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NEW HAMPSHIRE
AGRICULTURAL EXPERIMENT STATION,

DURHAM, N. H.

BULLETIN NO. 22.

Prevention of Potato Blight.

IVIAY, 1894.

N. H. COLLEGE OF AGRICULTURE AND
THE MECHANIC ARTS

Durham, N. H.

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POTATO BLIGHT.

The potato crop is liable to serious losses from two sources,
which losses are to a great extent preventable. These two
sources of loss are the Colorado beetle, or potato bug, and the
blight or rust. Every farmer knows how to deal with the potato
bug. Paris green is the sovereign remedy, but it is only within
a comparatively few years that a successful treatment has been
found for the blight. It seems now, however, that an effective
remedy has been found in the Bordeaux mixture.

The blight is caused by the growth in the potato vines of
minute parasitic plants, known as fungi. There are two dis-
tinct species of fungi which produce the blight in potatoes, — one,
known botanically as Macrosporiian sohuii, causes what is now
known as the early blight, the other, Phytophthora infestans,
causes the late blight which has been the most common and
destructive. Blight, rust, and rot are the various names by
which it has been called. This disease seems to have been
especially prevalent during the last few years, doubtless owing
to the peculiar conditions presented by the weather, a warm and
moist season being most favorable to its development.

It usually makes its first appearance during the month of
August when the vines of the later varieties are in full growth.
Early varieties often escape its attacks altogether. The leaves
are the first to show its effects. They become more or less dis-
colored, then begin at the edge to turn brown and curl up, or, if
the weather be very damp, to rot. If a leaf which is only partly
dead is closely examined there will be seen on the under surface,
especially along either side of the line separating the dead por-
tion from the living, a very fine white fuzz ; this consists of
the spore stalks of the fungus which is growing within the
tissues of the leaf and which constitutes the sole cause of the
disease. If a portion of the leaf thus affected be placed under

a suitable power of the microscope, the fine white branching
stalks may be plainly seen growing out of the breathing pores
or stomata in the epidermis, sometimes one and sometimes sev-
eral growing from a single opening ; on the sides and tips
of their branches will be seen little white egg-shaped bodies ;
these are the spores or seed of the fungus causing the blight.
If the interior of a leaf be examined there will be found run-
ning in all directions among the cells, especially those of the
under side of the leaf, fine white threads which are the myce-
lium or body of the fungous plant. These threads absorb from
the cells among which they run the nutriment which has been
elaborated for the use of the potato itself, and cause them to
die and shrivel up or decay. The spores as soon as they are
mature are very readily broken off from their stalks, and being
so light are easily borne by the wind to healthy vines where
they germinate in any moisture they may find upon the leaves
or stems and make their w^ay through the skin or epidermis in-
to the succulent tissues beneath, where the threads develop and
in their turn send out a crop of spores which help to spread the
disease.

If the weather is sufficiently warm and damp this develop-
ment and spread is very rapid, so that a large field of apparently
healthy vines may be entirely killed in a very few days. If this
destruction of the tops occurs early, the tubers cannot mature
and the yield will be a light one and of inferior quality ; more-
over, the fungus is not confined to the tops but makes its way
into the tubers also. The potatoes thus attacked are very likely
to decay, and great loss occurs, not only before they are dug but
even after they are stored in the cellar, if the conditions there
should be favorable. The threads of the fungus live over winter
in the tubers which, if used for seed, serve to start the disease
again the following summer.

Figure i represents a magnified section of a potato leaf affected
with the blight. The leaf like all parts of the plant is made up
of cells of various sizes and shapes. The upper and lower surfaces
are covered by single layers of cells called epidermis shown at a
and d. The upper side of the leaf consists of elongated cells
arranged perpendicularly to the surface and closely packed

together b ; the lower side c consists of irregular cells, loosely
packed, having air spaces between them. Communicating with
the air spaces are little openings in the epidermis, called breath-
ing pores or stomata, one of which is shown at e. In the left half
of the section the cells are represented filled with the chlorophyU
bodies which contain the green coloring matter. Running among
the cells are seen the threads or mycelium of the fungus which
causes the blight ; and growing from the breathing pores the
branching spore stalks bearing the spores.

Fig. I.

Treatment. — If it can be avoided, potatoes which are known
to be diseased or which have been grown in a field attacked by .
the blight should not be used for seed. However, a remedy has
been found which if used in season seems to be very efficient in .
checking or preventing the disease. This remedy is the Bordeaux

6

mixture, which, when applied to the vines, destroys the vitality of
the spores with which it comes in contact and thus prevents the
infection of healthy plants. It should be applied, if possible,
before the disease makes its appearance, at least by the last of
July, and the applications should be repeated at intervals of ten
to fourteen days, and oftener if the mixture is washed off by
rains, until the tubers have matured. Usually three applica-
tions will be sufficient but a fourth may sometimes be required.

In experiments made in the summer of 1892, the vines on
the sprayed plots remained green from one to four weeks longer
than those on the unsprayed plots.

In one case the yield of merchantable potatoes from the
sprayed plot was about one third greater than that from a simi-
lar unsprayed plot, while on two other sprayed plots the yield
was one half greater than on the unsprayed plot.

In another case, in a different field, a small sprayed plot
yielded three times as many merchantable potatoes as the cor-
responding unsprayed plot, while the weight of rotten potatoes on
the unsprayed plot was three times as great as on the sprayed
plot.

A photograph of the potatoes from these two plots is repro-
duced in Fig. 2,

EARLY BLIGHT.

The early blight {Macrosporium solani) has come to be recog-
nized as a serious disease of the potato only within the last two
or three years. It was first noticed at this station in the sum-
mer of 1892. As its name indicates, it makes its appearance
earlier than the ordinary blight — by the middle of July or earlier.
It first appears, as sharply-defined, rather irregular, dark brown
or blackish spots on the leaves. The surface of the spots is
usually marked with minute wrinkles, which take roughly the
form of numerous circles with a common centre ; this has been
called the " target marking," from its resemblance to the con-
centric rings of a target. The spots increase gradually in num-
ber and size till the whole leaf is destroyed.

The fungus producing this disease differs considerably
in the details of its structure from the late blight, but

like it, consists of mycelial threads which grow in the
tissues of the leaf and send out spore-bearing stalks on
the surface. The early blight may be distinguished from the
late blight by its characteristic spots, by the absence of the
mould-like fuzz of the latter, by its earlier appearance and
slower growth, and from the fact that it does not affect the
tubers. Both diseases may be present in a field at the same
time.

Our experiments have not yet definitely decided the question
whether the Bordeaux mixture is as effective a remedy for the
early blight as for the late, but the indications are favorable.
Our experiments last summer were planned for the ordinary
blight, but that did not make its appearance in this vicinity and
it was rather late when our attention was directed to the new
disease. Several plots were sprayed about the middle of
August, and while the vines were finally killed, those which
were sprayed resisted the disease about two weeks longer than
the unsprayed.

One of the sprayed plots yielded twenty bushels per acre
more than a similar unsprayed plot, while in another field a
small sprayed plot yielded nearly double the weight of merchant-
able potatoes that the unsprayed plot beside it did. In treating
the early blight the spraying should be commenced by the mid-
dle of July, or earlier, if signs of the disease make their appear-
ance.

BORDEAUX MIXTURE.

Blue vitriol (copper sulphate) i lb.

Fresh lime . . . . . . . . . i lb.

Water logals.

or

Blue vitriol • • 5 lbs.

P"resh lime . . . 5 lbs.

Water 50 gals, or i barrel.

Dissolve the blue vitriol in several gallons of water, say one
fourth the quantity to be used, or it may first be dissolved in a
smaller quantity of hot water and then several gallons of cold
water added. Slake the lime and make a thin whitewash, then
pour this slowly into the solution of blue vitriol, stirring

8

thoroughly ; then add water sufficient to make up the required
quantity. Before using the mixture it should be strained
through sacking or a sieve of some kind, with about a 1-16 inch
mesh, to remove the coarser particles, which are liable to clog
the nozzle of the pump. While spraying, the mixture should be
kept thoroughly stirred up.

Iron vessels should not be used in preparing this mixture.

This formula is one fourth stronger than that recommended
for spraying fruit trees.

For destroying potato bugs, Paris Green may be added to the
Bordeaux mixture in the proportion of one half pound to fifty
gallons.

The Bordeaux mixture may be applied to the potato vines
with an ordinary watering pot, but one of the forms of spraying
pump described in Bulletin No. 19 is much to be preferred.
For small fields, and where the work is to be done by one man,
the Knapsack pump is a good form ; but for general use one of
the force pumps which can be attached to a barrel will be found
more satisfactory. Tlie pump should be provided with a nozzle
which will give a fine spray, the form known as the Vermorel is
one of the best. Spraying pumps should be brass lined. They
may be obtained through any hardware dealer or direct from the
manufacturers. W. & B. Douglas, Middletown, Conn. ; George
Tyler & Co., 43 South Market St., Boston ; Field Force Pump
Co., Lockport, N. Y. ; Nixon Nozzle and Machine Co., Dayton,
Ohio, are reliable firms. Blue vitriol may be obtained from any
druggist, and in quantity ought not to cost more than ten or
twelve cents, and may be obtained for even less at wholesale.
Twenty-five to fifty pounds for three applications per acre will
be required.

The barrel with pump attached may be mounted on a cart or
wagon which will straddle two rows of potatoes, while the horse
walks between them. One man will be required to pump, and
another to hold the nozzle. With fifteen or twenty feet of hose
five or six rows can be sprayed on either side of the team, so
that little damage will be done in the necessary driving over
the field.

H. II. LAMSON.

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