Petersenia palmariae n. sp. (Oomycetes): a pathogenic parasite of the red alga Palmaria mollis (Rhodophyceae)
Specimens of Palmaria mollis (Setchell and Gardner) van der Meer and Bird collected from Vancouver Island, Canada, were found to harbour a pathogenic fungal parasite. When infected fronds were put into culture, the algal thallus became completely covered with small white lesions. These were most concentrated in young tissue and soon killed the apices of the fronds. The parasite completed approximately one infection cycle per week at 10 °C. It died at temperatures near 15 °C, even though the host remained healthy at 20 °C. The parasite was identified as a species of Petersenia (Oomycetes). It most resembled P. lobata (H. E. Petersen) Sparrow but differed sufficiently in detail to consider it a new species, Petersenia palmariae. The new species does not infect Ceramium rubrum (Hudson) C. Agardh which serves as host for both P. lobata and P. pollagaster, the only other members of the genus known to parasitize marine algae. Nor does it infect Palmaria palmata (L.) O. Kuntze, a species related to the host on which it was discovered, which suggests Petersenia palmariae may be host specific.