LIFE HISTORIES OF BUSTS. 23 



The number of successful inoculations of stem rust of oats to 

 wheat and rye has been insufficient to make absolute statements 

 concerning them, but there is little doubt that under highly favorable 

 conditions they can be made. On the other hand, there is no doubt 

 that the oat rust can be carried to barley and from barley to either 

 oats or barley, as a large number of successful trials by Derr have 

 shown. In all cases the pustules obtained in the course of the 

 inoculations were small and weak and the rust was very evidently 

 not on a congenial host. The oat rust is thus seen to be the most 

 closely specialized of the biologic forms of Puccinia graminis on the 

 small grains, but in its ability to infect the other species under 

 rarely occurring conditions still shows its close affinity to the other 

 rusts. Of all the stem rusts on the small grains that of oats is the 

 most distinctive and individualistic in appearance, having larger 

 pustules of uredo spores which are formed very commonly both on 

 stems and leaves (as in barley), in sharp contrast with the more 

 restricted location of the pustules in the rusts of wheat and rye. 

 As a biologic form, the stem rust of oats may be said to be generally 

 confined to oats. It can at times be carried to barley, but never 

 produces large or vigorous pustules. It is only rarely that the 

 transfers to wheat and rye can be made. 



EXPERIMENTS WITH BIOLOGIC FORMS OF LEAF RUST. 



Fewer experiments have been made with the biologic forms of 

 leaf rusts than with the stem rusts, but these experiments indicate 

 that, as a rule, the leaf rusts are not as versatile as the stem rusts, 

 being confined more closely to the original hosts. 



Diagram 17 presents a summary of inoculation experiments with 

 Puccinia rubigo-vera tritici (leaf rust) from wheat. 



Diagram 17. — Summary of inoculation experiments with leaf rust from wheat. 



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The leaf rust of wheat was carried directly to wheat, rye, and 

 barley, but in 47 inoculations it would not transfer to oats. It is 

 similar to Puccinia graminis tritici, which can easily be transferred 

 to the first two cereals, to rye rarely, and to oats only in very excep- 

 tional instances. But the leaf rust of wheat will not infect barley 

 nearly as readily as the stem rust of wheat, but seems to transfer 

 to rye more easily than the stem rust. No experiments were made 



216 



