Late blight, caused by Phytophthora infestans, is one of the most destructive potato diseases in the world. In Yemen, identification of P. infestans still depends on a visual survey and external examination of late blight symptoms. The objective of this study was to isolate and identify P. infestans by using advanced methods. We collected 71 disease samples and isolated the pathogen using the tuber slice method. To identify an isolated pathogen, we performed morphological characterization and gene sequence analysis of the coding genes for internal transcribed spacers. We used Koch's hypotheses to confirm the previous results. In our study. The morphological characters of the mycelium pattern of P. infestans isolates in Yemen were profusely branching, fluffy, and white. The sporangia showed remarkable limoniform papillate sporangial shape. with average length and width of 30.6 and 28.6 µm, respectively. The sequences analysis showed high homology with a degree of identity ranging from 98 to 100% to the database sequences on GenBank. Pathogenicity tests showed that the P. infestans was the causal agent. To our knowledge, this is the first study of the isolation and characterization of P. infestans in Yemen.
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