26 Potato Disease and Allied Diseases [CH. 
possible to prevent it to a certain extent, as we shall 
see later. 
It spreads in the field by means: of conidia which 
form zoospores. If we can prevent the formation of 
spores or their germination we shall put a check on 
the spread of the disease. A large number of sub- 
stances in the form of liquids and powders have been 
put on potato plants with a view to checking the 
damage done by this fungus, and some of them have 
proved successful. An application of this kind to 
check the spread of a fungus is known as a “fungicide.” 
That which has proved the most valuable in the case 
of potato disease is known as “Bordeaux mixture.” 
Bordeaux mixture is made from copper sulphate 
(blue stone), lime and water. Ordinary agricultural 
blue stone must not be used as it does not contain 
sufficient copper sulphate. The blue stone used must 
contain 97-98 per cent. of copper sulphate. The lime 
most suitable for this purpose is “fat” lime, or as it is 
often called, ““white lime.” This is the kind used by 
plasterers. It is important that the lime should be 
freshly burnt and in lumps. Soft water makes a 
better mixture than hard water. 
For making 50 gallons of the mixture the following 
are required : 
A tub (of about 60 gallons capacity), 
A tub (of about 30 gallons capacity), 
A wooden pail, 
6 Ibs. of copper sulphate, 
4 lbs. of lime, 
50 gallons of water. 
Put 25 gallons of water into the large tub and weigh 
out 6 lbs. of the copper sulphate (this should be in a 
