Aug. 21, 1916 



Infection of Timothy by Puccinia Graminis 



815 



It is quite evident both from the percentage of successful infections 

 and from the character of the infection that timothy can not be con- 

 sidered a congenial host for P. graminis avenae. The total number of 

 leaves inoculated was 3,270 and only 57 became infected, only 1.47 per 

 cent. The pustules were always small, ranging in size from mere dots to 

 pustules 0.3 mm. in diameter. On the older leaves they were often 

 surrounded by a small dead area, indicating a certain degree of hyper- 

 sensitiveness, while on younger leaves they often appeared to develop 

 quite normally except in size. Four or five pustules sometimes developed 

 on the same leaf, giving the appearance of fairly successful infection. 

 The incubation period varied from 8 to 12 days. The spores were con- 

 siderably smaller in size than those of P. graminis avenae, but they were 

 larger than those of P. phleipratensis. The spores of P. graminis avenae 

 are also reduced in size on barley; the character of infection is somewhat 

 the same as that on timothy and the spores become almost identical 

 in size. Comparative measurements of spore lengths are given in 

 Table II. 



Table II- — Length of uredinios pores of P. graminis avenae and P. phleipratensis 



Rust organism. 



Host on which measured. 



Length limits. 



Mode. 



P. graminis avenae 



Do 



Do 



P. phleipratensis. .. 



Avena sativa 



Phleum pratense . 

 Hordeum vulgare 

 Phleum pratense . 



2 4 to 35. 52 



20. 16 to 32.64. . . 



20.80 to 32.96 



16 to 28.80 



29.44 

 25.60 

 25. 60 



21. 76 



Although the size of the spores is decreased on timothy, it becomes 

 normal the first generation when the rust is transferred back to oats. 

 The decrease in size is probably to be regarded only as a stunting due to 

 unfavorable environment, since it has been previously shown that spores 

 of P. graminis produced on an uncongenial host tend to become smaller 

 than on a congenial host (7, p. 31). The color of the spores remains 

 constant on oats and timothy and they can thus be distinguished very 

 easily from spores of P. phleipratensis. Spores of P. graminis avenae are 

 a bright cadmium-yellow in color, while those of P. phleipratensis are 

 much duller, sometimes almost gray. 



The fact that P. graminis can infect timothy raises the question as to 

 whether P. phleipratensis may not have developed from some biologic 

 form of this rust. Only speculation is possible at the present time, and 

 a discussion of the possibilities is therefore probably useless. Neverthe- 

 less it is significant that P. graminis avenae, which now seems a possible - 

 source of the rust, produces urediniospores of very different shapes and 

 sizes on the same plant and in the same pustules, thus conceivably indi- 

 cating a tendency toward instability. The rust also has a wide range of 

 hosts, in that while occurring commonly on one cereal, oats, and being 



