WINTERING OF THE UEEDO GENERATION. 45 



The most important ])oint brought out by these experiments is 

 that for 52 generations there is no apparent diminution of the vitality 

 of the uredo genei-ation (hie to continuous cidture and the absence 

 of the ax'itUo or tekuito generations. For this k^igth of time, at h;ast, 

 there is no need for a sexual generation. How long successive uredo 

 generations can continue without lowered vitality has not been de- 

 termined, but these experiments indicate that they may continue 

 for a very long period and that the uredo generation may be all 

 suflicient. It must be noted that the number of successive inocula- 

 tions in these experiments far exceeds the probable number under 

 ordmary field conditions because they were continued throughout 

 the winter months. Since 8 to 12 days (even up to 20 days and over 

 in unfavorable weather) are necessary for infection and since prob- 

 ably not more than 5 or 6 successive infections follow each other 

 annually in field conditions, the inoculations in the above experi- 

 ments are equivalent to 7 or 8 years of successive infection in the 

 field. 



WINTERING OF THE UREDO GENERATION. 



HISTORY OF THE INVESTIGATIONS. 



The question whether or not the uredo stage of rusts lives over 

 winter either as mycelium or in the spore form has been a much 

 mooted one ever since De Bary demonstrated the heteroecism of 

 Puccinia graminis in 1865. This problem has been investigated by 

 many scientists in different countries and localities. 



Germany. — De Bary (12, p. 23) was one of the first of these investi- 

 gators. He looked for the wintering of the myceliuna of Puccinia 

 graminis on Agropyron re pens and Poa pratensis, but although 

 heavily covered with rust in the field tlie same i)laiits "in the fol- 

 lowing spring and- summer remained rust free. He concluded that 

 the rust mycelium is annual only, even in peremiial grasses. 



Ktihn (66, p. 401) found the uredo of Puccinia coronata in all stages 

 of development on IIolcus lanatus in the middle of winter and main- 

 tamed that it developed without hindrance in the spring; on this 

 account he considered a similar wintering in Puccinia graminis and P. 

 rnhigo-vera very possible. 



According to Eriksson and Henning (39, p. 38), Blomeyer (20, p. 

 405) believed that Puccinia graminis was able to winter over m the 

 uredo stage at Leipzig on account of the early appearance of P. 

 graminis in the spring (latter part of May) at that place. 



Klebahn (63, p. 64) says that neither does Puccinia graminis SLppear 

 to whiter in the.uredo stage nor P. coronifera avenae nor P. simplex, 

 because oats and barley rarely, if ever, are grown as winter grains in 



216 



