278 



Minnesota Plant Diseases. 



of the tree interfered with but the tree is usually stunted 

 and may eventually die. The leaves of the diseased shoots 

 contain less leaf-green and are paler in color than the ordi- 

 nary leaves. The exact method of the infection by spores in 

 this country is not yet known. A similar disease on the silver 

 fir in Europe has recently been thoroughly investigated. It is 

 found that the cluster-cup spores of the silver fir brooms can- 

 r- -^ not infect the silver 



fir leaves but can 

 cause infection on 

 certain plants be- 

 longing to the pink 

 family, e. g., the 

 common chick- 

 w e e d s. Here the 

 fungus gives rise to 

 the summer- and 

 winter-spore stages. 

 From the winter- 

 spore forms infec- 

 tion of the silver fir 

 takes place. The 

 commencement of 

 the characteristic 

 broom-like growth 

 of the branches 

 takes place in the 



7 



FIG. 137. Poplar leaf rust (Melampsora populina). A lOllOWing year. No 

 poplar leaf showing the fine, black spots of the crust- experiments h a V e 

 like clusters of winter spores on the under surface. 



Original. yet been carried on 



to determine wheth- 

 er or not the American fungus on the balsam fir is or is not 

 identical with the European form on the silver fir. 



All brooms should be removed and burned especially before 

 the formation of the cluster-cup spores in the spring. (See 

 Fig. 23.) 



The poplar rust [Melampsora populina (Jacq.) Wint.}. The 

 poplar rust is exceedingly abundant in Minnesota and in some 

 cases probably effects considerable damage of young poplars 



