DISEASE RESISTANCE IX PL.AJXTS. 125 



farming to obtain additional labor of the right sort at any time, he 

 can spray his various crops with profit, but when labor is limited 

 and the grower must depend on his steady labor to do the spraying, 

 he will usually find that to carry on the work ]5roperly other im- 

 portant things must be neglected. Spra^-ing must be done at the 

 right time, or it is of no value. The third reason is that there are 

 many diseases which experimentally can be controlled, but not 

 profitably. x\s an example we may mention the rust of wheat, 

 and many other diseases of field crops. The fourth reason is that 

 certain very serious diseases have never been satisfactorily con- 

 trolled even by experimenters. To this class belong such diseases 

 as peach yellows, crown gall, and various soil rots and wilts. Most 

 bacterial diseases of plants belong to this class. 



Methods. 



In order to develop varieties resistant to disease, several methods 

 of work are open to the investigator. 



(a) Selection of individuals. 



(b) Selection of varieties. 



(c) Hybridization followed by selection of varieties and individ- 

 uals. 



Selection of Individuals. 



Every grower of plants has doubtless observed that in a field of 

 a crop suffering from an epidemic disease, certain individual plants 

 may show more or less resistance to the disease. By selecting seed 

 from such plants we would expect, if the resistance is an actual 

 one, to have ofPspring which would inherit the character of resist- 

 ance. By repeated selection through several generations, always 

 saving seed from the most healthy plants, we would expect to 

 gradually develop a variety or strain much more resistant than the 

 ordinary one. This has been practically tested in a number of 

 cases and found to be successful. ^Nlr. P. K. Blynn, of the Colorado 

 Experiment Station, has obtained most satisfactory results by this 

 method in selecting a strain of cantaloupe known as the Pollock 



