RUST-RESISTANT VARIETIES OF WHEAT 33 



somewhat larger, were deeper brown, and seemed to be shedding 

 spores in greater abundance. On einkorn, in one experiment, the re- 

 sults were directly opposed to this. The differences were not especially 

 striking, the aecidial infections being perhaps slightly more virulent. 

 There is considerable evidence that the virulence of the rust attack 

 when carried by aecidiospores or primary uredospores is exceptionally 

 virulent. The results of these experiments, however, would not justify 

 such a conclusion in this particular case. 



METABOLISM OF THE HOST AND RUST RESISTANCE 



There seems to be no question but that weather and soil condi- 

 tions, determining the metabolism of the host plants, exert an influence 

 un the prevalence of rust in the field. Little (1883, p. 634) states that 

 weather is the determining factor and adds that high manuring, espe- 

 cially with nitrogenous manures, predisposes wheat plants to rust. 

 Bolley (1889), Anderson (1890), and many others since have held that 

 this is the case. Bolley suggests as a possible cause the increased 

 evaporation and consequent raising of the relative humidity. Jones 

 (1905) shows that Phytophthora rot of potatoes tends to be more seri- 

 ous after a heavy application of nitrogenous fertilizers to the land. 

 Miss Gibson (1904) concluded as the result of experiments with the 

 chrysanthemum rust that in an almost immune form normal develop- 

 ment of rust does not depend on the state of health of the plant, but 

 that a luxuriant state of growth favors the development of the fungus. 

 Hennings (1903, pp. 41-45), on the other hand, states that in observ- 

 ing plants infected with perennial smuts and with rusts he found that 

 the disease disappeared when the plants were placed under the most 

 favorable cultural conditions. This is not in accord with Arthur's gen- 

 eralization (1903, p. 13) that "so intimate is the association of host 

 and parasite that as a rule the vigor of the parasite is directly propor- 

 tional to the vigor of the host." Apparent discrepancies may, how- 

 ever, be explained by the fact that different plants and different para- 

 sites react quite differently. As far as the rusts of cereals are con- 

 cerned, Arthur's generalization would seem to be correct. Biffen 

 (1912, pp. 4^21-429) shows that Puccinia glumarum is most virulent 

 when a complete fertilizer is used and that the virulence decrease? 

 with the decrease in amount of fertilizer. 



Less work has been done to determine the exact manner in which 

 these causes operate. De Bary (1887, p. 359) says, "The physiological 

 reason for these predispositions cannot in most cases be exactly stated ; 

 but it may be said in general terms to lie in the material composition of 

 the host, and therefore to be indirectly dependent on the nature of its 

 food.'' Marshall Ward (1902, p. 145), in experiments with Puccinia 

 dispersa on bromes, tried the effect of mineral starvation and concluded 



