POPLAR DISEASES 299 



the other is confined to the Rocky Mountains. Injury to the 

 trees results from frequent defoliation. In regions of little 

 rainfall where poplars are important because of their adapta- 

 bility to such conditions, the rust often causes the death of 

 younger trees and seriously interferes with their propagation. 



Symptoms. 



The three leaf-rusts are very similar in appearance. Yellow- 

 ish, powdery pustules appear on the leaves in the summer. 

 These pustules may be scattered over the leaf or they may be 

 crowded over the entire surface, giving the leaves a golden 

 yellow tinge that may be seen from a distance. In some 

 regions the foliage may . be generally infected and the entire 

 trees are yellow. Later in the season, many small orange- 

 yellow or purplish brown slightly raised spots appear on the leaf. 



Cause. 



The leaf-rusts of poplars are caused by three species of the 

 rust-fungi, Melampsora Meditsw, M. abietis-canademis and M. 

 albertensis. The first two species affect many kinds of poplars 

 throughout the United States, while the latter occurs only in the 

 Rocky Mountains. Urediniospores, formed in yellowish powdery 

 pustules, infect other poplar leaves. The darker colored and 

 slightly raised spots on the infected leaves, later in the autumn, 

 contain teliospores. These spores germinate the following spring 

 and produce basidiospores. The basidiospores of M. MediiscB on 

 poplar were shown several years ago to infect young larch 

 leaves (see page 212). Recently also it has been demon- 

 strated that the basidiospores of the second fungus on the large- 

 tooth aspen, at least, cause the infection of young hemlock 

 needles in northeastern United States (see page 182). The 

 basidiospores of the rust common on poplars in the Rocky 

 Mountains cause the infection of the young needles of Douglas 

 fir (see page 159). The proximity to poplars of either larch, 



