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

Abstract

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’