212 INSECTICIDES, FUNGICIDES, AND WEED KILLERS. 



preventinjf the potato disease. However, numerous ex})eriments 

 with this end in view gave less decisive and less concordant results 

 than with mildew of the vine, but there is now no doubt that the 

 methods used to prevent mildew of the vine are equally efficacious to 

 avoid the Pltj/topJithora. Petermann, after three treatments, only ob- 

 tained 85"960 kilogrammes of potatoes instead of 46"370 on an un- 

 treated plot amongst which there were only 2-5 per cent of diseased 

 potatoes against 11'3 per cent in the check plot. Montanari found 

 that there were as many diseased tubers in the fields, sprayed with 

 0-25, 0'5, and 1 percent solutions of blue vitriol, as in those untreated. 

 These results show that the method of application has a great influence. 

 Frank and Kruger have shown, on the contrary, that a general \n^e- 

 ventive and judicial treatment of the potato increases its assimilative 

 capacity, which is manifested by an over-production of potatoes rich 

 in starch, but that it is necessary to choose other compounds than 

 blue vitriol to attain this end, the scorchings occasioned by sprayings 

 of blue vitriol, to which the potato is more sensitive than the vine, 

 being often the cause of smaller crops of tubers. 



To produce this favourable result it must not be forgotten that the 

 infection of the potato may be produced either by conidia, brought on 

 to the leaves by the wind, or by previous infection of the tubers. To 

 be complete, therefore, the treatment ought to be double. (1) Dis- 

 infection of the seed tubers, four to five weeks before planting, so as to 

 prevent the propagation of the disease by the tubers. (2) Eepeated 

 preventive spraying with copper preparations against the aerial in- 

 vasion. The cupric treatment which can prevent the rapid progress 

 of the disease, and its extension in potato fields, by annihilating the 

 conidia produced during summer, can in no way destroy the mycelium 

 which lives in the interior of the leaves and the stems, nor prevent 

 the progress of the disease on the infested tubers. The treatments 

 must therefore be preventive and capable of killing the conidia or the 

 zoospores which fall on the leaves, so as constantly to oppose the inva- 

 sion of the plant by this disease. By observing all these necessary con- 

 ditions the cupric treatments are crowned with the same success as 

 the preventive treatment for mildew of the vine. This point will be 

 dealt with more fully in describing the treatment of rhyUyphtJiora by 

 bouillie bordelaise. This latter, used judiciously, is so superior to blue 

 vitriol that the latter has been abandoned. Sorauer reeommends 

 " cupric-sulfosteatite," a mixture of 10 per cent of blue vitriol and 

 90 per cent of talc, to combat the disease of the tomato. This pre- 

 paration is added to the manure and sprayed at intervals of four to 

 five days, and after transplanting every eight to ten days. Care must 

 be taken not to use too much, for it may scorch the plant. Used 

 against the potato disease, sulfosteatite has given results varying from 

 one species to another. Whilst Liebscher declares that its use lowered 

 the crop 31 per cent and Steglich found it without action, Schoyen, 

 Hollrung, and Strebel obtained in many cases superior yields up to 

 26' 3 per cent. All are, however, agreed that its i-esults are not so 

 regular as those obtained with neutral bouillie bordelaise. 



Peronosj)ora viticola, De By. (mildew of the vine). — Owing to 



