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Plumeria rust samples were collected from five provinces in Thailand, including Bangkok, Nakhon Pathom, Rayong, Chonburi and Yala. All five isolates produced the uredial stage but only the isolates from Bangkok and Yala also underwent the telial and basidial stages. The morphological characteristics of all three stages present in the life cycle of the isolates were studied under stereo, compound and electron microscopes. Ribosomal DNA (rDNA) sequences at 28S and ITS (internal transcribed spacer) regions were analyzed with those in the GenBank database by Nucleotide BLAST and phylogenetic analyses. Coleosporium plumeriae was identified as the causal agent of plumeria rust by structure morphology and rDNA sequences that revealed genetic variation of the fungus as well. In general, there were significant differences in the morphological characteristics of uredospores, teliospores and basidia among the isolates. However, the variation of spore morphology was not related to the sampling locations. According to the phylogenetic analysis of 28S rDNA sequences, the UPGMA tree grouped all C. plumeriae from Thailand and foreign countries in the same clade as they shared identical sequences. On the other hand, the UPGMA tree inferred from ITS rDNA sequence data detected genetic variation of the isolate from Chonburi and separated it into the distinct tree branch. In this study, structure morphology and ITS rDNA were suitable genetic markers for both interspecific and intraspecific taxonomy of C. plumeriae.
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