112 MANUAL OF TREE DISEASES 



waxy-yellow and finally become brown and almost black. They 

 may be abundant and thickly cover the under side of the leaves. 



CaiLse. 



The rust of birch leaves is caused by Melampsoridium helulce. 

 This fungus occiu-s also in Europe, where it is known that the 

 basidiospores produced from the over-wintering teliospores on 

 the birch leaves cause the infection of the young needles of 

 larch and produce a blister-rust. The stage on larch has been 

 found rarely in this coimtry but very probably exists more 

 generally and has been confused with the other rusts of larch 

 (see page 212). 



Yellow Leaf-Blistek 



Caused by Magnusiella flava (Farlow) Sadebeck 



This leaf-blister disease occurs on white and paper birch in 

 northeastern United States. Small light yellow blisters are 

 formed on the leaves. The mycelium of the pathogene enters 

 the tissue of the leaf and causes a stimulus which results in an 

 increase in number and size of the cells. The increased size 

 of the affected tissue results in the bulging blisters in the 

 leaf. The fungus produces asci containing ascospores on the 

 surface of the blisters. 



For the control of this disease, the same methods used for 

 peach leaf-curl should give equally good results (Hesler, L. R., 

 and Whetzel, H. H., Manual of fruit diseases, p. 277). 



Red Leaf-Blister 



Caused by Exoascus hacteriospermus (Johanson) Sadebeck and Ta- 

 phrina carnea Johanson 



Two species of the leaf-blister fungi are found on Betula 

 nana. The first mentioned has also been found on Betida 

 glandvlosa. Although these pathogenes have been described 



