ORDER PERONOSPORALES 135 



are mutually exclusive. Schroter (1897), recognizing the similarity to the 

 Saprolegniales, placed Pythium in that group, but retained Phytophthora 

 in the Peronosporales. This cannot be upheld in view of the closeness 

 of the two genera, but merely shows the difficulty of drawing sharp 

 delimiting lines in some cases. A number of other genera have been de- 

 scribed in this family, some being soil-inhabiting or aquatic saprophytes 

 and others being serious parasites of various economic plants. By some 

 mycologists they are merged with the genus Phytophthora and by others 

 are maintained as distinct genera. 



Pyihiogeion, described by von Minden (1916), is an aquatic saprophyte 

 whose elongated or sac-like zoosporangium has its axis more or less 

 transverse to the hypha on which it is borne terminally or in an inter- 

 calary position. The zoospores are expelled in a mass which seems to lack 

 the tough plasma membrane characteristic of the vesicle of Pythium and 

 soon breaks up into the individual zoospores. Where known the oospores 

 have very thick walls. (Fig. 43.) 



Other genera assigned to this family are Diasporangium (Hohnk, 

 1936), a soil-inhabiting parasite, Trachysphaera (Tabor and Bunting, 

 1923), etc. Pythiomorpha was described by Petersen (1910) and con- 

 sidered by him to be worthy of the establishment of a separate family. 

 Several species have been ascribed to this genus but recent studies by 

 Blackwell, Waterhouse, and Thompson (1941) seem to indicate that 

 these all represent various species of Phytophthora growing in water. 



Family Albuginaceae. This family consists of the single genus 

 Albugo or, as it is often called, Cystopus. In view of the fact that the 

 former name was given in 1821 and the latter in 1847 the latter must be 

 abandoned. The species number about twenty-five. In contrast with the 

 members of the foregoing family the species are strictly parasitic, never 

 occurring as saprophytes. They do not lend themselves to cultivation on 

 culture media. Within some species — e.g., A. Candida (Pers.) Kuntze — 

 there are numerous specialized races that are adapted only to certain 

 host species or groups of species. The mycelium is strictly intercellular 

 except for the small globular haustoria which are borne on the ends of 

 short, very slender processes which pierce the host cell wall. Melhus 

 (1915) has shown that the mycelium of A. Candida may overwinter in the 

 tissues at or below the crown of such host plants as are winter annuals 

 or biennials, growing out into the new shoots in the spring. The conidio- 

 phores are formed on ends of short sympodially branching hyphae which 

 arise from a mass of mycelium gathered in a limited area underneath the 

 epidermis of the host (the so-called sorus). They are club-shaped and 

 stand, closely packed together, perpendicular to the surface of the epi- 

 dermis, between it and the subepidermal cells. From the apex of each 

 conidiophore are abstricted successively the spherical or ovoid pluri- 



