Pythium porphyrae
Scientific classification
Domain:	Eukaryota
Clade:	Diaphoretickes
Clade:	SAR
Clade:	Stramenopiles
Phylum:	Oomycota
Order:	Peronosporales
Family:	Pythiaceae
Genus:	Pythium
Species:	P. porphyrae
Binomial name
Pythium porphyrae M. Takah. & M. Sasaki, 1977[1]
Synonyms[2][3][4]
Pythium chondricola De Cock, 1986

Pythium porphyrae, is a parasitic species of oomycete in the family Pythiaceae.^[5] It is the cause of red rot disease or red wasting disease, also called akagusare (赤ぐされ) in Japanese.^[1][6] The specific epithet porphyrae (πορφυρα) 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.^[7] However, many of its hosts have been moved from the genus Porphyra to Pyropia.

Pythium porphyrae can destroy an entire crop of nori within three weeks.^[8][9][6] It prefers low salinity and warm water (24-28 °C).^{[10][11][12][6]} It will only grow in the 15-35 °C range.^[13] Mild winters correlate with higher infestations and lower crop yields,^[14] possibly due to decreased temperatures inducing the development of sex organs in the oomycete.^[1] Losses can be combated by destroying diseased fronds and exposing thalli to the air for 3–4 hours daily.^[6] The oospores can be spread in contaminated organic matter and the sporangia can spread through the water.^[15]

Pythium porphyrae has a mycelial thallus that is eucarpic, meaning only part of the thallus turns into sporangia.^[15] It is primarily a facultative parasite of algae, but can also be saprobic.^[15]

Its hyphae can grow up to 4.5 μm wide,^[13][15] and are not septate.^[1] On algae, the hyphae will extend through the cell wall.^[1] It does not have haustoria not chlamydospores.^[15] The appressoria are club-shaped.^[15] It has sporangia that are unbranched, filamentous,^[1][3] and non-inflated,^[3] typically forming 6-17 zoospores per vesicle.^[1][13][15] Encysted zoospores are 8-12 μm in diameter.^[15] Hyphal swellings are intercalary and globose, from 12-28 μm in diameter.^[15] Oogonia average 17 μm in diameter and are also intercalary and globose, but rarely are terminal.^[13][15] In each oogonium are 1-2 diclinous antheridia^[3][15][1] coming out far away from the oogonial stalk.^[13] The antheridia's cells are clavate (club shaped) or globose.^[1][13][15] The antheridia will be apical to the oogonial wall.^[15] Sometimes there will be two antheridial cells on one stalk.^[13] The yellowish oospores average 15 μm in diameter, have thick (~2 μm) walls, and are plerotic (fill the whole oogonium).^[13][15] Conidia are spherical at 8.8-30.8 μm diameter, but rarely produced.^[1]

Pythium porphyrae shares many physical traits with P. marinum and P. monospermum,^[13] and appears to be most closely related to P. adhaerens.^[2][3] 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.^[13] P. monospermum and P. marinum also have oogonia terminally on short branches, yet in P. porphyrae they tent to be intercalary.^[13]

In a laboratory it will grow 5 mm per day^[3] on seawater-cornmeal agar with low aerial mycelium^[15] and colorless colonies,^[1] but will not grow at all on potato-carrot agar.^[13]

Pythium porphyrae has been found in Japan,^[1] Netherlands, New Zealand,^[2] and Korea, but this range is likely underreported.^[15] 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.^[2]

• Bangia atropurpurea^[15]
• Callophyllis adhaerens^[15]
• Chondrus crispus^[2][3]
• Gelidium elegans^[15]
• Gloiosiphonia capillaris^[15]
• Gracilaria spp.^[15]
• Grateloupia turuturu^[15]
• Griffithsia subcylindrica^[15]
• Lomentaria hakodatensis^[15]
• Mastocarpus papillatus^[16]
• Polyopes affinis (Carpopeltis affinis)^[15]
• Polysiphonia morrowii^[15]
• Pterocladiella capillacea^[15]
• Pyropia cinnamomea^[2]
• Pyropia plicata^[2]
• Pyropia suborbiculata^[2]
• Pyropia tenera^{[2][10][15][1]}
• Pyropia virididentata^[2]
• Pyropia yezoensis^{[13][3][15][1]}
• Rhodymenia intricata^[15]
• Stylonema alsidii^[15]
• Wrangelia tanegana^[15]

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.^[17]

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^[3] Clade A has two clusters, and P. porphyrae shares one with the species also originating on algae, P. adhaerens.^[3]

A 2017 study of Pythium species in Clade A showed the following phylogenetic tree.^[2] It further demonstrated that P. porphyrae and P. chondricola are the same species.^[2][3] 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.^[2]

• Pythium in turfgrass
• Pythiosis

• Data related to Pythium porphyrae at Wikispecies