 2013; Bodnar, 2012; Yao et al., 2013).
The cassava cultivars can be divided into two major groups, the sweet ones and the bitter ones according to the content of hydrocyanic acid which is very high in bitter varieties (Akpingny et al., 2017). This characteristic plays a role in their adaptive resilience to environmental to conditions. For instance, according to Perrin et al. (2015), some bitter varieties can be grown in some places of the northern part of Côte d'Ivoire whereas the sweet ones cannot. Different cassava varieties, local and improved cultivars are grown in Côte d'Ivoire. Examples of locally wellknown varieties Akama (also called 'Six mois' or Kaman), Yace, Tambou and Bonoua (Dje Bi et al., 2018;Perrin et al., 2015). The improved ones are Yavo or TME07, Bocou 1, IM8 and TMS4 (2) Cassava cultivation is threaten by Cassava bacterial blight (CBB), one of the major worldwide threat to cassava production (Ogunjobi et al., 2010). CBB is responsible for high economic losses up to 100 % of the total production (Restrepo et al., 2000;Mamba-Mbayi et al., 2014). The disease is caused by Xanthomonas phaseoli pv. manihotis (Xpm), (Constantin et al. 2016). Losses of fresh roots, planting material, low accumulation of starch in edible roots and leaves which affect the availability of leafy vegetables for humans and reduces cash income in communities where cassava leaves are sold, have also been observed and can be high under favourable environmental conditions (Fanou et al., 2018). CBB is present in countries where cassava is produced, but its incidence and severity are variable (Fanou et al., 2017;Fanou et al., 2018). The characteristic symptoms of the disease are the wilting of leaves, blighting, angular leaf lesions and stem cankers, stem and leaf exudates production and dieback of stems (Jorge et al., 2001). Its occurrence depends on the interaction between a susceptible plant, a virulent pathogen and a conducive environment (Ghini et al., 2008;Rana and Randhawa, 2014).
The geographical distribution, incidence and severity of the disease have been studied in some countries like Colombia, Togo and Guinea (Verdier and Restrepo, 1997;Banito et al., 2007;Bamfeka et al., 2011). In Côte d'Ivoire, CBB constitutes a threat in the agro-ecological zones (AEZ), (Affery et al., 2016) with various severities and incidences. Since cassava is an important staple food crop as well as for the producers and for consumers, and the susceptibility of the varieties in the different agro-ecological zones has not yet been assessed, it seemed necessary to identify the most grown varieties and to establish their distribution maps, to highlight their behavior regarding the disease in the different AEZ. It also appears important to classify them according to their level of susceptibility by taking into account their repartition.

Material and methods:-Survey
Surveys were carried out in 2017 during the rainy seasons from July to the beginning of November in different cassava producing areas of the seven Ivorian agro-ecological zones. These zones ( Fig 1A) were identified by Halle and Bruzon (2006) as shown in the Table 1. They were defined according to the edapho-climatic conditions in Côte d'Ivoire In each AEZ, three fields/area were considered for the high cassava production areas. For areas with very low cassava production, one to two fields were considered. They were chosen at the entrance, in and outside of the locality and their geographical coordinates were recorded using a GPS GARMIN OREGON 550.

1222
Observations:-Each field was assessed for CBB presence/absence taking into account thirty cassava plants. The meeting point of two diagonal lines was taken as a reference point for the plants assessment. The severity and incidence were evaluated based on assessment sheets. Ten samples of leaves, stems and leafstalks per field showing CBB symptoms were collected. The rating scale of CBB severity described by Wydra and Msikita (1995) was used. The ratings ranged from 1 to 5 and describe as followed: 1: no symptom, 2: only angular leaf spot, 3: angular leaf spots, wilting, blighting, defoliation, and some exudates on stems/leafstalks, 4: blighting of leaves, wilting, defoliation, exudates, and tip die-back, 5: blighting of leaves, wilting, defoliation, exudates, tip die-back, and plant stunting ( Figure 5). The severity index (SI) and disease incidence (DI) were calculated for each parcel using following formulas below,

Results:-
The results of the surveys highlighted that three cassava varieties, Akama, Yace and Yavo were the most grown.

Geographical distribution of the cassava varieties
A total of 249 fields was recorded for the presence of the three varieties during the surveys. The variety Akama was the most encountered with a frequency of 46.59 % (116 fields), followed by Yace with a frequency of 38.55 % (96 fields) and by Yavo with a frequency of 14.86 % (37 fields).
The distribution of these varieties according to the AEZ was not the same. While Yace and Akama were mostly found in the AEZ1 respectively with 55.21 % and 36.21 %, Yavo was more present in the AEZ4 with 48.65 %. Yace was the only variety found in the AEZ7 with 1.04 % considered as the lower rate of presence of the variety. The AEZ3 and 7 were characterized by the absence of Yavo. The lower presence of Akama with 4.31 % was in the AEZ3; while the AEZ6 was characterized by the lower presence of Yavo (5.41 %) ( Fig. 1 B, C and D).

Disease repartition on the varieties in the agro-ecological zones Akama
On the 116 fields where Akama is grown, 67 fields (57.76%) were healthy while 49 fields (42.24%) were affected by CBB. The AEZ4 recorded the most diseased fields followed by the AEZ1 while in the AEZ3, there was no diseased field (Table 2).

Yace
Out of the total of the 96 fields obtained, 62 fields (64.58 %) where Yace is grown were healthy and 34 fields (35.42%) were affected by CBB. The AEZ1 recorded the most diseased fields followed by the AEZ2 whereas the AEZ4 and AEZ5 fields where CBB free (Table 3).

Yavo
Out of the 37 Yavo fields sampled, 15 fields (40.54 %) were healthy while 22 fields (59.46 %) were affected by CBB. The AEZ4 recorded the most diseased fields followed by the AEZ1. However in the AEZ3, there was no diseased field ( In the AEZ3, Akama did not showed a susceptibility to CBB (Fig 2 A and B). SI and DI showed a significant difference between the AEZ with p SI = 0.03 and p DI = 0.04.

Yace
The average SI and DI of Yace was respectively 9.  (Fig 3 A and B). There was no significant differences between the susceptibility of Yace in the AEZ with p SI = 0.45, DI with p DI = 0.41.

Yavo
Yavo presented overall means of SI and DI respectively equal to 19 (Fig 4 A and B). There was no significant differences between the susceptibility of Yavo in the AEZ with p SI = 0.45, DI with p DI = 0.44. SI and DI of the varieties were significantly different with respectively p SI = 0.007, p DI = 0.009. Although Akama was more widespread than the others, it was not found in the AEZ7 while Yace was found in all the AEZ and Yavo was not found in both AEZ3 and 7. Yace has been described as a bitter variety while Yavo has been described as a sweet variety by Akpingny et al. (2017). Akama was described as a sweet variety by the farmers surveyed. These facts could explain their distribution. Indeed, Perrin et al. (2015) stated that the varieties' ability to adapt themselves to the climatic conditions was also related to their bitterness feature. According to these authors, some bitter varieties can be cultivated in some northern parts of Côte d'Ivoire while the sweet ones cannot be grown there. This fact could explained the presence of Yace, a bitter variety in the AEZ6 and 7 and the absence of Akama and Yavo, sweet varieties in the AEZ7. However, unlike to what they said, Akama and Yavo were grown in the AEZ6 even if it was at lower rates. This finding could be explained by the fact that Akama and Yavo are in a process of adaptation to a new and hostile environment as said by Coakley et al. (1999). According to these authors, the repartition and development of the plants were going to change under climate change.

Discussion
Akama was mostly found in the AEZ1 followed by the AEZ4 and the AEZ2. Its lower occurrence was in the AEZ3 followed by the AEZ6. Yace distribution was mostly concentrated in the AEZ1, followed by AEZ2 and the AEZ3. Its lower rates were found respectively in the AEZ7, AEZ5 and AEZ4. Yavo was more widespread in the AEZ4 then 1224 in the AEZ1 and AEZ2. It was less widespread in the AEZ6 and AEZ5. Akpingny et al. (2017) mentioned that Yace has high production zones were in the southern (predominantly) and the central parts of Côte d'Ivoire; however, in this study, Yace was more widespread in the western part than in the central part. Yavo distribution is consistent with Akpingny et al. (2017); it was more present in the central, eastern and southern parts.
Concerning the susceptibility of the varieties, Yavo described as resistant to cassava mosaic virus (Perrin et al., 2015;Akpingny et al., 2017) was the most susceptible to CBB. Akama was the second susceptible variety while Yace showed a lower susceptibility.
The higher rates of Yace and Yavo diseased fields were found in AEZ where they mostly occurred: AEZ1, AEZ2 and AEZ3 for Yace and AEZ4, AEZ1 and AEZ2 for Yavo. This findings are consistent with the statements of Coakley et al. (1999) who stated that the distribution of the pathogen would followed those of the host. However, the AEZ6 and AEZ3 displayed the same rate of Yace diseased fields whereas the AEZ5 and AEZ2 showed the same rate of Yavo diseased fields. These results could be explained by the rapid change in Xpm strains distribution as highlighted by Shaw and Osborne (2011). Although Akama was mostly found in the AEZ1 and then in the AEZ4, the majority of diseased fields were found in the AEZ4, then secondly in the AEZ1 and lastly followed by the AEZ2. The AEZ5 and AEZ6 had the same rate of diseased fields. This may be due to the fact that the pathogen was able to be quickly widespread in the AEZ4 than the AEZ1 leading to a higher rate of diseased fields in the AEZ4. By considering all the AEZ, the AEZ6 recorded the higher rates of varieties susceptibility in term of SI and DI. It could be explained by the fact that the pathogen achieved its complete cycle. In fact, according to Fanou et al. (2018), Xpm goes through a survival stage during the dry season for the establishment of the primary inoculum and a parasitic stage during the rainy season where the disease symptoms occur. In Côte d'Ivoire, the AEZ6 has two seasons, a six months dry season and a six months rainy season. The pathogen could have hence had conductive environmental conditions to cause the infection on non-resistant varieties. In the AEZ4 which got the higher SI and DI rates after the AEZ6, there are two dry and two rainy seasons, corresponding to a set of conditions less favourable for the pathogen survival. However, the pathogen was able to cause disease. This finding could be attributed to a reduction of the incubation time as mentioned by Ahanger et al. (2013). The rates of CBB on the varieties in the AEZ5 could be justified by the stressful environmental conditions on both the varieties and the pathogen. According to Yáñez-López et al. (2012), the incidence and severity of a plant disease depend on the deviation of the climatic parameters taken separately into the best conditions for the disease occurrence. According to Shaw and Osborne (2011), the persistence of plant pathogens can be infrequent or regular with a low severity in regions without being a threat for producers in these zones. This fact could explained the low impact of CBB on the varieties in the AEZ3 but also the adverse climatic conditions that prevailed there. This AEZ is characterized by a long rainy season and a short dry season that would have reduced the survival and the quantity of primary inoculum, hence the expression of the disease.
Akama and Yace were mostly susceptible in the AEZ6 but Akama was more susceptible than Yace in this AEZ. Although Yavo was also susceptible in the AEZ6, its SI and DI were lower than those of the two other varieties. Nevertheless, it was mostly susceptible to the disease in the AEZ5. It could be explained by the compatible host interaction where the bacteria strains penetrate into cassava and overcome host defense barriers causing the characteristic symptoms of the disease (Hamza, 2010). It seemed in this AEZ that this interaction was strong to cause hence a high CBB severity on the variety. In fact, Yavo susceptibility reached the higher levels of susceptibility while Yace didn't show a susceptibility to CBB. The absence of Yace susceptibility to CBB in the AEZ4 and AEZ5 could be due to the incompatible host interaction where bacteria strains would have been unable to overcome cassava varieties defense reactions (Fargier, 2007;Hamza, 2010). Yavo and Akama susceptibility to CBB was secondarily higher in the AEZ4 with the high rates recorded in Yavo fields. While Yace was secondarily susceptible in the AEZ1, Yavo was thirdly susceptible in the same AEZ with higher rates. The level of each variety susceptibility varied according to the AEZ. This behaviour regarding the disease in each AEZ could be explained by the interaction between the environment and the genotype as described by Zinsou et al. (2005). In fact, according to Elad and Pertot (2012), plants proceed to the regulation of their genes due to the modifications of their environment patterns. Though in the AEZ6, Yace and Akama had the higher susceptibility rates than those of Yavo, its susceptibility was very high than the two other varieties in the AEZ where they were all together. This is in contradiction to what Tindo et al. (2016) found in their study which showed that local varieties where most attacked and susceptible to CBB than improved varieties. 1225

Conclusion:-
This study showed the geographical repartition of the three cassava varieties assessed and also the most widespread in Côte d'Ivoire. It also showed that the local ones (Akama and Yace) were still very much accepted than the improved one (Yavo). The presence of Akama and Yavo considered as sweet varieties in the AEZ6 help to understand that the geographical distribution of the varieties is changing. The varieties are in a process of adaptation in a new environment previously defined as unfavourable for their growth. Except Yace which didn't show a susceptibility to CBB in the AEZ4 and AEZ5, the others were susceptible in all the AEZ where they were found at different rates. Yavo was the most susceptible in all AEZ excluding in the AEZ6 where it was less susceptible than Akama and Yace; however, the varieties susceptibility differed from one AEZ to another. The AEZ6 was characterized by the high level of Akama and Yace susceptibility while the AEZ5 was characterized by those of Yavo susceptibility. These zones were followed by the AEZ4 for Akama and Yavo and the AEZ1 for Yace. This behaviour in the AEZ pointed out an interaction between the varieties, the pathogen and the environment. Since these factors seems to affect the relationship between the disease and the varieties, it would be important to test these varieties for other control strategies in order to prevent yield losses due to the strong pressure of CBB. Ethics approval and consent to participate'Not applicable' for this section.

Availability of data and materials
All data generated or analysed during this study are included in this published article