Begomovirus disease complex: emerging threat to vegetable production systems of West and Central Africa

Abstract

Vegetables play a major role in the livelihoods of the rural poor in Africa. Among major constraints to vegetable production worldwide are diseases caused by a group of viruses belonging to the genus Begomovirus, family Geminiviridae. Begomoviruses are plant-infecting viruses, which are transmitted by the whitefly vector Bemisia tabaci and have been known to cause extreme yield reduction in a number of economically important vegetables around the world. Several begomoviruses have been detected infecting vegetable crops in West and Central Africa (WCA). Small single stranded circular molecules, alphasatellites and betasatellites, which are about half the size of their helper begomovirus genome, have also been detected in plants infected by begomoviruses. In WCA, B. tabaci has been associated with suspected begomovirus infections in many vegetable crops and weed species. Sequencing of viral genomes from crops such as okra resulted in the identification of two previously known begomovirus species (Cotton leaf curl Gezira virus and Okra yellow crinkle virus) as well as a new recombinant begomovirus species (Okra leaf curl Cameroon virus), a betasatellite (Cotton leaf curl Gezira betasatellite) and new alphasatellites. Tomato and pepper plants with leaf curling were shown to contain isolates of new begomoviruses, collectively referred to as West African tomato-infecting begomoviruses (WATIBs), new alphasatellites and betasatellites. To study the potential of weeds serving as begomovirus reservoirs, begomoviruses and satellites in the weed Ageratum conyzoides were characterized. Sequence analyses showed that they were infected by isolates of a new begomovirus (Ageratum leaf curl Cameroon virus) that belong to the WATIBs group, a new betasatellite (Ageratum leaf curl Cameroon betasatellite), an alphasatellite and two types of defective recombinants between a begomovirus and an alphasatellite. Putative recombinations were detected in begomovirus genomes for all four plant species studied, indicating that recombination is an important mechanism for their evolution. A close relationship between the begomoviruses infecting pepper and tomato and A. conyzoides and the detection of the same alphasatellite in them support the idea that weeds are important reservoirs for begomoviruses and their satellites. With this high diversity, recombination potential and transmission by B. tabaci, begomoviruses and ssDNA satellites pose a serious threat to crop production in West and Central Africa.

Connecting smallholder tomato producers to improved seed in West Africa

“…Samples positive for begomovirus infection were further analyzed by SB-PCR (polymerase chain reaction) and DNA sequencing to identify the specific begomovirus(es) and betasatellites involved [19]. This involved using a combination of degenerate and species-specific primers, followed by sequencing of PCR-amplified fragments [19,42] [4,43]. In general, the majority of samples collected during the survey of West African countries and provided by the NARS partners were positive for begomovirus infection, whereas betasatellites were detected in a relatively small number of plants (~5-10%).…”

“…Recently, outbreaks of the tomato leaf miner (Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae) have caused substantial damage to tomato crops in some West Africa countries [17,18]. However, in recent years, tomato growers throughout West Africa have struggled with an emerging viral disease known generically as tomato leaf curl disease (ToLCD) [4,19,20]. ToLCD in West Africa and other parts of the world is caused by a complex of whitefly (Bemisia tabaci)-vectored begomoviruses and betasatellites [4,21].…”

“…However, in recent years, tomato growers throughout West Africa have struggled with an emerging viral disease known generically as tomato leaf curl disease (ToLCD) [4,19,20]. ToLCD in West Africa and other parts of the world is caused by a complex of whitefly (Bemisia tabaci)-vectored begomoviruses and betasatellites [4,21]. Begomoviruses (genus Begomovirus) are in the family Geminiviridae, characterized by a circular single-stranded DNA genome encapsidated within twinned isometric virions [22,23].…”

“…South of the Sahara Desert, north of the humid Guinean region, the Sudano-Sahelian zone presents difficult environmental conditions including poor soil fertility [1], extreme heat and insufficient or unpredictable water supply [2], all of which have only become more precarious due to climate change [3]. Biotic stresses create additional losses, with newly emerging plant diseases and pests causing decreased yields and serious reductions in produce quality [4][5][6]. In the developed world, these challenges can often be managed by technologies such as drip irrigation or pesticides.…”

“…Begomoviruses (genus Begomovirus) are in the family Geminiviridae, characterized by a circular single-stranded DNA genome encapsidated within twinned isometric virions [22,23]. ToLCD has become the most serious constraint to tomato production in several West African countries over the last decade, in some cases causing up to 100% yield losses [4,24,25]. In West Africa, ToLCD has caused such devastating losses that in many cases, farmers have abandoned tomato production entirely [26].…”