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a genus of lower fungi of the family Phytophthoraceae (Pythiaceae), order Peronosporales. Reproduction is asexual (by means of conidia) or sexual (by means of oospores). The genus embraces more than 40 species, which are found on all continents; the USSR has approximately 20 species. Some species cause serious plant diseases called phytophthoroses. The most harmful species is P. infestans, which infects the potato, tomato, and many other plants of the family Solanaceae.