The marine red alga Porphyra yezoensis (‘Nori’ in
Japanese), is extensively cultured in Japan and
other Asian countries as one of the most popular
edible seaweeds. The retail value of ‘Nori’ produced
in Japan is approximately ¥100 billion per
annum. However, ‘Nori’ farming has frequently
suffered from great damage due to diseases, such
as red rot disease,1 chytrid disease,2 and green spot
disease,3 which are caused by pathogenic microorganisms
or unusual weather. Among them, red rot
disease is known as the biggest threat to ‘Nori’
farming, causing an approximate loss of ¥5–7
billion per annum in Japan.
The causative agent of red rot disease belongs
to the fungal genus Pythium in oomycetes.4,5 This
disease spreads by biflagellate zoospores that are
released into seawater. Therefore, once the outbreak
occurs, the damage to ‘Nori’ spreads widely
all over the culturing area and becomes a very
serious situation because ‘Nori’ farming forms a
simple community comprising a single species.
To date, Porphyra thalli have been treated with
citric or phosphoric acid as a means of countermeasure
against this disease. However, there may
be the issue of environmental pollution with this
chemical treatment. By using selective breeding
methods, farmers have made a constant effort to
obtain those Porphyra seedlings with beneficial
characteristics; however, it is difficult and takes a
long time to establish a new breed using traditional
methods. Thus, a biotechnological approach is
necessary for effective breeding.