Minnesota Plant Diseases. 



39i 



posed of three cells arranged in clover-leaf fashion. The spore 

 clusters are usually very dense and vary in size from a pin- 

 head's width to three-quarters inch broad. The leaf under the 

 spores colors black, so that the groups can readily be seen 

 from the upper surface of the leaf. Cluster cups and summer 

 spores are not known for this rust. 



Rust of sedges [Puccinia caricis (Sghum.) Reb.]. This is 

 an exceedingly common rust on many Minnesota sedges. The 

 summer and winter spores are produced on the sedge plant and 

 the cluster-cup form is very common on the vacant lot weed, 

 _^ ^ stinging nettle. The attacked nettle- 



plant parts are usually deformed and 

 swollen, where the mycelium of the 

 rust develops. When the stem is at- 

 \ tacked, it is usually much bent and a 

 \ swollen cushion arises on one side. 

 Similar cushions arise on the leaf stalks 

 and on the leaf blades. On these 

 cushions which are usually orange to 

 yellowish in color are found the clus- 

 ter cups in great abundance. The 

 winter-spores are found on the sedge 

 leaves in long black rows, similar to 

 the black rust of wheat. Not all of 

 the rusts on sedges are of the same 



FIG. 208. Rust of "wild sarsaparilla; S P edeS ' Though the winter 



shows clusters of the winter spores. ma y k e so similar in appearance that 



they can scarcely be distinguished 



from each other, even with the microscope, they may form their 

 cluster cups on different host plants. The common rust of 

 sedges, however, is that one which forms its cluster cups on the 

 nettle. The white cluster cups of wild black currant are forms 

 of another sedge rust. 



Rust of the dark green rush (Puccinia angustata Ph.). One 

 of our commonest broad-leaved marsh rushes is frequently at- 

 tacked by a rust, which occurs in great abundance on the leaves 

 of the plant. The winter spores are particularly conspicuous, 

 forming long, black lines, in which are found the densely crowd- 

 ed winter-spores. The latter are two-celled. 



