Pythium porphyrae

Pythium porphyrae, is a parasitic species of oomycete in the family Pythiaceae. It is the cause of red rot disease or red wasting disease, also called () in Japanese. The specific epithet "porphyrae" () stems from the genus of one of its common hosts, "Porphyra", and the purple-red color of the lesions on the thallus of the host. However, many of its hosts have been moved from the genus "Porphyra" to "Pyropia".
Economic impact.
"P. porphyrae" can destroy an entire crop of nori within 3 weeks. It prefers low salinity and warm water (24-28 °C). It will only grow in the 15-35 °C range. Mild winters correlate with higher infestations and lower crop yields, possibly due to decreased temperatures inducing the development of sex organs in the oomycete. Losses can be combated by destroying diseased fronds and exposing thalli to the air for 3–4 hours daily. The oospores can be spread in contaminated organic matter and the sporangia can spread through the water.
Description.
"P. porphyrae" has a mycelial thallus that is eucarpic, meaning only part of the thallus turns into sporangia. It is primarily a facultative parasite of algae, but can also be saprobic.
Its hyphae can grow up to 4.5 µm wide, and are not septate. On algae, the hyphae will extend through the cell wall. It does not have haustoria not chlamydospores. The appressoria are club-shaped. It has sporangia that are unbranched, filamentous, and non-inflated, typically forming 6-17 zoospores per vesicle. Encysted zoospores are 8-12 µm in diameter. Hyphal swellings are intercalary and globose, from 12-28 µm in diameter. Oogonia average 17 µm in diameter and are also intercalary and globose, but rarely are terminal. In each oogonium are 1-2 diclinous antheridia coming out far away from the oogonial stalk. The antheridia's cells are clavate (club shaped) or globose. The antheridia will be apical to the oogonial wall. Sometimes there will be two antheridial cells on one stalk. The yellowish oospores average 15 µm in diameter, have thick (~2 µm) walls, and are plerotic (fill the whole oogonium). Conidia are spherical at 8.8-30.8 μm diameter, but rarely produced.
"P. porphyrae" shares many physical traits with "P. marinum" and "P. monospermum", and appears to be most closely related to "P. adhaerens". However it has up to four diclinous antheridia and sometimes two antheridial cells per stalk; "P. monospermum" has 1-4 either diclinous or monoclinous antheridia and "P. marinum" has only a single diclinous antheridium. "P. monospermum" and "P. marinum" also have oogonia terminally on short branches, yet in "P. porphyrae" they tent to be intercalary.
In a laboratory it will grow 5 mm per day on seawater-cornmeal agar with low aerial mycelium and colorless colonies, but will not grow at all on potato-carrot agar.
Ecology.
"P. porphyrae" has been found in Japan, Netherlands, New Zealand, and Korea, but this range is likely underreported. It has a parasitic relationship with the following hosts. Though some species of "Pyropia" and "Porphyra" are susceptible to infection in their gametophytic phase, they are resistant in their "Conchocelis" (sporophytic) phase.
Taxonomy.
A 2005 study concerning a case of Pythiosis from a related species ("P. insidiosum") indicated that "P. porphyrae" is related to "P. dissotocum", "P. myriotylum", "P. volutum", and "P. vanterpoolii".
In 2004, molecular analysis of "Pythium" determined that "P. porphyrae" is in "Clade A" along with "P. adhaerens", "P. deliense", "P. aphanidermatum", and "P. monospermum" Clade A has two clusters, and "P. porphyrae" shares one with the species also originating on algae, "P. adhaerens".
A 2017 study of "Pythium" species in Clade A showed the following phylogenetic tree. It further demonstrated that "P. porphyrae" and "P. chondricola" are the same species. "P. adhaerens" may also be conspecific based solely on genetic comparison, but showed a number of physical differences that show it may be a separate but very closely related species.