154 INTERNATIONAL CONGRESS OF VITICULTURE 



While the toxic effect of alkaline washes is not very serious, as would 

 be inferred from the general silence of the literature, it is sufficiently marked 

 when it occurs, as on the Clinton for example, to arrest the attention of a 

 keen observer. The intervenium portions of the leaf are the most sensitive 

 and the affected tissues in older leaves not infrequently fall away, which 

 gives the injured foliage a somewhat ragged appearance, while the recently 

 expanded leaves are not only scorched but more or less distorted and curled. 



The fact that alkaline washes are injurious while "acid" and "neutral" 

 washes are innocuous indicates that the notion, especially current in the 

 horticultural literature of the United States, that increasing the lime content 

 of a given Bordeaux mixture reduces its injuriousness is not, however 

 justifiable it may be in some instances, a practice to be universally recom- 

 mended; in the case of the grape vine, injury as I have indicated, may 

 result. The injurious action of the alkaline Bordeaux mixtures, however, is 

 hardly sufficiently marked to warrant disregarding these washes should they 

 be found to possess valuable compensating qualities. 



IV. 



The literature is unanimous in saying that "acid" Bordeaux mixtures are 

 more efficient than "neutral" or alkaline washes. It should, however, be 

 noted that the literature grounds the distinction on the more rapid solubiliza- 

 tion of the copper in "acid" Bordeaux mixture and not on the ground of 

 greater toxicity of the unit copper. We must, therefore, in deciding whether 

 an "acid" Bordeaux mixture is more efficient than a neutral or alkaline wash 

 take under consideration the time when they severally become active and 

 not the total time throughout which they remain active. 



Let us first consider the relative rapidity with which the copper becomes 

 toxic in "acid," "neutral" and alkaline Bordeaux mixtures. In order for any 

 copper wash to have a fungicidal value the copper must be in the form of 

 a soluble salt, or, if insoluble when applied, become slightly soluble on 

 weathering. In the case of Bordeaux mixtures, the copper is always insolu- 

 ble when applied, 5 but gradually yields on weathering small amounts of 

 soluble copper. The rapidity with which the copper becomes soluble will 

 depend on the degree of alkalinity of the mixture. The excess of calcic 

 hydrate present must be carbonated before any copper can go into solution, 

 but it seems doubtful that the interval between the time of appearance of 

 soluble copper in mixtures of various degrees of alkalinity is quite so great 

 as Millardet and Gayon6 found in one of their experiments. These authors 

 obtained the results shown in the following table: 



r >It should be noted that "acid" Borbeaux mixtures may contain soluble 

 copper. 



6 Millardet, A. and Gayon, U. Les divers precedes de traitement du 

 mildiou par les composes cuivreux. Journ. Agr. Pratique 1, 1887. 



7 Millardet, A. and Gayon U., loc. cit., ante. 



