PHYCOMYCETES 



163 



an acute stage. Older onions are apparently more susceptible 

 than young, and recovery in the former case is seldom. 



The fungus. The mycelium is considerable, and it penetrates 

 practically all parts of the leaf. The minute haustoria are numerous, 



Fig. 57. Onion Mildew 

 (Photograph by H. H. 

 Whetzel) 



Fig. 58. Mature Conidiophore, Germinat- 

 ing CONIDIUM, AND MYCELIUM OF ONION 

 Peronospora. (c after Whetzel) 



thread-like, and often branched at the tip. The conidiophores 

 arise through the stomates. They are of the characteristic type, 

 often 320/4 in height, and bear large elliptical conidia (44-52 x 

 22-26 fj) which germinate promptly by a side tube and effect 

 penetration through the stomates. The time required for infec- 

 tion and the production of conidiophores again is extremely 



