14 Bordeaux Mixture and Plants 



uninjured leaves, and both he and Crandall and other workers have 

 drawn attention to the fact that, in practice, foliage which is badly at- 

 tacked by insect or fungus pests or otherwise badly injured is specially 

 liable to serious damage by scorching. Pickering also refers to the 

 effect of injuries in intensifying scorching^. 



There is then no doubt whatever that even slight injuries to the 

 leaf cuticle, if they have not had time to dry up, play an important 

 part in determining the extent of scorching, following spraying with 

 Bordeaux mixtures. Summer foliage, known certainly to be un- 

 damaged, as far as our experiments go, shows no scorching. 



In order to explain the increased scorching due to leaf injuries, 

 it is necessary to account for the production of copper in a soluble form. 

 Until lately the view has been generally accepted that atmospheric 

 agencies, and in particular carbon dioxide, are responsible for the pro- 

 duction of soluble copper sulphate from the insoluble basic copper sul- 

 phate which is deposited on the leaf. Both the fungicidal action and the 

 scorching are attributed to the copper sulphate thus formed. It has, 

 however, been shown by one of us^ that from the chemical standpoint this 

 view is not tenable ; the fungicidal action of Bordeaux mixture cannot 

 be put down to copper sulphate liberated by the action of atmospheric 

 carbon dioxide ; and the experimental evidence for this statement is 

 equally applicable with reference to the scorching action. The authors 

 have further shown^ that the fungicidal action of Bordeaux mixture is 

 very largely, if not entirely, due to an inter-action between the fungus 

 and the particles of the insoluble basic sulphate with which it comes in 

 contact ; the fungus dissolving and absorbing enough copper to kill 

 itself. In the same way the simplest explanation of the enhanced 

 scorching of damaged, as compared with undamaged, foliage is, that 

 soluble copper compounds* are produced by the solvent action of exuda- 

 tions from the injured cells and from those underlying, which are exposed 

 by the injury, and that these substances are then absorbed through the 

 thin- walled cells of the internal tissues of the leaf. It is then easy to 

 understand the gradual spreading of the spots from a centre which is 

 observed in most cases of Bordeaux scorch. Serious scorching occurring 

 several days or even weeks after the actual spraying is probably to be 



1 Wth Rep. Wohurn Exptl. Farm, p. 123. 

 - Journ. Agric Sci. iv, p. 69. 

 3 Ibid. IV, p. 76. 



* Soluble copper produced in siicli manner may. as previously suggested, also act 

 fungicidally. 



