168 ANNUAL REPORTS OF DEPARTMENT OF AGRICULTURE. 



cloubtedly will throw much light on the relationships between early 

 and late maturing varieties and species and their natural distri- 

 bution. 



PLANT PHYSIOLOGICAL INVESTIGATIONS. 



Evidence has accumulated that a low water requirement, although 

 an important factor in comparative resistance to drought, is by no 

 means the only factor involved. The water requirement has now 

 been determined for practically all important species and varieties of 

 field crops, and the data obtained during the past several years make 

 it possible to answer decisively the question to what degree water 

 requirement is correlated with adaptability to drought conditions. 

 Attention was focused during the past year upon the increase and 

 distribution, in cooperation with the congressional seed distribution, 

 of improved strains of sorgo and millet adapted to conditions in the 

 northern Great Plains region. Drought-resistance breeding work 

 w^ith other forage plants, especially alfalfa and smooth brome-grass, 

 was continued, and promising strains of the two crops mentioned are 

 being increased for distribution to farmers in the central and north- 

 ern Great Plains. 



CEREAL PATHOLOGY. 



BLACK OR STEM RUST. 



Black-rust epidemics. An intensive nation-wide investigation of 

 black rust has been made during the year. Special attention has 

 been given to the factors which may contribute to epidemics of this 

 rust. Among these factors are weather conditions, soil drainage, air 

 drainage, barberry bushes, the winter hardiness of the urediniospores, 

 or red stage, and of the teliospores, or black stage, and the carrying 

 of spores by the wind. 



In the investigation of the overwintering of the black rust it has 

 been found that survival varies with the severity of the winter. In 

 average years very few of the red spores, which start the rust directly 

 on wheat in the spring, are able to survive winter conditions north of 

 about latitude 35 N. South of this general latitude, however, they 

 were able to overwinter abundantly. In the exceptionally mild win- 

 ter of 1918-19 they survived as far north as Wisconsin. On the other 

 hand, the black spores, w^hich start the rust on barberry bushes, nor- 

 mally live through even the severe winters of our most northern 

 States and start the rust in the spring. They can not start the rust 

 directly on wheat, however, so that destroying common barberries 

 makes these spores harmless. A thorough investigation of the infec- 

 tion of wheat seed by black rust has been completed. The study has 

 covered several years and involved the growing of many thousands 

 of plants from rust-infected seed ; no infection of seedlings from such 

 infected seed has been produced. 



Varieties of wheat almost completely resistant to some of the bio- 

 logical forms of black rust have been discovered or developed by 

 breeding and are being used in other breeding experiments in coop- 

 eration with the agricultural experiment stations of Minnesota and 

 Kansas. 



Barberry eradication. The campaign for the control of the black 

 rust of wheat through the eradication of the common barberry is in 



