16 PROTECTION OF PLANTS, 1920-21 



Lime- Sulphur. — Lime sulphur has already been mentioned among the 

 insecticides, but it is also one of the best fungicides. For a time after the dis- 

 covery of Bordeaux mixture it was discarded, but in 1906 its importance as a 

 fungicide was rediscovered, and ever since it has been employed extensively for 

 the control of many fungous diseases of the orchard. 



In Nova Scotia, however, the fruit growers observed that the calyx spray 

 and especially the following spray were responsible for a heavy fall of fruit on 

 account of its action on the leaves. For this reason they substituted in 1918 and 

 1919 a modified Bordeaux for the lime-sulphur. 



Lime-sulphur is in the main a mixture of polysulphides of calcium. The 

 fungicidal properties of lime-sulphur lie in the free sulphur that is formed on the 

 surface of the leaves and fruit on the evaporation of the water and the oxidation 

 of the polysulphides. 



When trees have been drenched with the mixture, injury is liable to occur, 

 but under proper conditions little or no injury follows, except in the case of 

 potatoes, American grapes and some varieties of peaches. 



Bordeaux Mixture. — From the time of its discovery by Prof. Millardet, 

 of France, in 1883 up to 1910, Bordeaux mixture was the standard fungicide for 

 summer spraying. But with the rediscovery of lime-sulphur in 1906 as a sum- 

 mer spray, Bordeaux has been relegated to second place in commercial orchard- 

 ing on account of the russetting of the fruit and the yellowing of the foliage that 

 followed the standard application, made according to the formula 4 :4:40. 



The stucture and composition of Bordeaux mixture has been recently, 

 ascertained. It consists of colloidal membranes composed of a complex mix- 

 ture of basic copper sulphates, enclosing a solution of calcium hydroxide and 

 calcium sulphate holding lime particles in suspension. The fungicidal prop- 

 erties depend on the number and size of the colloidal membranes and on the lime 

 particles. The smaller the membranes, the more effective does the mixture 

 cover the sprayed surface. When evaporation of water occurs, the minute 

 membranes dry down and attach themselves firmly to the surface. Then the 

 copper in the membrane is slowly dissolved when the leaves become moist, 

 and either kills or inhibits the germinating spores which lodge on the leaf. More- 

 over, it is believed that the lime particles are also fungicidal. 



It will be seen, therefore, that the quality of the Bordeaux depends upon 

 the method of preparation. 



The minute membranes will never be formed when concentrated solutions 

 of Copper sulphate and milk of lime are brought together. On the other hand, 

 ,hey will be formed when vigorous stirring takes place when the dilute solutions 



brought together. 



