CONTROL OF ROOT FORM OF WOOLLY APHIS. 



33 



INJURY TO THE TREE. 



Apple trees in the first year of growth were treated at the height 

 of the growing season with varying dosages of carbon disulphid in 

 water. 1 One ounce of carbon disulphid to 4 gallons of water caused 

 considerable injury, as evidenced by the drying of the foliage, but 

 did not kill the tree, while 2 ounces to 4 gallons of water killed the 

 tree in three days. 2 Since one-half ounce of carbon disulphid to 4 

 gallons of water is ample for control there is no necessity for increas- 

 ing the strength. 



In the same way apple trees in their second year of growth were 

 treated with varying dosages of carbon disulphid in water. 1 One 

 ounce carbon disulphid to 4 gallons of water caused no injury, 1^ 

 ounces caused con- 

 siderable injury, and 

 2 ounces killed the 

 tree outright in 10 

 days. 3 



The increased re- 

 sistance of the 

 older trees is ex- 

 plained by the 

 greater area in- 

 volved by the root 

 mass, a large part 

 of which was not 

 subjected at all to 

 the fumes of the 

 carbon disulphid. 



During the ex- 

 tended field tests 

 about 500 apple trees of varying ages were treated at the rate of 

 one-half ounce of carbon disulphid to 4 gallons of water and no 

 apparent injury resulted. 



Injury to the tree by carbon disulphid in large doses is first in- 

 dicated by a drying of the foliage. This, however, is merely direct 

 evidence that the small roots and rootlets have been injured, thus 

 cutting off the normal supply of water to the leaves and causing 

 them to turn brown. With medium doses rootlets killed by carbon 

 disulphid are rapidly replaced by the otherwise unimpaired root 

 system. With the dosages of one-half ounce to 4 gallons of water 

 the rootlet injury is negligible, and no killing of the foliage or check 

 in growth will result. 



Fig. 1. — Diagram illustrating method of determining the lateral 

 diffusion through the soil of carbon disulphid in water. For 

 explanation see p. 32. (Original.) 



1 Liquid employed at rate of three-fourths gallon per square foot of soil area treated (p. 31). 



2 Diameter of treated areas in these tests 30 inches. 

 3 Diameter of treated areas in these tests 48 inches. 



