Proceedings of Seventeenth Normal Institute 23T 



In other words, a mixture of closely related varieties belonging 

 to the same group is permissible within rather wide limits, but a 

 mixture of varieties belonging to different groups should be per- 

 mitted only within very narrow limits. 'No field showing over 

 15 per cent of varietal mixtures should be accepted' for seed even 

 if the owner is willing to rogue it. Fields showing less than 15 

 per cent mixture of varieties of different groups may be accepted 

 if thoroughly rogued. 



DISEASES 



The extent to which potatoes mav be diseased and yet qualify 

 for seed depends, of course, on the nature of the disease. There 

 must be different standards for different diseases. 



The wart, being a dangerous disease not yet established in the 

 United States, should be carefully guarded against. Seed pota- 

 toes should be absolutely free from wart. 



Just what may be expected of powdery scab is not clear at 

 present. Recent observations indicate that it may be unimpor- 

 tant in the United States except, possibly, in the extreme north- 

 ern part. However, for the present, it seems best to require 

 that seed potatoes be absolutely free from powdery scab. 



Of the common scab it may be said that a small amount of it 

 on seed potatoes is not seriously objectionable, because the germs 

 of the disease are readily destroyed by the corrosive sublimate 

 treatment, which should generally be given to seed potatoes any- 

 way. Of course, no very scabby tubers should be included. I 

 agree with Mr. Sands that a tolerance of 10 per cent slight scab 

 infection should be allowed, provided that of this amount the 

 infections on any one tuber do not cover more than 10 per cent 

 of the surface thereof. 



The same may be said of Rhizoctonia as of common scab. 



Even a severe attack of late blight need not, necessarily, dis- 

 qualify a field of potatoes for seed purposes. Frequently, little 

 or no rot follows severe attacks of blight. A severe attack of 

 rot, on the other hand, may warrant disqualification. Moreover, 

 when conditions are favorable for rot, shipment from blighted 

 fields for seed purposes should not be pennitted for at least a 

 month after digging. The object of this requirement is to give 

 affected tubers a chance to show tbemsielves so that they may be 



