East African cassava mosaic virus

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.

There are two major pathways for the introduction of EACMV and other CMGs to new areas, namely through distribution of planting materials and by the naturally transmitting whitefly vector, Bemisia tabaci. Infected cassava vegetative cuttings contribute to both long and short distance spread of EACMV and other CMGs. Whitefly mediated spread generally occurs over short distances because B. tabaci is a ‘weak’ flier with an estimated flight speed of ca 0.2 m/sec (Chant, 1958; Dubern, 1994) unless aided by the prevailing wind. Moreover, for every population of adult whiteflies feeding on infected cassava plants, up to 13% would acquire the virus and become infective (Legg et al., 2011). It would therefore require a huge whitefly population to cause widespread transmission. But the population dynamics of B. tabaci is influenced by many factors including environmental and biological factors and an abundance of susceptible cassava varieties. High populations are therefore highly likely in countries with a biannual rainfall pattern where cassava plants at the right foliar stage for feeding exist continually. In countries where whitefly populations are low, such as Zambia (Chikoti et al., 2013), Malawi (Mbewe et al., 2015) and Mozambique (Cossa, 2011), the pathway supporting introduction of EACMV to other areas is the movement of uncertified planting materials by farmers, NGOs and other stakeholders. Thus, cutting-borne infections probably contribute more to the spread of EACMV and other CMGs to new field and regions. Once introduced, eradication is difficult because of the nature of continuous cassava cropping and the presence of alternative weed or crop plant hosts of CMGs (Ogbe et al., 2006; Alabi et al., 2008; 2015; Monde et al., 2010).

EACMV is widespread across cassava fields in sub-Saharan African countries and adjoining islands. 

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.

EACMV and other EACMV-like viruses can been diagnosed through various methods. The successful purification of CMGs (Bock et al., 1977) led to the development of various forms of enzyme-linked immunosorbent assay (ELISA) including double- (DAS) and triple- (TAS) antibody sandwich ELISAs capable of detecting the viruses in leaf extracts. Distinguishing between EACMV and other CMGs via ELISA could occur using a panel of discriminating monoclonal antibodies (Thomas et al., 1986; Harrison and Robinson, 1988; Ogbe et al., 1997). The different forms of ELISA are versatile and can be used to screen large field samples. However, ELISA is incapable of discriminating between the different EACMV-like viruses and their recombinant variants in mixed infections due to similarities in coat protein epitopes within CMGs (Thottapilly et al., 2003). Species specific oligonucleotide primers have been designed to target different genes encoded in the DNA A component of CMGs making it possible to differentiate between these viruses. During the early development of the PCR technique, viruses were detected in single-plex PCR but recently multiplex PCR assays capable of differentiating EACMV in mixed infections with other CMGs and cassava brown streak virus have been developed (Ndunguru et al., 2005; Ogbe et al., 2006; Alabi et al., 2008; Abarshi et al., 2012; Aloyce et al., 2013).

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.

EACMV and other CMGs are indistinguishable from each other on the basis of foliar symptoms. Serological assays also have limitations in distinguishing CMGs. EACMV-type viruses can be distinguished from each other and from other CMGs using molecular diagnostics with virus-specific oligonucleotides.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

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. 

09/10/17 Original text by:

Olufemi Joseph Alabi, Department of Plant Pathology & Microbiology, Texas A&M University AgriLife Research & Extension Center, Weslaco, USA

Rabson Mulenga, Zambia Agriculture Research Institute, Mount Makulu Central Research Station, Chilanga, Lusaka, Zambia