Chytridiomycota A phylum of microscopic organisms, the chytrids, that live in soil or fresh water and have affinities with true fungi. The body (thallus) is unicellular or a coenocyte and gives rise to threadlike hyphae or rhizoids; some species form a branching network (mycelium). The cell walls contain chitin, and some also have cellulose. Chytrids feed by secreting enzymes to digest material extracellularly in order to absorb the nutrients. They produce motile stages (zoospores) equipped with a single undulipodium. Sexual reproduction is the norm; fusion of the gametes results in a zygote, which either produces motile zoospores or germinates directly into a new thallus. Chytrids are sometimes classified as protoctists, sometimes as fungi.

Oomycota (water mould, downy mildews) Fungus-like organisms in which the zoospores have two flagella, one of the whiplash type and one of the tinsel type. The cell walls of oomycetes are atypical of fungi in that in most species they contain cellulose but not chitin. The majority of species are aquatic, but some are important pathogens of terrestrial plants. The class contains 4 orders.

Oomycota A phylum of the Protoctista that includes the water moulds, downy mildews, and potato blight (Phytophthora), formerly classified as a class of fungi (Oomycetes). They are coenocytic and the cell wall is made of cellulose. Oomycotes are either saprotrophic or parasitic; they feed by extending hypha-like threads into the food source or host's body. Asexual reproduction is by means of flagellated zoospores, which are released from a sporangium. Sexual reproduction involves the fusion of an antheridium and an oogonium and results in the production of a zygote, which can develop a wall of chitin and become a resistant oospore.

water mould An aquatic fungus; sometimes specifically a fungus of the order Saprolegniales.