Occurrence and pathogenicity of Pythium (Oomycota) on Ulva species (Chlorophyta) at different salinities

Pythiumspecies are ubiquitous organisms known to be pathogens to terrestrial plants and marine algae. While sev-eralPythiumspecies (hereafter,Pythium) are described as pathogens to marine red algae, little is known about thepathogenicity ofPythiumon marine green algae. A strain of aPythiumwas isolated from a taxonomically unresolvedfilamentousUlvacollected in an intertidal area of Oslo fjord. Its pathogenicity to a euryhalineUlva intestinaliscollectedin the same area was subsequently tested under salinities of 0, 15, and 30 parts per thousand (ppt). ThePythiumisolatereadily infectedU. intestinalisand decimated the filaments at 0 ppt. Mycelium survived onU. intestinalisfilaments for atleast 2 weeks at 15 and 30 ppt, but the infection did not progress. Sporulation was not observed in the infected algal fila-ments at any salinity. Conversely,Pythiumsporulated on infected grass pieces at 0, 15, and 30 ppt. High salinity retardedsporulation, but did not prevent it. OurPythiumisolate produced filamentous non-inflated sporangia. The sexual stagewas never observed and phylogenetic analysis using internal transcribed spacer suggest this isolate belongs to the cladeB2. We conclude that thePythiumfound in the Oslo fjord was a pathogen ofU.intestinalisunder low salinity