phylum of fungi in the kingdom Chromista that is distinguished by its production of asexual reproductive cells, called zoospores. Zoospores move through the use of one or two whiplike swimming structures (flagella). New fungi may germinate from these spores, or mature fungi may reproduce sexually, with the resulting fertilized eggs being converted into nonmobile spores, or oospores, which then also germinate into mature individuals. Oomycetic fungi may occur as saprophytes (living on decayed matter) or as parasites living on higher plants. Among the various aquatic, amphibious, or terrestrial species, one played an important role in modern history: the species Phytophthora infestans destroyed Ireland's potato crop and caused the famine of 1845, which resulted in a mass migration of the Irish to the United States. Some other economically destructive genera include Saprolegnia (water molds), Aphanomyces (the cause of root rot of pea), Plasmopara (downy mildews), and Albugo (white rusts).
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