KEYS TO THE FAMILIES AND MORE IMPORTANT GENERA OF PERONOSPORALES 145 



si on of the nuclei in the sporangium may be a reduction division. The 

 cells that divide into four spores each have been called asci and the whole 

 sporangium a "synascus" but the absence of nuclear fusion in these 

 "asci" before the formation of the spores seems to exclude that possi- 

 bility. The cellulose nature of the cell wall, it has been suggested, speaks 

 for the phycomycetous relationship. If related to the Lagenidiaceae we 

 must assume the loss of motility of zoospores or gametes. Relationship 

 to the Chytridiales is less likely because of the presence of well-developed 

 mycelium with cellulose walls. Baker, Mrak, and Smith (1943) suggest 

 that Coccidioides immitis Rix. and Gil., the fungus causing the disease of 

 man called coccidioidomycosis may possibly belong in the Protomycetales. 



Keys to the Families and More Important Genera of Peronosporales 



Key to the More Important Genera of Family Pythiaceae 



Zoospores not preformed in the zoosporangium but developing in an extruded 

 mass which may or may not be enclosed in a plasma membrane. 

 Extruded mass of protoplasm not in a definite vesicle, zoosporangia elongated 



transversely to the supporting hypha. Pythiogeton 



Zoospores developed in a definite vesicle. 



Hyphae bearing short lateral branches adapted -to the capture of the rotifers 



upon which the fungus feeds. Zoosporangia hyphal. Zoophagus 

 No special hyphal branches for capturing prey. Zoosporangia hyphal or tor- 

 uloid or more or less spherical. Pythium 



Zoospores preformed in the mostly more or less spherical zoosporangium (con- 

 idium) and escaping individually or sometimes in a temporary vesicle. 



Phytophthora 

 No zoospores. External warted spherical conidia. Oogones with amphigynous 

 type of antherid. Of doubtful relationship. Trachysphaera 



Key to the Genus of Family Albuginaceae 



Only genus. Conidia catenulate on subepidermal conidiophores. 



Albugo 



Key to the More Important Genera of Family Peronosporaceae 



Conidiophores clavate or somewhat cylindrical, somewhat swollen above with 

 numerous short sterigma-like branches. Conidia without appendages, ger- 

 minating by formation of zoospores. Parasitic on Composites. 



Basidiophora 

 Conidiophores not much branched. Conidia with prominent beak and a basal 

 appendage consisting of the adhering upper end of the conidiophore. 

 Conidia germinating by production of zoospores. Possibly belonging to 

 the Pythiaceae. Kawakamia 



Conidiophores with prominent branches. 



Conidiophores stout, with heavy branches clustered near the apex, quickly 

 fugacious. Conidia germinating by germ tubes or by the formation of 

 zoospores. Parasitic on grasses. Sclerospora 



Conidiophores more slender, branching monopodially, usually nearly at right 

 angles. Tips of branches obtuse. 



