The spray chemicals used are those which are best adapted to destroy the various forms of plant and animal parasites which it is desired to attack. Sometimes it is possible to combine two forms of chemicals in one spray and thus make one operation serve two purposes. Those sprays intended for leaf-eating insects are classified as poisons and act like all poisons on being absorbed into the digestive tract of the insect. It is generally necessary only to secure an even distribution of such a spray, which usually contains some form of arsenic, over the leaves of the plant. Those sprays intended for insects, such as the scale insects, which do not eat leaves or green parts of plants, are classified as contact insecticides. They kill by suffocating the insect or by their direct physical action upon the body of the insect. Thus they may also be useful against leaf-eating insects as well. These sprays may be merely some powder in a form so minute that it can enter the breathing apparatus of the insect or, in the case of the sucking insects, some oil which can be held in suspension in water long enough to allow it to be sprayed over the plant. Other well-known contact insecticides are tobacco extract (nicotine sulphate) and various soaps, such as "whale oil" soap, made from fish oil. Sometimes poisonous gases are used for this purpose but that is, properly speaking, fumigation and not spraying. The third classification of sprays according to chemicals is that intended for fungous diseases, caused by low forms of vegetable parasites. Anthracnose, rust, mildew, canker, blight, and numerous other descriptive names are given to these diseases but they are nearly all treated alike by the application of a fungicide which is generally some spray containing sulphur, either lime-sulphur mixture or Bordeaux mixture, which is a lime and copper sulphate mixture. The lime-sulphur mixture also acts as a contact insecticide in some cases. Fungicides should be applied very evenly over the whole of the plant from top to bottom and may be used much stronger during the dormant season than would be safe during the growing season.
The types of spray classified according to the form in which they are used are two. The most generally used form is the liquid, which generally means a chemical in suspension in water. Most sprays used in liquid form are chosen because of their ability to stay in suspension in water for a considerable length of time without either going into solution or gathering in lumps. This property of staying in suspension is a very valuable one, because it insures an even distribution of the spray material without an excessive amount of agitation to keep the mixture even. There is an increasing tendency to use the dust form of spray in which the chemicals are blown upon the plants in the form of a very fine powder which is largely dependent upon atmospheric moisture to make it stick to the plant. Dust sprays are used more often in a commercial way than by the average amateur when protecting ornamental plants, and this form of spray is not adapted to all the chemicals used, as, for instance, the oil and tobacco extract sprays. Sometimes poisons are mixed with bait and spread upon the ground near the plants to be protected, but this again is not spraying in the generally accepted sense of the word.