240 IXSECTICIDES, FUNGICIDES, AND WEED KILLEES. 



working on a field of 0"'i28 hectare, about 1"06 acres, obtained with one 

 spraying :— 



TABLE XLI. — Shoiving the Effect of One Spraying of Bouillie Bordelaise on 

 the Weight of the Crop of Magiium Bonum Potatoes. 



The cost of the treatment came to 3s. per acre, and the net profit to 

 57s. per acre. Mark, who experimented on fifty varieties, got an in- 

 creased weight of crop of from 30-50 per cent. Stroebel found a 

 higher jaeld of 48-7 per cent, and only 0-28 per cent of diseased tubers, 

 against 5-8-23-3 per cent in the untreated field. Honeyball found 

 a greater yield of 7-30 per cent. Finally in America and Switzerland 

 the same effects have been realized, and this treatment is applied on 

 the large scale in these countries as well as in France, in the whole 

 valley of the Garonne, where it gives full satisfaction. Doubts as to- 

 the efficiency of the treatment of the phytophthora by the bouillie 

 bordelaise are now no longer permissible ; too many experiments show 

 that applied at the moment of the evolution of the disease, it com- 

 pletely prevents it from spreading, thus preserving the crop so gi'eatly 

 compromised when the disease spreads normally. It is recommended 

 not to apply the first spraying except at the moment when the first 

 spots of the disease appear on the leaves. But it is better to make 

 a first pi'eventive spraying. The number of sprayings to be practised 

 during the year is only regulated by the prevailing weather. If the 

 year be moist and warm, conditions essentially favourable to its 

 evolution and to very rapid spreading of the disease, it will be well to 

 make several sprayings, only eight to fifteen days apart. During dry 

 periods spraying is useless, and m the case of a very dry year, during 

 which the disease does not assume a disquieting extension, a single 

 preventive spraying may be regarded as sufficient. Scientists do not 

 agree as to whether spraying is beneficial when the disease is not to 

 be feared. Sorauer, an advocate of spraying at the time of the appear- 

 ance of the diseases, has shown that if the infected fields treated in 

 time yield a much greater number of tubers than untreated fields, it 

 is no longer so when a field which is neither threatened nor invaded 

 by the disease is treated. Sorauer experimented on perfectly sound 

 tubers, which gave, before the appearance of the disease, a crop 

 absolutely exempt from diseased tubers, whilst these same plants 

 yielded, after the evolution of the disease, 59-4 per cent of diseased 

 tubers. These experimental fields gave before and after treatment : — ^ 



1 Part of the potatoes of both plots were lifted before the disease appeared. 



