12 A STUDY IN CEREAL RUSTS 



EXPERIMENTS WITH PUCCINIA GRAMINIS HORDEI 



GENERAL STATEMENT 



Freeman and Johnson (1911, p. 20) state that the barley stem 

 rust is more versatile than any of the other biologic forms of the cereal 

 rusts, and, further, that the range of infection of a given form is in- 

 creased after having been transferred to barley. The results of the 

 author, in general, agree with those of Freeman and Johnson, although 

 the percentages of infection are in some cases quite different. 



INOCULATIONS ON RYE 



In the first series 20 leaves were inoculated, after which the pots 

 were kept under bell jars for 48 hours. In 8 days very distinct signs 

 of infection began to appear and on the tenth day distinct pustules 

 were quite numerous. Eventually 16 leaves out of 18 showed distinct 

 pustules. Two of the leaves had been killed. Very evidently not all 

 of the mycelial wefts developed pustules, since pustules were often 

 intermingled with typical, yellowish rust flecks and green islands. The 

 pustules were in all cases small, most of them less than a millimeter in 

 size, but they were fairly numerous. There were some fairly large 

 areas in which no pustules developed, but which contained a large 

 number of green islands. 



In the second series 40 leaves were inoculated, 10 of which eventu- 

 ally produced pustules. Of the remaining 30 leaves 29 were very 

 strongly flecked in such a manner as to show conclusively that infec- 

 tion had taken place although no pustules had been formed. 



The results of both series indicate that rye is easily infected by 

 the stem rust from barley. Out of 58 leaves, 26 were infected, indi- 

 cating greater ease of infection than in case of the strain used by 

 Freeman and Johnson (1911, p. 19) in their experiments. The 29 

 leaves which are indicated as having been strongly flecked would at 

 first sight have been counted as having pustules. However, close 

 examination showed that there were no ruptured pustules. 



Although a fairly high percentage of successful infections can be 

 obtained, rye is by no means a congenial host for Puccinia graminis 

 from barley. This is shown by the fact that the pustules are always 

 small. Long areas of the leaf may be killed and in this area there may 

 be many unruptured pustules. Often an area extending across the 

 entire leaf and one centimeter or more in length may be completely 

 killed. (See Plate I, A.) This may not contain a single pustule, but 

 close examination reveals the fact that there are many green islands, 

 some of which have a yellowish tinge in the center. These latter look 

 very much like unruptured pustules. Histological examination re- 

 veals the fact that the mycelium has spread, the host cells have died, 

 and wefts of mycelium have formed, either directly under, or a slight 



