Cassava is an important crop in Africa, Asia, Latin America, and the Caribbean. Cassava can be produced adequately in drought conditions making it the ideal food security crop in marginal environments. Although cassava can tolerate drought stress, it can be genetically improved to enhance productivity in such environments. Drought adaptation studies in over three decades in cassava have identified relevant mechanisms which have been explored in conventional breeding. Drought is a quantitative trait and its multigenic nature makes it very challenging to effectively manipulate and combine genes in breeding for rapid genetic gain and selection process. Cassava has a long growth cycle of 12–18 months which invariably contributes to a long breeding scheme for the crop. Modern breeding using advances in genomics and improved genotyping, is facilitating the dissection and genetic analysis of complex traits including drought tolerance, thus helping to better elucidate and understand the genetic basis of such traits. A beneficial goal of new innovative breeding strategies is to shorten the breeding cycle using minimized, efficient or fast phenotyping protocols. While high throughput genotyping have been achieved, this is rarely the case for phenotyping for drought adaptation. Some of the storage root phenotyping in cassava are often done very late in the evaluation cycle making selection process very slow. This paper highlights some modified traits suitable for early-growth phase phenotyping that may be used to reduce drought phenotyping cycle in cassava. Such modified traits can significantly complement the high throughput genotyping procedures to fast track breeding of improved drought tolerant varieties. The need for metabolite profiling, improved phenomics to take advantage of next generation sequencing technologies and high throughput phenotyping are basic steps for future direction to improve genetic gain and maximize speed for drought tolerance breeding.

Cassava (Manihot esculenta Crantz), a highly heterozygous crop, is devastated by cassava mosaic disease (CMD). The discovery of the CMD2 dominant gene is helpful in the genetic analysis of CMD resistance. Molecular markers for CMD2 gene were used to introgress CMD resistance into Latin American cassava genotypes and validated in the field for 4 yr for stability of resistance conferred by CMD2. Field screening identified 64 Latin American genotypes with stable resistance to CMD. Resistance to CMD of two Nigerian cassava cultivars (TMS 97/2205 and TMS 98/0505) was analyzed with markers and in the field. Molecular data indicated that CMD resistance in the two Nigerian cultivars was mediated by the CMD2 gene. Results showed TMS 97/2205 to be highly resistant to CMD in three ecological zones in Nigeria. Further genetic analysis of this genotype as a source of high level of resistance to CMD using a segregating F1 population derived from a TMS 97/2205 × NR 8083 cross was initiated using 530 simple sequence repeat (SSR) markers to identify quantitative trait loci (QTL) for CMD resistance. A marker (NS198) associated with a QTL for CMD resistance, explaining 11% of the phenotypic variance observed, was identified. The combined effect of this QTL and CMD2 may account for the high level of resistance of TMS 97/2205. The resistance profile of the evaluated CMD2 genotypes in growth cycle was not uniform and was affected by genetic background. The discovery of a new QTL (CMD3) for CMD resistance in TMS 97/2205 offers new opportunities for pyramiding CMD genes for enhanced durability of CMD resistance in cassava.

The importance of cassava as a food security crop in Africa and the world Cassava, originally from South America, is the fourth most important source of calories in the developing world after the cereal crops wheat, maize, and rice. Worldwide, it feeds an estimated 700 million people directly or indirectly. Cassava production has increased steadily for the last 50 years, with 242 MT harvested in 2012. The increase is likely to continue as farmers in more than 105 countries come to recognize the crop's advantages. A semi-perennial root crop, cassava can stay in the ground for up to 3 years. This makes it an excellent food security crop: when all other crops have been exhausted, cassava roots can still be harvested. It is naturally drought resistant and resilient to climatic changes, high temperatures, and poor soils, and in addition, cassava responds extremely well to high CO 2 concentrations, making it a very important crop for the 21st century. Africa alone accounts for more than 55 % of the world's production, and cassava is the first food crop in fresh tonnage before maize and plantain in sub-Saharan Africa. Cassava is also an important source of income, especially for women in sub-Saharan Africa (SSA). Furthermore, cassava is the second most important source of starch in the world. Cassava is thus a highly valuable crop for the world today and in the future. It is critical that it should not be compromised by viral diseases.

The gene pools for breeding cassava (Manihot esculenta Crantz) in Africa currently contain only a fraction of the existing genetic variation found in Latin America where the crop originates. Our research aimed to broaden the genetic base in Africa by introducing Latin American (LA) germplasm. The first set of introductions comprised sexual seeds that led to the evaluation of 20,032 seedlings in Nigeria between 1990 and 1994. A second set comprised in vitro cultures, where the dominant CMD2 gene for cassava mosaic disease (CMD) resistance was introgressed into LA germplasm through marker‐assisted selection (MAS). Through MAS 156 genotypes were preselected for the gene and evaluated in Nigeria between 2004 and 2006. Initial results from the first set of introductions indicated that LA germplasm was highly susceptible to CMD, minimizing its usefulness in African cassava‐breeding programs. In the second set of introductions from LA, introgression of the CMD2 gene resulted in high CMD resistance under African field conditions. Now at advanced stages in the African breeding program, 14 genotypes combining CMD resistance and high yield are being evaluated. Marker‐assisted introgression of CMD resistance into LA germplasm has improved the potential value of LA germplasm for Africa and enhanced the prospect of elite LA genotypes being released as improved varieties in Africa.

Microsatellites or simple sequence repeats (SSR) are the markers of choice for molecular genetic mapping and marker-assisted selection in many crop species. A microsatellitebased linkage map of cassava was drawn using SSR markers and a F 2 population consisting of 268 individuals. The F 2 population was derived from selfing the genotype K150, an early yielding genotype from an F 1 progeny from a cross between two noninbred elite cassava varieties, TMS 30572 and CM 2177-2 from IITA and CIAT respectively. A set of 472 SSR markers, previously developed from cassava genomic and cDNA libraries, were screened for polymorphism in K150 and its parents TMS 30572 and

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The genetic basis of early bulking in cassava was studied in a replicated, multi-locational trial using 144 F1 progeny derived from an intra-specific cross between two non-inbred parents. A second, sequential harvest experiment examined the relative importance of eight yield-related traits on early bulking and their QTLs during the crop growth cycle. Our objectives were to identify traits, and genes controlling them, strongly associated with early yield as a first step to marker-assisted improvement of the trait. Multiple linear regression analysis and stepwise regression of early yield on eight yield-related traits revealed harvest index, dry foliage weight and root diameter as the most important factors associated with early yield. A total of 18 QTLs controlling early yield were identified in the first and second experiments and 27 QTLs, 2 for dry foliage weight, 8 for harvest index and 17 for root diameter, in the second experiment. The individual effects of alleles at these QTLs identified ranged from 7% to 33% of the phenotypic variance explained. Seven of 18 QTLs found for early yield (39%) coincided with QTLs associated with one or more traits with significant influence on early yield. The results show that sink and source capacities are very important in determining early yield. The identification of a number of QTLs with positive effect for increased early yield provides an opportunity for marker-assisted selection and improvement of early bulking potential in cassava.

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