160 SYSTEMATIZED PLANT-BREEDING 



materials naturally present in well worked soils. 

 Increasing these supplies by the application of 

 so-called artificial manures has marked effects on 

 the intensity of a rust attack. Phosphates and 

 to a less extent potash salts tend to restrict its 

 severity, but the application of the essential nitro- 

 genous manures, whether in the form of nitrate 

 of soda, sulphate of ammonia, guano or any other 

 form, tends to increase its severity to such an extent 

 that disease becomes an exceedingly important 

 limiting factor. None of the methods tried up to 

 the present offer the slightest promise of checking 

 this tendency. But direct experiments on this 

 point show that the high degree of resistance, 

 amounting practically to immunity, possessed by 

 American Club and some of its hybrids is retained 

 to the full even when far larger applications of 

 nitrogenous manures are made to the soil in which 

 they are growing than any one farming for profit 

 would think of using. 



Unfortunately resistance to the attacks of one 

 rust does not necessarily imply resistance to the 

 attacks of other species, so that our resistant 

 wheats are of no value in countries where the 

 prevalent species is other than the common yellow 

 rust. Even in countries such as India where this 

 species is excessively abundant our wheats are 

 useless, for they fail to mature with sufficient 

 rapidity. If sown at the same time as the Indian 

 wheats they come into flower a month later so 



