The genera Phytophthora and Pythium include many economically important species which have been placed in Kingdom Chromista or Kingdom Straminipila, distinct from Kingdom Fungi. Their taxonomic problems, basic biology and economic importance have been reviewed. Morphologically, both genera are very similar in having coenocytic, hyaline and freely branching mycelia, oogonia with usually single oospores but the definitive differentiation between them lies in the mode of zoospore differentiation and discharge. In Phytophthora, the zoospores are differentiated within the sporangium proper and when mature, released in an evanescent vesicle at the sporangial apex, whereas in Pythium, the protoplast of a sporangium is transferred usually through an exit tube to a thin vesicle outside the sporangium where zoospores are differentiated and released upon the rupture of the vesicle. Many species of Phytophthora are destructive pathogens of especially dicotyledonous woody trees, shrubs and herbaceous plants whereas Pythium species attacked primarily monocotyledonous herbaceous plants, whereas some cause diseases in fishes, red algae and mammals including humans. However, several mycoparasitic and entomopathogenic species of Pythium have been utilized respectively, to successfully control other plant pathogenic fungi and harmful insects including mosquitoes while the others utilized to produce valuable chemicals for pharmacy and food industry.

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A diagnostic key has been developed to facilitate the identification of 48 species of Pythium recorded in Taiwan so far. Photomicrographs are provided for the 10 most common or economically important species to highlight their major morphological characteristics.

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