406 PEAR GROWING IN CALIFORNIA. 



No. 19. Hydrocyanic Acid Gas. 



This gas is generated by the treatment of either potassium or sodium 

 cyanide with dilute sulphuric acid in the proportions of 1 ounce of 

 potassium cyanide, 1 fluid ounce of sulphuric acid and 3 fluid ounces of 

 water or | ounce sodium cyanide with H fluid ounces of sulphuric acid 

 and 2 fluid ounces of water. It is necessary to confine this gas in a 

 tight box or house in order to get results. It is very light and rapidly 

 dissipates in the air unless confined. It is very valuable in the fumi- 

 gation of pear trees that are affected with root aphids or scale insects. 

 Being extremely poisonous, the greatest care must be exercised in its 

 use. It is generated usually in an earthenware jar, the water being 

 first put in the jar and the sulphuric acid then poured into the water, 

 after which the weighed amount of cyanide necessary for the particular 

 building is dropped into the solution, with the resultant formation of 

 the hydrocyanic acid gas. 



No. 20. Carbon Bisulphide Gas. 



This gas is sometimes used with nursery stock instead of hydrocyanic 

 acid gas. Being much more expensive and in most cases less effective 

 than the hydrocyanic acid gas it is not generally recommended for use 

 in. the fumigation of nursery stock. The gas is very heavy, flowing like 

 water, and if used it must be remembered that it is so heavy that it will 

 settle to the bottom of any box or house in which it is used. Thus it 

 becomes necessary to use a sufficient amount to practically fill the box 

 or house, providing that same is filled with trees. 



No. 21. Whitewash. 

 Wickson* recommends the following formula for whitewash : 



Lime 30 pounds 



Tallow . 4 pounds 



Salt 5 pounds 



Water sufficient to make it flow well. 



In cases where rabbits are troublesome to trees the salt should be 

 eliminated, as it has a tendency to attract the rabbits, which frequently 

 injure the trees by eating the bark from the trunks. In place of salt it 

 would be well to use a small amount of commercial aloes dissolved in 

 the whitewash, as this material is exceedingly distasteful to rabbits. 

 One pound of aloes to 5 gallons of whitewash would be sufficient to give 

 good results. 



MIXING INSECTICIDES AND FUNGICIDES. 



The pear tree is subject to the attack of so many different kinds of 

 insect pests and diseases that at times it is practical and economical to 

 mix certain insecticides in order that more than one pest may be con- 

 trolled, or to mix an insecticide and a fungicide in order that certain 

 insect pests and fungous diseases may be controlled. For example, it is 

 always customary in sections where pear scab and codling moth are 

 troublesome on pears to mix arsenate of lead with Bordeaux mixture, 



* "California Fruits," seventh edition, page 85. 



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