BREEDING DISEASE-RESISTANT PLANTS 413 



biologic form sometimes differ in degree of virulence on the same host; 

 and that all gradations in susceptibility occur among the hosts, from 

 complete immunity to complete susceptibility to various biologic forms. 



Finally, it must be remembered that but little is yet known about 

 the nature of rust resistance. That it is in no wise dependent upon 

 morphological characters appears to be well established. Carleton has 

 pointed out that biochemical investigations are needed in connection with 

 this problem. The recent investigations of Wagner on hydrogen ion 

 concentration and natural immunity in plants representing four genera 

 including the potato resulted in the conclusion that the variation in 

 hydrogen ion concentration in plant tissues is a phenomenon of reaction 

 to the injection of pathogenic bacteria. Also that the course and end 

 results are related to the susceptibility of the plant in question and to the 

 character of the disease as acute or chronic. An investigation now in 

 progress at the University of California (by W. W. Mackie) seems to 

 indicate that there is positive correlation between degree of acidity as 

 indicated by the concentration of hydrogen ions and degree of resistance 

 to P. graminis in wheat. A similar investigation of the species of 

 Bromus in relation to the physiological races of the Corn and Grass 

 Mildew, Erysiphe graminis DC., as reported by Salmon would be highly 

 desirable. Whatever the nature of the resistant quality may be, there 

 is no question regarding its heritability. But in view of the complicated 

 nature of the problem which we have briefly outlined it would appear to 

 be inevitable that the utilization of resistant varieties of wheat must be 

 confined to limited areas in which adequate tests have proven their 

 adaptability. 



Inheritance of Disease Resistance in Other Plants. The conclusions 

 we have reached in respect to rust resistance hold good in a general way 

 for other parasitic plant diseases. In addition to the typical cases 

 already described brief reference may be made to other notable examples 

 of the successful creation of disease resistant varieties by hybridization 

 and subsequent selection. The next case, however, will be considered 

 somewhat in detail because it serves as an excellent model in method of 

 procedure. The ravages of a group of wilt diseases caused by closely 

 related fungi of the genus Fusarium have been checked through the 

 successful efforts of the United States Department of Agriculture. As 

 reported by Orton these are the cotton wilt, Fusarium vasinfectum Atk., 

 the cowpea wilt, F. tracheiphilum Erw. Sm., and the watermelon wilt, 

 F. niveum Erw. Sm. It is clear that these fungi possess a high degree of 

 adaptation to the parasitic mode of existence. Also that, while the cause 

 of resistance in certain varieties of the host species is not fully established, 

 yet the resistance itself is a physiological quality. No constant mor- 

 phological differences have been detected between immune and suscepti- 



