1890.] PUBLIC DOCUMENT — No. 33. 219 



red varieties were available for the comparison. 3. The 

 potatoes raised on barn-yard manure were markedly more 

 scabby and more deeply scabbed than the rest. 4. Tobacco 

 dust in the drill had no appreciable efiect in increasing or 

 diminishing the scab. 5. Scabby "seed" produces a crop 

 neither better nor worse than that grown from smooth 

 potatoes. None of these results are new, but they may serve 

 as further material on which to base general conclusions, and 

 as confirmatory of the results of most previous similar experi- 

 ments. But all such results are comparatively without 

 significance, so long as the cause of the troul)le remains 

 unknown, and we are as much as ever in the dark, so far as 

 any basis of rational experimentation or treatment is con- 

 cerned ; therefore the most attention has been given to the 

 study of the development of the scab. 



From the time when tubers began to be formed till the 

 crop was dug, plants were taken up at intervals, and care- 

 fully examined. The first suspicious spots were found on 

 some small tubers June 20, and the first unmistakable scab 

 was noticed on the 28th. After this time abundant specimens 

 were obtainable. It is worthy of note that the first examples 

 of afiected tubers were obtained from sections 1 and 14, on 

 which barn-yard manure was used, and that they always 

 furnished the most and scabbiest material. 



The scab always begins in very small spots, and spreads 

 from these. When quite small, the spots usually show dark- 

 brown centres from which the lighter marginal portions seem 

 to have spread. These dark central spots mark the posi- 

 tion of the lenticels of the tuber, in which the disease 

 originates. The microscopic structure of the diseased spots 

 is the same at all stages of their development. The first 

 suspicious spots, detected June 20, on very young tubers, 

 proved, on microscopic examination, to be young scab-spots, 

 and could not be distinguished in minute structure from the 

 large patches on a full-grown tuber. The characteristic 

 change which produces the appearance and condition known 

 as scab consists in the browning, drying and shrivelling of 

 the walls of a few layers of the surface cells of the tuber, 

 which produces a hard and rough crust. The difierence 

 between a very small spot and a large patch of scabby surface 



