68 THE KUSTS OF GRAINS IN THE UNITED STATES. 



would be very high. It would seem, however, that with modern 

 macliinery and the many and varied formulae for spraying solutions 

 in existence, interesting results might be obtained with further spray- 

 ing experiments; particularly would this be true in the case of pre- 

 vention of stem rust of wheat (Puccinia gmminis), as we now know 

 the critical period for its attack, namely, the heading time of the 

 grains. It would seem possible to limit spraying operations to this 

 period, particularly in years when it falls in a prolonged cold season, 

 thus concentrating the spraying operations. Even under these 

 conditions there is considerable doubt that spraying would ever be 

 of practical value in preventing rust, but the possibilities justify 

 further experiments. 



The literature on spraying experiments for the prevention of rusts 

 in foreign countries is extensive and can not be reviewed in this 

 bulletin for want of space. Within the knowledge of the authors, 

 no such experiments have been successful from an economic stand- 

 point, though a few have shown some promise. 



SOIL AND SOIL TREATMENTS. 



That an excess of some elements in the plant food may predispose 

 a plant more or less to an attacking disease, or that an excess of some 

 other elements may have the opposite effect, rendering the plant 

 more resistant, has not been firmly estabhshed. On the contrary, 

 Ward (100, p. 138) has performed experiments to show that nutrition 

 alone does not make for or against predisposition or immunity on 

 the part of the host or virulence or impotence on the part of the 

 parasite. That cereals will absorb sufficient quantities of any ele- 

 ment originally in the soil, or wliich has been applied as fertilizer, 

 to render them resistant to rust attack is thus problematical. If 

 this were possible it would be a difficult matter to explain just how 

 this resistance is obtained, whether from changed physiology, modi- 

 fied morphology of the host, or from some toxic effect against the 

 fungous parasite. We know, for instance, that excess of certain salts 

 in the soil will change not only the morphology but the physiology 

 of cereals. Harter (54, p. 134) has shown that wheat plants grown 

 in soils made saline by the addition of 0.7 to 1.4 per cent of sodium 

 chlorid "modified their structure by depositing bloom on the leaf 

 surface, by thickening the cuticle, and by reducing the size of the 

 epidermal cells." In other words, the plants assumed xerophytic 

 characters. Physiologically, transpiration was decreased in plants 

 on soil sufficiently saline to cause increase in thickness of the cuticle, 

 and was increased in plants in soil containing soluble salts in pro- 

 portions too small to affect the measurements of the cuticle. 

 Although, as will be discussed later. Ward has shown that the 



216 



