246 SCIENCE AND FRUIT GROWING 



according to the heaviness of the' dressing ; as a consequence, 

 the yield of sound tubers (col. Ill) was less in the unsprayed 

 plots dressed with dung than in those dressed with artificials, 

 being, indeed, not much greater than in those with no dressings 

 at all. 



Passing on to the results in the sprayed sections, it will be 

 seen that the spraying had in every case increased the weight of 

 diseased tubers present (col. V, compared with II), though it 

 had increased the total yield (col. IV, compared with I) still 

 more, so that there was a large balance to the good of sound 

 tubers as a result of the spraying (col. VI, compared with III). 

 When the percentage, instead of the actual weight, of diseased 

 tubers in the crops is examined, it is found that the increase had 

 occurred only in cases where the proportion of disease was low ; 

 where it was high, as in the dunged plots, no increase had 

 occurred. 



Percentage of diseased tubers. 



Unsprayed. Sprayed. 



No manure .... 7-1 11-2 



Artificials, light . . .5-6 11-2 



heavy ... 5*5 H'9 



Dung . . . . . 28-3 28-4 



Just as the prejudicial effect of spraying, in increasing the 

 proportion of disease, depends on this proportion being otherwise 

 low, so the beneficial effect, in increasing the total yield of sound 

 tubers, depends on the lowness of this yield. In this series the 

 increase was phenomenally large 150 per cent. and the yield 

 was phenomenally small 3 tons to the acre in the unsprayed 

 plots; whereas in two other series carried out in the same 

 season elsewhere, and in which the yield was eleven tons to the 

 acre, the effect of spraying was to produce an increase of only 

 9 and 19 per cent., respectively. An examination of the various 

 results in other seasons (XIV, 27) helped to confirm this view : 

 but no connection could be observed between the benefit pro- 

 duced and the amount of disease in the crop : taking the two 

 instances out of eight series where the benefit was least, in one 

 of them the percentage of disease was the lowest recorded, in 

 the other, the highest. 



From these facts, it would appear that the whole question 

 of potato spraying should be considered from a wider point 

 of view than that generally adopted. Spraying tends to kill, 

 not only the fungus which causes the potato disease, but other 

 fungi as well, and these other fungi are probably important 



