Rediscovery of Saprolegniales in the Philippines Reveals New Phylogenetic and Ecological Insights

Saprolegniales (Phylum Oomycota), a group of fungus-like oomycetes, include economically important opportunistic pathogens responsible for disease outbreaks in aquatic organisms, yet their occurrence in tropical regions remains poorly documented. In this manuscript, five species of Saprolegniales were identified using an integrative taxonomic approach combining morphological characterization and phylogenetic analyses of nuclear (ITS, LSU) and mitochondrial (cox1, cox2) gene markers. Here we report the first Philippine records of Achlya glomerata and Saprolegnia ferax, and the second records of Achlya bisexualis, Achlya klebsiana, and Achlya prolifera. All species were isolated from both natural and artificial aquatic habitats, indicating broad ecological occurrence and potential relevance to aquatic health. Growth assays conducted at 15–35 °C demonstrated contrasting thermal responses, i.e., Achlya species exhibited enhanced growth with increasing temperature, whereas S. ferax showed reduced growth at warmer conditions. These patterns suggest that warm lowland aquaculture systems may be more vulnerable to Achlya-associated infections, while cooler highland environments may favor potential Saprolegnia-related outbreaks. Further, this study provides the first DNA sequence records of Philippine Saprolegniales, expanding molecular reference data and improving understanding of their diversity and distribution in tropical ecosystems.