The Republican journal. [volume] (Belfast, Me.) 1829-current, 1918-09-12
Late Blight Of Potatoes.
How Does 1t Live Over Winter and How
Does 1t Spread?


cultural Experiment Station indicate
that there is need of wider publicity rela-
tive to what has been learned in Maine
and elsewhere regarding the above ques-
tons. A recent inquiry indicated that
the writer thought the disease lives over
winter in the soil. This is a natural in
ference, since the organisms that cause
certain other diseases of potatoes not
only are carried by the seed tubers but
persist in the soil for varying periods af-
ter they are once introduced. Since at
the present writing late blight can be
found to some extent in all but the most
thoroughly sprayed fields over a consid-
cradle area of the potato growing sections
OF the State, discussion of the manner
in which the disease spreads and the
agencies concerned therein is timely.
AS is well known late blight, often in
correctly called crust" is caused by
fungus which not only destroys the foil
age but causes destructive rot of the
tuber as well. Frequently the greatest
loss comes from tuber decay. Typically
this is dry rot, causing a rusty brown
discoloration of the outer portions of the
tuber. In severe outbreaks, occurring in
the field at harvest time and soon there
after, the disease on the tuber progresses
with great rapidity and the entrance of
secondary organisms produces a soft,
mushy, sinking decay which is too fa-
miliar to the experienced potato grower
to require description.
Fungi as rule reproduce themselves
by means of relatively simple bodies
called spores which serve similar pur-
pose to that of the seeds of higher plants.
The late blight fungus belongs to class
of which several representatives produce
reproductive body capable of living
over winter out of doors. The results of
certain investigations have indicated a
tendency on the part of the late blight
fungus to do this, but there is no con-
elusive evidence that these bodies ever
develop sufficiently to function in this
way.
The only way that late blight lives
over winter is by means of tubers affect
ed with dry rot. Repeated observations
show that partially decayed seed pieces
from such tubers, when planted, are
stimulated to early termination. The
fungus attacks the young sprouts and
later spreads to the parts above ground.
1t then throws out branches on the sur
face upon which are produced large num-
bers of delicate, thin-walled spores. The
spores fall On nearby leaves of healthy
plants, terminate and send fungus
thread into the tissues. Here the fungus
branches and ramifies through the leaf,
killing the tissues and eventually pro-
ducing the characteristic brownish
blotches on the leaf. The under sides of
the outer portions of these diseased areas,
more especially in the morning or on
rainy or cloudy days, show delicate
frosty covering made up of hundreds Of
fungus branches, each carrying numer-
Gus spores. All of the above takes place
before the owner of the field is aware
that the disease is there If the lower
leaves of the healthy plants are thor
oughly coated with bordeaux mixture
early in the season and spraying is fol-
lowed up to protect new growth the dis
ease cannot get beyond the first stage
mentioned above, for the protective
spray is on the leaf in advance of the
spore of the fungus.
The spores of the late blight fungus are
also readily killed by drying. Hence hot
sunshiny weather checks the spread of
the disease while moderately cool, cloudy
or rainy weather favors it. The spores
have no means of getting from plant to
plant of themselves but they are light and
are very likely readily carried by the
wind. Once the upper leaves are dis
eased it is evident that those below may
be infected as the result of spores falling
on them Rainfall both by washing and
scattering is undoubtedly an important
factor. Moreover, if temperatures are
right the spores of the fungus, instead of
terminating directly, NAV each produce
several free, swimming bodies which
move about in films of rain or dew on the
eaf. After they come to rest they also
throw out germ tubes which penetrate
The leaf tissues and thus start new centers
Of disease. Simple contact of diseased
and healthy parts either caused by being
blown by wind or movement produced by
any other agency is also factor. Pota-
10 tubers become infected as the result of
he spores being washed down into the
soil from the diseased leaves.
There is common notion that insects
are an. important factor in the spread of
his disease and this seems to be a reason
able supposition. Some unpublished OBJ
'ervatlonS made by the pathologlst OF
his Station as the result of an outbreak
If late blight in an experimental green
house bear on this point. Circumstantial
evidence strongly indicated that either J
he flea beetle or the potato aphid were
he responsible agents in carrying the
disease from the field to the greenhouse,
,ut positive proof was not obtained. No
positive evidence was obtained that the
spores of the fungus adhered to the bodies
If the aphids. On the other hand it was
apparent that the flea-beetle might act as
l carrier, but this was by no means the
universal rule where they were taken
rom plants on which an abundance of
he spores of the late blight fungus were
being produced.
CHARLES D. MOODS. Director.