This section is from the book "Commercial Gardening Vol3", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
A considerable part of the expenses incurred in the maintenance of the plantation will be those incident to the combating of insect and fungoid pests. Judging by the number of specifics that have quite recently come upon the market, and the prodigality with which money is spent on advertising them, one is driven to the conclusion, either that nature has all of a sudden adopted many new inventions in the pest line, or that some keen commercial men have discovered in fruit growers a source of income ripe for tapping, One can find many growers, hale and hearty, who say that in their younger days spraying and spray fluids were all unknown, and, blissfully ignorant of such worries, and their bill files unburdened by their charges, they obtained better crops of fruit than we do now. It may be so; one knows that it is the multiplication of hosts that gives opportunity to an epidemic, and perhaps it is the great increase in the number of fruit trees in cultivation which has given opportunity for such a multiplication of the various organisms, fungoid and insect, that live upon them, as to make it a matter of necessity for the grower of to-day to call in the aid of the chemist and engineer to put into his hands weapons with which to check their ravages. There is this to be said, that in the, indiscriminate slaughter of all things living on the trees with toxic sprays, the innocent suffer with the guilty, and nature's own antidote to what is a pest to the fruit grower is swept along with the pest into a common grave.
If it be true, and there is no doubt it is, that a modern fruit plantation is not fully equipped without spray machines and the materials for spraying, it is also true that judgment is needed in the use of them or much money may be thrown away. A spray should be looked upon as a remedy for a certain disease, only to be used when that disease appears. Some people talk as though spraying should be done after the manner of the man who on leaving home on Monday whipped his children all round because they would be sure to deserve it before he came home again! The pests, insect and fungoid, that affect fruit trees form a horrible list as set out by the entomologist and the mycologist, but so do the diseases that human nature is subject to when one is silly enough to read a doctor's book. As a matter of fact those that are responsible for serious injury are few in number in both cases. (See Vol. I., p. 170 et seq.).
Dealing with Apples first, the principal are: (a) The Canker Fungus; (b) The Apple Sucker; (c) The Codlin Moth; (d) The Maggots of the Winter and March Moths; (e) The Woolly Aphis or American Blight; (/) The Scab Fungus.