18 A STUDY IN CEREAL RUSTS 



greater length of time, no pustules were developed on any of the 40 

 leaves inoculated. Nine of these leaves were, however, quite strongly 

 flecked. The use of chloroform did not result in the formation of 

 pustules, although there was a distinct advantage over the check plants 

 both in number and definiteness of flecks. In all, 40 plants were inocu- 

 lated and of these 19 became distinctly flecked. After the use of ether, 

 then, 3 out of 90 leaves were successfully infected, and 56 were sharply 

 flecked, and these, added to the totals after the use of chloroform, give 

 3 pustules and 75 infected leaves which developed no pustules out of 

 130 trials. 



Effect of manure 



Wheat was planted in two pots containing a rich loam very heavily 

 fertilized with rich barnyard manure. Of the 20 leaves inoculated, 6 

 developed pustules and the rest were strongly flecked. In this case, 

 however, there was a possibility that the pustules developed as a result 

 of accidental infection. The flecks were apparently due to the arti- 

 ficial inoculation, since such flecks have never been observed after 

 direct inoculation with spores from either wheat or barley, or after 

 the inoculation of wheat with wheat or barley rust. It is therefore 

 quite certain that the pustles, which were normal, were the result of 

 accidental infection, while the flecks, which were exactly like those 

 developed on a semi-immune form, were the result of artificial inocula- 

 tion. 



Effect of leaf injury 



In one experiment, 16 leaves were pricked full of holes in an 

 area of one centimeter or more. They were then inoculated, and 4 

 became flecked, but no pustules developed. In another experiment the 

 epiderm was stripped from 29 leaves immediately before inoculation. 

 Although 10 became flecked, the flecks were extremely minute and no 

 pustules were developed. Histological examination showed that the 

 spores had sent out germ tubes in large numbers. These tubes grew 

 among the host cells, but true infection did not take place. Sections 

 of these plants were made and examined. It was clearly evident that 

 leaf injury did not increase the chances for infection. The hyphae did 

 not develop better than did those in normally inoculated plants. 



Summary of inoculations on wheat 



Out of a total number of 283 leaves inoculated under varying 

 conditions only 4 developed pustules and 113 became flecked, showing 

 that, although the rust of oats can sometimes develop on wheat, it can 

 seldom attain to pustule formation. The severity of infection, always 

 very slight, can be increased somewhat by exposing plants to be in- 

 oculated to anesthetics. The experiment with high fertilization of 



