Comparative analysis of the temporal abundance of the epiphytic pest, Melanothamnus thailandicus, affecting farmed Kappaphycus striatus: Visual and molecular detection approaches

<i>Kappaphycus striatus</i> is a widely cultivated red eucheumatoid seaweed in Southeast Asia. However, infestations of epiphytic filamentous algae (EFA) have become a significant challenge, reducing biomass yields during harvest and resulting in income loss for farmers. This study evaluated and identified the EFA affecting farmed <i>K. striatus</i> in northern Iloilo, Philippines. Bi-monthly collections of representative individuals from a single site in a shallow water area were conducted for nine months (June 2019‒February 2020). Seaweed samples from different time points, including both visibly healthy and infested algal thalli, were further analyzed using molecular techniques to assess the effectiveness of visual examination in detecting these epiphytes. Visually, red filamentous epiphytic pests were observed during the rainy months, particularly between September and December. In <i>K. striatus</i> showing clear signs of infestation, these appeared as “goosebump-like” structures with epiphytic filaments protruding from the host. Molecular analysis revealed that this EFA was present in different regions of the seaweed thallus. Interestingly, the target epiphyte was also detected in thallus tissues that appeared visually healthy. PCR detection of EFA yielded higher results (25.7%) than visual examination (20.0%) (<i>p</i> &lt; 0.001). Phylogenetic analysis identified the filamentous epiphyte as <i>Melanothamnus thailandicus</i>‒ a species not previously recorded affecting farmed eucheumatoid seaweed in the Philippines. These findings suggest that molecular identification of epiphytes is more reliable than visual inspection. Therefore, it is recommended that molecular detection methods be incorporated into biosecurity protocols, farm management practices, and seedling movement to prevent the spread and unintentional introduction of epiphytic pests in seaweed farming areas.