International Laboratory for Tropical Agricultural Biotechnology (ILTAB/ORSTOM-TSRI), Division of Plant Biology-BCC 206, 10550 N. Torrey Pines Road, La Jolla, CA 92037
Department of Microbiology, University of Witswa-tersrand, Johannesburg, South Africa
ILTAB/ORSTOM-TSRI, Division of Plant Biology-BCC 206, 10550 N. Torrey Pines Road, La Jolla, CA 92037
Cassava mosaic disease (CMD) occurs in all cassava-growing regions of Africa, India, and Sri Lanka. Characterized by mosaic and distortion of cassava leaves and reduced plant growth, causing high yield losses, CMD is caused by geminiviruses (genus Begomovirus, family Geminiviridae) transmitted through infected cuttings or by the whitefly, Bemisia tabaci. Three such geminiviruses have been described: African cassava mosaic virus (ACMV) occurs in most of the cassava-producing zones of Africa; East African cassava mosaic virus (EACMV) in East Africa; and Indian cassava mosaic virus (ICMV) in the Indian subcontinent (1). The two components of ACMV and ICMV genomes, DNA-A and DNA-B, have been sequenced; only DNA-A of EACMV has been identified and sequenced. Variations in symptom expression and severity within the same cassava variety have been observed in Cameroon. To determine the nature of the virus species inducing such variations, 50 samples were collected from CMD-infected plants in the savannah and rainforest zones of Cameroon: 2 from the sahel/savannah plain, 13 from the western highland savannah, and 35 from the main cassava-producing belt of the southwestern rainforest. There is a high incidence of CMD in the rainforest region, with some farms completely infected, while in the savannah regions farms generally have less than 25% incidence. Variation in symptom expression was more common in the rainforest region. Samples were collected from plants with distinct symptoms and/or different extents of symptom severity, then analyzed with the polymerase chain reaction (PCR) with specific primers: JSP1, ATG TCG AAG CGA CCA GGA GAT; JSP2, TGT TTA TTA ATT GCC AAT ACT; and JSP3, CCT TTA TTA ATT TGT CAC TGC. Primer JSP1 anneals to the 5′ end of the coat protein (CP) of ACMV and EACMV; primers JSP2 and JSP3 anneal to the 3′ ends of ACMV and EACMV, respectively. Virus identification was based on presence of an amplified fragment of either virus. ACMV was detected in all 50 samples; EACMV was detected in 8. All samples infected with EACMV were from the southwestern rainforest of Cameroon and were more severely affected by the disease than single infected plants. Previous reports have limited occurrence of EACMV to East Africa (1). This is the first report of the occurrence of EACMV in West Africa. The CP gene of three isolates of EACMV from Cameroon (EACMV/CM) was sequenced from cloned PCR products. There was a high CP nucleotide sequence identity (>99%) with only two amino acid differences among all three EACMV isolates. In contrast, there was a rather low sequence identity (94%) with EACMV/TZ from Tanzania (2), suggesting they may belong to a previously undescribed West African strain of EACMV. This indicates the geminiviruses causing CMD in Africa are more widely distributed than previously reported. None of the Cameroon isolates showed the type of recombination of the EACMV isolate from Uganda (EACMV/ UG) (having the CP core segment the identical to the corresponding ACMV CP sequence) (2). This emphasizes the need for characterization of the viruses causing CMD in different cassava-growing regions of Africa since appropriate control strategies depend on adequate knowledge of disease etiology.
References: (1) Y. G. Hong et al. J. Gen. Virol. 74:2437, 1993. (2) X. Zhou et al. J. Gen. Virol. 78:2101, 1997.