342 



MANUAL OF TREE DISEASES 



common and is found generally distributed throughout the 

 northern states. The other two are apparently rare and 



are kliown only 

 in Colorado and 

 Utah. The leaf- 

 rusts are impor- 

 tant on yoimg trees 

 because defoliation 

 often results. 



Symptoms, 



The three rust- 

 diseases are very 

 similar in appear- 

 ance. In early simi- 

 mer the affected 

 leaves show yellow- 

 ish areas on which 

 are borne small 

 orange-yellow pus- 

 tules. These pus- 

 tules appear pow- 

 dery, due to the 

 mass of spores they 

 contain. Later in 

 the season on the 

 The 



Fig. 70. — Powdery mildew on willow leaves. 



same areas, small dark brown raised spots are formed, 

 heavily infected leaves may drop from the tree. 



Cause. 



The common willow rust is caused by Melampsora Bigelowii 

 Thiim. This fungus also causes a rust of the leaves of larch 

 (see page 212). The spores (seciospores) borne in the pus- 

 tules on the larch leaves infect the willow leaves. The orange- 



