416 GENETICS IN RELATION TO AGRICULTURE 
selection should receive more attention from plant pathologists and 
horticulturists in the future. 
Breeding Disease-resistant Plants by Selection.—Selection alone is a 
powerful means of improving plants with respect to disease resistance 
when used either in variety tests or in the improvement of asingle variety. 
The testing of varieties for disease resistance is an exceedingly important 
service which can be done most satisfactorily by experiment stations and 
commercial seedsmen in connection with their routine work. However, 
the geographical variability in many parasitic organisms and the impor- 
tance of local adaptation of varieties in many economic plants make it 
imperative that each important agricultural region should have its own 
station for variety testing. 
The diversity between varieties in respect to disease “‘resistance”’ is 
sometimes due to morphological or anatomical peculiarities which prevent 
the invasions of parasites. For example, pubescence or waxy excretions 
on the surface sometimes prevent disease; the number of stomata or the 
arrangement of cells beneath them may condition fungus infection. 
Also some varieties escape certain diseases by virtue of their seasonal 
adaptation or because of precocity. Thus certain grains are less troubled 
with smut than others because they germinate more quickly. A differ- 
ence of 2 days in time required for germination may be the deciding 
factor in smut infection. Certain varieties of potatoes are able to form 
a corky layer in about 6 hours after being cut while others require 3 
or 4 days. Bacteria require from 12 to 24 hours to commence 
putrefaction through enzyme action. In addition to these and 
many other varietal differences there is always the possibility of real 
immunity which is due to some specific physiological character of the 
variety. A probable instance of considerable importance is the immunity 
of milo to the smut fungus which is infectious to all other sorghums. 
Although there are numerous valuable reports on the disease relations 
of certain varieties of our important crop plants, much remains to be done 
in the way of both extensive and intensive testing. The following 
citations merely illustrate the kind of information that is now available. 
Recent observations at the Kansas Station on 119 varieties of winter wheat 
showed infection with orange leaf rust, Puccinia rubigo-vera tritica 
Carleton, varying from 5 to 90 per cent. According to Orton there are 
varieties of the potato which are partially resistant to late blight and 
probably also to scab, a matter which has received considerable attention 
abroad but very little in this country, although there are undoubtedly 
great possibilities in this work. Orton’s success in producing wilt resist- 
ant varieties of cowpea was made possible by the discovery of one immune 
variety, the Iron, which was apparently of chance origin. In future 
breeding work much time and effort might be saved if agronomists and 
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