First report of cassava mosaic disease and cassava mosaic geminiviruses in Gabon

Cassava mosaic disease (CMD), arguably Africa's greatest plant protection problem, has been known to occur in central/West Africa for more than 70 years. There is, however, no published record of the occurrence of CMD or the cassava mosaic geminiviruses (CMGs) that cause it from the central African country of Gabon. Significantly, however, the severe Uganda variant of East African cassava mosaic virus (EACMV‐UG), associated with the African CMD pandemic, has been recorded from the Republic of Congo (Neuenschwander et al., 2002), bordering Gabon to the west. Cassava fields were examined at 55 sites throughout Gabon in April and July 2003, in order to identify the CMGs associated with CMD and to determine if the pandemic‐associated EACMV‐UG was present. At each site, 30 plants were examined and assessments made of CMD incidence, symptom severity and the abundance of the whitefly vector Bemisia tabaci. At least two virus‐diseased leaf samples were collected from each field for subsequent virus diagnosis. DNA was extracted from these samples on the day of collection using the method of Dellaporta et al. (1983). Virus diagnoses were subsequently made from DNA samples using both specific primer PCR (Zhou et al., 1997) and RFLP analysis involving restriction digestion by EcoRV and MluI of near full‐length DNA‐A fragments amplified using abutting primer PCR with universal geminivirus primers (Briddon & Markham, 1994). ACMV was the most widely distributed CMG species, occurring at 53 of the 55 sites. EACMV was identified from a single site just south of Tchibanga in southern Gabon. EACMV‐UG was detected in samples collected from 12 sites, all of which were in the eastern provinces of Haute‐Ogooué and Ogooué‐Ivindo. Of the 17 samples infected with EACMV‐UG, 16 were mixed infections with ACMV. Symptom severity of plants infected by EACMV‐UG (4·1) was significantly greater (χ2 = 44·4, d.f. = 3, P < 0·001) than that of plants infected by ACMV alone (2·8). Additionally, four of the five virus‐sampled plants having current season whitefly‐borne CMD were dual ACMV + EACMV‐UG infections. These data comprise the first published record of CMGs in Gabon. The results also describe an early stage of spread of the EACMV‐UG‐associated CMD pandemic into eastern Gabon, which now represents the pandemic's westernmost ‘front’

Viruses associated with cassava mosaic disease in Senegal and Guinea Conakry

A survey in Senegal and Guinea Conakry established the presence and incidence of cassava mosaic virus disease (CMD) in both countries. CMD occurred in all the fields surveyed, although its incidence was higher in Senegal (83%) than in Guinea (64%). Populations of the whitefly vector, Bemisia tabaci, were low in both countries averaging 1.7 adults per shoot in Guinea and 3.2 in Senegal. Most infections were attributed to the use of infected cuttings, 86 and 83% in Senegal and Guinea, respectively, and there was no evidence of rapid current‐season, whitefly‐borne infection at any of the sampled locations. Disease severity was generally low in the two countries and averaged 2.5 in Guinea and 2.3 in Senegal. No plants with unusually severe CMD symptoms characteristic of the CMD pandemic in East and Central Africa were observed. Restriction fragment length polymorphism (RFLP)‐based diagnostics revealed that African cassava mosaic virus (ACMV) is exclusively associated with CMD in both the countries. Neither East African cassava mosaic virus (EACMV), nor the recombinant Uganda variant (EACMV‐UG2) was detected in any sample. These survey data indicate that CMD could be effectively controlled in both countries by phytosanitation, involving the use of CMD‐free planting material and the removal of diseased plants

A distinct Bemisia tabaci (Gennadius) (Hemiptera: Sternorrhyncha: Aleyrodidae) genotype cluster is associated with the epidemic of severe cassava mosaic virus disease in Uganda

During the 1990s, an epidemic of cassava mosaic virus disease caused major losses to cassava production in Uganda. Two factors associated with the epidemic were the occurrence of a novel recombinant begomovirus, EACMV-Ug, and unusually high populations of the whitefly vector, Bemisia tabaci. Here we present molecular evidence for the occurrence of two cassava-colonizing B. tabaci genotype clusters, Ug1 and Ug2, one of which, Ug2, can be consistently associated with the CMD epidemic in Uganda at the time of collection in 1997. By contrast, a second genotype cluster, Ug1, only occurred 'at' or 'ahead of' the epidemic 'front', sometimes in mixtures with Ug2. Comparison of mitochondrial cytochrome oxidase I gene sequences for Ug1 and Ug2 and well-studied B. tabaci reference populations indicated that the two Ugandan populations exhibited approximately 8% divergence, suggesting they represent distinct sub-Saharan African lineages. Neither Ugandan genotype cluster was identified as the widely distributed, polyphagous, and highly fecund B biotype of Old World origin, with which they both diverged by approximately 8%. Within genotype cluster divergence of Ug1 at 0.61 +/- 0.1% was twice that of Ug2 at 0.35 +/- 0.1%. Mismatch analysis suggested that Ug2 has undergone a recent population expansion and may be of nonUgandan origin, whereas Ug1 has diverged more slowly, and is likely to be an indigenous genotype cluster

Incidence and severity of cassava mosaic disease in the Republic of Congo

Diagnostic surveys were conducted in 2002 and 2003 in order to provide a comprehensive and detailed assessment of the status of cassava mosaic disease (CMD) in the Republic of Congo (ROC) and to determine if the disease was spreading. In 2002, 105 farmers’ fields were assessed in the four major cassava-producing regions ofthe country. In 2003, 163 fields were sampled in nine regions and Brazzaville Commune. Incidence of cassava mosaic disease was generally high, averaging 80 and 86 % for 2002 and 2003, respectively; while damage was moderate to severe. In 2002, disease incidence was moderate in Pool (73%) but high in Brazzaville (81%),Cuvette Centrale (82%) and in Plateaux Region (84%). Pool region still had the lowest incidence (78%) in 2003, while Sangha (95%) had the highest incidence. The greatest disease severity was recorded in Niari region in the south and Cuvette Ouest region in the north in 2003. East African cassava mosaic virus Uganda variant (EACMVUG) occurred virtually throughout the country, commonly in dual infections with African cassava mosaic virus. The high incidence of disease in plants considered to be the results of cutting infection (74% in 2002 and 82% in 2003), relatively low incidence of disease in plants considered to be infected by whiteflies and wide distribution of EACMV-UG points to the fact that the CMD pandemic is a chronic in the country and the areas sampled are currently in a stable post-epidemic phase. This situation is comparable to that in areas of East Africa affected by a pandemic during the 1990s, including Uganda, parts of western Kenya and north-western Tanzania. These findings clearly verify the assertion that the CMD pendemic has expanded across Central Africa and provide abasis for designing interventions and control strategies for the entire region

Cassava mosaic geminiviruses associated with cassava mosaic disease in Rwanda

Cassava mosaic disease (CMD), and cassava mosaic geminiviruses (CMGs), were investigated in a country-wide survey conducted in six cassava-producing prefectures (administrative regions) of Rwanda in 2001. CMD occurred throughout the country at an average incidence of 30%. Incidence was highest in Kigali (44%) and lowest in Butare (11%). Cutting infection resulting from the use of CMD-affected planting material was the main type of infection. Disease symptoms were generally severe, with little difference between cultivars or locations. Most affected plants lacked the ‘candle-stick’ symptoms characteristic of severe CMD associated with infection by the ‘Uganda Variant’ of East African cassava mosaic virus (EACMV-UG2). PCR analysis detected two viruses in CMD-affected plants: African cassava mosaic virus (ACMV) and EACMV-UG2. ACMV predominated and occurred throughout the surveyed areas, whereas EACMV-UG2 was restricted to the prefectures of Byumba, which border south-western Uganda and Kibungo in south-eastern Rwanda. No dual infections were detected

The effect of cassava mosaic geminiviruses on symptom severity, growth and root yield of a cassava mosaic virus diseasesusceptible cultivar in Uganda

A study was carried out to assess the effect of different cassava mosaic geminiviruses (CMGs) occurring in Uganda on the growth and yield of the susceptible local cultivar ‘Ebwanateraka’. Plants infected with African cassava mosaic virus (ACMV), ‘mild’ and ‘severe’ strains of East African cassava mosaic virus-Uganda (EACMV-UG2) and both ACMV and EACMV-UG2 were grown in two experiments in Kabula, Lyantonde in western Uganda. The most severe disease developed in plants co-infected with ACMV and EACMV-UG2 and in those infected with the ‘severe’ form of EACMV-UG2 alone; disease was least severe in plants infected with the ‘mild’ strain of EACMV-UG2. ACMV-infected plants and those infected with the ‘mild’ strain of EACMV-UG2 were tallest in the 1999–2000 and 2000–2001 trials, respectively; plants dually infected with ACMV and EACMV-UG2 were shortest in both trials. Plants infected with ‘mild’ EACMV-UG2 yielded the largest number and the heaviest tuberous roots followed by ACMV and EACMV-UG2 ‘severe’, respectively, whilst plants dually infected with ACMV and EACMV-UG2 yielded the least considering the two trials together. Reduction in tuberous root weight was greatest in plants dually infected with ACMV and EACMV-UG2, averaging 82%. Losses attributed to ACMV alone, EACMV-UG2 ‘mild’ and EACMV-UG2 ‘severe’ were 42%, 12% and 68%, respectively. Fifty percent and 48% of the plants infected with both ACMV and EACMV-UG2 gave no root yield in 1999–2000 and 2000–2001, respectively. These results indicate that CMGs, whether in single or mixed infections, reduce root yield and numbers of tuberous roots produced and that losses are substantially increased following mixed infection

Incidence and severity of cassava mosaic disease in the Republic of Congo

Diagnostic surveys were conducted in 2002 and 2003 in order to provide a comprehensive and detailed assessment of the status of cassava mosaic disease (CMD) in the Republic of Congo (ROC) and to determine if the disease was spreading. In 2002, 105 farmers’ fields were assessed in the four major cassava-producing regions of the country. In 2003, 163 fields were sampled in nine regions and Brazzaville Commune. Incidence of cassava mosaic disease was generally high, averaging 80 and 86 % for 2002 and 2003, respectively; while damage was moderate to severe. In 2002, disease incidence was moderate in Pool (73%) but high in Brazzaville (81%), Cuvette Centrale (82%) and in Plateaux Region (84%). Pool region still had the lowest incidence (78%) in 2003, while Sangha (95%) had the highest incidence. The greatest disease severity was recorded in Niari region in the south and Cuvette Ouest region in the north in 2003. East African cassava mosaic virus Uganda variant (EACMVUG) occurred virtually throughout the country, commonly in dual infections with African cassava mosaic virus. The high incidence of disease in plants considered to be the results of cutting infection (74% in 2002 and 82% in 2003), relatively low incidence of disease in plants considered to be infected by whiteflies and wide distribution of EACMV-UG points to the fact that the CMD pandemic is a chronic in the country and the areas sampled are currently in a stable post-epidemic phase. This situation is comparable to that in areas of East Africa affected by a pandemic during the 1990s, including Uganda, parts of western Kenya and north-western Tanzania. These findings clearly verify the assertion that the CMD pendemic has expanded across Central Africa and provide a basis for designing interventions and control strategies for the entire region