DISEASES OF WHEAT 165 



of them may dry sufficiently to be carried by the wind, yet 

 not enough to be injured. In the early spring months the 

 damp straw and the ground and surface waters of an old wheat 

 field may be swarming with countless millions of these sporidia, 

 and clouds of them are wafted by the winds to distant points. 

 No evidence has yet been secured, however, that the sporidia 

 directly infect the wheat plant. Those of the stem rust infect 

 the fruit and leaves of the barberry bush, and, as far as 

 known, no other plants. Little cups and clusters of cups of 

 yellow spors are formed. The floors of the cups start pali- 

 sade-like chains of the spores, which, when mature, again take 

 wings with the wind. These are the wheat infecting aecidio- 

 spores with which we began, and the life cycle is thus complete. 



At the present writing the life history of all wheat rusts is 

 not perfectly and certainly known in all its phases, but it is 

 quite conclusively known that in some cases certain stages of 

 the life cycle which is given above (and which describes the 

 stem rust more correctly and fully than any other) are not 

 essential. The cluster cup form of rust, found on barberry 

 bushes in black stem rust, forms on common wild plants of 

 the borage family in the case of orange leaf rust. In Europe 

 it forms on hound 's-tongue, but this stage of the rust seems 

 to be absent in the United States. In crown rust of wheat the 

 cluster cup stage commonly forms on the buckthorn. 



It is now established that the uredospores of a number of 

 the important rusts, including Puccinia graminis, can pass the 

 northern winter in viable form. Dried and scattered by the 

 August and July winds, a very large per cent of these rust 

 spores germinated after a dry fall and a North Dakota winter. 1 

 In the warmer climates the leaf rust not only survives the 

 winter in the red spore stage, but forms new pustules every 

 month of the year. Viable spores of both rusts successfully 

 pass the winter frozen in snow and ice. The very early general 

 infection by rust can hardly be explained by the wintering 

 uredospores, however. Experiments by Bolley show quite con- 

 clusively that the infection comes by way of the air and not by 

 way of the soil. It is thought possible that rust filaments 

 passing the winter in green plants, and also broken particles 

 of the mycelium from the crop of the previous year, may aid in 



1 Bolley, Science, N. S. 22:50-1. 



