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 a single 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, Pucdnia rubigo-vera tritici 

 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 



