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, Hrysiphe 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. trachetphilum 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- 
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