Fondong V N, Pita J S, Rey C, Beachy R N, Fauquet C M
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.
Plant Dis. 1998 Oct;82(10):1172. doi: 10.1094/PDIS.1998.82.10.1172B.
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.
木薯花叶病（CMD）在非洲、印度和斯里兰卡所有种植木薯的地区均有发生。CMD以木薯叶片出现花叶和畸形以及植株生长受抑制为特征，会导致产量大幅损失，它由双生病毒（菜豆金色花叶病毒属，双生病毒科）引起，可通过受感染的插条或烟粉虱传播。已描述了三种此类双生病毒：非洲木薯花叶病毒（ACMV）出现在非洲大部分木薯产区；东非木薯花叶病毒（EACMV）出现在东非；印度木薯花叶病毒（ICMV）出现在印度次大陆（1）。ACMV和ICMV基因组的两个组分，即DNA - A和DNA - B，已完成测序；仅EACMV的DNA - A已被鉴定和测序。在喀麦隆，同一木薯品种内症状表现和严重程度存在差异。为确定引发此类差异的病毒种类的性质，从喀麦隆草原和雨林地区感染CMD的植株上采集了50个样本：2个来自萨赫勒/草原平原，13个来自西部高地草原，35个来自西南雨林的主要木薯产区。雨林地区CMD发病率很高，一些农场完全感染，而在草原地区，农场发病率一般低于25%。症状表现的差异在雨林地区更为常见。从具有明显症状和/或不同症状严重程度的植株上采集样本，然后用特异性引物通过聚合酶链反应（PCR）进行分析：JSP1，ATG TCG AAG CGA CCA GGA GAT；JSP2，TGT TTA TTA ATT GCC AAT ACT；JSP3，CCT TTA TTA ATT TGT CAC TGC。引物JSP1与ACMV和EACMV外壳蛋白（CP）的5'端退火；引物JSP2和JSP3分别与ACMV和EACMV的3'端退火。病毒鉴定基于是否存在任一病毒的扩增片段。在所有50个样本中均检测到ACMV；在8个样本中检测到EACMV。所有感染EACMV的样本均来自喀麦隆西南雨林，且比单一感染的植株受病害影响更严重。先前的报告将EACMV的发生局限于东非（1）。这是EACMV在西非发生的首次报道。对来自喀麦隆的三种EACMV分离株（EACMV/CM）的CP基因进行了克隆PCR产物测序。所有三种EACMV分离株的CP核苷酸序列同一性很高（>99%），仅存在两个氨基酸差异。相比之下，与来自坦桑尼亚的EACMV/TZ的序列同一性相当低（94%）（2），这表明它们可能属于此前未描述的EACMV西非株系。这表明在非洲引起CMD的双生病毒分布比先前报道的更为广泛。喀麦隆的分离株均未表现出来自乌干达的EACMV分离株（EACMV/UG）的重组类型（其CP核心区段与相应的ACMV CP序列相同）（2）。这强调了对非洲不同木薯种植区引起CMD的病毒进行特征描述的必要性，因为合适的防治策略取决于对病害病因的充分了解。参考文献：（1）Y.G. Hong等人，《病毒学杂志》74:2437，1993年。（2）X. Zhou等人，《病毒学杂志》78:2101，1997年。
