FIR DISEASES 159 



Leaf-Rusts 



Caused by Melampsora arciica Rostrup and M. albertensis Arthur 



Two species of rust-fungi cause pale yellowish or white 

 pustules on fir needles. These rusts are very similar to the 

 blister-rusts described above except that no bladder-like cov- 

 ering is developed over the pustules. 



Willow rust. 



A rust of balsam fir needles similar to the blister-rusts above 

 mentioned is found in eastern Canada and may occur in the 

 extreme northeastern part of the United States. This disease 

 is caused by Melampsora arctica Rostrup. The uredinial and 

 telial stages are formed on arctic species of willow. The ba- 

 sidicspores are formed in the spring from over-wintering telio- 

 spores and infect the new fir needles. White pustules are 

 produced on the imder surface of the fir needles in midsummer. 



Poplar rust. 



A rust of poplar leaves in northwestern United States and 

 the adjacent parts of Canada has its alternate stage on fir 

 (Pseudotsuga) needles causing white pustules. This fungus is 

 known as Melampsora albertensis Arthur and was previously 

 called Cwoma occidentalis Arthur on Pseudotsuga mucronata. 



Leaf-Cast 



Caused by Lophodermium nervisequum Fries 



The needles of balsam fir are often killed by this disease. 

 Trees of all ages are affected and complete defoliation may 

 result. This is especially true of small trees. The leaves be- 

 come yellow and brown during the summer and autumn. The 

 fruiting-bodies of the causal fungus break through the epi- 

 dermis of the brown needles in long black lines. The spores are 

 mature the following spring and infection may occur during 



