Insects, Diseases and Frost 271 



plants may be sprayed at any time. The chief reliance for freedom 

 from leaf-blight is the selection of resistant varieties. Some sorts, 

 as Warfield, Beder Wood, Bubach and Gandy are very susceptible; 

 others, as Brandywine, Michel and Thompson, rarely are affected 

 seriously. Varieties that are susceptible in one place may not be in 

 another; much depends on air drainage and soil drainage. Set 



Leaf-spot, or rust. 



only healthy plants. Mowing and burning the plants after harvest 

 helps to some extent. 



There are at least two other closely related forms of leaf-bhght. 

 One of these (AposphcBria, sp.) is distinguished from Sphcerella 

 by the shape of the spot, which usually begins at the margin of the 

 leaf and extends toward the center, in the shape of the letter V. 

 The Ascochyta commonly attacks the hulls and fruit-stalks. Neither 

 responds readily to treatment. 



Powdery mildew (Sphcerotheca humuli). 



Powdery mildew curls the leaves and the white mycelium of the 

 fungus is conspicuous on the under surface. It rarely is troublesome 

 in the field, but may be on plants forced in the greenhouse. Bor- 

 deaux spraying for leaf-blight controls this disease, also. In the 

 greenhouse, the plants may be dusted with flour of sulfur every 

 ten days. 



