The Relative Resistances of Porphyra Species (Bangiales, Rhodophyta) to Infection by Pythium porphyrae (Peronosporales, Oomycota)

Abstract

The relative resistances among cultivated (Porphyra yezoensis T-14) and wild Porphyra yezoensis strains collected from natural populations, and the non-cultivated species of Porphyra (P. tenera, P. tenuipedalis, P. lacerata, P. suborbiculata, P. seriata, P. kanyakumariensis and P. pseudolinearis) to infection by the microbial pathogen Pythium porphyrae was investigated. Resistance was assessed by exposing the isolates to equivalent amounts of infective material (zoospores) and measuring two parameters, supported fungal biomass and amount of disease produced. The cultivated and the wild strains of Porphyra yezoensis were highly susceptible to infections by Pythium porphyrae with relative infection levels of 1.00 and 0.88, respectively. The noncultivated species were relatively resistant and displayed fewer infection sites and infected thallus areas. Porphyra suborbiculata (0.13) and Porphyra tenuipedalis (0.19) were most resistant to infections by Pythium porphyrae. Minimal amount of fungal biomass was supported by resistant strains in terms of mycelial proliferation rate and oospore production. Initial steps of host-pathogen interactions observed with scanning and epifluorescence microscopy on a susceptible (Porphyra yezoensis T-14) and relatively resistant (P. tenuipedalis) species revealed significant differences. Relatively high numbers of Pythium porphyrae zoospores attached and encysted on to the susceptible strains relative to other resistant strains. Only a transient relation between encystment rate and degree of resistance was observed among different Porphyra species. The encysted zoospores immediately germinated, formed appressoria and penetrated the algal cells near the site of encystment on the susceptible cultivar strains. On relatively resistant Porphyra tenuipedalis thallus surface, cysts with germ tubes growing on the thallus surface without attempting to penetrate the host were observed. Induction of appressoria, an essential prelude of penetration, following cyst germination, was not observed. Rarely very few long germ tubes penetrating the algal cells, with delayed appressoria formation was observed on Porphyra tenuipedalis thalli.