Soybean (Glycine max L. Merr.) is an important leguminous crop for Colombia, given the growing demand from the livestock, poultry, and aquaculture industries. About 80 percent of Colombian soybean production is in the State, or Department, of Meta, located in the Eastern Plains region, or Llanos Orientales, where the crop has an average yield of 2.5 t.ha-1 (FENALCE 2020). In July 2017, foliar symptoms, including rounded to irregular reddish-brown spots surrounded by a yellowish halo progressing to irregular spots with concentric rings, and in severe cases defoliation, were observed in a production field of Soyica P-34 soybean cultivar in Puerto López, Meta (Colombia). Dark brown lesions on stems and dark-brown to black spots on pods were also observed, and the incidence of symptomatic plants was recorded as 50%. Four infected plants were arbitrarily sampled from different locations across the field, and used for pathogen isolation. Specimen collection was conducted according to the permit conferred to AGROSAVIA under ANLAS' resolution No 1466 of December 03, 2014, Colombia. Symptomatic leaf pieces (~ 5 mm2 sizes) were rinsed with tap water for 1 min, dipped in a 0.5% sodium hypochlorite for 2 min and rinsed with sterile distilled water for 1 min, and then plated on potato dextrose agar (PDA, 39 g.L-1). Plates were incubated at 25°C for two weeks with a 12/12 h light/dark cycle using dark light. Four monoconidial isolates, with similar morphological characteristics, were obtained. Observations under the light microscope showed that conidia (n=50) were hyaline, elongated, 65-120 × 9-12 μm, with 3 to 10 pseudosepta, corresponding with those characteristics described for Corynespora cassiicola (Berk. & M. A. Curtis) C. T. Wei (Ellis and Holliday, 1971). Colonies, on PDA medium, were dark gray with abundant aerial mycelia growth. To confirm the morphological identification, extracted DNA from isolates AGSV13 and AGSV15 was used as a template to obtain partial sequences of the internal transcribed spacer (ITS) region of ribosomal DNA using the primer pair ITS 5/ITS 4 (White et al. 1990). Results from an NCBI-BLASTn search revealed that the ITS sequences (GenBank accessions MN298749 and MN298751) were 100% identical to those of C. cassiicola in GenBank (Accessions MN945374, AB873045). A phylogenetic analysis, using the maximum likelihood method, based on ITS sequences from voucher specimens of Corynespora spp. available at GenBank, revealed that the two isolates were placed in the same clade as C. cassiicola. Pathogenicity tests were conducted by spraying a conidial suspension (1 x 104 conidia.ml-1) of C. cassiicola AGSV13, onto young leaves (two to four true leaf stages) of 10 soybean plants (cv. Soyica P-34). Five plants were sprayed with sterile distilled water and served as non-inoculated control. All plants were incubated at high humidity for seven days at 28°C. Fungal inoculated plants showed typical foliar symptoms of brown spots surrounded by a yellowish halo, similar to those observed in the field. Disease symptoms were not observed on plants of the non-inoculated control. Fungal cultures were recovered from symptomatic leaves of all inoculated plants and verified as similar in morphology to the original C. cassiicola isolates, thus fulfilling Koch's postulates. Based on morphology, pathogenicity, and sequence data results, this is the first report of C. cassiicola causing Target spot on soybean in the Eastern Plains of Colombia and expands our knowledge of this disease. Target spot poses a threat to the expanding soybean crops in this country, as the range of yield losses due to this disease, in South America, has been estimated to be 8% to 42%, depending on the cultivar (Edwards Molina et al. 2019). These findings are significant to the soybean industry in Colombia and will be useful to provide better recommendations to growers for disease monitoring and management.

中文翻译：

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哥伦比亚东部平原首次报告由 Corynespora cassiicola 引起的大豆目标斑病。


鉴于畜牧业、家禽和水产养殖业的需求不断增长，大豆 (Glycine max L. Merr.) 是哥伦比亚重要的豆科作物。哥伦比亚约 80% 的大豆产量产自东部平原地区的梅塔州或省，即东方利亚诺斯，该地区的平均产量为 2.5 t.ha-1（FENALCE 2020）。 2017 年 7 月，在洛佩斯港 Soyica P-34 大豆品种的生产田中观察到叶面症状，包括圆形至不规则红棕色斑点，周围有淡黄色晕圈，进展为同心环不规则斑点，严重时出现落叶。 ，梅塔（哥伦比亚）。茎上也观察到深棕色病斑，豆荚上也观察到深棕色至黑色斑点，有症状植物的发生率为50%。从田间不同地点任意取样四株受感染的植物，用于病原体分离。样本采集是根据哥伦比亚 2014 年 12 月 3 日 ANLAS 第 1466 号决议授予 AGROSAVIA 的许可进行的。有症状的叶片（约 5 mm2 大小）用自来水冲洗 1 分钟，在 0.5% 次氯酸钠中浸泡 2 分钟，用无菌蒸馏水冲洗 1 分钟，然后铺在马铃薯葡萄糖琼脂（PDA，39 gL）上-1）。将板在 25°C 下孵育两周，使用暗光进行 12/12 小时的光/暗循环。获得了四种具有相似形态特征的单分生孢子分离株。光学显微镜下观察显示，分生孢子 (n=50) 透明，拉长，65-120 × 9-12 μm，有 3 至 10 个假隔膜，与 Corynespora cassiicola (Berk. & MA Curtis) CT Wei 描述的特征相对应（埃利斯和霍利迪，1971）。 PDA 培养基上的菌落呈深灰色，具有丰富的气生菌丝体生长。为了确认形态学鉴定，从分离株 AGSV13 和 AGSV15 中提取的 DNA 用作模板，使用引物对 ITS 5/ITS 4 获得核糖体 DNA 内部转录间隔区 (ITS) 区域的部分序列（White 等人，1990） 。 NCBI-BLASTn 搜索结果显示，ITS 序列（GenBank 登录号 MN298749 和 MN298751）与 GenBank 中 C. cassiicola 的序列（登录号 MN945374、AB873045）100% 相同。使用最大似然法，基于棒孢菌属凭证标本的 ITS 序列进行系统发育分析。 GenBank 提供的数据显示，这两个分离株与 C. cassiicola 属于同一进化枝。通过将 C. cassiicola AGSV13 的分生孢子悬浮液 (1 x 104 分生孢子.ml-1) 喷洒到 10 株大豆植物 (cv. Soyica P-34) 的幼叶（2 至 4 个真叶期）上进行致病性测试。用无菌蒸馏水喷洒五株植物并作为未接种的对照。所有植物均在 28°C 高湿度下培养 7 天。接种真菌的植物表现出典型的叶面症状，即被淡黄色光环包围的棕色斑点，与在田间观察到的类似。在未接种对照的植物上没有观察到疾病症状。从所有接种植物的有症状叶子中回收真菌培养物，并证实其在形态上与原始 C. cassiicola 分离株相似，从而满足科赫假设。根据形态、致病性和序列数据结果，这是哥伦比亚东部平原大豆上由 C. cassiicola 引起的第一份报告，扩大了我们对该病害的认识。 目标斑点对该国不断扩大的大豆作物构成威胁，因为在南美洲，这种疾病导致的产量损失范围估计为 8% 至 42%，具体取决于品种（Edwards Molina 等，2017）。 2019）。这些发现对哥伦比亚的大豆产业具有重要意义，并将有助于为种植者提供更好的病害监测和管理建议。