PERONOSPORALES 



The Peronosporales comprise primarily terrestrial microscopic fun- 

 gi parasitic for the most part on flowering plants. However, one 

 family, the Pythiaceae, is transitional so far as habitat is concerned, 

 some of the species are distinctly aquatic, others amphibious, and still 

 others terrestrial. Probably all of its parasitic members are able to lead 

 a saprophytic existence. Although certain monographers, notably 

 Schroeter (1893), placed the Pythiaceae in the Saprolegniales, it unques- 

 tionably belongs in the Peronosporales (Fischer, 1892). The discussion 

 is restricted to this family, since it contains the only aquatic species of 

 the order. 



All genera of the Pythiaceae have aquatic representatives. The 

 species of Pythiomorpha, maintained separate in the first edition, are 

 now placed under Phytophthora, since Buisman (1927), Drechsler (1930, 

 1932), and especially Blackwell, Waterhouse, and Thompson (1941) 

 showed that Petersen (1909, 1910) based his genus upon a saprophytic 

 species of Phytophthora with proliferous zoosporangia. Matthews 

 (1931) and Middleton (1943) have brought together the material on the 

 aquatic as well as the terrestrial species of Pythium; hence, the reader 

 is referred to their monographs as well as to the older classic by Butler 

 (1907). Here it is deemed sufficient to provide a comprehensive key to 

 the valid aquatic and soil-inhabiting species of Pythium. The key given 

 (p. 1033), however, contains more than the usual amount of data and is 

 based on and includes descriptive matter taken from Middleton (op. 

 cit.). 



Species of Pythium live as parasites and saprophytes of algae and 

 microscopic animals and are found on bits of debris of plant and animal 

 origin. Although the genus has been regarded as confined to fresh-water 

 hosts, soils, and vascular plants, one species, P. marinum, has been 

 isolated on decaying marine red algae (Sparrow, 1934c), another, P. 

 maritimum, obtained from living Ceramium (Hohnk, 1939), and an 



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