All over the world an incessant war is waged against animal diseases which attack cattle and sheep, and the fungus growths, moulds, microbes and insect pests that decimate crops. In this struggle many kinds of treatment are needed, for what is effective in some diseases may be quite useless in others. Spraying and dusting are the most common remedies, and copper compounds are a constituent of many of the powders and solutions used.
Much information on this vital subject will be found in the C.D.A. Publication No. 41, Copper Compounds in Agriculture and Industrial Microbiology, from which the following statement is taken:
"The earliest commercial use of copper was in the form of sulphate as a seed dressing to destroy cereal diseases. Much later it was discovered that copper sulphate also prevented foliage diseases. If applied in too strong a solution it damaged the foliage but, by mixing with lime, Bordeaux Mixture was formed, and this has excellent adherence to foliage. This mixture enables plants to be provided with a protective coating of copper which prevents the penetration of the spores into the tissues. As long as the copper deposit remains on the tissue protection is maintained."
The most serious disease treated in this manner is potato blight, and neglect to spray the crop can be disastrous. Tomatoes are also sprayed against blight, raspberries and currants against leaf-spot; also stone-fruits, hops and vines, citrus fruits, bananas, tea (against blister blight), coffee (for rust and blight), and tobacco (for wildfire).
Probably, about 200,000 tons of copper sulphate are used in the world every year for these and similar purposes, in addition to smaller quantities of other copper compounds such as copper oxide and oxychloride and the copper-arsenic compound known as Paris Green.
Minute quantities of copper are essential to life; hence it is the practice on poor peaty or sandy soils deficient in that element to add copper sulphate to the usual fertilizer, thereby increasing the yield of the crops. For similar reasons 'salt licks' are provided for sheep to prevent the disease of lambs known as 'sway-back', and for cattle and other grazing animals. The dangerous tropical disease of bilharzia, which is due to a minute animal parasitic on snails, is also controlled by treating infected streams and lakes with a copper sulphate solution. Liver rot in sheep, due to flukes in another water-snail, is also treated by applying copper sulphate to the infected ground.
Thus, in the 20th Century we have turned the complete circle. Industry began with man, the agriculturist, picking up shining pieces of copper and wondering what they were; and we conclude with man, the horticulturist, putting back the same element in solution out of a watering-can.