In treating for the potato-rot, "an ounce of prevention is worth a pound
of cure;" for when leaves or vines are once dead, they ever remain so.
All that can be done for potatoes infested is to stop the mildew from
spreading, by destroying it where it is, and by strengthening "those
things which remain." The writer was led to the adoption of the remedy
proposed by experiments made upon fruits.
Every one who has an apple or pear-orchard must have observed that
mildew of fruit supervenes after some sudden change of temperature,
especially when accompanied by rain. Spots of mildew invariably form on
the young fruit immediately after a cold night, when the thermometer has
indicated a change of twenty to twenty-five degrees. This growth of
mildew takes place when the apples are of various sizes, from the
earliest formation to the size of large marbles. These fungous growths
appear as dark-colored spots, which arrest the growth of the apple
immediately beneath, causing it to become distorted, while the expansion
and contraction bring on diseased action, which results in the cracking
and general scabbiness of the fruit.
Knowing that dry-rot (_Merulius Lachrymans_, Schum,) another species of
fungus, was remedied by an application of sulphuric acid, I thought it
might possibly destroy the fruit mildew. An application of plaster,
(gypsum,) which is composed of lime and sulphuric acid, was made with
the happiest results. It was found that an apple dusted with ground
plaster at its first formation remained free from mildew and came to
maturity, while apples growing by it, but not so treated, became scabby
and worthless. It was also ascertained that a thorough application of
plaster destroyed the mildew after it had formed, and that such fruit
came to maturity. On the potato mildew, so far as the writer's
experience extends, plaster, if applied early, is a perfect prevention,
and if not delayed too long after the disease appears, is a certain
remedy.
The vines should be watched closely, and on the first appearance of the
disease plaster should be applied; not merely sowing it broadcast, but
dashing it over and under the vines, bringing it in contact with the
stalks, using a handful to three or four hills. Plaster for this purpose
should be very dry and powdery, and should be applied when the air is
still. One application is seldom sufficient; it should be renewed as
often as circumstances require. Examine the vines about three days after
a cold night, or about the same length of time after a heavy rain. If
the leaves begin to curl and wither, apply plaster at once; and, in
short, whenever the vines show any signs of drooping, be the cause bites
of insects, excessive aridity, or excessive humidity of the atmosphere,
or sudden change of temperature, drooping from any cause whatever
indicates the approach of mildew, which should be promptly met with an
application of plaster. As before stated, plaster the vines as soon as
they are up, again after the last plowing and hoeing; after that, one,
two, or three times, as circumstances indicate.
By this method the vines are kept of a bright lively green, and the
tubers are kept swelling until growth is stopped by frost. Another point
gained is, potatoes so grown are so sound and free from disease as to be
easily kept for spring market without loss by rot.
Whether the surprising effects of plaster on the potato mildew is
attributable to the sulphuric acid, to the lime, or to its simply being
a dust, has not been determined. It is well known that the fruits of a
vineyard or orchard in close proximity to a dusty and much frequented
highway are remarkably free from mildew, which can only be due to dust
settling on the trees and fruit. But in the case of plaster, the writer
is inclined to believe its efficacy is mainly due to the sulphuric acid,
probably assisted by the lime in a state of dust. Be this as it may, it
matters not. The result is all that can be desired; the remedy is easily
applied, costs but a trifle, and a single season's trial is all that is
needed to convince the most skeptical grower of its merits.