This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
The purest 98-per-cent copper sulphate should be used, as cheaper brands contain impurities, chiefly iron sulphate, which may injure the foliage. One pound to 10 gal. water may be used as a winter wash.