Small Fruits 



123 



Oregon, and California, it is regarded as the most important 

 grape disease. 



Powdery-mildew attacks all parts of the plant except 

 the roots, showing first upon both the upper and lower 

 surface of young leaves as whitish circular spots, which ap- 

 pear somewhat as though dusted with flour. These spots 

 enlarge, coalesce, and may cover the whole leaf. The af- 

 fected leaf is retarded in growth and becomes distorted. 

 Mildewed blossoms fail to set their fruit, and fruit that 



Fig. 62. — Black-rot as it shows on the leaves. 

 After Anderson. 



is mildewed soon ceases growth and falls, or develops ir- 

 regularly and fails to ripen. 



The mildew fungus is strictly superficial and may be 

 rubbed off, leaving a discolored spot. In late stages perithecia 

 may be recognized as very small black bodies, about 0.1- 

 0.12 mm. in diameter, lying within the diseased areas. The 

 disease develops in most destructive form in shaded, damp, 

 poorly ventilated situations. Rain and fogs favor it, while 

 very dry weather inhibits it. The American vines are less 

 susceptible than the European varieties. 



