This section is from the book "Commercial Gardening Vol4", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
This may be made up in a 40-gal. paraffin barrel also, using four times the quantity of material. Proceed as follows: "Dissolve the sulphate of copper exactly as has been described in Recipe 1, viz. dissolve 8 lb. sulphate of copper in 35 gal. of water in the paraffin barrel. Next prepare the milk of lime. For this purpose procure a wooden tub holding 5 gal., and also a bucket. Put into the bucket 4 lb. of good freshly burnt unslaked lime. Sprinkle it with sufficient water to change it to a powder. Then add sufficient water to fill the bucket. This, when it has been well stirred up, will make a thin milky fluid. Pour this into the tub, and add thereto sufficient water to cool the mixture and to bring the quantity up to 5 gal. After being thoroughly stirred it may be slowly poured through a fine sieve - such as is usually sold with spraying machines - into the barrel containing the copper sulphate solution. The contents of the barrel should be continuously stirred while the milk of lime is beinof added to it.
"The mixture should then be of a bluish colour and ready for use, but in order to secure the best results the blue-litmus-paper test should also be applied to it. If the paper turns red, a further quantity of milk of lime should be prepared, and added in small quantities at a time to the mixture until fresh paper put into the solution remains blue. It should then be applied with as little delay as possible, and the mixture should be well stirred each time before the sprayer is filled".
It is better to dissolve the sulphate of copper and washing soda in hot water instead of cold. If kept separate they will last for several days, but once mixed they should be applied immediately. If the mixture is kept even for a day it rapidly deteriorates, and is then much more readily washed off the plants by rain. All vessels coming in contact with sulphate of copper (which is poisonous) should be of wood, and not of metal. Spraying with a high-pressure sprayer is recommended three times during the season, the first time being early and before the appearance of any blight. Dry weather is obviously best for the purpose.
Potato-leaf Curl is a disease caused by Macrosporium Solani, which causes the stems to become more or less stunted, the leaves small, and the leaflets much curled, and in many cases the shoots do not appear above ground, thus causing gaps in the rows. The fungus appears to spread upwards from the tubers, thus preventing the rise of the sap and causing the stems and leaves to "flag" or wilt (fig. 486). As a rule the leaf-curl disease causes the tubers to remain hard and firm, and prevents the natural fermentation for proper growth. Perhaps the best remedy against this disease is to obtain immature or at least not over-matured "seed"; to sprout all tubers before planting; and to dress the deeply dug ground with slaked lime or chalk - about 1/2 bus. to 1 rod - if much stable manure has been used.
Mr. W. P. Lasham (Messrs. Sutton & Sons' potato expert at Reading) is strongly of opinion that Potato Curl is due to over-maturing of the seed, because scarcely a crop grown from Scotch seed suffers from the disease. He has always found that immature seed did not suffer from "curl". In the case of mature or over-mature seed, the old set is found nearly always as sound as when planted. This is never the case with immature seed. Mr. Lasham thinks that there is not sufficient moisture in mature seed to enable the tuber to decay quickly enough to yield its nourishment to the sprouting stems and root fibres.