Species delimitation in Pythium species complexes: the case of Pythium myriotylum Drechsler and Pythium zingiberis Takahashi

Abstract

Pythium myriotylum and P. zingiberis have both been implicated in soft rot of ginger (Zingiber spp.). The status of these two taxa as distinct species follows original descriptions of physiology, morphology and pathogenicity. However, their status has been questioned by phylogenetic analyses. In this study, putative P. zingiberis isolates recovered from edible ginger (Z. officinale) rhizomes with Pythium soft rot (PSR) disease sampled from Queensland, Australia, were compared with reference isolates of P. myriotylum from peanut in Israel (CBS254.70) and P. zingiberis from ginger in Japan (NBRC30817). All isolates differed slightly in temperature optima for growth and produced similar sizes of oogonia and oospores. The sequence homology of 20 gene fragments retrieved from nuclear and mitochondrial genomes ranges from over 99 to 100% to each other. In vitro pathogenicity assays were conducted on excised carrot, ginger, potato, radish and sweet potato tuber/root sections and on seeds and seedlings of cucumber, cauliflower, millet, rye, sweet corn, tomato and wheat with each of the isolates. The reference isolate P. zingiberis NBRC30817, which was previously believed to have a narrow host range, was pathogenic on a number of tested plant species. Analysis of this comprehensive set of data allowed us to assign all tested isolates, including the isolate P. zingiberis NBRC30817, to the taxon P. myriotylum, thus confirming that the causal pathogen of PSR disease of ginger in Australia is P. myriotylum.