Variation in qualitative and quantitative traits of cassava germplasm from selected national breeding programmes in subSaharan Africa

An improved understanding of phenotypic variation within cassava germplasm in southern, eastern and central Africa will help to formulate knowledge-based breeding strategies. Thus, the overall objective of this study was to examine the phenotypic variation in cassava germplasm available within six breeding programmes in Africa, namely Uganda, Kenya, Tanzania, Rwanda, Democratic Republic of Congo and Madagascar. In each country, single-row plots were used for assessment of 29 qualitative traits and evaluation of four quantitative traits: root dry matter content (DMC), harvest index (HI), leaf retention (LR) and root cortex thickness. Qualitative traits provided limited discrimination of cassava germplasm. However, differences in DMC, HI, LR and root cortex thickness were observed among the germplasm indicating scope for genetic improvement. Highest average DMC was registered in Uganda (39.3%) and lowest in Tanzania (30.1%), with the elite genotypes having a relatively higher DMC than local genotypes. Highest average HI was observed in Uganda (0.60) and lowest in Kenya (0.32). Cassava genotypes displayed varied root peel thickness (0.34–4.89 mm). This study highlights variation in agronomic traits that could be exploited to increase cassava productivity

Comparing the regional epidemiology of the cassava mosaic and cassava brown streak virus pandemics in Africa

The rapid geographical expansion of the cassava mosaic disease (CMD) pandemic, caused by cassava mosaic geminiviruses, has devastated cassava crops in twelve countries of East and Central Africa and is well documented. Region-level epidemiological studies continue to reveal a more-or-less regular pattern of annual spread along a contiguous ‘front’. More recently, outbreaks of cassava brown streak disease (CBSD) were reported from Uganda and other parts of East Africa that had been hitherto unaffected. Recent survey data reveal several significant contrasts between the regional epidemiology of the two pandemics: i) severe CMD spreads in an ‘expanding concentric rings’-like manner, whilst CBSD seems to be spreading from independent hot-spots; ii) the severe CMD pandemic has arisen from virus recombination and inter-species synergy, whilst current knowledge suggests that the CBSD pandemic is a ‘new encounter’ situation; iii) CMD pandemic spread has been tightly linked with the appearance of super-abundant B. tabaci whitefly populations, in contrast to CBSD, where outbreaks have occurred 3-12 years after whitefly population increases; iv) the cassava mosaic geminiviruses (CMGs) causing CMD are transmitted in a persistent manner, whilst cassava brown streak viruses (CBSVs) appear to be semi-persistently transmitted; and v) different patterns of symptom expression mean that phytosanitary measures are easy to implement for CMD but have limited effectiveness whilst the same measures are hard to do for CBSD but potentially very effective. One important similarity between these two cassava virus pandemics is that the viruses occurring in pandemic-affected areas are also found elsewhere, indicating that contrary to earlier published information, the viruses per se are unlikely to be the key factors driving the two pandemics. A model is proposed for the temporal relationship between B. tabaci abundance and changing incidences of both CMD and CBSD in the Great Lakes region. This emphasizes the pivotal role played by the vector in both pandemics and highlights the urgent need to identify effective and sustainable strategies for controlling whiteflies on cassava

Spatiotemporal patterns of genetic change amongst populations of cassava Bemisia tabaci whiteflies driving virus pandemics in East and Central Africa

The greatest current threat to cassava in sub-Saharan Africa, is the continued expansion of plant virus pandemics being driven by super-abundant populations of the whitefly vector, Bemisia tabaci. To track the association of putatively genetically distinct populations of B. tabaci with pandemics of cassava mosaic disease (CMD) and cassava brown streak disease (CBSD), a comprehensive region-wide analysis examined the phylogenetic relationships and population genetics of 642 B. tabaci adults sampled from cassava in six countries of East and Central Africa, between 1997 and 2010, using a mitochondrial DNA cytochrome oxidase I marker (780 bases). Eight phylogenetically distinct groups were identified, including one, designated herein as ‘East Africa 1’ (EA1), not previously described. The three most frequently occurring groups comprised >95% of all samples. Among these, the Sub-Saharan Africa 2 (SSA2) group diverged by c. 8% from two SSA1 sub-groups (SSA1-SG1 and SSA1-SG2), which themselves were 1.9% divergent. During the 14-year study period, the group associated with the CMD pandemic expansion shifted from SSA2 to SSA1-SG1. Population genetics analyses of SSA1, using Tajima's D, Fu's Fs and Rojas’ R2 statistics confirmed a temporal transition in SSA1 populations from neutrally evolving at the outset, to rapidly expanding from 2000 to 2003, then back to populations more at equilibrium after 2004. Based on available evidence, hybrid introgression appears to be the most parsimonious explanation for the switch from SSA2 to SSA1-SG1 in whitefly populations driving cassava virus pandemics in East and Central Africa

Genetic diversity of cassava (Manihot esculenta Crantz) landraces and cultivars from southern, eastern and central Africa

Studies to quantify genetic variation in cassava germplasm, available within the national breeding programmes in Africa, have been limited. Here, we report on the nature and extent of genetic variation that exists within 1401 cassava varieties from seven countries: Tanzania (270 genotypes); Uganda (268); Kenya (234); Rwanda (184); Democratic Republic of Congo (DRC; 177); Madagascar (186); Mozambique (82). The vast majority of these genotypes do not exist within a formal germplasm conservation initiative and were derived from farmers' fields and National Agricultural Research Systems breeding programmes. Genotypes were assayed using 26 simple sequence repeat markers. Moderate genetic variation was observed with evidence of a genetic bottleneck in the region. Some differentiation was observed among countries in both cultivars and landraces. Euclidean distance revealed the pivotal position of Tanzanian landraces in the region, and STRUCTURE analysis revealed subtle and fairly complex relationships among cultivars and among landraces and cultivars analysed together. This is likely to reflect original germplasm introductions, gene flow including farmer exchanges, disease pandemics, past breeding programmes and the introduction of cultivars from the International Institute of Tropical Agriculture – Nigeria. Information generated from this study will be useful to justify and guide a regional cassava genetic resource conservation strategy, to identify gaps in cassava diversity in the region and to guide breeding strategies

Successful innovations and lessons learnt in cassava improvement and deployment by IITA in Eastern African Region

The International Institute of Tropical of Agriculture (IITA) established its East and Southern Africa Regional Research Centre (ESARC) in Uganda at the former Namulonge Agricultural and Animal Research Institute (NAARI), presently Namulonge Animal and Crops Research Institute (NaCRRI), to address issues of cassava, banana, and plantain development; coordinate all related activities, and work closely with the national agricultural research institutes (NARS). IITA-ESARC began extensive cassava germplasm development to counter the pandemic of African cassava mosaic disease (ACMD) in the region in 1995 through the Eastern Africa Root Crops Research Network (EARRNET). More than 100,000 seeds were evaluated through the conventional plant breeding scheme. Selected genotypes were kept in in-situ conservation from where the regional cassava national programs selected clones for further evaluation in their own countries. Burundi, Democratic Republic of Congo (DRC), Kenya, Rwanda, Tanzania, and Uganda benefited immensely. Through EARRNET, the region gained significantly from the large germplasm base to mitigate the scourge of ACMD and the production of cassava was restored. A new joint effort that was established between Catholic Relief Services and IITA in collaboration with the NARS and other stakeholders promoted the adoption of improved germplasm through participatory variety selection. The breeding approach used enabled to reduce selection period for NARS to release new varieties to farmers as they received elite materials for evaluation. However, the spread of cassava brown streak disease (CBSD) in mid altitude threatened the achievements already made as the new disease attacked most of the ACMD-resistant and high yielding varieties. Its spread in the region calls for more effective collaborative action than before from IITA and its partners to develop new resistant materials to mitigate the effects of both ACMD and CBSD. The present paper attempts to summarize the breeding work efforts made and demonstrate how the germplasm development at this regional center has been useful to the region through effective partnership