S66 I)IS]£ASES OF ECONOMIC PLANTS 



are of more brownish tinge. The sori are most common 

 upon the stems and leaf sheaths, more rare upon the leaf 

 blade. As with some of the other cereal rusts, the uredo- 

 spores may remain viable over winter, and thus perpetuate 

 the rust, even without the aid of other forms of spores. 



Following upon this stage the black teleutosori appear 

 upon the stems, and leaf sheaths, constituting the most con- 

 spicuous and predominating symptom. 



The damage by this disease is due to the inroads of the 

 fungus upon the green leaf tissue, destroying the starch- 

 producing power, diminishing the vigor of the plant, and 

 showing its effects finally in the shrunken, shriveled grain. 



Since this rust usually appears late in the season, the 

 varieties of wheat that mature early are likely to escape its 

 attack to a greater extent than those varieties which are 

 late in maturing. For a similar reason any climatic condi- 

 tions which conduce to slow growth and late maturing of 

 the crop favor the rust. Speaking of the resistance of the 

 durum wheats, Carleton ^ says : — 



" Numerous reports indicate that the loss to the wheat 

 crop by rust amounted to as much as 50 or 60 per cent over 

 large areas in North Dakota, South Dakota, and Minnesota 

 (in 1904), while in all cases the loss to durum wheat through 

 the same source seldom reached more than 10 per cent, 

 being usually 3 to 5 per cent, and sometimes nothing. Con- 

 servative estimates have given the decrease in the entire 

 production of wheat in the three states mentioned at from 

 25,000,000 to 40,000,000 bushels, or a loss in the farm value 

 of the wheat of at least $25,000,000. There is Httle doubt 

 that if all wheat grown in these three states during 1904 had 



1 Carleton, M. A., U.S. Dcpt. Agr. Farmers' Bui. 219. 



