INSECTICIDES AND FUNGICIDES. 23 



Chemical and Physical Properties. — When the dissolved copper sulfate 

 and milk of lime are poured together, a reaction takes place between them 

 resulting in the formation of a voluminous, gelatinous colloidal precipitate 

 which does not settle rapidly to the bottom, but remains evenly distril)uted 

 throughout the liquid and begins to settle only after standing undisturl:)ed 

 for several hours. As seen under the microscope it is a mass of very thin 

 precipitation membranes, each in the form of a minute closed bag. After 

 a few hours the gelatinous precipitate gradually becomes crystalline, the 

 copper salt then appearing in the form of blue sphserocrystals (32) which 

 do not remain afloat but settle to the bottom. In the best mixtures these 

 membranes are most fully and abundantly formed, and as a result they 

 "stand up" longest. In poor mixtures they settle to the bottom quickly. 

 The ability of the mixture to perfectly cover the surface of the plant and 

 to adhere tenaciously is dependent on the thoroughness of development of 

 these precipitation membranes. It is therefore customary to gauge the 

 excellency of the mixture to no inconsiderable extent by the length of 

 time required for the precipitate to settle. Since even the best of mixtures 

 will begin to settle after a few hours, it is essential that, to get the l^est 

 results, Bordeaux be applied when freshly prepared. In the 4-4-50 Bor- 

 deaux there is a considerable excess of lime (as calcium hydroxide). When 

 the spray dries on the leaves, the membranes which are spread over the 

 surface conform tightly to every irregularity, much as a piece of thin wet 

 tissue paper does when dried on a flat surface, and are not washed away 

 readily by rains or removed by winds or other agencies. Bordeaux mixture 

 surpasses all other fungicides in its ability to adhere to the host. Copper, 

 its only active fungicidal agent, is in these dry membranes in a form almost 

 insoluble in pure water. Since it cannot affect the fungus in an insoluble 

 form, it must be brought into solution by some other agency. The follow- 

 ing agencies have been found more or less active in this direction : — 



1. Carbon dioxide from the air or from the plant, in solution in dew or rain 

 drops, very gradually brings the copper into solution after the excess lime has been 

 carbonated. 



2. Ammonia and nitrous or nitric acids, present in small amounts in rain water, 

 cause some solution of the copper. 



3. Organic substances such as sugars, excreted in small quantities from the host 

 cells, bring about verj' gradual solution. 



4. There is some evidence that excietions from the fungus itself biing into solu- 

 tion enough copper to kill it. 



Bordeavx Injury. ■ — Bordeaux mixture falls short of the requirements 

 of a good fungicide in that it frequently causes injur}' to the plant. The 

 copper, brought into a soluble form in one or more of the waj^s enumerated 

 above, enters the tissues of the plant directly through the epidermal walls 

 by a process of osmosis, or through the stomates, lenticels or wounds. 

 The invaded cells are killed by the toxic copper. On leaves this results in 

 definite dead spots or irregular areas on margins or tips. More or less 

 defoliation may result on fruit trees. On fruits, the death of some of the 



