164 Minnesota Plant Diseases. 



the common rusts of wheat forms its summer and winter spores 

 on the wheat plant, but the pycnidium and cluster-cup spores 

 are formed on the common buckthorn. 



The fungus which forms its winter spores on the cedar ap- 

 ples of the red cedar produces its cluster-cups on apples and 

 pears, having no summer spores in its life-history, but it pro- 

 duces its cluster-cups throughout the summer. The life-story 

 of such a rust fungus which possesses five kinds of spores, four 

 of which are produced from a parasitic mycelium and two of the 

 latter on one host and the other two on another host, offers an 

 exceedingly complex history. It may be remarked in passing 

 that to combat such accomplished parasites requires an intimate 

 knowledge of their histories. 



The question of a breeding-act in the rust life-history is 

 still, perhaps, an open one. A fusion of two elements in the 

 young winter spore has been interpreted as such an act by some 

 botanists. Recently, the beginning of the association of these 

 sexual elements has been discovered just preceding the forma- 

 tion of the cluster-cup. 



The rusts are all parasites — true parasites, unable to live ex- 

 cept in the tissues of their hosts. The mycelium grows inside 

 of the tissues and the spores are in almost all cases borne at the 

 surface of the plant, whence they can best be shed; but some, 

 buried in the tissues, depend on the decay of the host plant for 

 their release. 



The rusts vary greatly, also, in their location on the host 

 plants. Most commonly they are found upon the leaves but 

 in many forms the stem is also attacked and even the under- 

 ground portions may be invaded. Floral parts are seldom 

 directly attacked. The rusts also possess in some cases the 

 power of stimulation of the host to extraordinary effort, thus 

 increasing the available food supply for the fungus parasite. 

 The cedar apples of the red cedar are merely enlarged twigs 

 of the cedar tree in which a rust mycelium is at work. Some 

 rusts on pines produce great swellings on the stem and still 

 other cases might be cited of the deforming and stimulating 

 effect upon the host by rust parasites. Witches'-brooms are 

 very often of rust-fungus origin. Such is the common birds'- 



