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Cassava plants infected by EACMV and other CMGs display diverse foliar symptoms, the type and severity of which are determined by a number of factors. Symptoms include yellow or green mosaic, mottling, and misshapen and twisted leaves that may be reduced in size. Although these symptoms are characteristic of all CMGs, they differ in distribution in fields, from plant to plant, and even on the same plant. In some cases, two branches emerging from the same cassava plant may show varying phenotypes, with one branch being symptomless and the other exhibiting typical CMD symptoms. Symptom severity also varies with variety, environment and infection type. Plants that are infected by mixed CMGs typically express more severe symptoms than those with single infections. For example, plants that are co-infected with ACMV and EACMV-UG show severe foliar symptoms, as observed in the pandemic movement of a severe form of cassava mosaic disease in East Africa (Zhou et al., 1997). In addition, so-called ‘sequences enhancing geminivirus symptoms (SEGS)’ can enhance cassava mosaic symptoms and break CMD resistance when they interact synergistically with CMGs in cassava plants (Ndunguru et al., 2016). Symptoms-based field diagnosis of EACMV and other CMGs is impracticable due to similarities of induced symptoms in infected plants regardless of the causative CMG. Consequently, it is imperative to confirm virus presence using PCR and/or ELISA methods with species-specific oligonucleotides and discriminating antibodies, respectively. PCR diagnosis is the method of choice for confirmation due to the high serological relationship among EACMV-type viruses and the cross reactivity of their antibodies.
Management of EACMV-associated CMD follows the same approach as other viral diseases on crops. Vector control through the use of insecticides is cost prohibitive and impracticable especially as cassava is considered as a minimal input crop in Africa. Two main approaches to CMD management are considered feasible, phytosanitation and breeding for resistance (Thresh and Otim-Nape, 1994). In theory, the two approaches applied singly or in combination should result in effective management of CMD caused by CMGs. However, there are bottle necks in their implementation. Phytosanitation was used effectively in Uganda to manage CMD over a period of time (Jameson, 1964). It involved releasing large quantities of virus-free plant materials to farmers to replace the rogued diseased plants accompanied by a heightened awareness campaign. Central to the success of this approach was the availability of a farmer-preferred cultivar multiplied on a large scale. However, this lapsed with the passage of time and the clean stock was lost in the process leading to the resurgence of CMGs and the associated pandemic.
The use of resistant/tolerant materials to control CMD coupled with phytosanitation and awareness is a more reliable control strategy. Many cassava varieties with demonstrable resistant traits have been used in various parts of Africa where cassava is grown. In Tanzania, studies on breeding for resistance started in the 1930s and 1940s and later spread to Madagascar and West Africa. In West Africa, the International Institute of Tropical Agriculture (IITA) pioneered the work and resistant lines from there were produced and shared with national breeding programmes across sub-Saharan Africa (Mahungu et al., 1994). But resistance breeding is a long term endeavor and sometimes promising resistant/tolerant lines may lack farmer-preferred traits due to linkage drag. Another major issue is effective deployment of resistant cultivars, when available. Consequently, CMD management becomes ineffective and slow. Besides, resistance could break down over time due to high disease and vector pressure, the emergence of resistance-breaking recombinant virus variants, etc. Recently, it has been shown that CMGs can produce virulence factors such as the so-called ‘Sequences enhancing geminivirus symptoms (SEGS)’ that help break down resistance in cassava.
In view of the benefits and limitations of each approach, a combination of both methods is encouraged in integrated pest management.