The Taxonomy and Biology of Phytophthora and Pythium

Members of the genera Phytophthora de Bary and Pythium Pringsheim are economically important “fungal-like-organisms” or “pseudofungi” which have been widely studied throughout the world but their taxonomic treatment has been highly controversial. The production of globose oogonia in the sexual reproduction has placed them in the phylum Oomycota. Although members of Oomycota are similar to all filamentous fungi they have been excluded from Kingdom Fungi and placed in the Kingdom Chromista [1] along with several photosynthetic algal groups: brown algae (Phaeophyta), yellow green algae (Xanthophyta), golden algae (Chrysophyta) and diatoms (Bacillariophyta), that produce heterokont zoospores with two laterally inserted flagella, the anterior being tinsel-like and the posterior being smooth and whiplash, inserted at the same point. In order to better distinguish the colorless oomycetes from the pigmented algal allies, the term “straminenopile” was introduced by Patterson [2], based on the tubular hairs (mastigonemes) on the anterior flagellum of a heterokont zoospore as “straw with hairs”. It was formalized as a new kingdom Straminipila by Dick [3]. The diploid (2N), cellulosic mycelium of oomycetes differs from the true fungi in producing oospores in sexual production and heterokont zoospores in asexual reproduction, whereas true fungi have chitinase, haploid (N) or dikaryotic (N+N) mycelium producing zygote, zygospores, ascospores or basidiospores in sexual reproduction and no zoospore or zoospore with single posterior whiplash flagellum in asexual reproduction.