Oxford Democrat., 1920-05-25
Dislnfect Seed Potatoes.

Dislnfect Seed Potatoes.
For some years the pathologists of tht
Maine Agricultural Experiment Station
have given more or less attention to tht
Rbizoctonis disease Of potatoes and tht
methods of its control. Ample evidence
has been secured to show that this disease
is one Of much economic Importance IL al
parts of the state, but some of the wors
cases of injury have been seen in the
southern and central parts. There Is nc
common name for this trouble. Rhizoc
lonia IL the generic name Of the fungus,
as originally described, which causes It.
Every housewife, who prepares pota
toes for the table, and every observant
potato grower is familiar with one form
of this fungus, although 1t Is not usually
recognized by them as such. The little,
black or brownish- black specks or larger,
raised patches often found closely ad
herring to the surfaces of potato tubers,
but which can be removed readily by the
thumb nail or by vigorous scrubbing with
q stiff brush when the potatoes are wet,
are not PARTICLES or Jumps OF dirt, as Ia
often supposed, but compact masses of
threads of the Rhizoctonia fungus, called
"sclerotia." The condition which they
cause is often referred to as "black
scurf" or black scab". These sclerotia
serve the same purpose for the fungus
that the tubers upon which they occur
serve for the propagation of the potato
plant. As soon as, and probably some
time before, the planted potato tuber
begins to sprout they begin TO throw out
fungous threads which grow with consld-
erable rapidity If conditions are right.
Rhizoctonia manifests itself on the be-
low-ground parts OF the potato 10 various
ways, but by far the most important Of
these is in the form of an injury or brown.
ing where the fungus attacks the stems.
Young, tender sprouts are much more
seriously injured, often being entirely
cut of, either before they reach the sur
face or when the plants are still small.
Hence the fungous threads that grow from
the sclerotia on the potato seed pieces
are advantageously placed for they have
young, succulent potato sprouts right sr
hand upon which to grow and feed and
cause destruction.
While the Rhizoctonia fungus is a nor.
NAL inhabitant Of all of our potato soils
there is probably no Maine potato field
entirely free from tit is obvious that
if the sclerotia upon the surfaces of the
seed tubers can be destroyed an impor-
tant step has been taken toward remov.
ing the chief source of injury from it. II
has been recognized for some time that
both the formaldebyde and corrosive
sublimate seed treatments for potato
scab were In varying degree effective In
killing these sclerotia. It has been
claimed, and certain unpublished expert.
ments conducted by this station agree
with this claim, that the corrosive subli.
mate treatment is the more effective of
the two for Rhizoctonia.
New or modified "quick" and "easy"
methods of seed tuber disinfection have
from time to time. One of
these originated largely with this sta
ton and. while it was very promising at
first, difficulties arose In the practical
application of 1t which caused 1t to be
abandoned. Hence, even at the risk of
being classed as non-progressive we have
been advising our correspondents to stick
to the old, established methods of rela-
lively long soaking in corrosive subli-
mate or formaldehyde solution. Infor.
nation just received furnishes q very
justification for this position.
For the past four years Professor B. L.
Richards of the Utah Experiment Sta.
ZION, first In Utah and later working with
DR.. L. R. Jones at the University Of Wis.
consin has been trying to determine the
factors which control the amount OF in
jury caused by Rhizoctonis, as we'll as
the merit of the different methods of
seed potato disinfection. II Is believed
that his conclusions are 80 we'll founded
and 80 important that Maine potato
growers should at once have the benefit
of them. He says that tho results have
convinced him and also the pathologists
and horticulturists who have followed
this work in Utah and Wisconsin that
until further evidence Is secured the one
safe, practical procedure is to regard
Rhizoctonia seriously and for it's control
to treat seed potatoes In pretty nearly
the "old-fashioned" way with rather long
soaking In corrosive sublimate solution.
The following is taken from a state
ment recently issued by Professor Rich
ards, giving a summary of bis work and
the conclusions therefrom. In this state.
ment we have taken the liberty to sub.
stitute the term corrosive sublimate for
mercuric chloride. Both mean the same
chloride is the name
used by chemists, but corrosive subli.
mate is more commonly employed by the
public. Corticium vagum is the name
mycologists now give to the Rhizoctonia
fungus, and mycelium is the term for
fungous threads in general.
Four ounces of corrosive sublimate in
80 gallons of water makes approximately
a 1:1000 solution, or solution of proper
strength for seed tuber disnfection. The
standard method of treatment Is to soak
the uncut seed tubers for 1.2 hours In
this solution. As will be seen below 1t
is better to do this before the tubers
sprout. Corrosive sublimate solution
must be used in wooden tubs or barrels
and not In metal containers. 1t should
be remem bered that 1t is strong poison
and should be handled accordingly.
Tubers disinfected with 1t, unlike those
treated with formaldehyde, are not safe
for food.
"The results clearly show that: (D
these serious losses occasioned by tuber-
borne organisms can be almost COM
pletely prevented by proper methods of
seed tuber disinfection; (2) 2 hours
treatment of tubers in solution of cor
rosive sublimate Q:100O) gave plants
practically free from LESSONS and from
the mycelium of Corticium uagun,; (9
the average yield from tubers variously
treated with corrosive sublimate was in
certain plots twice that obtained from
unlreated seed; (4) shorter time treat.
ments Of 80 to go minutes In q corrosive
sublimate solution Il :1OO0) and 2 hours
treatment in formaldehyde solution
Il pint to 80 gallons) was found to exer-
else decidedly protective function
against 'Rhizoctonia, but were not ef-
feotive In controlling this fungus; (5)
A.