Natural biocontrol of a Porphyra sp. pest on farmed Gracilaria chilensis by a pythiosis outbreak

Gracilaria chilensis (a.k.a. pelillo) is the most produced seaweed in Chile and Latin America, yet its cultivation has historically faced lots of pest-associated constrains that threat its profitability and sustainability. Pests show temporal cycles of recruitment, growth and death/senescence, variation normally linked with sharp changes in environmental factors occurring in estuarine areas whereby Gracilaria is cultivated. Here we report the appearance of a bladed Bangiales species epiphytic on long-line farmed Gracilaria and identified as Porphyra. This species recruits to cover up to 50–72% of G. chilensis early in a suspended set-up in spring, until a filamentous fungal-like organism colonizes Porphyra blades, infecting a wide proportion of its tissue. After this outbreak, Porphyra recruits collapse, disappearing in few weeks from farmed Gracilaria. Observations of diseased individuals, and subsequent isolation and marker-assisted taxonomy of the pathogen, provide evidence for the identification of this organism as Pythium porphyrae, the aetiological agent for the red rot disease in commercial nori/gim in Asia. This is the first reported case for P. porphyrae in Chile and the Southeastern Pacific as well as for a disease-driven natural biocontrol of a Gracilaria pest alga, suggesting an unknown -yet considerable- cryptic biodiversity acting as natural regulators of natural pests during a Gracilaria cultivation cycle.