174 



Minnesota Plant Diseases. 



occurs on other more or less closely related plants. The fungus 

 mycelium gains entrance to the plant in the spring and first pro- 

 duces cluster cups. These are followed by the summer spores, 

 throughout the summer months, forming small red-brown patches 



FIG. 199. Leaf rust of roses. The cluster-cup stage on the stems and leaves. On left is 

 a stem distorted by the cluster-cup cushions. Photograph by H. Cuzner 



upon large, dark, red-brown spots in great abundance, on the 

 lower surface of the leaves. The summer-spores are followed 

 toward autumn by the winter-spores, which are formed in similar 

 but darker groups. The winter-spores are two-celled. Where 



