Paris Green is being rapidly displaced by arsenate of lead for several reasons. It is a compound of white arsenic, copper oxide, and acetic acid. The commercial form is a crystal which in suspension settles rapidly, a serious fault. It is more soluble than arsenate of lead and hence there is greater danger of burning the foliage with it. Moreover, it costs from twenty to twenty-five cents a pound, and the arsenate of lead can be purchased for from eight to ten cents a pound.
The amount which it is safe to use in fifty gallons of water is from one-half to three-quarters of a pound. When paris green is used alone as a poison lime should be added. Both these arsenicals should be thoroughly wet up by stirring in a smaller receptacle before they are put into the spray tank, in order to get them in as complete suspension as possible. They may be used in the same mixture with Bordeaux or lime sulphur.
Contact Sprays.—Four compounds are used as contact sprays in combating sucking insects, namely, lime sulphur, soaps such as whale oil soap, kerosene emulsion, and tobacco extract. Of these lime sulphur is the most used and for winter spraying is probably the best. This preparation is made by boiling together for one hour or until they unite, twenty pounds of quick lime, fifteen pounds of flower of sulphur, and fifty gallons of water. Although the home made mixture is much cheaper than the commercial form which may be purchased on the market, many people prefer the latter because of the inconvenience and trouble of preparing the mixture, although there is nothing difficult about it.
This contact spray is used chiefly for the San José scale and the blister mite, and in order to control these must be applied strong on the dormant wood. The strength necessary will vary from one part of the mixture above mentioned or of the commercial preparation, to from seven to ten parts of water, according to the density test of the material, which should be around twenty-eight per cent. Beaumé (a scale for measuring the density of a liquid) for home made, and thirty-two per cent. for the commercial mixture.
Any good soap is effective in destroying soft bodied insects such as plant lice. The fish oil soaps, although variable in composition, are often valuable, especially the one known in the trade as whale oil soap. This soap dissolved in water by boiling at the rate of two pounds of soap to one gallon of water, makes a good winter spray for scale but should be applied before it gets cold as it is then apt to become gelatinous. For a summer contact spray against lice, one pound of soap to seven gallons of water is strong enough to be effective. It is objectionable because of its odor and because it is disagreeable to make and handle. Lime sulphur is to be preferred as a winter spray, but the soap spray is often necessary and valuable for summer sucking insects.
Kerosene emulsion was formerly more commonly used than now against the scale and plant lice. It is a mixture of one-half pound of soap and two gallons of kerosene in one gallon of water—preferably in hot water. For dormant trees one gallon of this mixture should be diluted with six gallons of water. While this spray is effective it is no more so than lime-sulphur and is quite difficult and disagreeable to handle. As a summer spray, however, it is often necessary. Several preparations of petroleum known as the miscible oils are sometimes used. Their use is the same as that of lime-sulphur and they are not as good.
Within the last few years a tobacco concoction known as black leaf tobacco extract (nicotine sulphate) has come into quite common use. It can be purchased commercially under various brand names, and should be diluted according to its strength, but usually about one part to fifty of water. It may be made by boiling one pound of good tobacco stems in two gallons of water for one-half-hour. Objections to it are that it evaporates very quickly, although it is supposed to be non-volatile, and that it is expensive, but it is very convenient to use, can be readily mixed with other summer sprays, and is very effective against plant lice and mites.
Bordeaux Mixture. Fungicides are mixtures of chemical compounds made up for the purpose of controlling plant diseases caused by a class of plant weeds known as fungi. There are three commonly well known and used fungicides, Bordeaux mixture, commercial lime sulphur, and the self-boiled lime-sulphur. The Bordeaux mixture is the best all-around fungicide known. It is a mixture of three pounds of copper sulphate (blue vitriol or bluestone) with three or more pounds of fresh burned stone lime in fifty gallons of water. The two compounds should be put together as fruit growers say "with water between," that is each should be diluted with the water separately before the two are mixed.
The best plan is to have stock mixtures of each in barrels, fifty gallon cider or vinegar barrels making good receptacles for the purpose. Place the bluestone in an old fertilizer or meal sack and suspend it about midway in the barrel of water. In a few hours it will all be dissolved and will remain in suspension for some length of time very well. If say fifty pounds of the copper sulphate are dissolved in fifty gallons of water, each gallon of water will contain one pound of the bluestone, which makes a very convenient way to measure it. So also fifty pounds of fresh burned stone lime should be placed in a barrel—in this case in the bottom of the barrel rather than in a sack—just covered with water and allowed to slake, more water being added as required up to fifty gallons. If too much water is added to the lime at the first it will be "drowned" and its slaking checked. These two stock mixtures, each gallon containing one pound of the copper sulphate or one pound of the lime, are then mixed together.
It is well to fill the tank about half full of water, then put in the required amount of the copper sulphate, and after stirring well add the lime milk. It is a good plan to add an excess of lime as it minimizes the danger of burning and aids the mixture in sticking to the leaves well. If one is sure that he has at least as much lime, or an excess of lime, it will not be necessary to test the mixture, but if he is not, a simple test may be made with ferro-cyanide of potassium, obtained at a drug store. A few drops of this mixture will disappear if the lime is equal or in excess of the copper sulphate, that is, it will be neutralized, but if it is not, they will remain a bright purplish red. Bordeaux mixture is used in strengths varying from three to five pounds each of bluestone and lime in fifty gallons of water, but the former is usually sufficient.