Host range and salinity tolerance of Pythium porphyrae may indicate its terrestrial origin

One of the most devastating diseases in Pyropia sea farms in Korea and Japan is red-rot disease caused by Pythium porphyrae. Outbreaks occur yearly causing losses in millions of dollars. Little is known about the source of the inoculum initiating these epidemics. Outbreaks of red-rot disease usually start in sea farms nearest river mouths. We hypothesized that the initial inoculum for red-rot disease may come from land. To test this, we determined the growth of P. porphyrae in different salinities. Growth experiments showed that it grew better in lower salinity (15 ‰) than in full seawater, but also grew in 0 ‰. When the seedlings of 11 land plants were exposed to hyphae of P. porphyrae, eight species showed root rot. The seedlings of carrot, Napa cabbage, radish, and rice died due to this infection. A hypersensitive response was observed in carrot seedlings; the root hairs died in 5 h after inoculation, and root damping off occurred within 24 h. The seedlings of cucumber, onion, and pumpkin showed retarded growth of root, shoot, and leaves. Pythium porphyrae zoosporangia and oocyte formation occurred on the roots of infected seedlings. Phylogenetic analysis, using several markers, showed that P. porphyrae is genetically identical or nested with isolates or species collected from terrestrial environments and different locations around the world. Our results suggest that terrestrial runoff could be an additional source of the inoculum initiating red-rot disease in Pyropia farms.