The Principles And Practice Of Spraying
The spraying of fruit trees in the United States is of comparatively
recent origin, having been a general commercial practice for less than
two decades. It involves the principle of applying with force and in
the form of a fine rain or mist,
water in which a poison or a
substance which kills by contact is suspended. The first application
of the principle was against chewing insects with hellebore. Pure
arsenic was early used and soon led to the use of other arsenicals.
Our greatest fungicide, Bordeaux mixture, was discovered by accident
in 1882 when it was found to control mildew in France. Up until about
five years ago Bordeaux mixture as the fungicide and paris green as
the poison were almost universally used. Within the last few years,
however, there have been developed two substitutes which, although
known and used to some extent for twenty years, have only recently
come into such general use as practically to replace the old sprays.
These are lime and sulphur as the fungicide and partial insecticide
and arsenate of lead as a partial insecticide.
The necessity for and the advisability of spraying have already been
pointed out. There is an increasing demand for fine fruit the
supplying of which is possible only with thorough spraying. In the
humid East especially the competition of more progressive sections in
the West is demanding more and better spraying. There is no cure-all
in this process. It does not make a tree more fruitful except as it
improves its general health, but it does bring a larger percentage of
the fruit to perfection. Certain knowledge is fundamental; the grower
must know what he is spraying for, when and with what to combat it and
how to accomplish the desired result most effectively.
Spraying is an insurance against anticipated troubles with the fruit,
and the best and most successful growers are those most completely
insured. It has many general advantages also. It stimulates the grower
to a greater interest in his business because of the extra knowledge
and skill required. It compels thoroughness. It necessitates spending
money, therefore a return is looked for. To be sure, it is only one
of the operations necessary to success, but it enables us to grow a
quality of fruit which we could not obtain without it.
SPRAY MATERIALS are conveniently divided into two classes,
insecticides and fungicides. An insecticide is a poison by which the
insect is killed either directly by eating it, or indirectly by the
caustic, smothering, or stifling effects resulting from closing its
breathing pores. Direct poisons are used for insects which eat some
part of the tree or fruit and are called stomach poisons. Sprays which
kill indirectly are used for insects which suck the sap or juice from
the tree or fruit and are called contact sprays. Arsenical compounds
have supplanted practically all other substances used to combat
external biting insects. Two stomach poisons are commonly used,
namely, arsenate of lead and paris green, but the former is rapidly
replacing the latter.
ARSENATE OF LEAD is prepared by mixing three parts of crystallized
arsenate of soda with seven parts of crystallized white sugar
(acetate) of lead in water, but it will not as a rule pay the grower
to mix his own material, as arsenate of lead can be purchased in
convenient commercial form at a reasonable price. The preparation on
the market is a finely pulverized precipitate in two forms, one a
powder and the other a paste. These are probably about equally good
and are readily kept suspended in water. Less free arsenic is
contained in this form than in any other compound of arsenic, making
it safer to use, especially in heavy applications. Arsenate of lead
may be used without danger of burning the foliage as strong as five or
six pounds to fifty gallons of water, but three pounds is the usual
and a sufficient amount for the control of any apple insect for which
it is efficacious.
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.
Next: Contact Sprays
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