.Family Cruciferae 213 



turnips (Brassica rapa), black mustard {Brassica 

 nigra), rutabaga {Brassica campestris), shepherd's 

 purse {Cap sella bursa-pastoris) , garden cress {Le- 

 pidium sativum) , wild pepper grass {Lepidium virgini- 

 cum), hedge mustard {Sisymbrium officinale and S. 

 altissimum) , candy- tuft {Iberis umbellata), water cress 

 {Nasturtium officinale), and wall flower {Cheiranthus 

 cheiri). From the above experiments, it would seem 

 that in dealing with the white rust fungus, Cystopus 

 candidus, it is possible that there exist distinct races 

 or strains, all of which are specialized to certain 

 special hosts of the various crucifers. The best 

 infection is secured when the seedlings of the host 

 plant are chilled. This is why white rust is more 

 prevalent in cool seasons. 



The Organism. Cystopus candidus has two fruit- 

 ing stages. The summer or conidial stage is made up 

 of simple chains of spores (fig. 33 c). The latter are 

 separated one from the other by a minute beak-like 

 projection. Each spore or zoosporangium germi- 

 nates by six or more swarm spores, or zoospores. 

 These, when set free, swim around, then come to rest 

 and germinate by means of a germ tube. The 

 oospore or sexual spore of the Cystopus is formed 

 later in the season. The oogonia and antheridia (fig. 

 33 d) are developed within the infected host tissue. 

 Fertilization proceeds in the same way as in Pythium. 

 The mature oospore has a thick, sculptured wall, and 

 is brown in color. The oospores germinate in the 

 same way as the zoosporangium, i. e., by the forma- 

 tion of zoospores (fig. 33 e). 



