Root and Stem Rot by Phytophthora sojae Kaufm. & Gerd. in the eastern high plains of Meta Department.

ABSTRACT The oomycete Phytophthora sojae has been reported to cause “Damping off” during pre and post-emergence, as well as root and stem rot in soybean crops worldwide. The increase in the soybean planted area in Colombia, in the region of eastern high plains of Meta Department, and the presence of highly favorable environmental conditions led to the appearance of seedling damage symptoms, which could be caused by a pathogen complex that includes P. sojae. This study aimed to identify the causal agent of root and stem rot in soybean crops in the region of Meta Department through microbiological and molecular diagnosis of 84 affected tissue samples. Morphological characterization of the causal agent and macroscopic characterization of colonies were performed in modified V8 medium. Molecular identification was done by amplification of the ITS region. Phylogenetic comparison employed ITS sequences reported in previous studies and deposited in the GenBank for different species of the genus Phytophthora, isolates of the species P. sojae from different geographical regions and the sequences of P. sojae obtained in this study. Of the 84 samples analyzed during 2015-2017, seven (7) samples corresponding to 8% showed typical symptoms of root and stem rot, which were attributed to P. sojae, confirming Koch’s postulates. The remaining 77 samples corresponding to 92% presented symptoms of the “Damping off” complex and were associated with Pythium spp. and Fusarium spp. This is the first report of the presence of P. sojae and its participation as a causal agent of root and stem rot in commercial areas of soybeans in the eastern high plains of Meta Department.

摘要
据报道，卵菌大豆疫霉菌（Phytophthora sojae）可在大豆出苗前后引发猝倒病（Damping off），还可导致全球范围内的大豆作物发生根腐与茎腐病。哥伦比亚梅塔省（Meta Department）东部高平原地区大豆种植面积持续扩大，加之当地环境条件极为适宜病害发生，致使该区域出现大豆幼苗受害症状，推测其致病原为由大豆疫霉菌等组成的病原复合群体。本研究通过对2015至2017年间采集的84份大豆患病组织样本开展微生物学与分子生物学鉴定，旨在明确该区域大豆根腐与茎腐病的致病菌。研究采用改良V8培养基（modified V8 medium）对致病菌进行形态学特征分析，并对菌落进行宏观形态观察；通过扩增内转录间隔区（ITS）序列完成分子鉴定。系统发育比较分析所用序列包含：既往研究报道并提交至GenBank的疫霉属不同物种的ITS序列、不同地理来源的大豆疫霉菌分离株ITS序列，以及本研究获得的大豆疫霉菌ITS序列。经检测，84份样本中有7份（占比8%）表现出典型的根腐与茎腐病症状，经鉴定致病菌为大豆疫霉菌，且符合科赫法则（Koch’s postulates）。剩余77份样本（占比92%）表现出猝倒病复合症状，其致病菌为腐霉属（Pythium spp.）与镰孢属（Fusarium spp.）物种。本研究为首次在哥伦比亚梅塔省东部高平原地区的商业大豆种植区报道大豆疫霉菌的存在，并证实其为大豆根腐与茎腐病的致病菌之一。