COPPER SULPHATE (BLUE VITRIOL). 213 



its purely aerial propagation the mildew of the vine is much easier to 

 destroy than the pliytophtliora. It suffices to prevent the disease to 

 stop the zoospores from germinating by poisoning them. There can 

 be no question of destroying the mycelium in the tissue of the leaves, 

 and if the disease has invaded the plant the treatment must be con- 

 fined to preventing the propagation of the disease to neighbouring 

 plants by the conidia coming from the plant attacked. Cupric prepara- 

 tions, by depositing on the leaves an adherent and slightly solub'e 

 layer, creates an insurmountable barrier to the spores of this fungi. 

 As 2-3 ten-millionths of this salt soluble in the dew suffice to prevent 

 the zoospores from germinating, it will be seen that the layer of the 

 cupric compound lying on the leaves will always cede this amount to 

 the water deposited on the leaves. The most active and efficient pre- 

 parations are, therefore, those which possess a very weak solubility 

 with perfect adherence. Blue vitriol is the copper salt which reahzes 

 these conditions to the least extent ; it has therefore been replaced by 

 the bouillies which give much better results and never injure the vine. 

 When in 1884 it was seen in Burgundy and Maconnais vineyards, 

 ravaged by the Peronospora, that certain stocks preserved a normal ap- 

 pearance, it was found that those which were exempt from the disease 

 were attached to new props, impregnated with blue vitriol to preserve 

 them. This observation was communicated to the Academy of Science 

 by Perry, who observed it himself in the department of Saone et Loire. 

 Experiments were made the following year by Paulin, using vitrio- 

 lized props throughout a whole vineyard, and by Pi'osper de Lafite, by 

 simply steeping the ropes of straw or willow skins in a bath of blue 

 vitriol. But these experiments wex'e soon followed by a bolder and 

 more rational treatment, consisting of spraying with more or less dilute 

 solutions of Ijlue vitriol on the foliage of the vine. These treatments, 

 which enabled the invasion of mildew to be victoriously contended 

 against, were, however, far from giving entire satisfaction. The pro- 

 portions of blue vitriol used in the beginning were from 1-15 per cent. 

 In 1885 Muntz recommended the treatment of this disease by 10 per 

 cent solutions of blue vitriol. But there followed scorchings of the 

 leaves ; it is true they were not serious, but all the same alarmmg. 

 The doses were reduced, owing to these mishaps, to 3 per cent, 

 then to 1 per cent, and then by Bouchard and Beaume to 0'3 per 

 cent. With this dose Ricaud got the same preservative result as with 

 the larger doses. In spite of this small dose of blue vitriol the vines 

 showed the same characteristic burning, especially when the spraying 

 was done in hot, warm weather, because the drops of this dilute solu- 

 tion deposited on the leaves concentrated rapidly, and then acted like 

 a concentrated solution. Blue vitriol, used against vine diseases, pos- 

 sesses two defects : (1) injuring the foliage, (2) possessing no adher- 

 ence on the leaf ; rain removed all the blue vitriol deposited thereon. 

 To obtain with these solutions the same effect as with bouillies, it 

 would be unceasingly necessary to multiply the treatments, and in 

 these conditions the advantage of blue vitriol being cheaper, as compared 

 with bouillies, entirely disappears. Blue vitriol sprayings have also the 

 drawback of leaving no visible trace on the leaves, whilst bouillies 



