Histology and cytochemistry studies of the resistance mechanisms in cassava during Xam infection showed callose deposition that act as a barrier in cortical parenchyma cells and phloem to block bacterial multiplication and dispersion (Kpémoua et al., 1996 Sandino et al., 2015). These mechanisms have been extensively studied in model plants, but knowledge generated in cassava is relatively scarce. Plants have evolved several mechanisms to defend themselves against pathogens. The analysis of 65 Xam genomes revealed that this pathogen harbors 14–22 effector genes, from which nine are conserved in all the strains (Bart et al., 2012).Īlthough some measures such as planting of disease-free material can be applied in the cassava fields in order to protect the crop, the safest and most efficient strategy to control CBB is to take advantage of natural plant genetic resistance to develop resistant cultivars for cultivation in CBB prone regions. The Colombian Xam populations remain highly dynamic and exhibit a high genetic diversity (Trujillo et al., 2014). The advancement of CBB epidemics has been reported in many countries, with Burkina Faso, being one of the most recent ones (Wonni et al., 2015). CBB has been reported in all regions where cassava is grown (López and Bernal, 2012 Taylor et al., 2012), including 56 countries distributed over Asia, Africa, Oceania and North, Central, and South America ( ) and the number of countries affected by the disease is increasing. Xam has been described among the top 10 most important plant pathogenic bacteria (Mansfield et al., 2012). The disease has a very high destructive power causing losses between 12 and 100% in affected areas (Lozano, 1986 López and Bernal, 2012). The major bacterial vascular disease affecting this crop is Cassava Bacterial Blight (CBB), caused by Xanthomonas axonopodis pv. Cassava tolerates drought, therefore it has been considered as one of the best alternatives for providing food for the world population in the context of climatic change (Howeler et al., 2013). This crop represents an important source of calories for about one billion people (Ceballos et al., 2010). DNA sequence analysis of the QTL intervals revealed 29 candidate defense-related genes (CDRGs), and two of them contain domains related to plant immunity proteins, such as NB-ARC-LRR and WRKY.Ĭassava, Manihot esculenta Crantz, is a starchy root crop and one of the main staple food crops over the world due to its essential role for food security in tropical regions. Genotype by environment analysis detected three QTL by environment interactions and the broad sense heritability for Xam318 and Xam681 were 20 and 53%, respectively. Four of them show stability among the two evaluated seasons. Based on composite interval mapping analysis, 5 strain-specific QTL for resistance to Xam explaining between 15.8 and 22.1% of phenotypic variance, were detected and localized on a high resolution SNP-based genetic map of cassava. The phenotypic evaluation of the response to Xam revealed continuous variation. The evaluation was conducted in rainy and dry seasons and additional tests were carried out under controlled greenhouse conditions. An F1 mapping population of 117 full sibs was tested for resistance to two Xam strains ( Xam318 and Xam681) at two locations in Colombia: La Vega, Cundinamarca and Arauca. Here, we report the detection and localization on the genetic map of cassava QTL (Quantitative Trait Loci) conferring resistance to CBB. manihotis ( Xam), it is the most destructive disease causing heavy yield losses. Cassava production is constantly threatened by several diseases, including cassava bacterial blight (CBB) caused by Xanthomonas axonopodis pv. Cassava, Manihot esculenta Crantz, has been positioned as one of the most promising crops world-wide representing the staple security for more than one billion people mainly in poor countries.
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