CHAPTER XXXII 



WILLOW DISEASES 



The many species of willow (Salix), which are the common 

 trees and bushes in lowlands and along stream banks, are not 

 subject, so far as known, to many important diseases. A few 

 leaf diseases are known which may cause defoliation. The 

 leaf-rusts are very common. Wood-rots are often found in 

 willow but no studies have been made of them. In general, 

 but little attention has been paid to willow diseases because of 

 the slight economic importance of the trees. 



Powdery Mildews 



Caused by Uncinula salicis (D.C.) Winter, and Phyllactinia corylea 



(Pers.) Karst. 



Two powdery mildew fungi attack the leaves of willows in the 

 United States. Both species are found throughout the country. 

 They are of little importance so far as damage to the trees is con- 

 cerned. Without the aid of the microscope, the two cannot be 

 distinguished from one another. The one, Uncinula salicis, 

 however, may occur on both sides of the leaf, while Phyllactinia 

 corylea is usually confined to the under side (Fig. 70). The 

 black fruiting-bodies of the former are smaller than those of the 

 latter. The life history and methods of control of the powdery 

 mildew fungi are discussed on page 37. 



Leaf-Rusts 

 Caused by fungi of the genus Melampsora 



Three species of rust-fungi are known to affect the leaves of 

 various willows in the United States. One of these is very 



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