The valuable papers given below, by Professor Johuson of the 

 Agricultural Department of the Sbeflfield Scientific School, Yale 

 College, New Haven, first appeared in the columns of the " Coun- 

 try Gentleman" during the present year. They have deservedly 

 attracted much attention and have been copied into the leading 

 Agricultural Journals of Great Britain, but nowhere will they be 

 read with greater interest and profit ihan here, where the potato is 

 not only the leading root crop, but a staple article of export also : 



THE TRUE CAUSE OF THE POTATO DISEASE. 



BY PROF. S. W. JOHNSON, 



Our heading reads the true cause for the reason that so many 

 false causes have been lafd at the foundation of this disease, that to 

 say simply the cause, would merely imply another, no better, per- 

 haps worse, than those already familiar to the reader and alike 

 unsatisfactory. 



But why is it that we have had so many causes for this disease ? 

 It is on account of the difiSculty of investigating the matter. 

 Strange as it may appear, the true cause is the one suggested first 

 of all, nearly as soon as the evil showed itself It was not how- 

 ever, proven to be the cause. The earliest observers saw the cause, 

 described it, figured it and gave their opinion that it produced the 

 disease, but did not demonstrate the fact. That a parasitic fungus 

 or mould plant was always associated with the potato rot, was the 

 first microscopic observation made. But to the suggestion that it 

 caused the decay of the potato, it was replied by Liebig and his 

 school, the fungus is not the cause but the result of the decay. 

 Liebig's theory of decay and fermentation, at that time a new, 

 plausible, and incontrovertible theory, did not allow a fungus to 

 originate a rot, but only to feed upon it. To this, the fungus the- 

 orists made but lame replies, and other " causes" were shortly dis- 

 covered in appalling numbers. Smee found a sort of a louse or 

 aphis grazing among the fungi, and he decided it to be the cause. 



