New reports of Achlya and Saprolegnia in tropical biomes of Luzon, Philippines, reveal ecological insights

Research on Saprolegniales, an aquatic fungal-like group harboring pathogenic species, often centers on their economic impact as agents of aquatic disease outbreaks. However, they remain underrepresented in tropical regions, creating significant research gaps. This study investigated Saprolegniales in lowland and highland aquatic ecosystems of Luzon, Philippines. Five species were identified through morphological and phylogenetic analyses with nuclear (ITS, LSU) and mitochondrial (cox1 and cox2) markers. This study reports the first Philippine records of Achlya glomerata and Saprolegnia ferax, and the second records of Achlya bisexualis, Achlya klebsiana, and Achlya prolifera. The successful isolation of these species from natural and man-made environments raises concerns about their potential pathogenicity. This implies an increased risk of disease outbreaks such as saprolegniasis and Epizootic Ulcerative Syndrome (EUS), which may affect wild fish and amphibian populations as well as local aquaculture systems. Moreover, their colony growth measurements on different culture media (i.e., PDA, CMA, and MP5) across varying temperatures from 15 to 35 C revealed that temperature is a key factor influencing growth, wherein Achlya species showed increased growth with rising temperatures, while S. ferax exhibited the opposite trend. This suggests that lowland aquaculture areas, which are characteristically hot, are susceptible to Achlya-associated infections, while highland areas with low ambient temperatures, are at risk of Saprolegnia-associated infections. This is the first study to retrieve and deposit DNA sequence data of Philippine Saprolegniales, contributing valuable information to global databases and advancing knowledge on tropical saprolegniaceous fungi.