Birithia R, Subramanian S, Pappu H R, Sseruwagi P, Muthomi J W, Narla R D
Plant Health Division, International Centre of Insect Physiology and Ecology, P.O. Box 30772-00100, Nairobi, Kenya.
Department of Plant Pathology, Washington State University, Pullman 99164.
Plant Dis. 2011 Sep;95(9):1195. doi: 10.1094/PDIS-01-11-0057.
Onion (Allium cepa L.) is one of the key vegetables produced by small-holder farmers for the domestic markets in Sub-Saharan Africa. Biotic factors, including infestation by thrips pests such as Thrips tabaci Lindeman, can inflict as much as 60% yield loss. Iris yellow spot virus (IYSV; family Bunyaviridae, genus Tospovirus) transmitted by T. tabaci is an economically important viral pathogen of bulb and seed onion crops in many onion-growing areas of the world (2,4). In Africa, IYSV has been reported in Reunion (1) and South Africa (3). In September 2009, symptoms suspected to be caused by IYSV were observed on onions and leeks cultivated in Nairobi, Kenya. Symptoms consisted of spindle-shaped, straw-colored, irregular chlorotic lesions with occasional green islands on the leaves. The presence of the virus was confirmed with IYSV-specific Agdia Flash kits (Agdia Inc., Elkart, IN). Subsequently, surveys were undertaken in small-holder farms in onion production areas of Makueni (January 2010) and Mwea (August 2010) in Kenya and Kasese (January 2010) and Rwimi (January 2010) in Uganda. The incidence of disease in these locations ranged between 27 and 72%. Onion leaves showing symptoms of IYSV infection collected from both locations tested positive for the virus by double-antibody sandwich-ELISA with IYSV-specific antiserum (Agdia Inc). IYSV infection was confirmed by reverse transcription-PCR with primers IYSV-465c: 5'-AGCAAAGTGAGAGGACCACC-3' and IYSV-239f: 5'-TGAGCCCCAATCAAGACG3' (3) as forward and reverse primers, respectively. Amplicons of approximately 240 bp were obtained from all symptomatic test samples but not from healthy and water controls. The amplicons were cloned and sequenced from each of the sampled regions. Consensus sequence for each isolate was derived from at least three clones. The IYSV-Kenya isolate (GenBank Accession No. HQ711616) had the highest nucleotide sequence identity of 97% with the corresponding region of IYSV isolates from Sri Lanka (GenBank Accession No. GU901211), followed by the isolates from India (GenBank Accession Nos. EU310287 and EU310290). The IYSV-Uganda isolate (GenBank Accession No. HQ711615) showed the highest nucleotide sequence identity of 95% with the corresponding region of IYSV isolates from Sri Lanka (GenBank Accession No. GU901211) and India (95% with GenBank Accession Nos. EU310274 and EU310297). To our knowledge, this is the first report of IYSV infecting onion in Kenya and Uganda. Further surveys and monitoring of IYSV incidence and distribution in the region, along with its impact on the yield, are under investigation. References: (1) L. J. du Toit et al. Plant Dis. 91:1203, 2007. (2) D. H. Gent et al. Plant Dis. 88:446, 2004. (3) H. R. Pappu et al. Plant Dis 92:588, 2008. (4) H. R. Pappu et al. Virus Res. 141:219, 2009.
洋葱（Allium cepa L.）是撒哈拉以南非洲小农户为国内市场生产的主要蔬菜之一。包括烟蓟马（Thrips tabaci Lindeman）等蓟马害虫侵害在内的生物因素，可造成高达60%的产量损失。由烟蓟马传播的鸢尾黄斑病毒（IYSV；布尼亚病毒科番茄斑萎病毒属）是世界上许多洋葱种植区洋葱鳞茎和种子作物的一种具有经济重要性的病毒病原体（2,4）。在非洲，留尼汪岛（1）和南非（3）已报道发现IYSV。2009年9月，在肯尼亚内罗毕种植的洋葱和韭菜上观察到疑似由IYSV引起的症状。症状表现为叶片上出现纺锤形、稻草色、不规则的褪绿病斑，偶尔有绿色小岛。使用IYSV特异性Agdia Flash检测试剂盒（Agdia公司，美国印第安纳州埃尔克哈特）确认了病毒的存在。随后，在肯尼亚马库埃尼（2010年1月）和姆韦亚（2010年8月）以及乌干达卡塞塞（2010年1月）和鲁维米（2010年1月）的洋葱生产区小农户农场进行了调查。这些地点的发病率在27%至72%之间。从这两个地点采集的表现出IYSV感染症状的洋葱叶片，通过使用IYSV特异性抗血清的双抗体夹心ELISA检测，结果显示病毒呈阳性（Agdia公司）。通过逆转录PCR，分别使用引物IYSV - 465c：5'-AGCAAAGTGAGAGGACCACC-3'和IYSV - 239f：5'-TGAGCCCCAATCAAGACG3'（3）作为正向和反向引物，确认了IYSV感染。从所有有症状的测试样品中均获得了约240 bp 的扩增子，但健康样品和水对照未获得。对每个采样区域的扩增子进行了克隆和测序。每个分离株的一致序列来自至少三个克隆。肯尼亚IYSV分离株（GenBank登录号HQ711616）与来自斯里兰卡的IYSV分离株（GenBank登录号GU901211）的相应区域具有97%的最高核苷酸序列同一性，其次是来自印度的分离株（GenBank登录号EU310287和EU310290）。乌干达IYSV分离株（GenBank登录号HQ711615）与来自斯里兰卡的IYSV分离株（GenBank登录号GU901211）的相应区域具有95%的最高核苷酸序列同一性，与来自印度（与GenBank登录号EU310274和EU310297的同一性为95%）的相应区域也具有95%的最高核苷酸序列同一性。据我们所知，这是IYSV在肯尼亚和乌干达感染洋葱的首次报道。目前正在对该地区IYSV发病率和分布及其对产量的影响进行进一步调查和监测。参考文献：（1）L. J. du Toit等人，《植物病害》91:1203，2007年。（2）D. H. Gent等人，《植物病害》88:446，2004年。（3）H. R. Pappu等人，《植物病害》92:588，2008年。（4）H. R. Pappu等人，《病毒研究》141:219，2009年。
