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COMPARATIVE ACTION OF CERTAIN FUNGICIDES ON THE SCLEROTIA OF

RHIZOCTONIA some-:1 Kim: on POTATO TUBERS

 

By

George Lewis arnes

A THESIS

Submitted to the School of Graduate Studies of Michigan
State College of Agriculture and Applied Science
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Botany and Plant Pathology

1950

1:15.515

 

 

ACKI‘TOWLEDGTTENT

The author is grateful to Dr. J. H. Muncie for
the advice and suggestions given throughout this in-
vestigation, for the use of his equipment, chemicals,
and library facilities and for the criticism and cor-
rection of the manuscript. '

Acknowledgment is made to both Dr. J. H. Muncie
and to Mr. M. R. Hatfield for the photographic work.

**********
********
******
##1##

*1!

*

TABLE OF CONTENTS

Page

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Economic Importance of the Potato Crop..................
Potato Diseases of Nichigan.............................
History of the Causal Fungus of the Rhizoctonia Disease.
Taxonomy and Forphology of Rhizoctonia Solani K3hn......
Early History of the Disease in America.................
Economic Importance of the Disease......................
Symptoms of the Disease................................. 1

 

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INTRODUCTION

Economic Importance of the Potato Crop

 

The production of potatoes is limited largely to the areas of
the north temperate region of the world where the climate is fairly
cool and moist. The potato plant is grown largely for its edible
subterranean tubers which supplies a cheap, nutritious, carbohydrate
food to at least a third of the world's total population. Hardenburg
(15) states that the potato ranks first in terms of volume (23.4 per
cent) of the fresh product and is outranked only by Rice in terms of
population fed.

Eighty per cent of the total potato crop of the entire world is
grown in Europe alone due to the cool growing season and cheap hand
labor in that region. By contrast, according to Hardenburg (13), the
United States produces annually only about 4.4 per cent of the world
total.

On a state level, Vichigan, producing around 19,000,000 bushels
per year, ranks sixth and ”aine ranks first by producing around
54,000,000 bushels annually. There is presented in table I, a pro-
duction comparison of the fifteen leading potato states according to
a recent 12 year average from the United States Department of Agri-
culture publication, Crops and markets (1).

Of the fifteen leading potato states there were several which,
during the 12 year period 1937-1948, produced 115 bushels or less
an acre. They are in order of decreasing yield per acre; Ohio, North

Carolina, Nfichigan, Nfinnesota, and'Wisconsin.

-1-

TABLE I

ACREAGE AND PRODUCTION IN THE 15 LEADING POTATO GROWING STATES

 

 

 

 

Production Yield

Acreage (Thousand Per Acre

State (Thousands) Bushels) ‘ (Bushels)

Average Average Average

1937-48 1937-48 1937-48
Heine 213 54,220 300
Idaho 148 35,204 238
NeW'York 186 31,073 388
California 89 29,661 325
Pennsylvania 151 19,614 132
Michigan 182 19,231 109
Minnesota 190 18,881 104
North Dakota 139 17,512 121
Colorado 80 15,933 200
Wisconsin 153 13,290 91
New Jersey 61 11,073 184
Nebraska 71 10,229 143
North Carolina 83 9,250 113
Virginia 72 9,245 128
Ohio 76 8,445 115

 

 

The relatively low yield per acre for Michigan, prior to 1948,
is due to several limiting factors, each of which must be taken into
account in any attempt to increase the yield. Some factors to con-

sider are as follows: climate, soil, varieties, fertilizer, pest

control, and disease free "seed" stock. Although for the period
1937-48 the average yield in Michigan was 109 bushels per acre; for
1948 and 1949 it had risen to 150 and 165 bushels per acre respec—
tively. This increase in yield per acre is due to several factors.
They are: increased use of irrigation, increased use of high quality

fertilizer, and the federal farm price support plan.

Potato Diseases of Michigan

 

The potato, being an established crop in Michigan and grown under
a wide range of soil and climatic conditions, is subject to a wide
variety of diseases. Some of these diseases are of only slight eco-
nomic importance while others assume serious proportions under certain
conditions and at other times caused very little loss to the crop.
Some, such as scab and Rhizoctonia, cause serious reductions in yield
and quality every year.

Each one of the diseases present in Michigan is not in itself a
limiting factor in potato production in this state, but all of them
together are responsible for annual losses amounting to many thousands
of bushels.

Almost all of the diseases common to the potato are present in
Michigan. Therefore, many are of considerable importance; scab,
Rhizoctonia, wilts, late and early blight, and various virus diseases.
They are discussed fully in various technical publications and bulle-

tins of the Michigan.Agricultural Experiment Station and Extension

Service. All of the above diseases are discussed in detail by
Nuncie (20) in one of these publications which should be very useful

to all potato growers in this state.

History of the Causal Fungus of the Rhizoctonia Disease

 

Root destroying fungi of the genus Rhizoctonia were first dis-

covered by De Candolle in 1815 (9). He named two species: Rhizoctonia

 

medicaginis, occurring on Medicago, Trifolium, and related hosts; and

 

 

Rhizoctonia crocorum, destructive to crocus bulbs. Little important

 

work was published on Rhizoctonia until 1851, when the Tulasne brothers
(9) gave a comprehensive summary on all the described species of the
genus and the various host plants. They concluded that all these
species were not to be regarded as distinct from each other but rather

as one species which they called Rhizoctonia violacea. However, in

 

1858 (9) Kuhn discussed more at length certain forms of economic impor-

tance and described a new species which occurred on the potato, Solanum

 

tuberosum L., and which he named Rhizoctonia solani.
Many attempts have been made to connect the sterile fungus

Rhizoctonia solani Kfihn with a perfect stage of various fungi. Several

 

fungi have been suggested as being the perfect stage through their
frequent association with the sterile stage but evidence of genetic
relationship was always lacking. However, Rolfs in 1903 (22), while
working with the Rhizoctonia disease of potatoes in Colorado, con-
stantly found a basidiomycete on the stems associated with the presence

of Rhizoctonia solani Kuhn on the roots, stolons, and tubers.

 

.4-

Dr. E. A. Burt identified the fungus as Corticium vagum.B.& C.var.

 

solani but later dropped var. solani because he believed that there

is no varietal distinction between Corticium.vagum.8. & C. and the

 

fungus occurring on potatoes. Rolfs was able to trace the connection
between.the two fungal forms and finally completed his evidence when

he obtained cultures of Rhizoctonia solani Kuhn from single spores of

 

Corticium vagum.B. & C. For certain mycological reasons, Rogers (25)

 

in 1943 proposed that the name Corticium vagum B. & C., be changed to

 

Pellicularia filamentosa (Pat) comb. nov. Since the name Pellicularia

 

 

filamentosa (Pat) Rogers has been used in the recent literature to

 

denote the sexual stage of the fungus, it is evident that the name has

become accepted and valid.

Taxonomy and Uorphology of Rhizoctonia Solani Kfihn

Taxonomy

Rhizoctonia solani Kfihn is the imperfect or sterile phase of

 

the basidiomycete, Pellicularia filamentosa (Pat.) Rogers, of the

 

family Thelephoraceae, order.Agaricales. As Rhizoctonia solani Kuhn,

 

the fungus is a member of the form genus Rhizoctonia, order’vycelia
Sterilia of the Deuteromycetes (Fungi Imperfecti).
Peltier (22) states that Duggar has given the following synonomy

under Rhizoctonia solani Kuhn:

 

Rhizoctonia napaeae'West (1846)

 

Rhizoctonia rapae West (1852)

 

Rhizoctonia solani Kfihn (1858)

Rhizoctonia betae Eidam (non.K3hn) (1887)

 

The following names are now synonyms for Pellicularia filamentosa
(Pat.) Rogers:

Hypochnus solani Prill. & Del. (1891)

 

Corticium'vagum B. & C. (1903)

 

Hypochnus solani Prill.& Del. has been accepted by the Europeans

 

in general as the perfect stage of the fungus. However, Peltier (22)
states that "in his monograph on the Thelephoraceae, Burt limits
Hypochnus to resupinate species with colored, echinulate spores, while
under Corticium he includes species always resupinate, which have color-
less spores and lack cystidia. According to Burts' classification, 32227

chnus solani Prill. & Del. becomes a synonym.under Corticium.vagum B. & C.."

 

 

Both are now synonyms for Pellicularia filamentosa (Pat.) Rogers.

 

-6-

1

forpholory

The morphological characters of Rhizoctonia solani Kuhn,vary

 

with the age of the mycelium. The young hyphae branch from the parent
hypha at an acute angle and then grow parallel with it. A constriction
is present at the union of a hyphal branch with the parent hypha and

a septum is usually laid down.a few microns from the point of union.
The young hyphae are frequently strongly vacuolate and are light in
color. With age the hyphal branches tend to form.at right angles to
the parent hypha and are darker in color and less strongly vacuolate
than the young hyphae.

The formation of sclerotia in nature is common, but they vary
considerably in size and shape. 0n potatoes they are generally flat
and measure from.about one to ten millimeters in width on the average.
Occasionally giant flat sclerotia are formed. Sclerotia are formed
by the grouping together of many hyphal filaments to form.a compact
mass of brown hyphae which soon turn black and become very resistant.
Apparently they form in response to moist conditions in the environ-
ment. They appear on potato tubers in the fall, especially if the
tubers have been left in the ground long beyond the normal harvesting
time.

Less frequently, during the summer the Pellicularia (Corticium)
stage is formed. This stage is formed by the dark-brown hyphae of
the sterile stage gathering about the base of a plant and giving rise
to an ashy gray mycelium which forms a loosely attached fine network

about the stem. The outer hyphae of the ashy gray'nycelium bear

club-shaped basidia with four sterigmata and basidiospores. The
basidiospores are colorless, oval to ovate, and have pointed bases.

The usual basidiospore measurement varies from 9 to 14" by 6 to 8/1.

Early History of the Disease in America

 

Although a Rhizoctonia disease of potato has long been common
in Europe, especially in Germany, where it is known as "Grind" and
"Pockenkrankheit", there had been no report on Rhizoctonia in.America
until Pammel (9) in 1891 reported that a beet rot in Iowa was caused

by Rhizoctonia betae Eidam (R. solani Kuhn). The first record of

 

R. solani Kuhn occurring on potatoes in.America as reported by Duggar
and Stewart (9) was made by Sirrine and Rolfs who found it on potato
stems collected on Long Island, New'York in 1898. Later surveys in
1899, by Duggar and Stewart, demonstrated that the fungus could be
found on the stems and tubers of potatoes in many areas of the state
of New York. Specimens of tubers and stems bearing sclerotia of

Rhizoctonia solani Kfihn'were received by Duggar and Stewart during

 

1899 and 1900 from many states including Pennsylvania, Ohio, Michigan,
Alabama, Colorado, Iowa and Haryland. They suggested that the fungus
is probably very generally distributed. Time has proven their assump-

tion to be correct, because Rhizoctonia solani Kuhn is now found

 

wherever potatoes are grown in North.America.

Economic Importance of the Disease

 

The Rhizoctonia disease of potatoes is very widespread; i.e.,
being present in all of the continents of the world and the'West
Indies, Japan, and New Zealand. Apparently all commercial varieties
of potatoes are susceptible to the disease.

There is presented in tablelI a comparison of the reduction of

yield of potatoes due to Rhizoctonia solani Kuhn based on estimated

 

figures from certain issues of the Plant Disease Reporter (3).

TABLE II

REDUCTIOH IN YIELD OF POTATOES DUE TO LOSSES RESULTIVG FRO” THE
RHIZOCTONIA DISEASE

 

Per cent Reduction in Yield

 

 

 

 

 

Category
'51 '32 '55 '34 '35 '36 '37
111 Diseases 18.8 22.3 16.6 -- 21.0 15.7 18.3
(U.S.)
111 Diseases 24.7 12.0 12.3 18.8 10.3 18.0 31.8
(Uich.)
Rhizoc. Disease 2.3 2.5 2.7 -- 3.5 2.2 1.5
(U.s.)
Rhizoc. Disease 0.5 0.5 1.0 1.0 1.0 5.0 2.5
(Mich.)

 

 

For the seven year period covered in.tablelI, Michigan lost on
the average about 1.6% of the annual potato yield for the state.
During the same period the average yearly loss for the United States

as a whole was about 2.5%.

Symptoms of the Disease

 

The Rhizoctonia disease of the potato, Solanum tuberosum L.,

 

caused by Rhizoctonia solani Kfihn is very common and widespread; being

 

found anywhere in the world where potatoes are grown to any extent.
It is present, and severe under some conditions, in all the Northern
potato growing states of the United States.

The disease not only affects the potato, but also many vegetables,
field crops, ornamental plants, and certain weeds. In the case of
potatoes, losses resulting from this disease are of two types. First,
there is an actual loss due to poor stands, low yields, and a decrease
in marketable tubers. Secondly, there is an indirect loss due to re-
jection for certification for "seed" stock and also lowered quality
for table stock.

The disease is usually more severe on potatoes in the presence
of abundant soil moisture in all types of soil; especially those of
an acid nature, even though the fungus is frequently prevalent in neu-
tral or slightly alkaline soils. The fungus generally thrives under
those conditions favorable to the growth of the potato plant itself.

The Rhizoctonia disease of the potato has been given a wide
‘variety of common names, each usually according to an expression of
the disease on some particular portion of the plant. According to
Heald (14) the disease has been given such names as black speck, russet
scab, black pox, black scurf, and little potatoes, because of the
effect on the tubers; brown stems, stem rot, stem.canker, and collar

rot due to the nature of the stem attacks; and rosette, leaf roll,

-10..

and aerial potato because of accompanying physiological disturbances.
Because it is impossible to use a name which will describe the symp-
toms of all the different stages, the generic name of the fungus,
Rhizoctonia, has been applied to the disease as well and apparently

is the generally accepted common name for the disease now. In the
past, the terms rhizoctonosis and rhizoctoniose have been used by some

writers.

Top Symptoms

 

A severely injured root system or stem will result in certain
physiological responses in the aerial portion, of the plant.

A leaf roll similar to the effect of drought or to early symptoms
of the leaf roll virus disease is frequently the result of extensive
root injury. The leaflets at the top of the shoots roll upward and
may become yellow but the lower leaves do not become rolled, harsh,
and brittle later as occurs when.true leaf roll is present. Since
true leaf roll is present in most potato seed stocks, the presence of
Rhizoctonia can not be positively verified by this symptom.alone.

A rosette of yellow or green leaves often occurs if the base of
the stem is severely attacked after the plant has grown considerably.
This is due to a shortening of the new petioles of the stem so that
the top leaves are massed together in a rosette fashion. Since the
canker sometimes prevents the translocation of elaborated sugars from
the leaves to the tubers, the excess food causes the vine to become

unusually large. These large plants will set a large number of small

-11-

knobby and russeted tubers which will never increase in size and are
consequently unmarketable.

Leafy aerial tubers sometimes develop in the axils of the leaves
when the stem has been injured to the extent that translocation can
not occur. Aerial tubers can also result from injuries due to other
causes and are, therefore, not a positive evidence of the Rhizoctonia

disease.

Symptoms on Stems

 

According to Dana (8) injury to the stem.may take several forms:
stem cankers or lesions, growing point injury, corroded stems, or
the stem may be covered by the mycelium.of the fungus which does no
noticeable damage but which can serve as a sign by which the disease
may be identified.

The stem lesions are of varying size, and in extreme cases may
even girdle the stem. These lesions develop most commonly on shoots
from.very early or volunteer plants and as the season advances these
lesions may be largely outgrown.

Growing point injury is caused by lesions forming at the shoot
tips where the tissues apparently are very susceptible to attack by
the fungus. This type of injury is most serious in early planted
fields because of the low temperatures and abundant moisture at that
time. Every emerging shoot may be killed in this manner until the
seed piece is exhausted and produces no more shoots, resulting in a

missing hill.

-12-

Corroded stems are found on the older infected plants. This
corrosion by the fungus may even girdle the stem at its base and give
rise to the various physiological symptoms previously discussed.

Root and stolon development may be hindered or even prevented from
this area. Frequently a cluster of malformed tubers is formed about
the stems very close to the surface of the ground. Loss from this
type of injury will depend upon the extent to which the nourishment

of the plant and the development of the tuber is disturbed.

Symptoms on Roots

 

Injury by Rhizoctonia solani Kfihn occurs to roots of all sizes,

 

but is most destructive to the feeding roots. The larger roots may
shOW'cankers while the largest roots are affected to a serious extent
only when the base of the stem is seriously corroded in which case
they may be prevented from developing properly.

Injury to the feeding roots is one of the most serious effects
of the disease; because if the small rootlets are killed, the absorb-
ing capacity of the plant is materially reduced. This is particularly
harmful to the plant during dry situations and often results in poor
or abnormal top growth even when sufficient moisture is present.

Careful observations tend to show that feeding root injury by
R. solani Kfihn is common wherever potatoes are grown and constitutes

a serious phase of the disease.

\

Symptoms on Stolons

 

The stolons, or underground stems on which the tubers are borne

may be attacked by Rhizoctonia solani Kfihn in much the same way as

 

are the stems. Cankers on the stolons will interfere with the nour-
ishment of the young tubers and in severe cases may cut off the stolons
and prevent the formation of tubers. This type of injury occurs most
frequently in wet poorly drained soils and often results in consider-

able loss to the potato grower.

Symptoms on Tubers

 

Rhizoctonia solani Kuhn in the resting or dormant stage is common

 

on tubers as dark brown or black bodies (sclerotia) which vary in size
from more specks to giant sclerotia an inch or more in diameter. Since
these sclerotia are often hard to wash off they have been called by
some housewives as "the dirt that won't wash off." There may be only
one or two sclerotia on a tuber, or the tuber may be almost completely
covered by them. These sclerotia are sometimes called black scurfs.

The attack of the fungus on the tuber may take the form of a rus-
seting or rotting. This russeting, or roughening of the skin of the
tuber, resembles an infestation of the potato scab organism in many
cases. It may cover a part or nearly all of the tuber and render it
unattractive or subject to diseases in storage.

Rhizoctonia solani Kfihn seldom induces rotting in potato tubers,

 

but at times it, together with bacteria and certain fungi, notably

Fusarium spp., causes a so-called stem end jelly rot.

 

-14-

Under certain conditionis. solani Kfihn invades the lenticels
and produces very small dry pits that can be lifted from.the tuber
with the point of a knife. This type of infection has been desig-

nated sand scurf by Ramsey, Wiant and Smith (24).

-15..

REVIEW OF THE LITERATURE

Black scurf, or Rhizoctonia disease of the potato, caused by

the fungus Rhizoctonia solani Kfihn, is widespread in Michigan, and

 

other states, and sometimes causes considerable loss to the potato
grower. There is a definite need for control of this disease and
since it is impractical to treat large areas of soil, fungicidal
treatments of the infected "seed" potatoes has come into common usage.
These treatments may reduce both the damage by Rhizoctonia and seed
piece decay. Both results can increase the yield.

Seed treatments for the control of certain potato diseases was
first proposed by Bolley (5) in 1891 when he suggested the use of
disinfectants such as are used in medical practice. He recommended
the use of mercuric chloride, 2 oz. in 15 gallons of water. This
standard treatment for control of potato scab called for soaking
whole potatoes in HgClz(l-1000) for one and one-half hours. As a re-
sult of the research by Arthur in 1895 (5), the use of formalin
(1-240) almost completely replaced the use of corrossive sublimate
solution in general practice in.various states.

However, about this time the various workers investigating the
relative killing power of the two chemicals on the sclerotia of

'Rhizoctonia solani Kfihn began to fall into two schools of practice.

 

One group advocated the use of corrosive sublimate solutions while

the other group backed the use of formalin solution, either hot or

cold as the case may be.

-15-

The literature on the use of corrosive sublimate solutions and
formalin solutions is voluminous and will not be extensively re-
viewed here. However, excellent summaries on the use of corrosive
sublimate solutions are given.by Gloyer (ll), Coons (5), and Goss and
Werner (12); and on the use of formalin solutions by Helhus and Gilman
(17), Gilman and Melhus (10), and Nelhus, Gilman, and Kendrick (18).
The use of acidulated corrosive sublimate was first proposed by Cun-
ningham (16), modified by Leach, Johnson, and Parson (16), and com-
parative field trials were run by Plummer and Bonds (23), Sanford (26),
and‘Wheeler and Mbore (28) in comparison with the standard fungicides.

In reviewing the literature covering the use of these "old"
standard fungicides, several general conclusions stand out:

(a) Hot formalin solutions are more effective than the cold solu-
tions but are more phytotoxic, resulting in low or retarded
emergence of shoots from treated tubers if temperatures are
not strictly regulated. The use of the hot solutions re-
quires special equipment and sensitive temperature controls
which add considerably to the cost of treatments.

(b) If the treatment is applied for killing the tuber borne scab
organism alsone, either the standard corrosive sublimate
solution or the standard formalin solution may be used; but
when both Rhizoctonia sclerotia and scab are present the
corrosive sublimate solution has been found to be the more
effective fungicide. Apparently the formalin penetrates

the sclerotia with difficulty and sometimes doesn't even

-17-

kill certain fungi hand bacteria associated with the
sclerotia.

(c) The use of corrosive sublimate is a health hazard and it
is also phytotoxic. If the tubers are not dried rapidly
after treatment they may be injured to such an extent that
no or few sprouts will emerge after planting.

(d) Acidulated corrosive sublimate is more effective than non-
acidulated corrosive sublimate in killing the sclerotia of

Rhizoctonia solani Kfihn. Treated tubers must be dried

 

after treatment to avoid injury to the sprout primordia.

‘Within the past twenty years various organic mercury compounds
have been developed and used to some extent for tuber treatments.
Conflicting data with the use of the organic mercurials in the con-
trol of Rhizoctonia have been presented by various workers.

Goss and Werner (12), state that‘White in summarizing experiments
reported from various states and Canada in 1926, concludes that "In
general, although by no means consistentky so, the Semesan Bel com-
pounds used as a 10 per cent dip have given satisfactory control of
stem lesions, have resulted in.at least 20 per cent increases in yield
and have yielded a crop as free of sclerotia as the standard corrosive
sublimate treatment." Clayton (4) has stated that the organic mercur-
ials controlled potato scab as well as corrosive sublimate, but'were
less effective against black scurf (tuber borne Rhizoctonia). Miles
(12) in 1929 stated that in no case was there any effective field

control of Rhizoctonia, as measured by the sclerotia present on tubers

-18...

at harvest, with either Semesan Bel or Bayer Dip Dust, while mercuric
chloride gave good control. Michigan tests conducted in 1928 and

1929 by Hoore and Wheeler (19) indicated also that mercuric chloride
is more effective than the organic mercurials. Wheeler and Hoore (28)
in 1933 found that Semesan Bel, Bayer Dip Dust, and Sano seed were
less effective than corrosive sublimate in controlling scab and the
Rhizoctonia disease.

The tendency of the work on the effectiveness of the organic mer-
curials is to consider them as less effective against the Rhizoctonia
sclerotia than against scab. Cunningham (6,7) reports that treatments
with yellow oxide of mercury and The New Improved Semesan Bel caused
a delay of shoot emergence and decreased yield when used on certain
varieties of potato tubers.

From the foregoing review of the literature it is readily seen
that there is a need for new materials in the treatment of Rhizoctonia
infected potato tubers. The author will present his results with
laboratory experiments with certain "new" fungicides manufactured by

the Dow Chemical Company, Midland, Michigan.

-19-

CURREH T RESEARCH

Objective

The objective of this investigation was to study the comparative

action of certain fungicides for use in potato tuber treatments which

will meet the following desired specifications for the control of

black scurf of potato tubers:

(a)

(b)

(1‘)

The material must be as effective as the present standard
treatment in Michigan; i.e., soaking whole tubers in HgCl2
(1-1000) for 30 minutes.

The material must be effective in the same, or less, time
than the standard 30 minute soaking period.

The material must be effective when the solution is cold

so that the treating equipment and operating procedures

will be simplified.

The cost of the material should be much lower than the cost
of mercuric chloride.

The material should be non-corrosive so that metal containers
can be used if desired.

The material should be other than the mercurials (calomel,
corrosive sublimate, acidulated corrosive sublimate, yellow'
oxide of mercury, ethyl mercury iodide, and the organic mer-
cury compounds such as Semesan Bel, etc.). Ken.Knight (15)
has shown that where these compounds are used in Michigan,

and some other states, there is an increase of potato scab

-20-

due to the presence of mercury tolerant parasitic actinomy-
cetes in the soil. In addition, the use of mercurials
presents a health hazard wherever they are used.

(g) The solution must not lose significant strength after using
it for a reasonable time.- If the solution maintains its
strength then there is no need to frequently renew its
strength or to discard the solution and make up a new one.
Obviously, to discard a solution after only a few lots of
tubers have been treated or to renew its strength frequently
is very expensive. Because of these difficulties, many farm-
ers either ignore tuber disinfection or use easier, but less
effective treatments.

From the foregoing statements it is readily seen that there is

a real need for a tuber disinfectant that will meet the above speci-

fications.

-21-

materials and Methods

For the laboratory experiments potato tubers of the Katahdin,
Chippewa, Pontiac, and Russet Rural varieties were obtained from the
Lake City Experiment Station, Lake City, Michigan. Each tuber was
selected for the presence of abundant sclerotia and an effort was
made to obtain sclerotia of all sizes. These tubers were kept in stor-
age until selections were made for the various fungicide trials.

Various standard and new materials were employed in.a study of
the comparative fungicidal effect in killing the sclerotia of

Rhizoctonia solani thn at certain dilutions and exposure times.

 

These fungicides are: corrosive sublimate (HgClZ), acidulated corro-
sive sublimate, New Improved Semesan Bel, SCD 765 (Dow Chemical Com-
pany), a number of proprietary organic compounds called Dowicides.

The New Improved Semesan Bel is an organic mercurial fungicide
manufactured by the Dupont Semesan Company (Inc.). It is composed of
12 per cent hydroxymercurinitrophenol, 3.2 per cent hydroxymercuri-
chlorophenol, and 84.8 per cent inert ingredients. The mercury equiva-
lent of this compound is 9.5 per cent.

S.C.D. 765 is a proprietary compound produced by the Dow Chemical
Company. The chemical components of this fungicide cannot be re-
vealed at the present time.

The Dowicides are manufactured by the Dowicide Division, Dow
Chemical Company, Midland, Michigan. They are for the most part,
sodium salts of various chlorinated phenols. Some are sodium salts

of phenyl phenol compounds. They are differentiated by the addition of

-22-

one of the capital letters of the alphabet immediately following the
term Dowicide. Their known fungicidal properties and uses are dis-
cussed in a publication (2) of the Dow Chemical Company. They are as
follows:

Dowicide A. Sodium Salt of Orthophenylphenol.

Dowicide B. Sodium Salt of 2,4,5-Trichlorophenol.

Dowicide C. Sodium Salt of 2 Chloro-Orthophenylphenol.

Dowicide D. Sodium Salt of 2 Chloro-4-Phenylphenol.

Dowicide E. Sodium Salt of 2 Bromo-4-Phenylphenol.

Dowicide F. Sodium Salt of 2,5,4,6-Tetrachlorophenol.

Dowicide G. Sodium Salt of Pentachlorophenol.

As a consequence of the results obtained by the early workers,
both in the laboratory and the field, laboratory methods were devised
by which the field value of a given tuber treatment could be determined
without the necessity of field trials. A review of the earlier work
showed that the results of the laboratory methods were confirmed by
subsequent field trials.

Various investigators have made contributions to the methods of
aseptically growing Rhizoctonia from sclerotia on agar in Petri dishes.
Gloyer (11) states that Duggar and Stewart have noted that Rhizoctonia
grows well on acidulated agar without interference from bacteria. In
his work, Gloyer aseptically placed sclerotia on potato agar of one
per cent acidity, but it was found that the plates were soon overrun

by the potato bacillus, Bacillus vulgatus Trevisan, when untreated

 

sclerotia or those treated with formaldehyde were used. To keep down

-23-

these bacteria, he added two drops of 50 per cent lactic acid to each
10 cc. of the medium. Gloyer generally placed five sclerotia in each
Petri dish, allowed them to set at room temperature, and after three
to five days the results from the untreated sclerotia could be deter-
mined. In the case of the treated sclerotia, the cultures were fre-
quently examined and after ten days they were examined with the low
power of the microscope to determine if hyphae of Rhizoctonia were
growing from the sclerotia. Helhus, Gilman, and Kendrick (18) used
essentially the same method in their laboratory work involving the
fungicidal action of formaldehyde. In 1933, Sanford and marritt (27),
in a laboratory study of viability of sclerotia after certain treat-
ments, aseptically placed treated sclerotia on potato dextrose agar
which was acidified, while it was cooling after sterilization, by add-
ing 3 per cent by volume of 5 per cent lactic acid.

The method as used by Sanford and Marritt was employed in the
present investigations. It has proved very satisfactory in preven-
tion of contamination, especially by bacteria. Usually ten or more
sclerotia were placed in each Petri dish. The Petri dishes were stored
at room temperatures, examined macroscopically every few days, and
then examined with a low power binocular microscope after four to six
days to determine the number of sclerotia which lived through the
treatments and produced Rhizoctonia hyphae.

It has been observed by various investigators that the size and
density of the sclerotia play an important part in any tuber treatment

test either in the field or in the laboratory.

-24-

The first study on the relation of the size of the Sclerotia to
the required time necessary to kill them with various fungicides was
published by Sanford and Harritt (27) in 1933. They found, in count-
ing the sclerotia from representative lots of infected potato tubers,
that from a total of 4,732 sclerotia, 76.3 per cent were small, 17.6
per cent were of medium size, and 6 per cent large. From these figures
they suggested that the proportion of each size occurring in nature
was a ratio of 13:3:1. Extra small and extra large sclerotia were not
counted. They showed that all of the small and medium.sclerotia could
be killed in 15 minutes in acidulated mercuric chloride (1-500) and
only 20 per cent of the large ones survived. In reference to the very
small number of large Sclerotia occurring in nature, this percentage
is negligible. It can be assumed that any treatment which will kill
the small, medium, and most of the large sclerotia is an effective
one from both field and laboratory standpoints.

Coons (5), after treating infected tubers one half hour in corro-
sive sublimate (l-lOOO), without any regard to the sizes of sclerotia,
obtained a 98 per cent stand of plants with commercial control of the
disease.

In certain laboratory experiments by Muncie (21) in 1934 it was
necessary to determine the thickness of the sclerotia in order to
test the efficiency of certain tuber treatment fungicides. For this
purpose, be devised a very simple measuring instrument. This cone
sisted of a glass tube, 7.5 millimeters in internal diameter and about

four inches in length, within which was inserted a glass rod somewhat

-25-

longer than the tube. A millimeter scale was marked on a label and

the label in turn was glued to the glass rod so that the top line of
the graduation was aligned with the top of the tube. In measuring

the thickness of a sclerotium, the tube was set over the sclerotium

and the rod lowered until it rested on the sclerotium. The thickness
of the sclerotium was then determined by noting the distance between
the top of the tube and the top mark on the scale on the rod. In
practice the procedure followed was as follows: The thickness of each
sclerotium on a tuber was measured and the figure recorded on the

tuber with India ink near the sclerotium. The tubers were then treated,
rinsed with sterile tap water, and the sclerotia of each size range
'were removed with a flamed scalpel and plated together on potato dex-
trose agar containing 3 per cent by volume of 5 per cent lactic acid

as used by Sanford and Marritt (27). TableIIIpresents unpublished

data by Vuncie (21) in which is shown the relative efficiencies of cer-
tain tuber treatments in regard to the thickness of the sclerotia.

TablelIItends to show that any treatment which would kill both the
small and medium sized sclerotia would generally kill the large
sclerotia also.

From the foregoing investigations it seems that the size of the
sclerotia within narrow limits is not always the limiting factor in any
tuber treatment trials either in the laboratory er in the field. There-
fore, in the work reported by the writer, he considered the size of the
sclerotia as a negligible factor in Michigan and performed his experi-
ments using representative sclerotia from.all size ranges. The same
culture techniques were followed as used by the investigators previously

mentioned.

-25-

TABLE III

EFFECT OF CERTAIN FUNGICIDES ON VARIOES SIZES OF SCLEROTIA OF RHIZOC-
TONIA SOLANI KUHN

 

 

 

 

 

 

 

 

 

 

Time: Thickness Number Number Per Cent
material Minutes of Sclerotia Sclerotia Sclerotia
Sclerotia Treated Killed Killed
HgCl
1-1 00 30 0.5 mm 264 264 100
1.0 46 46 100
1.5 ~ 16 .16 100
HgCl
1-1 00 60 0.5 435 429 98.6
1.0 40 39 97.5
1.5
HgCl
1-1 00 90 0.5 264 264 100
1.0 40 40 100
1.5 8 8 100
Acid HgClz
l-lOOO' 10 0.5 504 504 100
1.0 50 50 100
1.5
HgCl
1-8 4 10 0.5 392 392 100
1.0 31 . 31 100
1.5
Semesan 10 0.5 264 264 100
Bel 1.0 24 24 '100
1-125 1.5 11 11 100
Check No 0.5 152 103 68
treat- 1.0 8 1 12.5
ment 1.5 B O O

 

 

-27-

Results and Conclusions

 

It was thought that the results of some of the laboratory experi-
ments of the earlier workers should be confirmed and substantiated by
laboratory experiments.

Table IV presents the results obtained by the author in determin-
ing the killing efficiency of corrosive sublimate, acidulated corro-
sive sublimate, and the New Improved Semesan Bel on the sclerotia of

Rhizoctonia solani Kuhn. The data presented here represents the sum—

 

marized results of five separate tests.

TABLE IV

cormmmmvs EFFECTS OF CERTAIN T-ERCURIAL FUI-TGICIDES ON THE SCLEROTIA
or mnzocrorm SOLANI Kifm:

 

AM“ “ ___._— u M

 

 

 

 

 

 

 

Time: Number Number Per Cent

Eaterial Min— Sclerotia Sclerotia Sclerotia
utes Killed Killed
Semesan Bel 1-125 10 1242 1155 92.99
HgCl2 l-lOOO 30 2198 1686 76.71
HgCl2 l-lOOO 60 1214 - 1127 92.83
HgClz l-lOOO 90 1008 942 93.45
Acid HgCl2 1-1000 10 2379 2187 91.93
HgCl2 1-834 10 423 423 100.00

 

 

-28-

Table IV shows that a 100 per cent kill of Rhizoctonia sclerotia
was obtained by soaking the infected tubers in HgCl2 (1-834) for ten
minutes. Because the number of sclerotia was rather low in this case,
this percentage may not be valid. The use of more sclerotia in this
test would have given figures which would be more indicative of its
efficiency. The data also shows that 30 minute treatment in HgClz
l-lOOO was not as effective as treatments of 60 and 90 minutes.

On the average, the percentages are high enough to consider these
mercurials as excellent fungicides from a laboratory standpoint. How-
ever, when the mercurials are used in actual field trials they may
prove to be phytotoxic if the tubers are not dried rapidly in the
shade after the treatments. Ken.Knight (15) has shown also that there
is an increase‘of potato scab after using the mercurials because of
the presence of certain mercury tolerant Actinomycetes in the soils of
several states, including Michigan. In addition, the mercurials
present a definite health hazard and are also corrosive in nature.

It can be concluded that although the mercurials are excellent
fungicides they do not meet all the desired specifications because of
possible phytotoxicity, high cost, and the health hazard involved.

To overcome these objections certain non-mercurial fungicides were
used in later tests.

Tables V, VI, and VII present the results of tests on the toxicity

values of certain new fungicides on the sclerotia of Rhizoctonia

 

solani Kuhn. The data presented here represent the summarized results

of eight separate tests.

—29-

TABLE V

COMPARATIVE TOXICITY OF CERTAIN DOWICIDES AT A 1-500 DILUTICN FOR.A
30 MINUTE PERIOD ON SCLEROTIA OF RHIZOCTONIA SOLANI KUHN

 

 

 

 

 

 

 

 

 

 

 

Number Number Per Cent

Material Sclerotia Sclerotia Sclerotia
Killed Killed
Dowicide A 93 93 . 100.00
Dowicide B 277 276 99.64
Dowicide C ' 367 367 . 100.00
Dowicide D 161 129 80.12
Dowicide E 154 99 64.29
Dowicide F 50 50 100.00
Dowicide G 295 269 91.19
NaOCl 1-2 Check 70 O 0.00

 

 

From.the results shown in table V it can be readily seen that of
the Dowicides used in this test the large majority of them are highly
toxic to the sclerotia of E. solani Kuhn.

The fungicidal properties of these compounds were not fully known
at the time of the initial tests. Therefore, the 1-500 dilution was
arbitrarily selected. Dowicides D and E are shown to be definitely

inferior, from a fungicidal standpoint, at this dilution. The sclerotia

-30..

exposed to Dowicides A, C, and F were killed in all cases. Dowicides
B and G showed over 90 per cent kill and therefore might, at this
concentration, be considered as effective fungicides.

Under the conditions of the experiment, Dowicides A, B, C, and F
might be considered as acceptable fungicides for tuber treatments.

From the foregoing results, it was decided that a series of tests,
involving a higher dilution, would be made. The results are presented
in table VI. The data presented represent the summarized results of
eight separate tests.

TABLE VI

COHPARATIVE TOXICITY OF CERTAIN DOWICIDES AT l-lOOO DILUTIQN FOR.A
3O EINUTE PERIOD ON SCLEROTIA OF RHIZOCTONIA SOLANI KERN

 

 

 

 

 

 

 

 

 

 

 

Number Number Per Cent
Naterial Sclerotia Sclerotia Sclerotia
Killed Killed
Dowicide A 984 815 82.82
Dowicide B 510 487 95.49
Dowicide C 183 180 98.36
Dowicide D 183 137 74.86
Dowicide E 119 98 82.35
Dowicide F 496 456 91.94
Dowicide G 496 474 95.57
NaOCl 1-2 Check 90 5 5.55

 

The results of the tests presented in table VI show that at a
dilution of l-lOOO, Dowicides B, C, F, and G are more effective in
killing Rhizoctonia sclerotia than Dowicides A, D, and E. At a dilu-
tion of 1-1000 Dowicide A was not as effective as it was at a dilution
of 1-500. However, Dowicides B, C, F, and G proved to be almost as
highly toxic at a l-lOOO dilution as at a 1-500 dilution. Of course,
Dowicides D and E still showed a low toxicity value at a 1-1000 dilu-
tion.

Dowicides B, C, F, and G are apparently excellent fungicides but
Dowicides D and E are inferior fungicides under the conditions of the
experiments. It is indicated that Dowicide A is an excellent fungi-
cide at a 1—500 dilution but is a rather poor one at l-lOOO.

While working with the Dowicide compounds it was observed that
Dowicide F gave high toxicity values, was highly soluble, and also
relatively safe to handle. It was thought that it would prove inter-
esting to determine its toxicity values at various dilutions. At the
same time, tests were made'with an instant dip compound designed to
prevent bacterial soft rot of potato tubers in storage. This compound
is known as S.C.D. 765. It is composed of one of the Dowicides and a
special detergent.

The results of these tests are presented in table VII. The data
presented represent the summarized results of a number of separate

trials.

-32-

TABLE VII

COMPARATIVE TOXICITY OF CERTAIN DONICIDES AT VARIOUS DILUTIONSdAND
DIFFERENT LENGTHS OF TINE ON SCLEROTIA OF RHIZOCTONIA SOLANI KUHN

 

 

 

 

 

 

 

 

 

Time: Number Number Per Cent

material Uin- Sclerotia Sclerotia Sclerotia
utes Killed Killed
Dowicide F 1-1000 30 910 900 98.90
Dowicide F 1-2000 30 436 433 99.31
Dowicide F 1-3000 30 126 126 100.00
Dowicide F 1-4000 30 170 168 98.82
SCD 765 1-500 15 500 416 83.20
SCD 765 1—500 30 50 35 70.00
NaOCl 1-2 Check 5 210 6 2.86

 

 

The results of the tests presented in table VII show that Dowicide
F has a very high toxicity value from.a dilution of 1-1000 to as high
a dilution as 1-4000. This indicates that this fungicide is highly
efficient and should prove to be an excellent compound for treating
potato tubers in field practice.

Despite the bactericidal value of SCD 765 its fungicidal value is
apparently inferior to those of the Dowicides.

During this investigation it was thought that the addition of a
detergent to an experimental fungicide would lower the surface tension

of the solution and possibly achieve the same, or improved, fungicidal

-33-

efficiency in a shorter soaking period. Vel, a neutral sulfated mono-
glyceride detergent, was used in this experiment at a concentration of
.05 grams per liter of solution.

The results of this investigation are presented in table VIII.
The data presented represents the summarized results of three separate
trials.

From the results shown in table VIII it can be concluded that in
the case of the Dowicides compounds the addition of Vel actually
lowered the fungicidal efficiency of each to a great extent; that is,
about 20 per cent in the case of Dowicides F and G and about seven per
cent for Dowicide C. However, the Dowicides have been shown to be
excellent fungicides without the addition of Vel.

In the case of the mercurials used in this experiment the addi-
tion of Vel at-the rate of .05 grams per liter increased the fungicidal
value of Semesan Bel and acidulated corrosive sublimate only about
one per cent but appeared to lower the standard corrosive sublimate
about 4 per cent.

It is apparent that the value of the addition of a detergent to
a fungicide depends upon the chemistry of the compounds used.

In an investigation of this type, the conditions of the labora-
tory experiments must simulate the conditions and procedures in the
field as closely as possible if the results are to prove valid and
applicable in future field practice.

Therefore it was deemed that because the common field practice is

to place the sacks or crates of treated potato tubers in a pile and

-34..

TABLE VIII

TOXICITY OF CERTAIN FUNGICIDES, USING VEL.AS A DETERGENT, TO SCLEROTIA
OF RHIZOCTONIA SOLANI KUHN

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Time: Number Number Per Cent
Material Min- Sclerotia Sclerotia Sclerotia

utes Killed Killed
NaOCl 1:2 5 190 0 0
HgClz 1—1000 * 50 55 51 92.72
93012 1-1000 50 155 149 96.13
Acid HgClz 1-1000 * 10 95 95 100.00
Acid HgCl2 1-1000 10 250 245 97.20
Semesan Bel 1-1000 * 10 60 60 100.00
Semesan Bel 1-1000 10 330 327 99.1
Dowicide c. 1-1000 * 50 100 91 91.00
Dowicide c 1-1000 50 183 180 98.36
Dowicide F 1-1000 * 15 75 55 75.55
Dowicide F 1-1000 15 496 456 91.94
Dowicide F 1-1000 * 50 80 65 79.75
Dowicide F 1-1000 50 910 900 98.90
Dowicide G 1-1000 * 50 190 I 142 74.74
Dowicide G 1-1000 50 496 474 95.57

 

 

* .05 g. VeL7L. added

 

-35-

allow them to set there until they were cut and planted, that an
experiment simulating this practice would be made in the laboratory.
In the field these treated tubers sometimes remain in a pile from
one half hour to three or four hours. It was thought that in the
.1aboratory a two hour setting period would represent the typical time
in the field. Therefore in tests performed in the laboratory the
tubers were treated for the desired exposure period, the fungicide was
then drained off and the tubers allowed to set for two hours in
covered containers.
The results of this investigation are presented in table IX.
From the results presented in table IX, it can be concluded
that the toxicity values of the mercurials, SCD 765, and Dowicides
C and G are decreased as a result of the common field practice in the
method of handling treated tubers after removal from the treatment
baths. Since this conclusion is based on a very low number of trials
and sclerotia used, it can be assumed that the conclusion may not be

valid. Further work is required along this line.

-35-

TABLE IX

COMPARATIVE STUDY OF COVERING TREATED POTATO TUBERS IN RELATION TO THE
TOXIC EFFECT ON SCLEROTIA OF RHIZOCTONIA SOLANI KUHN

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Tifie: Number Number Per Cent
material Nin- Sclerotia Sclerotia Sclerotia
utes Killed Killed
5g012 1-1000 * 50 194 92 47.4
Hg012 1-1000 50 525 515 96.9
*
Semesan Bel 1—125 10 299 299 . 100.0
Semesan Bel 1-125 15 315 290 92.4
Acid Hg012 1-1000 * 50 554 554 100.0
Acid HgCl2 1-1000 15 252 268 94.7
SCD 765 1-500 * 50 50 55 70.0
SCD 765 1-500 15 - 100 64 64.0
a:
Dowicide 0 1-1000 50 155 150 95.4
Dowicide 0 1-1000 15 140 125 59.2
Dowicide G 1-1000 * 50 496 474 95.6
Dowicide G 1.1000 15 6O 55 55.5
NaOCl 1:2 Check 5 50 50 57.5

 

* Not covered

 

 

-37-

Summary

Certain desired tuber treatment fungicide specifications have
been presented. These fungicides should be non-phytotoxic, non-
mercurial, low in cost, non corrosive, present no or very little
health hazard, and be as effective as the standard corrosive sublimate.

Sclerotia of R. solani Kahn from treated tubers were planted
aseptically on acidulated agar plates according to the method of
Sanford and Marritt (27) to prevent bacterial contamination.

Sanford and Narritt have suggested that the numbers of small,
medium, and large sclerotia occur in nature in an approximate ratio of
13:3:1. In tuber treatment experiments it has been indicated that
size of sclerotia is not always in itself a limiting factor in tuber
treatments.

The mercurials are excellent fungicides but may be phytotoxic,
tend to increase potato scab, are corrosive, and present a health
hazard.

Some of the new Dowicides were used in experiments by the author.
Laboratory results indicate that they are excellent fungicides at high
dilutions and do not have the undesirable characteristics of the mer-
curials.

The addition of a detergent, Vel, to the experimental fungicides,
decreased the effectiveness of the Dowicides but had little effect on
the mercurials.

It is concluded from the data presented that Dowicides B, C, F,

and G are excellent fungicides for control of Rhizoctonia on potato

tubers in the laboratory. Also in the absence of soil infestation
they should give satisfactory control in the field.

Of these four materials, Dowicides F (sodium tetrachlorophenol)
appears to be one of the better fungicides for potato tuber treat-

ment.

-39..

2.

3.

4.

5.

6.

7.

B.

10.

ll.

12.

13.

14.

LITERATURE CITED

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Clayton, E. E. Potato-seed treatment experiments on Long Island
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Coons, G. H. Seed tuber treatments for potatoes. Phytopath. 8:
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Cunningham, H. S. Yellow oxide of mercury treatment for seed
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Cunningham, H. S. A comparison of New Improved Semesan Bel and
different strengths of yellow oxide of mercury as materials
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Dana, B. F. The Rhizoctonia disease of potatoes. wash. Agr. Exp.

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Gilman, J. C., and I. E. Nelhus. Further studies on potato seed
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Heald, F. D. Manual of plant diseases. 2nd ed. New York.
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action of formaldehyde. Iowa Agr. Exp. Sta. Res. Bul. 59.
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Uoore, H. C. and E. J. Wheeler. Seed potato disease controls
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Fannie, J. H. Hichigan potato diseases and their control. Hich.
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Uuncie, J. H. Unpublished data.

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planting. Sci. Agr. 17:225-234. 1936.

 

Sanford, G. B. and J. W. Marritt. The toxicity of formaldehyde
and mercuric chloride solutions on.various sizes of sclerotia
of Rhizoctonia solani. Phytopath. 23:271-280. 1933.

 

Wheeler, E. J., and H. C. Moore. Potato seed treatment tests.
Hich. Agr. Exp. Sta. Spec. Bul. 246. 1933.

-41-

PLATE I

Sclerotia of Rhizoctonia solani Kuhn on Potato tubers.

 

Note the Variability of Size of the Sclerotia.

-42-

 

 

PLATE II

Effect of treatment on growth of sclerotia of Rhizoctonia

solani Kuhn.

A. Left:
Right:

B. Left:
Right:

 

Check (HaOCl - 5 minutes).
SCD 765 (1-500) - 15 minutes
Check (HaOCl - 5 minutes).

Dowicide G (1-1000) - 15 minutes.

-43-

 

PLATE III

Effect of treatment on growth of sclerotia of Rhizoctonia

solani Kfihn.

C. Left:
Right:

D. Left:
Right:

 

Check (NaCCl - 5 minutes).
Dowicide C (l-lOOO) - 15 minutes.
Check (HaOCl - 5 minutes).

Dowicide D (l—lOOO) - 2 hours.

-44-

 

PLATE IV

Effect of treatment on growth of sclerotia of Rhizoctonia

 

solani Kuhn.
E. Left: Check (NaOCl — 5 minutes).

Right: Hg012 (1-1000) - 30 minutes.

   
 

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