The year 1845 will ever be memorable by its giving birth to a disease
which threatened the entire destruction of the potato crop, and which
caused suffering and pecuniary ruin to an incredible extent throughout
Europe.
The potato, at the time of the appearance of the potato disease, was
almost the sole dependence of the common people of Ireland for food.
That over-populated country experienced more actual suffering in
consequence of the potato disease than has any other from the same
cause. Although this disease has never, in this country, prevailed to
the same ruinous extent that it has in some others, yet we are yearly
reminded of its existence, and in some seasons and localities its
destructive effects are seriously apparent.
The final or culminating cause of the disease known as the "potato-rot"
is _Botrytis (peronospora) infestans_. This may be induced by many and
various predisposing causes, such as feebleness of constitution of the
variety planted, rendering them an easy prey to the disease; by planting
on low, moist land, or on land highly enriched by nitrogenous manures,
causing a morbid growth which invites the disease; also by insects or
their larvæ puncturing or eating off the leaves or vines. But by far the
most wide-spread and most common cause of the disease is sudden changes
of atmospheric temperature, particularly when accompanied by rain.
Drought, though quite protracted and severe, unless accompanied by
strong drying winds, and followed by sudden and great reduction of
temperature, seldom affects the potato seriously. It is not uncommon in
the Northern States, during the months of August and September, for
strong westerly winds to prevail for many days in succession. These
winds, coming from the great American desert, are almost wholly devoid
of moisture, and their aridity is often such that vegetation withers
before them as at the touch of fire. Evaporation is increased in a
prodigiously rapid ratio with the velocity of wind. The effects of the
excessive exhalation from the leaves of plants exposed to the sweep of
such drying winds are at once seriously apparent.
When these winds finally cease, the atmosphere has a low relative
humidity, not enough moisture remains in the air to prevent radiation;
the heat absorbed by the earth through the day is, during the bright,
cloudless night, rapidly radiated and lost in space, and a reduction in
temperature of twenty to thirty degrees is the consequence.
In the first place, the potato-vines suffer by excessive exhalation; in
the second, by sudden reduction of temperature, and, though not frozen,
their functions are much deranged, and their vitality greatly enfeebled.
To use a common expression, the plant "has caught a violent cold that
has settled on the lungs."
The leaves (which are the lungs of plants) now fail to perform their
functions properly. The points of many of the leaves turn brown, curl
up, and die.
The ascending sap, not being fully elaborated by the diseased leaves,
oozes out through the skin of the stalk in a thick, viscous state, and
the plant to all appearance is in a state of consumption.
At this stage the ever-present minute spores of the _Botrytis infestans_
eagerly pounce on the sickly plant, fastening themselves on its most
diseased parts. The _Botrytis infestans_ is a cryptogamous plant, and is
included in the Mucidineous family, (moulds.) It is a vegetable parasite
preying upon the living potato plant, like lice or other animal
parasites upon the animal species.
At first this mould forms webby, creeping filaments, known in botanical
language as mycelium. These root-like fibres then branch out, sending
out straight or decumbent articulated stems. These bead-like joints fill
up successively with seeds or spores, which are discharged at the proper
time to multiply the species.
Under favorable conditions of warmth and moisture, the mycelium spreads
very rapidly. Spores are soon formed and matured, to be carried to
plants not yet infected. Rains also wash the seminal dust down the
plant, causing it to fasten and grow on the vine near the ground. The
roots of the parasite penetrate and split up the stalk even to the
medullary canal.
These roots exude a poisonous substance, which is carried by the
elaborated descending sap down to the tubers, and as the largest tubers
require the largest amount of elaborated sap for their development, they
will, consequently, receive the greatest quantity of the vitiating
principle, and will, on digging, be found a mass of rottenness, when the
smaller ones are often but slightly affected. The _Botrytis infestans_
can not gain a lodgment on vines that are truly healthy and vigorous,
high authority to the contrary notwithstanding.
Healthy varieties, growing in a sheltered situation on dry, new soil, to
which no nitrogenous manures have been applied, can not be infected,
though brushed with other vines covered with the fungus. Different
varieties, and sometimes different members of the same variety, are not
always alike affected by the disease, though growing in the same hill.
As will be noticed, the potato disease is rather an effect than a cause,
and appears to have been designed to prevent members enfeebled by
accident or otherwise from propagating their species by putting such
members out of existence. Ozone, supposed to be a peculiar form of
oxygen, is exhaled from every part of the green surface of plants in
health, and effectually repels the attacks of mildew; but it is found
that when the atmosphere is very dry, or, on the other hand, very humid,
plants cease to evolve ozone, and are therefore unprotected. Winds from
the ocean are strongly ozonic, and it is ascertained that plants growing
on soil to which salt has been applied evolve more ozone than others.
Hence the benefit derived from the use of salt on potato lands.
The "Black knot," another species of fungus that attacks the branches of
the plum and Morello cherry, operates very similarly to the potato
mildew. The roots of the parasite penetrate and split up the cellular
tissue of the branch on which it fastens, and if the limb be not
promptly amputated, the descending sap carries the deleterious principle
through the whole system, and the following year the disease appears in
a greatly aggravated form in every part of the whole tree. The remedy in
this case is prompt amputation of the part diseased on its first
appearance, and a judicious application of salt to the soil.
Common salt, to a certain extent, is as beneficial to some plants as to
animals; and every intelligent farmer knows that if salt be withheld
from the bovine _genus_ for any considerable length of time, the general
health droops and parasites are sure to abound. The object of nature in
bringing into existence the large family of mildews, each member of
which is a perfect plant in its way, and as capable of performing its
functions as the oak of the forest, was undoubtedly to prevent
propagation from sickly stock, and by the decomposition of feeble plants
to make room and enrich the soil for the better development of
healthier plants. But it by no means follows that, because a plant is
attacked by mildew, it must necessarily be left to die, any more than it
follows that, because an animal is infested with vermin, it should be
let alone to be eaten up by them.