Murithi H M, Beed F D, Soko M M, Haudenshield J S, Hartman G L
International Institute of Tropical Agriculture, Tanzania.
Bvumbwe Agriculture Research Station, Malawi.
Plant Dis. 2015 Mar;99(3):420. doi: 10.1094/PDIS-09-14-0924-PDN.
Soybean rust (SBR), caused by Phakopsora pachyrhizi, has become established in Africa since the first report in Uganda in 1996 (2). The urediniospores, as windborne propagules, have infested new regions of Africa, initiating SBR in many countries, including Ghana and Democratic Republic of the Congo in 2007 (4) and Tanzania in 2014 (3). No refereed reports have been published about rust in Malawi, but some people have indicated that soybean rust may have been observed as early as 2008. Typical symptoms and signs of SBR, including leaf yellowing and tan, sporulating uredinia, were observed on soybean in May 2014 during field surveys in the major soybean-growing areas of Malawi, including the central (Dowa, Mchinji, and Kasungu) and southern (Thyolo) regions in nine out of 12 sites surveyed. When microscopically examined, urediniospores were elliptical, echinulate, and hyaline to pale yellowish brown. Leaves exhibiting sporuliferous uredinia were sent by APHIS permit to the University of Illinois. To confirm the pathogen, symptomatic soybean leaf tissue of approximately 1 cm was excised from each of the samples, and DNA was extracted using the FastDNA Spin Kit (MP Biomedicals, Solon, OH), with further purification using the MicroElute DNA Clean-up Kit (Omega Bio-Tek, Norcross, GA). The resulting DNA was analyzed by quantitative PCR using published Taqman assays for P. pachyrhizi and P. meibomiae, with a multiplexed exogenous internal control reaction to validate negative results (1). P. pachyrhizi DNA was detected in excess of 180,000 genome equivalents/cm in all samples, indicating a substantial infection. P. meibomiae DNA was determined to be absent from all samples, within the limit of quantification of ~2 pg DNA/cm. Urediniospores dislodged from three leaves and inoculated onto susceptible soybean cultivar Williams 82 produced tan lesions after 2 weeks of incubation in a detached-leaf assay. This is the first confirmed report of P. pachyrhizi causing rust on soybean in Malawi, putting at risk 14,000 ha currently under soybean production. The reports of soybean rust in Malawi and adjoining countries will alter soybean production practices and research interests. In some cases, foliar application of fungicides has increased and planting dates have been changed to avoid conditions that are most conducive for rust development. Efforts to understand the virulence and genetic diversity of the pathogen in the region are needed in order to develop and deploy resistant cultivars. References: (1) J. S. Haudenshield and G. L. Hartman. Plant Dis. 95:343, 2011. (2) R. Kawuki, et al. Afr. Crop Sci. J. 11:301, 2003. (3) H. M. Murithi et al. Plant Dis. 98:1586, 2014. (4) P. S. Ojiambo et al. Plant Dis. 91:1204, 2007.
大豆锈病（SBR）由大豆层锈菌（Phakopsora pachyrhizi）引起，自1996年在乌干达首次报道以来，已在非洲定殖（2）。夏孢子作为随风传播的繁殖体，侵染了非洲的新区域，在许多国家引发了大豆锈病，包括2007年在加纳和刚果民主共和国（4）以及2014年在坦桑尼亚（3）。关于马拉维锈病尚无经同行评审的报道，但一些人表示，早在2008年可能就已观察到大豆锈病。2014年5月，在马拉维主要大豆种植区进行田间调查时，在包括中部（多瓦、姆钦吉和卡松古）和南部（蒂约洛）地区在内的12个调查点中的9个点，在大豆上观察到了大豆锈病的典型症状和病征，包括叶片发黄和呈棕褐色、产生夏孢子堆。经显微镜检查，夏孢子呈椭圆形、具刺，无色至浅黄褐色。带有产孢夏孢子堆的叶片通过美国动植物卫生检验局（APHIS）的许可被送往伊利诺伊大学。为确认病原菌，从每个样本中切取约1厘米有症状的大豆叶片组织，使用FastDNA Spin试剂盒（MP Biomedicals公司，俄亥俄州索伦）提取DNA，并使用MicroElute DNA纯化试剂盒（Omega Bio-Tek公司，佐治亚州诺克罗斯）进一步纯化。使用已发表的针对大豆层锈菌和梅氏层锈菌（P. meibomiae）的Taqman检测方法，通过定量PCR对所得DNA进行分析，并进行多重外源内对照反应以验证阴性结果（1）。在所有样本中均检测到大豆层锈菌DNA超过180,000基因组当量/厘米，表明感染程度严重。在所有样本中均未检测到梅氏层锈菌DNA，其定量限为约2皮克DNA/厘米。从三片叶子上抖落的夏孢子接种到易感大豆品种威廉姆斯82上，在离体叶片试验中培养2周后产生了棕褐色病斑。这是关于大豆层锈菌在马拉维引起大豆锈病的首次确诊报道，使目前正在种植大豆的14,000公顷土地面临风险。马拉维和邻国关于大豆锈病的报道将改变大豆生产实践和研究方向。在某些情况下，叶面喷施杀菌剂的情况有所增加，并且种植日期也已更改，以避免最有利于锈病发展的条件。为了培育和推广抗病品种，需要努力了解该地区病原菌的毒力和遗传多样性。参考文献：（1）J. S. Haudenshield和G. L. Hartman。《植物病害》95:343，2011。（2）R. Kawuki等人。《非洲作物科学杂志》11:301，2003。（3）H. M. Murithi等人。《植物病害》98:1586，2014。（4）P. S. Ojiambo等人。《植物病害》91:1204，2007。
