Press Bulletin No. 55.
UNIVERSITY OF FLORIDA.
LAKE CITY, FLA.
The loss to the potato industry of Florida, due to late
blight (Phytophthora infestans De Bary), during the past sea-
son, emphasizes the great necessity of using every possible
means to protect the potato plants from an invasion of this
APPEARANCE OF DISEASED PLANTS.-The first indication
of this disease is the development of small brown spots on the
leaves. Under favorable conditions these injuries are rapidly
extended, followed by the curling of the leaves, and if a spell
of warm damp weather follows, the stems and leaves turn
black and decay within a few days, giving off a disagreeable
INFECTED SEED.-Tubers obtained from a diseased plant
should not be used for seed, since this fungus also invades the
tubers, where it frequently passes into a latent condition over
winter, beginning active development with the growth of the
plants; or it may continue its work after the tubers are stored,
especially if they are placed in warm, damp bins, and thus
Feb. 7, 1905.
tubers touching diseased tubers may become infected. Where
the tubers have not passed beyond the first stage of the dis-
ease, they show no signs of external injury, but when such
tubers are used for seed, diseased plants grow from them.
HARDY VARIETIES -This disease is a great menace to
the potato industry of Europe and sume of the ablest foreign
investigators have given this malady careful attention,
and they have shown beyond a doubt that varieties vary
greatly in the susceptibility to this disease. European seed-
men have frequently advertised the immunity of certain new
varieties, but it is extremely doubtful if such a variety exists.
American investigators have given the variety phase of the
subject less attention, and have directed their energies mostly
toward spray mixtures with considerable success.
BORDEAUX MIXTURE.-This mixture has proved the most
efficient spray, but it is simply a preventive, and not a cure,
for the malady. It is very essential that the spraying be done
at the right time. If the treatment is. delayed until signs of
damage appear, very little good will result. When the leaves
turn brown at the edges and curl up, the disease has already
become thoroughly established inside the tissue, and beyond
the reach of the spray mixture. Spraying,to be effective, must
be done before any signs of disease appear,and it must be done
thoroughly, for unless the entire surface of the leaf is covered
by the mixture, some of the sports (seeds) of the fungus will
fall on the unprotected spots and gain entrance to the plant.
During warm, moist weather the plants grow rapidly,and new
surface is being constantly pushed out. Damp, warm weather
also favors the development of this disease; consequently dan-
ger from its attacks is greatest at such times. Frequent appli-
cations of this mixture are necessary. especially if sufficient
rain falls to wash off the mixture.
It is ordinarily considered best to begin spraying when
the plants are about four inches high, and repeat the treat-
ment at intervals of ten days. If plants show signs of blight,
it may be necessary to spray as often as once in seven days.
Usually about six applications will be sufficient.
PREPARATION OF BORDEAUX MIXTURE.-The following
is the standard formula for Bordeaux mixture:
Copper Sulphate .............. ........ 6 lbs.
Lime............ ............... 4 lbs.
W ater................................ 50 gals.
Potato vines require from 2 to 6 barrels of this mixture per
acre according to the size of the plants. When spraying is
done on an extensive scale it is usually convenient to make up a
stock solution of copper sulphate. Suspend a coarse sack con-
taining 45 lbs. of copper sulphate into a barrel containing 45
gals. of water. The copper sulphate should be placed near the
surface of the water a day or two before it is to be used. Stir
this solution thoroughly before using, and one gallon ot it will
contain one pound of copper sulphate.
A quantity of fresh lime may also be slacked and placed in
a barrel, and if it is covered with a few inches of water it can
be kept in an excellent condition for some time.
To prepare a mixture from these solutions, take six gallons
of copper sulphate and place it in the spray barrel and add 19 gals
of water. From the slacked lime take four lbs. of the paste,
dilute it with 25 gals. of water and add it to the copper sulohate
solution. All the material ought to be passed through a seive
so as to exclude the particles which might clog the nozzles.
If the '"bugs" are plentiful add one lb. of Paris green to 50
gals. of the above mixture. A oaste should be made of the
Paris green by mixing it with a small amount of water before
putting it into the sprayer.
POTASSIUM FERRO-CYANIDE TEST.-This test is used to
determine if enough lime has been added to combine with all
the copper sulphate. It is a yellow poisonous salt which dissolves
readily in water. Ten cents worth dissolved in ab.,ut ten
times its bulk of water will ordinarily be enough for one
season. In using this test; pour into your spray barrel 25 gals.
of the diluted copper sulphate solution and then add the 25 gals.
of the milk of lime. Stir the mixture thoroughly and add a
drop of the potassium ferro-cyanlde. If enough lime has
been added the drop will not change color when it strikes the
mixture; however, if sufficient lime has not been added it will
immediately change to a dark, reddish brown color and more
lime must be added. Even after the test shows no color it is
best to add a little more lime.
SUGGESTIONs,-Prevention is best promoted by choosing
varieties which show greatest resistance to the disease; use
every means possible to secure strong, healthy seed. Seed se-
lection, also, does much towards getting rid of the various
diseases which injure the subterranean parts of the'
plants. It is evident that no spray can save the top if the
subterranean parts of the plant are severely injured. The soil
should be well drained, and there should be free circulation of
air around the plants. Confined, warm air promotes the rapid
growth of disease. Seed tubers ought to be cut about two days
before planting, and stored in a dry place so as to allow the in-
jured surface to form a callus before the seed is planted. All
old tubers that show traces of the disease, and all dead stalks
and leaves, should be burned.
F. M. ROLFS,
Botanist and Horticulturist.
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