FUNGI 248 



and, just as the general effect of spraying may vary greatly from 

 year to year, so also does the relative effect of different spray- 

 fluids. Different varieties of potatoes, too, often show marked 

 differences in their behaviour towards spraying in different 

 seasons : thus, in 1896, out of seven varieties used in some 

 spraying experiments, the effect was considerable in the case 

 of only two of them ; but in the preceding season these saire 

 two varieties were amongst those which had been affected least 

 by the spraying. 



The effect of spraying may be measured in three ways : by 

 the appearance of the haulms, by the percentage of diseased 

 tubers in the total crop, or by the yield of sound tubers. The 

 last of these is the only sound and practical basis of measurement. 

 The appearance of the haulms is generally most misleading, 

 both as to the amount of disease in the tubers, and as to the 

 magnitude of crop of sound tubers. Thus, in 1912 the following 

 results were obtained 



Relative amount of disease Crop of 



according to the sound 



Haulms. Tubers. Tubers . 



No spraying . . . zoo 1 ioo 2 100 



One spraying ... 85 87 108 



Two sprayings . . 43 67 109 



Three sprayings . .26 62 114 



So that, whilst according to the haulms, three sprayings had 

 reduced the amount of disease by three-quarters (74. per cent.), 

 the actual reduction of the disease in the tubers had only been 

 38 per cent., and the increase in crop of sound tubers was as 

 small as 14 per cent. 



As a consequence of such results, the comparative effect of 

 different fungicides must never be judged by the appearance 

 of the haulms : thus, in 1910, certain results with ordinary 

 Bordeaux mixture and with Bordeaux paste gave 



No spraying 

 Mixture . 

 Paste 



so that, according to the haulms, the mixture had made a much 



1 Eighty per cent, of the total leaf surface was diseased. 



2 Fourteen per cent, of the total crop was diseased. 



3 Ninety per cent, of the total leaf surface diseased. 



4 Twenty-seven per cent, of the total crop diseased. 



