Spraying of plants as a scientific practice is, comparatively speaking, a modern procedure, but the necessity for doing something to protect plants against insects and plant diseases has been understood since antiquity. In their writings the Greeks, Romans, and Hebrews noted the existence of rusts and mildews, and the plague of locusts is of Biblical record.
Spraying is only one of several ways of protecting plants and among the others may be enumerated hand picking, fumigating, banding, burning, using fungous diseases as insecticides, crop rotation, soil sterilization and various other more or less practical methods. These other methods are important when understood and put into practice at the right moment and in the right way, but they are inexact compared to spraying and are seldom as efficient. Spraying, by which is meant the use of chemicals to poison or otherwise exterminate animal and vegetable parasites on plants, has been reduced very nearly to an exact science in this country, largely within the last century, and, while it is not the purpose here to go too deeply into this art, some broad rules may be laid down and some little understood points cleared up.
Our Government and State Experiment Stations have been largely responsible for the rapid strides taken in this art in this country. They have issued many bulletins and spray calendars containing exact directions for combating all the known insect pests and plant diseases and they always stand ready to help any one who asks for it. Yet much of their help comes too late and much money is wasted each year with consequent disappointment, because a few simple principles are not clearly understood. Some of the overlooked factors which are not taken into account are as follows: (1) a spray mixture must be the correct one as, for example, it does no good to use poison upon an insect at a period in its life history when it does not eat; (2) the spray mixture must not injure the plants, or else the cure will be worse than the disease; (3) a thorough job must be done or else the whole job may have to be done over at too late a season to secure the best results; (4) the correct time must be picked or a rain storm may undo the whole work within a few hours; (5) the spray must stick to the plant long enough to be of some use, especially in the case of poisons.
The various forms of sprays may be classified in four different ways: according to the season of spraying, the kind of chemicals used, the form in which the chemical is applied, and the kind of plants sprayed.