GHrape. 303 



Powdery Mildew (Uncinula spiralis, B. & C.). 1 Description. 

 The fungus causing the powdery mildew of the grape differs 

 from the preceding diseases in the fact that it is a surface 

 fungus, the body of the parasite not being found in the tissues 

 of the host-plant, but upon the surface of the green parts. 

 The vines are attacked throughout the growing season, but 

 contrary to the general rule, the disease develops more abun- 

 dantly during comparatively dry weather. It is therefore 

 found over a wide territory, but fortunately it is not as a 

 rule very serious. 



Young shoots attacked by the powdery mildew are checked 

 in their growth, and if the threads of the fungus are abun- 

 dant, they impart to the affected portion a grayish- white color. 

 This color is particularly noticeable upon the leaves (Fig. 61), 

 for the fungus almost invariably grows upon their upper sur- 

 face, and if allowed to develop unchecked, large patches soon 

 become covered so thickly that the green parts underneath can 

 no longer be seen. If these patches are firmly rubbed, the 

 white covering may be removed, and the browning of the parts 

 formerly covered will be seen. This is especially distinct when 

 the mildew has made an energetic growth. The discoloration 

 is due to the small suckers which the fungus has projected into 

 the leaf cells for the purpose of obtaining nourishment, and the 

 greater the number of these suckers, the more marked is the 

 discoloration. 



Affected berries show similar discolorations, and the whitish 

 covering may be removed as well. As the berry grows, the 

 injured parts assume a brown, scurfy covering which is com- 

 posed of dried epidermis. This golden-brown film is very 

 commonly seen upon green grapes ; it is often cracked so that 

 the green tissue is visible. Such disfigurements are, however, 

 not always caused by the powdery mildew, since any cause 

 which destroys portions of the epidermis will be followed by 

 similar discolorations. 



Treatment. The powdery mildew is not a serious disease, 

 and as the body of the fungus is not within the host-plant, 

 there is little occasion for treating the vines until the fungus 



1 The common surface mildew of European vines is O'idium Tuckeri. B. In 

 general appearance it resembles our powdery mildew. But it is more easily con- 

 trolled, the flowers of sulphur having long proved to be a specific. 



