Cotton leaf curl Gezira virus

Cotton leaf curl Gezira virus (CLCuGV) is endemic to the African Sahel region (Idris et al., 2000). It is an economically important cotton-infecting begomovirus, and poses a serious threat to cotton production. It causes yield loss in all affected cotton-growing areas in Africa. Losses are difficult to assess, but estimates range up to 20% when infection occurs early in the growing season and/or with highly susceptible cultivars.

Natural spread is mainly by the whitefly vector, Bemisia tabaci, which transmits the virus in a persistent, circulative manner. Viruliferous whiteflies on infested/infected plants harbouring CLCuGV imported to other countries are of concern for preventing introduction.

Leaf curl disease of cotton in Africa has been reported in all countries where studies could be conducted north of the equator (Golding 1930; Tarr, 1951; Nour and Nour, 1964; Cauquil and Follin, 1983; Brown, 1994) and most studies published between 1912 and the mid-1980s centred on leaf curl isolates from Nigeria and Sudan. The recent use of molecular virology tools to identify CLCuGV has confirmed the virus is widespread throughout the cotton belt in subSaharan Africa/Sahel region (see references in Tiendrébéogo et al., 2010; Leke et al., 2013; Leke et al., 2015) where it has been reported from symptomatic cotton, cucumber, hollyhock, okra, Sida spp., tomato and a number of wild hosts representing several plant families (Idris et al., 2002a, 2002b, 2013, 2014).

Molecular cloning and DNA sequencing has facilitated the identification and characterization of CLCuGV-satellite complexes in some of the locations where historically, leaf curl symptoms were observed in cotton. Although cotton is no longer produced in some of these countries/locations, the distribution of CLCuGeV (primarily in okra and vegetable crops and wild species, has been corroborated in Benin, Burkina Faso, Cameroon, Chad, Côte d'Ivoire, Egypt, Niger, Nigeria, Sudan, Togo and other African countries where cotton is grown and where it is thought to have originated, but now occurs in locales outside of Africa. Similar to scenarios involving the CLCuD-begomoviruses in South Asia, CLCuGV has been shown to infect a broad range of host plants, making it and the CLCuD-begomoviruses problematic in cotton-vegetable production systems often found in irrigated agriculture where cotton is grown.

Recently CLCuGV has been identified infecting okra and/or tomato plant hosts in the Arabian Peninsula including Oman, Saudi Arabia and United Arab Emirates (see references in Idris et al., 2014) but how widespread the virus is and whether the isolates represent endemic and/or introduced strains is not yet entirely certain. The genome sequences of CLCuGV isolates from Saudi Arabia proved to be divergent from the previously reported isolates from Sudan, at 88-93% shared nt identity, suggesting these isolates might be endemic or that they are introductions that have been present, but gone undetected for some time. The genome of the UAE isolate shared its highest nt identity (96 to 97%) with isolates from Egypt (AF155064), Pakistan (FR751142) and Jordan (GU945265). In contrast, the UAE isolate shared 93% identity with an isolate of CLCuGeV (HG530540) from the west coast of Saudi Arabia. The 1,356-bp betasatellite shared its highest nt identity, at 97%, with the Okra leaf curl Oman betasatellite (KF267444) reported from infected okra plants in a neighbouring country, Oman.

During 2005, a CLCuGV isolate sharing high nt sequence identity with CLCuGV from Egypt, was identified from symptomatic cotton from the Sindh Province southern Pakistan, providing robust evidence for a recent introduction from northern Africa (Tahir et al., 2011).

Apparently, the introduction occurred from North Africa to West Africa, and from the Middle East to South Asia. There is no evidence the virus has spread from where it is believed to have been introduced. The symptomatic plants infected by CLCuGV in Pakistan contained no detectable CLCuGB. Also, whitefly mitotypes from Africa likely required for CLCuGV transmission, are not known to occur in Pakistan, except for a B-like mitotype that is not widely distributed in cotton (Masood et al., 2017). The latter two factors may explain why the CLCuGV from Africa has not spread.

From the Middle East, CLCuGV was detected in DNA isolated from whiteflies sampled in Israel and Jordan during 2011. This discovery suggested the possible long-distance dispersal of the CLCuGV by whiteflies, or that it had been introduced some years earlier, and had not yet been detected in plants. Subsequently, CLCuGV was identified infecting tomato plants in Jordan (Anfoka et al., 2014) and Israel (Gelbart et al., 2020). Conduits for dispersal are thought to have occurred through the plant trade and/or by viruliferous whiteflies associated with the plant trade because the CLCuGV betasatellite (CLCuB) also occurs in the east coast of the Mediterranean Sea and Middle East. In Israel, CLCuB has been shown to transreplicate TYLCV-IL and TYLCV-Mld, species endemic to Israel and Jordan (Gelbart et al., 2020).

Most recently, during 2019, the complete monopartite genome sequence of the CLCuGV was determined from symptomatic okra plants in south Texas, USA. The genome (2,764 nt) and shared 99.5 to 99.6% nt identity with an isolate of CLCuGV from hollyhock plants in Jordan (CLCuGV-[JO:Hol:09]: GU945265). The closest relative of the CLCuGA okra isolate, of 1,354 nt (MN027203 to 05), was CLCuGA-[IL:IsSq-C171:11] (KT099176), an isolate sequenced from whiteflies collected from squash plants in Israel, at 99% identity. This strongly suggests the virus-betasatellite complex was introduced from the Middle East, perhaps through the transport of plants and/or viruliferous whiteflies associated with the plant trade.

Viruliferous whiteflies on infested/infected plants harbouring CLCuGV imported to other countries are of concern for preventing introduction under optimal circumstances. No seed transmission is known to occur.

Alternative hosts and their role in disease epidemiology are still poorly documented, and studies are needed to identify the most important reservoirs of CLCuD-begomoviruses that most directly affect cotton. Gossypium hirsutum and G. barbadense are the predominant economic species.

The natural host range of the leaf curl virus isolates studied from various locations on the African continent is more moderate than the CLCuD-begomovirus complex in South Asia, and includes a number of species within the Malvaceae and at least several other plant families. The natural and experimental host range of CLCuGV from Africa are reported to include Capsicum annuum, Cucumis pepo, Gossypium spp, Hibiscus spp., Lycopersicon esculentum, okra, papaya, and several species within the genera Althaea, Amaranthus and Asystasia. Among the wild hosts reported are: Corchorus fascicularis, Lavatera cretica, Malva spp., Malvaviscus arboreus, Pavonia hastata, Sida spp. and wild hollyhock (Anfoka GH, MF522207 GenBank database); (Tarr, 1951; Nour and Nour, 1964; Bink, 1975; Cauquil and Follin, 1983; Fauquet and Thouvenel, 1987; Brown, 1992; Gambley et al., 2020).

Cotton leaf curl Gezira virus is transmitted by the whitefly vector Bemisia tabaci cryptic species complex. Disease incidence is dependent on climatic factors that influence population levels of the whitefly vector. Periods of high rainfall prior to planting promotes the growth of weeds, which serve as hosts of the whitefly and as reservoirs of the virus. Cotton is the primary site of infection as the virus is transmitted by viruliferous whiteflies arising from infected weed hosts. Secondary spread occurs within infected cotton fields, and can be extremely rapid under moderate-to-high vector population levels. In Sudan, the greatest whitefly vector populations and the highest disease incidence has been reported to occur in October-November following August-September planting dates (Idris, 1990; Brown, 1992, 1994, 2002; Idris and Brown, 2002).

Whitefly vector transmission

Studies have shown that begomoviruses are transmitted by a number of phylogeographically-distributed cryptic species (and mitotypes, therein) of the whitefly vector B. tabaci (Brown, 2010; De Moya et al., 2019). In the Sahel region of Africa, the Q-type mitotypes are predominant in central and eastern Africa, while one or more B-types occur in Egypt and Arabia (Brown, 2010). Begomoviruses are transmitted in a persistent, circulative manner and once acquired, transmission can occur throughout the life of the vector. The virus is not transovarially transmitted.

Environmental factors such as the whitefly vector mitotype, itself differentially responsive to insecticides and prone to development of insecticide resistance, are thought to influence in part the fluctuations in virus composition from season to season. An exacerbating factor is that different mitotypes are known to exhibit differential transmission competency among members of the CLCuGV-begomovirus-satellite complexes (Bedford et al., 1994; Guo et al., 2015; Pan et al., 2018; Chen et al., 2019). Detailed studies of mitotype competency have not been carried out for the CLCuGV and its endemic whitefly vector mitotype(s).

Antisera raised to purified virions of a number of begomoviruses have been used for virus detection in symptomatic leaves. Serology has been useful for detection but not for species-level identification (Swanson et al., 1992; Swanson et al., 1993).

Begomoviruses can be detected by DNA-DNA hybridization with non-radioactively-labelled cloned begomoviral fragments from a panel of divergent species allowing for differential detection of a number of begomoviruses (Brown and Poulos, 1990; Brown and Poulos, 1991; Swanson et al., 1992).

Membrane-bound total DNA from leaf squashes onto FTA cards (Whatman International Ltd., Maidstone, UK) (Leke et al., 2007; Leke et al., 2013) has been used as template for PCR amplification using general begomovirus primers (Wyatt and Brown, 1996) and the viral genome and associated satellites can first be enriched using rolling circle amplification (RCA) (Inoue-Nagata et al., 2004).

Polymerase chain reaction (PCR) amplification of one of several conserved regions of the virus genome has been used to detect infection by CLCuD-begomoviruses and/or alphasatellites and/or betasatellites in cotton and other CLCuD-begomovirus plant hosts by a number of laboratories worldwide (Wyatt and Brown, 1996; Brown et al., 2001, 2017; Briddon et al., 2002; Bull et al., 2003; Radhakrishnan et al., 2004; Chakrabarty et al., 2005; Monga et al., 2005; Sharma et al., 2005; Kumar et al., 2018) and in whiteflies (Rosario et al., 2015; Rosario et al., 2016).

Amplification by PCR and cloning followed by primer walking, or by high-throughput DNA sequencing of viral genome and betasatellite components are used to detect and identify begomovirus isolates, strains and species (Idris et al., 2014). Validation of PCR amplicons from suspect-begomovirus infected plants is accomplished by cloning and sequencing, and comparative sequence analysis, initially by a BLASTn search of the NIH-NCBI GenBank database. Greater scrutiny is applied by alignment of selected sequences of interest, and pairwise distance analysis using the RDP algorithm (Muhire et al., 2013). Species demarcation criteria available at the ICTV website and other published references (see Taxonomy and Nomenclature).

Whitefly vector-mediated transmission from symptomatic plant samples suspected to be infected by CLCuD begomoviruses to cotton indicator plants is a reliable bioassay method when a virus-free, whitefly vector colony is available.

Leaf curling, vein-thickening, and minor veinal enations on undersides of leaves are common symptoms of Cotton leaf curl Gezira virus. Leaf curling, distortion and shortened internodes, with symptoms occurring particularly on the newest growth, are diagnostic for begomovirus-incited diseases. Foliar discoloration, sometimes bright or dull yellow and/or green mosaic and mottling, are symptoms accompanying foliar curling, particularly on the newest growth, are diagnostic for begomovirus infection.

The leaf curl viruses of cotton are most closely related to other begomoviruses (Geminiviridae), based on particle morphology, genome sequence and organization, and transmission by a whitefly vector, Bemisia tabaci.

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.

Resistant varieties XL1, L1530, X1730A, X1030 and B6L were developed for cotton growing areas in the African Sahel and adjacent locales through selection in the 1930s. These exhibit a mild mosaic but produce viable yields and high quality, marketable cotton. The mechanism of resistance was thought to involve reduction in virus levels in infected plants, and these lines have apparently proven durable.

Chemical control to reduce whitefly vector populations, and hence virus inoculum levels, is effective when population pressures are low; but recent upsurges in vector populations due to changing agricultural practices and development of insecticide resistance have made this a less than satisfactory solution (Sippell et al., 1987). No recent reports of new cotton varieties released for Africa could be located. One report indicated Sudan was experiencing a crisis in insect control in cotton (Eveleens, 1983) and another promoted the use of natural enemies for whitefly control in the Gezira (Abdelrahman, 1986). Elimination of weeds near cotton fields may have some advantage in reducing virus and vector reservoirs. Host-plant resistance is undoubtedly the most promising means of controlling this disease and renewed efforts are under way in several laboratories around the world to work toward this goal.

01/06/20 Original text by:

Judith K Brown, School of Plant Sciences, The University of Arizona, Tucson, AZ 85721 USA