52 



THE RUSTS OF GRAINS IN THE UNITED STATES. 



The uredospores of Puccinia graminis on Hordeum jubatum, Agro- 

 pyron repens, and A. tenerum obtained from the natural field habitat 

 have thus been demonstrated to retain their viability until April 15, 

 and material from the two former kept buried in the snow until 

 March 20 has also been shown to remain viable. Puccinia ruhigo-vera 

 on Agropyron repens and Triticum vulgare from the field have been 

 demonstrated to germinate as late as February 15, and Puccinia 

 simplex on Hordeum vulgare as late as December 27. After these 

 dates no material could be obtamed. A large per cent of the uredo- 

 spores collected in the fall and kept buried in snow since December 10 

 germinated as late as March 20, 1907. Bohey has shown that spores 

 of Puccinia ruhigo-vera collected in Minnesota April 9 and in North 

 Dakota April 13, 1905, were viable (28, p. 649). Together with 

 Bolley's and Christman's investigations cited above, these experi- 

 ments demonstrate conclusively that it is possible for the uredo- 

 spores of various stem and leaf rusts to retain their viability through- 

 out the winter in Minnesota, North Dakota, and Wisconsin. 



How commonly the wintering of the uredospore in these northern 

 States takes place is yet to be determined. Where snow remains 

 throughout the winter, preventing alternate freezing and thawing of 

 material thus covered, the wintering of the uredospore is, perhaps, 

 facilitated. Indeed, it is very probable that the uredospore sur- 

 vives the winter more easily in the north, where snow is continuous 

 during the winter, than in localities where snow covers the ground 

 only at intermittent periods. Then, there is probably as good a 

 chance, if not better, for the uredospore to winter in northern ^lin- 

 nesota or southern Canada, as in southern Alinnesota or Iowa. This 

 view is also held by Bolley and Pritchard (28, p. 643). 



From Kansas south, it has been proved by Hitchcock and Caiieton 

 (57, p. 11) that Puccinia ruhigo-vera winters very easily in the uredo 

 stage, and undoubtedly this also holds true for P. graminis. In the 

 springs of 1908 and 1909, the authors personally observed wheat 

 fields\i Texas and Oklahoma. During the latter part of April, 1908, 

 both Puccinia graminis and P. ruhigo-vera were extremely alnrn- 

 dant on wheats at San Antonio, Tex. Farther north, at Amarillo, 

 Tex., P. ruhigo-vera was well scattered April 30, though not 

 l)lentiful. At StiUwater, Okla., May 7, this rust was abundant. 

 Wheats at San Antonio, in 1909, were heavily rusted April 4, with 

 both P. graminis and P. ruhigo-vera, and the superintendent of 

 the San Antonio Experiment Fann said that a rust was abundant 

 in the grain plats in February. 



There is, then, an abundance of rust spores in southern wheat fields 

 ill the early spring, and, according to investigations cited in this 

 paper, there are also a large number of uredospores of Puccinia 



216 



