14 PROTECTION OF PLANTS, 1920-21 



clog both the spray pump and the nozzles. Moreover, it could be used only 

 for the dormant or semi-dormant stage. 



About 1912, a new method of preparation was discovered, which removed 

 the objectionable features of the old method. It consisted in making a concent- 

 rated solution by boiling together 50 lbs. of fresh stone lime, 100 lbs. of sulphur 

 and 40 to 50 gallons of water. The solution when properly made can be kept 

 for months, and for use can be readily diluted to any strength required for dor- 

 ant and summer spraying. 



The discovery by Sanders and Kelsall of the cause of the heavy drop of 'he 

 fruit after applications of lime-sulphur is worthy of note. They found that 

 when lime-sulphur (1 to 30) was applied only to the upper surface of the leaf 

 no injury followed, but when both sides, or only the lower surface wer6 sprayed, 

 injury was done. When sprayed leaves were examined miscroscopically it was 

 observed that the chlorophyll of the leaf-cells had been acted upon by an appre- 

 ciable amount of lime-sulphur absorbed through the lower surface. They ob- 

 served, however, that no such injury followed applications of Bordeaux and 

 soluble sulphur. 



The excessive dropping of the fruit, they maintain, is due to the injury to 

 the leaves whereby they fail to function as carbohydrate manufacturers for the 

 fruit. The greatest drop occurs after the fourth application, that is, the one 

 given two weeks after the petals fall, and has only become evident since the 

 advent of high-pressure pumps and greater capacity nozzles. 



Another important investigation by Sanders and Kelsall bore on the ques- 

 tion of the influence of fungicides on the killing value of poison insecticides 

 when the two are mixed together. They found that with Bordeaux the aver- 

 age poison is decreased in value by about 50 per cent. With soluble sulphur, 

 the value of the poison is increased from 10 to 15 per cent, and with lime-sul- 

 phur the decrease is about 20 per cent. 



Kerosene Emulsion. — A solution of Kerosene Emulsion has long been a 

 valuable insecticide against sucking insects. The standard formula (Riley- 

 Hubbard) was originated in 1884, although several modifications have also 

 appeared. When properly prepared Kerosene Emulsion can be applied with 

 safety on most plants, but of late years tobacco extracts have taken its place. 



Tobacco Extracts. — Tobacco dust and extracts have been used probably for 

 more than a hundred years for the control of aphis and other sucking insects, 

 but it is only within the last ten years that concentrated extracts of nicotine 

 have been placed on the market. The best known of these are "Black Leaf 40", 

 being a 40 per cent solution of nicotine sulphate, a non-volatile substance, and 



