Characterizing the microbial culprit of white spot disease of the conchocelis stage of Porphyra yezoensis (Bangiales, Rhodophyta)

White spot disease is the most frequent and harmful disease affecting the shell-boring conchocelis stage of the economically important red alga Porphyra yezoensis (= Pyropia yezoensis (Ueda) Hwang & Choi) (Bangiales, Rhodophyta). To date, its potential pathogens and pathogenesis remain poorly understood. In this study, we isolated 7 bacteria, 26 fungi, and 10 actinomycetes from sick conchocelis. Re-infection experiments revealed that only one fungus, GF014, caused disease. Morphological characteristics of the colony, mycelium, sporangium, and spore of GF014 suggested that the strain belongs to the order Pleosporales and the genus Phoma. Classification based on internal transcribed spacer (ITS) sequences supported the results of morphological identification. These results indicate that GF014 is a species of Phoma and a specific pathogen for Porphyra. GF014 preferred organic nitrogen sources, and 20–28 °C and low salinity water were its optimal living conditions. Our findings will play an active role in preventing and treating white spot disease in P. yezoensis.