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Spray Chemicals Used. The spray chemicals used are those whichare best adapted to destroy the various forms of plant and animalparasites which it is desired to attack. Sometimes it is possible tocombine two forms of chemicals in one spray and thus make one operation serve two purposes. Those sprays intended for leaf-eatinginsects are classified as poisons and act like all poisons on being ab-sorbed into the digestive tract of the insect. It is generally necessaryonly to secure an even distribution of such a spray, which usually con-tains some form of arsenic, over the leaves of the plant. Those spraysintended for insects, such as the scale insects, which do not eat leavesor green parts of plants, are classified as contact insecticides. They killby suffocating the insect or by their direct physical action upon thebody of the insect. Thus they may also be useful against leaf-eatinginsects as well. These sprays may be merely some powder in a formso minute that it can enter the breathing apparatus ot the insect or,in the case of the sucking insects, some oil which can be held in suspen-sion in water long enough to allow it to be sprayed over the plant.Other well-known contact insecticides are tobacco extract (nicotinesulphate) and various soaps, such as "whale oil" soap, made from fishoil. Sometimes poisonous gases are used for this purpose but that is,properly speaking, fumigation and not spraying. The third classifica-tion of sprays according to chemicals is that intended for fungousdiseases, caused by low forms of vegetable parasites. Anthracnose,rust, mildew, canker, blight, and numerous other descriptive namesare given to these diseases but they are nearly all treated alike by theapplication of a fungicide which is generally some spray containingsulphur, either lime-sulphur mixture or Bordeaux mixture, which is alime and copper sulphate mixture. The lime-sulphur mixture alsoacts as a contact insecticide in some cases. Fungicides should beapplied very evenly over the whole of the plant from top to bottomand may be used much stronger during the dormant season than wouldbe safe during the growing season. |
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