48 A STUDY IN CEREAL RUSTS 



That the important scientific questions involved in the specializa- 

 tion of biologic forms and that of rust-resistant varieties of wheat 

 are essentially the same seems obvious. The same phenomena can be 

 observed in both ; there are various degrees of resistance and suscep- 

 tibility in both and a thorough investigation with refined biochemical 

 methods will probably not only prove the similarity, but show the real 

 reason for resistance and susceptibility. 



SUMMARY OF PART II 



1. In making inoculations in the greenhouse on wheats resistant 

 to Puccinia graminis tritici it was found that only two, Khapli and 

 Kubanka 2094, especially the former, possessed a very marked degree 

 of real resistance, although a number of others were fairly resistant 

 in the field. 



2. It was observed that the more resistant a form proved, the 

 more pronounced was the tendency of the rust to kill small areas of 

 the leaf. The pustules developed in these areas were always very 

 small. 



3. The length of the incubation period of the rust is correlated 

 to a certain extent with the degree of immunity, the most nearly 

 immune forms, as a rule, having the longest incubation period. 



4. On the most resistant varieties, such as Khapli, the spores 

 are often small in size and sometimes abortive. 



5. Infection secured on partially resistant varieties as a result 

 of inoculations with aecidiospores and primary uredospores proved 

 only slightly more virulent than did that secured by means of inocula- 

 tion with long-time uredospores. 



6. Drought-resistant durum wheats grown in very dry soil 

 rusted more severely than those grown in soil with a higher moisture 

 content. Minnesota No. 163 did not show much difference, the plants 

 in wet soil being slightly more severely attacked. The conditions 

 normal for the host plant are probably also the conditions under 

 which the rust develops best. 



7. It was found that in general the absence or presence, in 

 excessive amounts, of various nutrient substances, such as nitrogen 

 and phosphorus salts, did not directly affect the immunity or suscepti- 

 bility of wheats. Conditions favoring a normal development of the 

 host were conducive to vigorous development of the rust. The action 

 of fertilizers, either natural or artificial, is probably indirect. Tem- 

 perature conditions and relative humidity of the atmosphere are prob- 

 ably more important than soil conditions. 



8. The addition of copper sulfate, copper carbonate, and iron 

 sulfate to nutrient media in which plants inoculated with rust were 

 grown did not markedly diminish the amount of rust when they were 



