EACMV-type viruses possess bipartite, circular, ss(+)DNA genomes that are encapsidated in twin (geminate), small, quasi-isometric particles measuring 20 x 30 nm. Both the DNA-A and DNA-B genome components are needed for efficient transmission of the virus to healthy cassava plants (Liu et al., 1997). Whereas this description is used sensu lato, there is considerable variability among EACMV-type viruses at the molecular level. Each species is named after its country of first discovery/characterization although the use of country names in geminivirus nomenclature is now being discouraged (Brown et al., 2015).
Symptoms caused by EACMV-like viruses are not distinguishable from those caused by other CMGs by visual inspection. However, mosaic patterns on cassava leaves indicate the presence of one or more of the CMD viruses, which can be discriminated using various diagnostic tools.
Related invasive species
- East African cassava mosaic virus
Related Farm Practice
- Diagnosis
- Transmission
- Movement
Like other CMGs, cassava is the primary host of East African cassava mosaic virus (EACMV) and related viruses, although the virus has been detected in other plant species (Ogbe et al., 2006;Alabi et al. 2015). Analysis of the genomes of different isolates of EACMV-type viruses show considerable genetic variability and genome plasticity relative to ACMV isolates. The primary means of virus spread is via movement of contaminated vegetative cassava cuttings and secondary spread occurs via the whitefly vector, Bemisia tabaci. Perhaps the most notable documentation of invasiveness of EACMV-type viruses is the regional pandemic of a severe CMD in East Africa caused by EACMV-UG which began in Uganda in the early to mid-1990s (Gibson et al., 1996;Otim-Nape et al., 1997) on popular and widely cultivated cassava varieties and soon spread to other countries in East Africa, including Kenya and Tanzania (Otim-Nape et al., 1997;Legg, 1999). The pandemic resulted in famine-related deaths (Otim-Nape et al., 1998) due to complete devastation of affected cassava farms in the region. EACMV is not on the IUCN or ISSG alert list.
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.
EACMV, like the other CMGs, is primarily borne in cassava vegetative cuttings. Emerging leaves from such cuttings may manifest CMD symptoms and serve as sources of virus inoculum for secondary spread within and across fields by the whitefly vector. True cassava seeds are not known to carry the virus (Dubern, 1994). Depending on the mode of infection, symptoms appear in the first emerging leaves for cutting and 12-20 days after inoculation by viruliferous whiteflies (Storey and Nichols, 1938) and are usually determined by varietal characteristics.