Minnesota Plant Diseases. 277 



but attacks the leaves instead of the stem. The attacked leaves 

 are often normal in appearance and not at all distorted. The 

 cluster cups are formed on the surface of the leaves in the 

 spring. They are large, swollen, sac-like affairs and contain 

 a powder of light-yellow spores. It is possible that this is the 

 same fungus which forms its summer and winter spores on 

 asters and goldenrods and which is there known as the aster 

 and goldenrocl rust. 



The leaf rust of pines is seldom present in sufficient quan- 

 tities to injure the trees seriously. 



Ash leaf rust [Pnccinia fraxinata (Lk.) Arthur.]. Ash leaves 

 are attacked by a cluster-cup rust which is not at all uncommon 

 in the state, though it does not seem to be abundant. Several 

 species, including the green ash, are the hosts. The cups are 

 formed on large yellow sp.ots on the leaf blade or petiole and 

 the infected portion is often considerably distorted. The dust 

 of spores is a bright orange red. Within recent years this clus- 

 ter-cup stage has been shown to be connected with winter 

 spores on the common grass plant, Spartina cynosuroides. 

 (See also Fig. 75.) 



Witches'-broom of balsam fir (Aecidium elatinum Alb. et. 

 Schw.). There are formed on the balsam firs throughout the 

 northern part of Minnesota peculiar bush-like branch-growths 

 known as witches'-brooms. The production of this bush is due 

 to the action of a rust fungus, which lives in the tissues of the 

 branches. The first result of the attack of the parasite is the 

 formation of a spherical swelling on the side branch of the 

 balsam fir tree. From this swelling arise a very large number 

 of branches, which grow very fast and become much longer 

 than the unaffected branches. They often have a somewhat 

 climbing, twining habit and grow upward instead of in a hori- 

 zontal direction as do the ordinary lateral branches. The dis- 

 eased branches do not hold their leaves through the whole 

 year but shed them every fall. The fungus forms cluster-cups 

 on the leaves of the broom shoots in great abundance in early 

 summer and when the spores are ripe, a cloud of yellow dust of 

 cluster-cup spores can be shaken from the broom. The broom 

 increases in size from year to year and often several brooms are 

 developed on the same tree. In some cases, almost the entire 

 tree may be broomed. Not only is the symmetry of the growth 



