GRAPE DISEASES 



239 



only a slight change in color ; within this area are found numer- 

 ous brown punctations, which do not unite to form a dead, 

 brown spot in the leaf-tissue. The disease on the lower surface 

 of the leaf is at first similar to that on the upper, but is very 

 soon covered by a downy, white growth (Fig. 61) which is 

 most pronounced just beneath the 

 greenish yellow portion mentioned 

 above. On the brown portion the 

 whitish felt, the mildew-pathogene, 

 soon disappears. The amount of 

 mildew on a leaf depends on weather 

 conditions and on the variety. The 

 lower portion of a vine may be de- 

 nuded of its foliage. 



The canes, leaf-petioles and ten- 

 drils are subject to attack, especially 

 in wild varieties. At first the af- 

 fected portion has a water-soaked, 

 shiny appearance, which is attended 

 by a local swelling of the tissue. 

 The lesion is bare at first, but soon 

 the mildew covers it. In older 

 cases the affected area becomes 

 brown and dead, and a depression 

 results. In cases of severe attack 

 the cane is dwarfed, the leaves 

 remain small, and the cane may 

 die (Fig. 62). 



The fruit (Fig. 63) and flowers of wild varieties commonly 

 show the downy-mildew. The attack may come at any time 

 from flowering to maturation. The first sign of the trouble 

 on the fruit is a hardening of the berry, together with a change 

 from its normal color to a grayish blue lead-color. It is during 

 this stage that the mildew appears. In later stages the berry 



FIG. 62. Downy-mildew on 

 grape-shoot. 



