REPORT OF COMMITTEE ON PUBLICATION 155 



Table III. Effect of increasing the alkalinity of Bordeaux mixture on the 

 time of appearance of soluble, copper. 



Cupric Sulphate Ratio Soluble Copper 



Taken Cupric Sulphate Present 



% Calcic Oxide After 



1.6 100:21.0 5 days 



1.5 100:22.4 7 days 



1.5 100:44.8 12 days 



1.5 100:89.6 13 days 



1.5 100.179.2 18 days 



Though the rapidity with which the copper in alkaline Bordeaux mix- 

 tures becomes soluble will of necessity always be less than in "acid" 

 mixtures, as no soluble copper can exist in the presence of calcic hydrate, 

 it seems however, quite unlikely that the retardation in its appearance is 

 as material as is sometimes thought. To be sure Millardet and Gayon" 

 observed that Bordeaux mixture spots might remain alkaline for five or six 

 six weeks in summer and the data presented in the above table unmistak- 

 ably show that increasing the lime content of a Bordeaux mixture beyond 

 the amount required to produce a neutral mixture delays the appearance of 

 soluble copper. Nevertheless alkaline Bordeaux mixtures lose their alka- 

 linity usually quite promptly, though the amount of spray applied per square 

 meter as well as the degree of atmospheric moisture prevailing at the time, 

 particularly the latter, are factors of paramount importance, as a glance at 

 the following table will show: 



Table IV. Time required for 1 per cent Bordeaux mixture 1:1 to become 

 neutral in dry air and in air saturated with water vapor. 



Amount of Spray Used Time Required to Reach Neutrality 



per 1000 cm2 In Moist Air In Dry Air 



0.108 grm 5- 7 hours (8) 



0.263 grm 11-13 hours 



From the -data presented in the above table we gather that the rate at 

 which Bordeaux mixture 1:1 becomes neutral is proportional to the amount 

 applied per square meter even under very favorable conditions for carbona- 

 tion, whereas in absolutely dry air little or no change occurs on long stand- 

 ing irrespective of the quantity used. It is, therefore, clear that the effective- 

 ness of alkaline Bordeaux mixtures will much depend on the weather condi- 

 tions prevailing during the interim between the time of their application and 

 the period at which protection must be afforded. 



While it cannot be questioned that "acid" Bordeaux mixtures are more 

 efficient than alkaline washes when immediate action is required, as in the 

 case of black-rot (Guignardia bidwellii) when the spraying has not infre- 

 quently to be carried out during or just prior to spore distribution and 

 germination, this advantage should not in itself be considered of capital 

 importance in preventive spraying, for alkaline Bordeaux mixtures are as 

 efficient as "acid" washes as soon as they have become carbonated. The 

 copper in an "acid" Bordeaux mixture and in a carbonated alkaline mixture 

 of the same age dissolve at approximately the same rate as the data pre- 

 sented in the following table shows: 



8 The experiment was discontinued at the end of 14-64 hours. 



