1891.] PUBLIC DOCUMENT — No. 33. 219 



neither the necessary accommodations nor equipment for such 

 investigations. At present other equally important studies 

 are in progress, and it does not seem advisable for us to take 

 up the subject, especially since the bacteriological investi- 

 gations are of a very special nature, and are best conducted 

 bv persons who devote their entire attention to that field. It 

 is to be hoped, however, that some European bacteriologist 

 will take up the subject, and repeat the investigations 

 already made in this country ; since, as was shown in our 

 last report, the same disease prevails there, at least in its 

 " surface" form, as well as here. 



From what has been already said, it is evident that the 

 term " scab," as commonly used, is a general and not a 

 specific one. The rough crusts "which give rise to the name 

 are cork formations produced by the growing tuber, in 

 response to, and as an attempted protection against, exter- 

 nal irritating influences. That this irritation is sometimes 

 due to parasitic or semi-parasitic organisms seems probable, 

 in the light of the investigations mentioned above ; but that 

 it is always so produced is by no means proved, since, in 

 practice, badly diseased tubers have too often given a 

 practically smooth crop. One who insists on the presence 

 of some specific organism as a necessary preliminary to 

 the development of scab must admit a very strong depend- 

 ence on peculiar conditions on the part of such organism. 



It is difficult to believe that the " deep " form of the scab 

 is entirely distinct from the " surface " form, as Dr. Thax- 

 ter's results would indicate, although he has found pota- 

 toes attacked only by the former near New Haven, Conn.,. 

 the past season ; for both forms have occurred abundantly 

 on potatoes from the same hill on our experimental plots, 

 and not uncommonly both on the same tuber. And the 

 "deep" form has shown, in our experience, such differ- 

 ences in depth as almost to furnish a series running up to 

 the " surface " form ; yet it is true that a certain darker and 

 more decayed appearance almost always distinguishes it 

 from the latter. 



•With regard to the prevention of these troubles, what- 

 ever their cause, our recommendation of last year can be 

 repeated with much stronger emphasis. The best land for 



