﻿COPPUR SPRAYS ON IRISH POTATO TUBERS. 21 



copper for all four varieties of potato plants. The three early vari- 

 eties of potato plants contained more copper than the Green Moun 

 tain, a late variety. An appreciable quantity of the copper \v;is 

 present in all of the check plants. 



GENERAL DISCUSSION. 



In the case of copper sprays it is generally recognized that a small 

 quantity of copper is gradually rendered soluble either by the juices 

 of the plant or by the carbon dioxid of the air, and this copper thus 

 rendered soluble may protect the plant and may stimulate its meta- 

 bolic activities. The intensity of action of the copper compounds 

 varies with the kind of plant used and with the quantity of copper 

 applied. This difference in the ability of plants to withstand the 

 action of copper sprays was illustrated by the drastic effect of Pick- 

 ering sprays (9) on the grape and apple compared with its favorable 

 action on the potato and cranberry. The influence of environment, 

 soil, climate, etc., must also be considered in this connection, as a 

 Bordeaux spray may be used on a certain plant in one section of the 

 country but can not be used without severe injury on the same plant 

 in another locality. The data show that increased growth and tuber 

 formation of the potato following the use of copper sprays may be 

 secured. The fact that a certain amount of hopperburn was present 

 in most of the fields where these tests were conducted is recognized. 

 It was not severe in any case and practically none was observed on 

 the plants at Arlington Experimental Farm. Some of the increased 

 yield of tubers from coj)per-sprayed potato plants was undoubtedly 

 due to the action of the copper sprays in controlling potato leaf- 

 hoppers and thereby reducing hopperburn. It is difficult to explain 

 it all on the basis of protective action alone. 



Analyses of grapes from copper-sprayed and from unsprayed vines 

 reported by the writer (9) showed that the composition of the grape 

 had apparently been altered by the application of the copper sprays. 

 Evidently copper sprays alter the composition of the potato and the 

 grape and there is no reason to believe that their action is restricted 

 to these two plants. 



Several theories have been advanced to explain the increased yield 

 of tubers from Bordeaux-sprayed potato plants : 



(a) Bordeaux spray increases the transpiration rate of potato 

 plants, according to Lutman, Duggar, and others. This effect would 

 apparently be an advantage in wet seasons or wet localities but a dis- 

 advantage in dry seasons or dry localities. Differences in humidity 

 cause differences in the transpiration of plants, and this may react 

 on the growth of the plant and the composition of the tubers. 



(b) It is possible that changes in the rate of respiration or in the 

 general metabolism of potato plants may follow the application of 

 copper sprays. A small quantity of copper may be absorbed by the 

 plant and stimulate it to increased activity. There are several ex- 

 amples of stimulation brought about by a small amount of a sub- 

 stance which in large quantities is toxic. There is evidence that 

 copper stimulates some plants and the fact that stimulation can not 

 be shown to exist does not prove that it is not there. Analyses of 

 the leaves, vines, and roots of sprayed and unsprayed potato vines 

 in most cases showed a higher proportion of copper in the leaves. 



