72 PROTECTION OF PLANTS — 1922-23 



Arsenate of lead is very widely used in Bordeaux mixture, the theory being 

 that it is mere adhesive than other poisons. The lime and the copper in Bor- 

 deaux break away two-fifths of the arsenate from ordinary hydrogen arsenate 

 of lead to form either calcium arsenate or double copper calcium arsenate. The 

 remainder forms basic lead arsenate. Lead arsenate is a very slow killer in Bor- 

 deaux. The sticking value of arsenate of lead in a Bordeaux is so surpassed by 

 the superior sticking value of the Bordeaux itself as to make the sticking value 

 of the arsenate of lead negligible in the combination. 



Paris Green is still the favorite in many sections of the country as a Bor- 

 deaux poison. After all the trouble that the manufacturer has in forming a 

 nice green copper aceto arsenite crystal, the Bordeaux extracts the acetic acid 

 to form either calcium acetate or copper acetate and leaves arsenite the same as 

 we get from pouring arsenite of soda into a copper sulphate solution. 



Arsenate of lime is probably the ideal poison for use in Bordeaux. It is 

 low in cost, the copper acts very slightly on it, if at all, and it should give as good 

 or better killing than any other arsenate in Bordeaux. 



Lime Sulphur and Arsenicals. 



Lead Arsenate is the most popular poison in lime sulphur solution. We 

 all know that a reaction takes place between the two. Various competent chem- 

 ists have explained this reaction to us in various ways. We know that there is 

 a reaction. If we puddle our lead arsenate with calcium caseinate we reduce 

 this reaction. If we puddle our lead arsenate with lime we do the same. We do 

 not know if an excess of lime is desirable in lime sulphur since after a short time 

 we have to depend on the SO2 formed from precipitated sulphur as our fungi- 

 cide, and we would expect the excess of lime to neutralize this SO2 and so reduce 

 our fungicidal value. 



Some arsenate of lead which contain certain spreaders, stickers, and sus- 

 pending agents, react much more completely with lime sulphur solution than 

 straight lead arsenate. Recent knowledge regarding the manufacture of cal. 

 cium arsenate helps to explain this reaction. 



Arsenate of lime has been quite widely used in lime sulphur and I believe 

 that the time is coming when we will all be recommending arsenate of Hme in 

 lime sulphur, but our present arsenates of lime vary too much for this recom- 

 mendation to be made at the present time. The addition of a couple of pounds 

 of high calcium lime for each pound of arsenate of lime in Kme sulphur makes 

 it safe, but we are there up against the danger of cutting down the fungicidal 

 value of our lime sulphur. 



