30 DISEASES OF ECONOMIC PLANTS 



Bordeaux mixture is dangerous to the foliage, while a 

 slight excess of lime is a point of safety. It is therefore 

 advisable, unless it is certain that sufficient lime of good 

 quality is used, to apply a test in this regard. If an excess 

 of lime be present in the mixture, the breath expired upon 

 it for a few moments, as held in a saucer or cup, will cause 

 a flaky film to form upon the surface ; if no such fibn forms, 

 more lime is needed. If an excess of copper sulphate is 

 present and a bright piece of iron or steel, a knife blade 

 or nail newly filed or sanded, be held in the solution for 

 a few moments, a distinct deposit of metallic copper will 

 appear upon the bright iron. It is well to practice each 

 of these tests with solutions known to have excess of copper 

 and lime respectively, to thus become familiar with the 

 working of the test. No metal cans or utensils should be 

 used in preparing Bordeaux mixture. 



Soda or potash Bordeaux mixture. — While the usual 

 lime-Bordeaux mixtures are very successful in preventing 

 plant diseases, slight difficulty is sometimes encountered in 

 using them, owing to the occurrence of small stones, im- 

 perfections in the lime. These, if not thoroughly strained 

 out, clog the spraying nozzle and are troublesome. To 

 escape this difficulty and to render the preparation and 

 apphcation of the Bordeaux mixture easier, as well as to 

 give a mixture which can be used as the fruit approaches 

 maturity, without spotting it, use of potash or soda as 

 a substitute for the lime has been recommended. Soda 

 is most acceptable for this purpose. For general use there 

 is no indication that these mixtures will supersede the 

 ordinary lime-Bordeaux. 



