The organism behind potato blight looks like a fungus but is closer to seaweed
Late blight of potato and sudden oak death are caused by oomycetes, microbes long filed with the fungi because they grow in the same threadlike way. Molecular studies moved them next to brown algae and diatoms. Their cell walls are made of cellulose, like plants, rather than fungal chitin.
Oomycetes are filamentous microorganisms that feed on other organisms rather than making their own food. The name means egg fungus, after the large round oogonia that hold their female sex cells. They reproduce in two ways. Sexually, male and female structures meet to form thick oospores that can survive winter as resting spores. Asexually, they make sporangia that release swimming zoospores. Some live by breaking down dead matter; others are among the most destructive plant pathogens known.
Their old nickname, water moulds, reflects both the earlier fungal classification and the damp, running-water habitats of the most basal groups. Yet most species are actually land-based plant pathogens. The traditional scheme recognised six orders. The Saprolegniales are the most widespread, many decomposing debris; the Rhipidiales anchor themselves with rootlike rhizoids to the beds of stagnant or polluted water; and the Peronosporales include many of the worst agricultural parasites. The fossil record is extremely thin, with one possible oomycete preserved in Cretaceous amber.
What gave them away as non-fungi was a list of differences. Beyond cellulose walls, oomycetes rarely have internal cross-walls in their filaments, and their cells carry two copies of each chromosome during growth, where fungi carry one. Their zoospores swim with two flagella, one smooth and whiplike, the other branched like tinsel, a type unique to their broader group; the few fungi with flagellated spores have a single whiplash. They also make the amino acid lysine by a different chemical route and have tubular rather than flattened folds inside their mitochondria.
That evidence places them among the stramenopiles, alongside photosynthetic brown algae and diatoms. Not every oomycete is a villain: Pythium oligandrum parasitises other fungi and is used as a biological control, attacking fungi that cause plant disease.
Source: Oomycete
