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CMGs cause a variety of foliar symptoms on cassava but no pattern is ascribed to any one single virus. Generally, symptoms include yellow or green mosaic, mottling, and misshapen and twisted leaflets (Thottappilly et al., 2003; Alabi et al., 2011). The display of the symptoms may vary in distribution in the fields, from plant to plant and even on the same plant. The pattern of foliar symptoms is influenced by the associated virus species, the presence of sequences enhancing geminivirus symptoms (SEGS) (Ndunguru et al., 2016) and the presence of single or mixed infections, age of the plant, variety responses to infection and environmental factors (Legg and Thresh, 2000; Maruthi et al., 2002; Ogbe et al., 2003). Studies have shown that severe symptoms are usually characteristic of plants infected by mixtures of CMGs, their recombinant variants and/or SEGS. This was evident in plants infected by ACMV/ East African cassava mosaic Uganda variant (EACMV-UG) first recorded in Uganda in the 1990s (Zhou et al., 1998). EACMV-UG is a recombinant virus that arose from recombination of portions of the genomes of ACMV and EACMV (Zhou et al., 1997). The time to symptom expression in an infected plant depend on the mode of infection. Planting material sourced from infected stock display ACMV symptoms in the first few emerging leaves (cutting-borne infection) whereas ACMV infections facilitated by whiteflies (vector-borne infection) require a lag period before the virus titre can build up to levels capable of eliciting symptoms.
Field diagnosis of CMD symptoms may be confusing especially if the fields are also infested with cassava green mites (CGMs); with symptoms looking more severe than normal. Although CMGs affect cassava plants to varying levels of severity, the symptoms produced cannot easily be ascribed to any one virus species by visual inspection of diseased leaves. This is because the mosaic symptoms do not form characteristic patterns associated with specific viruses. In infection complexes, therefore, it is important to confirm the causal species through PCR and ELISA methods.
Approaches to CMD management have been discussed in several review articles (Atiri et al., 2004; Thresh and Cooter et al., 2005; Vanderschuren et al., 2007). They include crop resistance, CMD avoidance and cultural control, vector management, monitoring and survey.
Over the years, conventional breeding for resistance to ACMV and other CMGs has been the main thrust for prevention and control of cassava mosaic disease (Thresh and Cooter, 2005; Dixon et al., 2001, 2010). In the initial stages of breeding efforts, various sources of resistance were identified but Manihoti glaziovii was initially the sole candidate resistance gene source (Jennings, 1994). Later efforts included cassava landraces in the resistance gene pool (Fregene et al., 2001). Crosses between different cassava varieties mainly from West Africa resulted in the generation of tropical Manihot species (TMS) and tropical Manihot esculenta (TME) that were considerably resistant to CMGs and were pivotal in the control of the severe form of CMD in East Africa (Legg et al., 2006). These materials were shared with many national breeding programmes in Africa for inclusion in local breeding programmes. Recently, to complement conventional breeding, efforts have been made in the development of transgenic resistance to CMD (Vanderschuren et al., 2007; 2009; Sayre et al., 2011)
Avoidance and Cultural Control
This is probably the cheapest way of managing ACMV and CMD in resource poor farmers’ fields. It involves planting of virus-free cassava cuttings obtained from a careful selection of plant materials from older crops observed to be disease-free in the previous season. Other approaches include roguing of diseased plants in the early stages of crop growth, disease avoidance by adjusting dates of planting, intercropping and varietal mixture. Results obtained from applying these management strategies have been viewed differently. Some have deemed the strategies effective (Sserubombwe et al., 2001; Fondong et al., 2002; Thresh and Otim-Nape, 1994), whereas others have disputed their effectiveness (Fargette and Fauquet, 1988; Otim-Nape et al., 1997). Despite disagreements, such methods have found application with varying levels of success.
Chemical control of whiteflies to limit the spread of ACMV has not been widely adopted by farmers in Africa mainly due to the cost of chemicals. Biological control using parasitoids has remained at an experimental level with little to show for effective delivery of whitefly control to minimize the spread of CMGs (Legg et al., 2014, and references therein).
Monitoring and Survey
In order to monitor changes in the CMD dynamics across sub-Saharan Africa, several surveillances have been and continue to be conducted (Ndunguru et al., 2005; Bull et al., 2006; Ogbe et al., 2006; Sserubombwe et al., 2008; De Bruyn et al., 2012; Harimalala et al., 2012; Muengula-Manyi et al., 2012; Zinga et al., 2012; Chikoti et al., 2013; Mulenga et al., 2016). Results from these efforts have informed decisions made by government authorities in addressing the CMD disease burden in farmers’ fields via deployment of disease-resistant cultivars.