i62 DISEASE IN PLANTS. 



are often serviceable, but great practical difficulties 

 are apt to interfere with their use in the open 

 field. 



The principal liquids employed against fungi 

 have been copper sulphate and other metallic 

 compounds (Bordeaux mixture, Eau Celeste, etc.), 

 various compounds of arsenic {e.g-. "Paris green"), 

 potassium sulphite, permanganate, etc., and emul- 

 sions of carbolic acid, petroleum, and such like 

 antiseptics, for the exact composition of which 

 the special treatises must be consulted. Some of 

 these, especially Bordeaux mixture, have been 

 experimented with on a very large scale, especially 

 in America, and various forms of spraying 

 machines have been introduced for dealing with 

 large areas. 



It is clear that these spraying operations are 

 more particularly adapted to field crops such as 

 Turnips, Hops, Vines, Potatoes, and to garden 

 and greenhouse plants than to woods and plan- 

 tations ; as a rule they cannot be applied to 

 forest trees though they have been used in 

 orchards or to roots, seeds, and other parts in 

 the soil, and many special forms of treatment 

 have been devised for particular cases of these 

 kinds. 



One of the oldest of these is the steeping of 

 grain in solutions of copper, or in hot water, just 

 before sowing, and the practical eradication of 

 Bunt and, partially, of Smut is due to this prac- 

 tice, which has lately been adapted to potatoes, 

 the principle being that the parasitic germs shall 



