Pethybridge and Laffekty — Dry-Rot of the Potato Tuber. 209 



were made. Into the exact details of all of these space does not permit us to 

 enter, and a general account of them must suffice. 



In every infection experiment undertaken controls were used, consisting 

 of tubers similar to those used for inoculation, and treated similarly to the 

 latter, except that no actual inoculation of them was made. In every case 

 these control tubers remained sound and free from rot. 



The general method adopted for practically all of the inoculation experi- 

 ments consisted, first, in carefully selecting healthy tubers with undamaged 

 skins. These were then well washed in water, and next steeped for a short 

 time in a weak solution of formaldehyde. Finally, they were washed in 

 sterile water and then allowed to dry. The inoculating material was 

 introduced through wounds specially made with a sterile scalpel. A more or 

 less tetrahedral or conical cut was made into the tuber, and the cone of 

 tissue, still attached by a small portion of skin at its base, was everted. The 

 inoculum was placed in the bottom of the cavity thus produced ; the apex of 

 the cone cut off and its base closed clown on the inoculum. By this means a 

 gaping wound was avoided, and the chance of contamination from stray 

 spores minimized. 



A positive result was only regarded as having been obtained when the rot 

 produced was extensive and progressive. In such cases the whole tuber 

 ultimately became rotten, if kept long enough. Cases where a certain amount 

 of rot at first proceeded from the inoculation wound, but did not progress 

 further owing to the formation of a layer of cork cutting off the infected area, 

 were regarded as negative. 



The inoculated tubers were kept in covered glass dishes, at least for a 

 short period after the inoculation had been made, at laboratory temperatures. 

 Later on the lids were sometimes removed, and, when once the rot had 

 definitely started, its rate of progress did not appear to be affected in any 

 very marked degree by varying conditions of dryness or moisture. 



Out of a total of 848 inoculated tubers there resulted in 577 eases 

 (i.e. sixty-eight per cent, of them) complete rotting of the tubers. The failure to 

 cause rot was sometimes due to non-maturity of the tubers used, or to the 

 variety used being more or less resistant. In the total mentioned we have 

 included all the trials made, and these factors are of considerable importance 

 in deciding whether infection will occur or not. One hunded and eighty-two 

 tubers were inoculated at various times with the strain of F. cwruleum obtained 

 from Amsterdam, and in eighty-four of them (i.e. forty-six per cent.) the 

 typical rot was produced. 



It was not considered necessary in more than a few cases to complete the 

 detailed proof that the fungus used for inoculation purposes was in reality the 



SCIENT. PROC. R.D.S., VOL. XV., NO. XXI. 2 h 



