PLANT DISEASES 



379 



No plants should be used which come from seed beds or 

 greenhouses in which damping off is known to have 

 occurred. 



383. Downy Mildew of the Grape. This disease appears 

 on young branches and leaves of both wild and cultivated 

 grapes and is not uncommon wherever 

 grapes are grown. It may at times 

 cause considerable damage, especially 

 to slow-growing varieties. The disease 

 was introduced into Europe from the 

 United States, and in a short time it 

 almost ruined the grape-growing in- 

 dustry over extensive areas. An af- 

 fected leaf shows irregular yellowish 

 areas on the upper surface, and at the 

 same time spore-bearing branches ap- 

 pear on the lower surface of the leaf 

 immediately below the discolored spots. 

 Irregular dark areas appear also on the 



stem, and at times the fungus is so . Fl . G " 2I *-~ s f por , e " 

 7 bearing branch of the 



abundant on the young fruit as to give downy mildew of the 

 it a downy appearance, whence the grape, making its ap- 

 name "downy mildew." It is only pearance through an air 

 J J pore on the under side 



when the fruit is diseased that any O f the leaf; o, a portion 

 considerable loss is caused in the United of the branch with spores 

 States. The fun^s lives between the ^VitacLe^ 

 cells of the host, sending short, rounded sp0 re; c, a spore, much 

 branches into the living host cells and enlarged, 

 thus robbing them of their food. The 

 spores are borne on peculiarly branched structures that 

 project beyond the surface of the host (Fig. 213) and 

 produce free-swimming cells like those of the damping-off 

 fungus, which in turn germinate and produce the new 

 growth upon the grape. The large thick -walled zygotes 

 are not very commonly found, although they do occur. 



