MAINTENANCE 77 



operation serve two purposes. Those sprays intended for leaf-eating 

 insects are classified as poisons and act like all poisons on being ab- 

 sorbed into the digestive tract of the insect. It is generally necessary 

 only to secure an even distribution of such a spray, which usually con- 

 tains 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 suspen- 

 sion 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 classifica- 

 tion 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. 



Forms of Sprays. 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 



