Bordeaux mixture for this purpose is made by using six pounds of copper sulphate and four pounds of quick lime, dissolved in separate wooden vessels, and the lime water poured into the dissolved blue stone. This should be diluted with water sufficient to fill a forty-five gallon barrel. Paris green to the amount of from one-quarter to three-quarters of a pound to the barrel should be added, to destroy beetles and other insects.
The vines should be sprayed five or six times, beginning when they are 6 inches high, at intervals of ten days or two weeks. During rainy weather the spraying should be more frequent than during clear weather. The object is to prolong the life and vigor of the vines. The cost of the five or six sprayings, including labor at $1.50 per day, is put at not above $6 per acre, while the crop at stake may be affected to the extent of scores of bushels.
Scab and Rot.—The evidence about scab and rot is still contradictory, but it is likely that these diseases will presently be under control.
At the New Jersey Station, Professor Halsted completely conquered scab with an application of 300 pounds of flowers of sulphur per acre scattered in the rows, while the same treatment at the Ohio Station was less successful. At the latter station benefit was found in the use of salt, kainit, sulphate of potash, etc.
The various experiments and observations on potato scab and rot seem to indicate that scab flourishes best on a soil inclined to be alkaline, while rot is most prevalent on a soil inclined to be acid. The use of lime increases scab, while the use of kainit diminishes it.
The best practice, therefore, under present knowledge, would be to use clean seed on new ground, avoiding fresh stable manure. Clean seed can be had by treating tubers with corrosive sublimate. This substance is dissolved to the amount of 2¼ ounces, in two gallons of hot water, and (after standing a day) diluted with water so as to make fifteen gallons. In this solution the uncut seed potatoes should be soaked for an hour and a half. All unplanted seed potatoes should be destroyed, as the corrosive sublimate is highly poisonous.
The use of sulphur, as recommended by Professor Halsted, will prove entirely satisfactory in some soils. In others, the use of kainit or sulphate of potash or acid phosphate would no doubt be found preferable.
Where soil is badly affected with disease germs, it is unquestionably better to seek a new field than to attempt to disinfect the old one. A rotation of crops will probably restore diseased land to health more cheaply and more thoroughly than any other process.
Profits.—Of potato profits it is not necessary to speak, except to remark that it costs but little more to produce 300 bushels to the acre than 100 bushels. There can be no doubt whatever that it pays handsomely to spray potato vines with the Bordeaux mixture.