Background: Plants, including marine algae, produce allelochemicals that influence the growth, survival, and reproduction of other organisms.
 Questions: To identify natural algicidal or antifouling allelochemicals, we screened 18 common seaweed extracts for suppression of rhizoid and blade production in a convenient Porphyra suborbiculata monospore assay.
 Species study and data description: Addition of extract from the most potent phaeophyte, Hizikia fusiformis , suppressed rhizoid formation, rhizoid number, rhizoid length, blade formation, and blade length.
 Study site and dates: Seaweed thalli for methanol extraction were collected on the coast of Korea from October 2012 to July 2015.
 Methods: Extracts were tested using the P. suborbiculata monospore assay system.
 Results: The 50 % suppression doses were 15 µ g ml-1 for rhizoid formation, 2.4 µ g ml-1 for rhizoid number, 13 µ g ml-1 for rhizoid length, 6 µ g ml-1 for blade formation, and 11 µ g ml-1 for blade length. The H. fusiformis extract also suppressed rhizoid and blade production in leafy green ( Ulva pertusa ) and brown ( Undaria pinnatifida and Ecklonia cava ) seaweed spores, as well as suppressing diatom settlement.
 Conclusions: The allelochemicals that suppressed or eliminated competing seaweed species may be efficacious for new seaweed control technologies, including the development of antifouling or algicidal agents based on natural products.