FUNGI 



245 



From every point of view it is probably better to use small 

 doses, and repeat the sprayings, than to use a double or treble 

 dose in one application. According to the principles governing 

 fungicidal action established by the Woburn results (see p. 192), 

 an increase in the dose beyond a certain point produces but a 

 comparatively small increase in the effect, and that was borne 

 out by the results on potato spraying itself, where the benefit 

 from a single and double dose amounted to 14-5 and 15 '5 per 

 cent., respectively (XIV, 15). There is also a very strong reason 

 for increasing the number of applications in the fact that the 

 effect of a single spraying does not generally last for more than 

 three or four weeks, and may have worn off before the disease 

 makes its appearance. 



Another set of experiments which afford interesting evidence 

 on the subject of potato spraying were made in 1912, when 

 potatoes were grown in sixty plots which had been manured 

 differently for the previous sixteen years': one' half of each 

 plot being sprayed, and the other half not. The ground was 

 not specially prepared for the crop, and the yield was conse- 

 quently a small one, but the effect of the spraying probably 

 as a result of the smallness of the. crop was very great. A 

 summary of the results is shown in the following table. 



Taking the results in the unsprayed plots, it will be seen that 

 manuring had, as might have been anticipated, a notable effect 

 on the total yield (col. I), but that, contrary to what might have 

 been expected, the effect was the same whether the manure was 

 dung or artificials. On the other hand, dung and artificials had 

 had a very different effect on the amount of disease present, for 

 artificials had not increased the proportion of diseased tubers at 

 all (col. II), whereas dung has increased it four- and eight-fold, 



