Front Cover
 Board of control and station...
 Description of bacterial soft...
 Economic importance
 The causal organism
 Experiments with washed and unwashed...
 Commercial drying of washed potatoes...
 Commercial pre-cooling of washed...
 Discussion and conclusions
 Control recommendations
 Literature cited

Title: Bacterial soft rot of potatoes in southern Florida
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00026763/00001
 Material Information
Title: Bacterial soft rot of potatoes in southern Florida
Physical Description: Book
Creator: Ruehle, George D.
Publisher: University of Florida Agricultural Experiment Station
Publication Date: 1940
Copyright Date: 1940
 Record Information
Bibliographic ID: UF00026763
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: aen5206 - LTUF
18229790 - OCLC
000924579 - AlephBibNum

Table of Contents
    Front Cover
        Page 1
    Board of control and station staff
        Page 2
        Page 3
    Description of bacterial soft rot
        Page 4
        Page 5
        Page 6
    Economic importance
        Page 7
    The causal organism
        Page 8
        Page 9
        Page 10
        Page 11
        Page 12
    Experiments with washed and unwashed potatoes
        Page 13
        Page 14
        Page 15
        Page 16
        Page 17
        Page 18
        Page 19
        Page 20
        Page 21
        Page 22
        Page 23
        Page 24
        Page 25
    Commercial drying of washed potatoes with heated air
        Page 26
        Page 27
    Commercial pre-cooling of washed potatoes
        Page 28
        Page 29
        Page 30
    Discussion and conclusions
        Page 31
        Page 32
    Control recommendations
        Page 33
        Page 34
    Literature cited
        Page 35
        Page 36
Full Text

Bulletin 348 September, 1940









Single copies free to Florida residents upon request to

John J. Tigert, M. A., LL.D., President H. P. Adair, Chairman, Jacksonville
of the University: W. M. Palmer, Ocala
Wilmon Newell, D.Sc., Director3 Chas. P. Helfenstein, Live Oak
Harold Mowry, M. S. A., Asst. Dir., R. H. Gore, Fort Lauderdale
Research N. B. Jordan, Quincy
V. V. Bowman, M.S.A., Asst. to the J. T. Diamond, Secretary, Tallahassee
J. Francis Cooper, M.S.A., Editor3 BRANCH STATIONS
Jefferson Thomas, Assistant Editor3 T STAT
Clyde Beale, A.B.J., Assistant Editor3 NORTH FLORIDA STATION, QUINCY
Ida Keeling Cresap, Librarian J. D. Warner, M.S., Agron. Acting in
Ruby Newhall, Administrative Manager3 Charge
K. H. Graham, Business Manager3 R. R. Kincaid, Ph.D., Associate Plant
Rachel McQuarrie, Accountant3 Pathologist
Elliott Whitehurst, B.S.A., Assistant An.
AGRONOMY Jesse Reeves, Asst. Agron., Tobacco
W. E. Stokes, M.S., Agronomist1 CITRUS, STATION, LAKE ALFRED
W. A. Leukel, Ph.D., Agronomist3 A. F. Camp, Ph.D., Horticulturist in
Fred. H Hull, Ph.D., Agronomist Charge.
G. E. Ritchey, M.S., Associates John H. Jefferies, Asst. in Cit. Breeding
W. A. Carver, PH.D., Associate Michael Peech, Ph.D., Soils Chemist
John P. Camp, M.S., Assistant L. H. Greathouse, Ph.D., Chemist
Roy E. Blaser, M.S., Assistant B. R. Fudge, Ph.D., Associate Chemist
ANIMAL INDUSTRY W. L. Thompson, B.S., Associate
A. L. Shealy, D.V.M., Animal Indus- W. W. Lawless, B. S., Asst. Horticulturist
tralist3 R. K. Voorhees, M.S., Asst. Plant Path.
R. B. Becker, Ph.D., Dairy Husbandman3 F. Cowart, Ph.D., Asso. Horticulturist
E. L. Fouts, Ph.D., Dairy Technologist'
W. Neal, Ph.D., Asso. in An. Nutrition EVERGLADES STATION, BELL GLADE
D. A. Sanders, D.V.M., Veterinarian J. R. Neller, Ph.D., Biochemist in
M. W. Emmel, D.V.M., Veterinarian3 Charge
N. R, Mehrhof, M.Agr., Poultry Hus- J. W. Wilson, Sc.D., Entomologist
bandman3 F. D. Stevens, B.S., Sugarcane Agron.
W. G. Kirk, Ph.D., Asso. An. Husband- Thomas Bregger, Ph.D,. Sugarcane
man3 Physiologist
R. .I. Crown, M.S.A., Asst. An. Husb3 Frederick Boyd, Ph.D., Asst. Agronomist
P. T. Dix Arnold, M.S.A., Asst. Dairy G. R. Townsend, Ph.D., Plant Pathologist
Husbandman' R. W. Kidder, B.S., Asst. An. Husbandman
L. L. Rusoff, Ph. D., Asst. in An. W. T. Forsee, Ph.D., Asso. Chemist
Nutrition B S. Clayton, B.S.C.E., Drainage En-
O. W. Anderson, M.S., Asst. Poultry gineer2
Husbandman3 F. S. Andrews, Ph.D., Asso. Truck Hort.
R. V. Allison, Ph.D., Chemist 3 W. M. Fifield, M.S., Horticulturist Act-
Gaylord M. Volk, M.S., Chemist ing in Charge
F. B. Smith, Ph.D., Microbiologist3 S. J. Lynch, B.S.A., Asst. Horticulturist
C. E. Bell, Ph.D., Associate Chemist Geo D. Ruehle, Ph.D., Associate Plant
H. W. Winsor, B.S.A., Assistant Chemist Pathologist
J. Russell Henderson, M.S.A., Associates W. CENTRAL FLA. STA.,
L. H. Rogers, M.S., Asso. Biochemist BROOKSVILLE
Richard A. Carrigan, B.S., Asst. Chemist W F. Ward, M.S., Asst. An. Husband-
C. V. Noble, Ph.D., Agricultural FIELD
Bruce McKinley, A.B., B.S.A.. Associate Leesburg.
Zach Savage, M.S.A., Associate
A. H. Spurlock, M.S.A., Assistant M. N. Walker, Ph.D., Plant Pathologist
in Charge
ECONOMICS, HOME K. W. Loucks, M.S., Assistant Plant
Ouida D. Abbott, Ph.D., Home Econ- Pathologist
omist1 Plant City
Ruth Overstreet, R.N., Assistant A. N. Brooks, Ph.D., Plant Pathologist
R. B. French, Ph.D., Asso. Chemist Cocoa
ENTOMOLOGY A. S. Rhoads, Ph.D., Plant Pathologist
J. R. Watson, A.M., Entomologist' Hastings
A. N. Tissot, Ph.D., Associate
H. E. Bratley, M.S.A., Assistant A. H. Eddins, Ph.D., Plant Pathologist.
HORTICULTURE E. N. McCubbin, Ph.D., Asso. Truck
HORTICULTURE Horticulturist
G. H. Blackmon, M.S.A., Horticulturist' Monticello
A. L. Stahl, Ph.D., Associate
F. S. Jamison, Ph.D., Truck Horticul- Samuel 0. Hill, B.S., Asst. Entomologist
tourist3 Bradenton
R. J. Wilmot, M.S.A., Fumigation Jos. R. Beckenbach, Ph.D., Truck Horti-
Specialist culturist in Charge
R. D. Dickey, M.S.A., Asst. Horticulturist David G. Kelbert Asst. Plant Pathologist
J. Carlton Cain, B.S.A., Assistant vi
Horticulturist Santord
Victor F. Nettles, M.S.A., Assistant R. W. Ruprecht, Ph.D., Chemist in
Horticulturist Charge, Celery Investigations
F. S. Lagasse, EAD., Horticulturist2 W. B. Shippy, Ph.D., Asso. Plant
H. M Sell, Ph.D., Asso. Horticulturist 3 Pathologist
PLANT PATHOLOGY E. S. Ellison, Meteorologist2
W. B. Tisdale, Ph.D., Plant Pathologistx 3 B. H. Moore, A.B., Asst. Meteorologist2
George F. Weber, Ph.D., Plant Path-
L. .O. Gratz, Ph.D., Plant Pathologist 1Head of Department
Erdman West, M.S., Mycologist "2n cooperation with U.S.D.A.
Lillian E. Arnold, M.S., Asst. Botanist 3Cooperative, other divisions, U. of F.


Page Page
DESCRIPTION OF BACTERIAL SOFT ROT 4 Effect of chilling washed pota-
toes -____ ------ 15
ECONOMIC IMPORTANCE -------- 7 Drying of washed potatoes under
ordinary atmospheric condi-
Isolations --_____ 8 Drying potatoes with heated air 22
Morphology and culture charac- POTATOE WITH HEATED A
Identity of the causal organism- 12 WASED POTATOES
Experiments with sodium hypo- SUMMARY ---34
chlorite solutions 14 LITERATURE CITED -_ __ 35

The practice of washing potatoes before packaging has increas-
ed in recent years because of a steadily increasing demand on the
market for table stock free of adhering soil. The improved ap-
pearance of the washed potatoes amply justifies the added ex-
pense of the cleaning process.
The Lower East Coast section of Florida specializes in growing
the Bliss Triumph variety. Most of the potatoes in this section
are grown on a highly calcareous marl soil which has a decided
tendency to adhere to the surface of the tubers as they are har-
vested, thus masking more or less the attractive red color of the
variety and interfering with grading. Practically all of the po-
tatoes grown in the section are washed before packaging.
The washing, even if accomplished under the best conditions
commercially practicable, is likely to be accompanied by con-
siderably more loss from decay than would occur if such pota-
Stoes had not been washed. Early potatoes are usually somewhat
immature, with especially tender skins, and it is almost impos-
sible to prevent a certain amount of bruising and abrading during
the packing operation. This is usually shown in the scuffed con-
dition commonly seen on such stock on the market. The pre-
Sdisposition for washed potatoes to decay is caused by the inocu-
lation which the surface of each potato receives from wash water
(containing decay organisms that may be brought in with the soil
or on potatoes already infected in the field. Every skin break

4 Florida Agricultural Experiment Station
on the tubers incident to handling is a possible point of entry for
decay organisms.
The usual method of washing potatoes in commercial packing-
houses prior to the 1937 shipping season was to run them in a
continuous stream through a soaking tank and then through a
rinsing spray, or directly through a spraying compartment where
practically all of the adhering soil was removed. An endless
conveyor system carried the potatoes from the washer over a
sizer and then along past sorters who attempted to remove all
decayed or otherwise undesirable tubers while the remaining
ones passed into the shipping containers. A bushel crate con-
structed of wood has become the standard container for this pur-
pose in southern Florida. The potatoes were still more or less
wet when packed. In spite of this, the condition of the stock on
arrival at destination was said to be excellent when the tubers
were washed and loaded during periods of low relative humidity.
It was considered unsafe by some shippers, however, to wash
and ship potatoes if the relative humidity of the atmosphere was
above 70 percent. The potatoes remained wet at loading time
when washed and packed under these conditions, and were liable
to show much decay on arrival at destination.
An investigation of the factors responsible for the development
of bacterial soft rot was begun in the 1936 shipping season at the
Sub-Tropical Experiment Station (10)1. Control experiments
were inaugurated at the same time, and eventually led to the
development of satisfactory methods for avoiding losses from
the disease in the larger packinghouses.


The decayed tissue varies considerably in color and consistency,
depending upon temperature, moisture, and light conditions, as
Swell as the presence of other organisms. Affected tissues are
typically white to cream colored, soft and somewhat watery if
the decay develops in a moist atmosphere and in dark situations.
A clear, amber-colored liquid often exudes from the decayed
area under these conditions. The line of demarcation between
the soft, disintegrated and the sound, firm tissues is sharp, and
frequently the outer portions of the tuber remain firm while the
inner are completely broken down. The decay develops a brown

lltalic figures in parentheses refer to "Literature Cited" in the back
of this bulletin.

Bacterial Soft Rot of Potatoes 5

color which begins at the margin and may gradually extend over
the entire surface after exposure to air and light. Affected tis-
sues lose their soft watery consistency in a dry atmosphere and

Fig. 1.-Soft rot caused by Erwinia carotovora, starting at cuts, bruises
and lenticels on washed and crated Bliss Triumph potatoes.

as they dry may become slimy, and finally, when completely dry,
may be reduced to grayish white chalky masses.
Bacterial soft rot in advanced stages.of natural infection usual-
ly is attended by foul, nauseating odors which are caused by
.secondary organisms. The decay produced by pure culture in-
oculations has only a slight odor which is not particularly offen-
Infection may start at cuts, cracks, bruises, or lenticels (Fig. 1).
It may spread by contact from decayed to healthy tubers when
potatoes are packed or piled together. The liquid which exudes

6 Florida Agricultural Experiment Station
from rotted tissues under moist conditions is teeming with the
causal organism, and as it comes in contact with sound tubers
may start infections at lenticels or skin breaks.
Much of the rot that develops in transit and storage has its
beginning in the field. Bacterial soft rot is frequently secondary,
following late blight tuber rot, Fusarium tuber rot, or other in-
fections. It often causes seed-piece decay in water-logged soils.
Seed-piece decay is more apt to occur in depressions in the field,
such as around sink-holes, or in the case of higher ground, in
seasons characterized by abundant rainfall shortly after planting
Lenticel infection of the new tubers also may occur in water-
logged soils. This type of infection frequently is seen, in early
stages, as raised watery swellings up to one-quarter inch in di-
ameter around the lenticels. Exposure of the harvested tubers
to a dry atmosphere stops the development of surface infection
of this type. The swellings disappear and the affected tissues
may dry to form shallow depressions at the lenticels (Fig. 2).


Fig. 2.-Lenticel infection on Bliss Triumph potatoes from water-
logged soil.
Vines also may be attacked late in the season by soft rot bac-
teria if rains are excessive. The first evidence of the trouble is
the sudden prostration of the normally erect vines due to the
development of soft rot just above the ground level. The de-
cayed area is typically greenish-brown in color at first but may
change to dark brown with age. The outer epidermis slips off
readily and the parenchyma tissues of the pith and cortex are

L .

I At

'ig. 3.-Stem rot and prostration of vines by Erwinia carotovora after a heavy rain.

reduced to a mushy consistency. The top of the plant above the
decayed area may remain turgid and green for many hours after
the stalk has fallen over (Fig. 3). This sudden prostration of the
plant and the lighter color of the decayed area serve to distin-
guish this type of stem decay from blackleg, which also occurs
in southern Florida. In contrast, the stems of plants affected
with typical blackleg are usually conspicuously erect and the
lower portions are characterized by relatively dry black lesions.
Bacterial soft rot is one of the major diseases of the potato in
the Lower East Coast section of Florida, and prior to the adoption
of improved methods of handling the crop it was the most im-
portant disease. Its destructiveness varies greatly in different
seasons, in different lots of potatoes, and in different parts of
the same season. It is difficult to estimate the amount of loss
due to a disease that occurs in such a sporadic manner.
The average annual loss of potatoes in transit probably has not
exceeded 2 percent in recent years, although estimates of annual
loss have been as high as 15 to 20 percent in earlier seasons. If
the losses were evenly distributed over the entire acreage, the
g. 3.-Ste rotand rostatio of inesby Ewini cartovr~e ev an

redce to ar~ mushy cnsisteny. Thetp F tepat bv h
decayed aramyreantr~gidangen o mny ous ate
the69 stl hsfaln vr Fig.L 3) hssde rotaino h
plntan te lght r clroftedcae ra ev o itn
gus hstp fse ea ro lclg hc loocr
in ~ I sotenFoia ncotat h tm fplant afcte
with tyical backleg re usully conpicuoulyr erc n h

owr poton r caateie b eltvlydyblc esos
Bateil of otisoe fthe mJodiessothptaon
the Lower East Coast section of loridadpirtoteaoto

( mrvdmtosl ofhadin tecr twa h ms m
porantdisase It de tructivees varie gral n irn
seasons, in,. difrn oso ptteadi dfeetprso

8 Florida Agricultural Experiment Station
disease doubtless would not be considered of great importance,
but such has not been the case. The decay might be negligible
one week and rise to 10 to 25 percent or higher in certain ship-
ments the following week. In a favorable season for rot develop-
ment, such as occurred in 1936, several shipments showed 50 to
75 percent decay, and a few instances occurred where carlot ship-
ments were beyond salvaging upon arrival at destination. Ob-
viously, this liability of early potatoes to decay interfered greatly
with the orderly marketing of the crop in southern Florida, and
growers and shippers alike sustained losses as a result.
The amount of loss in the field due to bacterial soft rot is also
difficult to estimate. Many of the infected tubers are so com-
pletely disintegrated by the time of harvest that they are left in
the field by the pickers. A count was made in 1936 of the soft
rot in potatoes as they were harvested on a commercial farm
near Homestead. Three percent by weight of the harvested
tubers in field crates were infected and at least as many more
were left in the field. Examination of crates harvested from
different parts of the Experiment Station farm in the same year
showed from a trace to 2.5 percent by weight of infected tubers
in individual crates, with an average for the farm of 1.5 percent.
Much of the bacterial soft rot on both farms followed late blight
tuber rot. The losses from soft rot may be much greater in low
spots where the soil is apt to be water-logged during rainy per-
Isolations.-Isolations were made from tubers which developed
soft rot in the field and in packed boxes. The method of isola-
tion in each case was the same. Specimens were selected that
showed typical soft rot in an advanced stage but which showed
little external evidence of contamination with other organisms.
They were washed with soapy water to remove the soil, plunged
momentarily into 95 percent alcohol, and then allowed to dry
thoroughly. The outer tissues of the rot were removed with a
flamed scalpel and a bit of decayed pith was transferred to a
sterile water blank. The bacteria were allowed to diffuse into
the water for several minutes and then a series of dilution
plates were poured, using commercially prepared nutrient agar
(A. P. H. A. "Standard Methods" formula) as the medium.
The resulting colonies were practically all of one type. They
were round, raised, glistening, grayish-white or slightly opales-

Bacterial Soft Rot of Potatoes 9
cent by transmitted light, with entire margins which often be-
came more or less irregular from the proliferation of finger-like
projections if the colonies were thinly sown.
Isolations also were made from potato stems showing soft rot
in early stages. Affected stems often showed a vascular brown-
ing extending 6 to 8 inches above the decayed area. Practically
pure cultures of bacteria which formed colonies on nutrient agar
similar to those produced by the strain from tubers, were ob-
tained by excising bits of tissue from the discolored vascular
area, and plating these on potato dextrose agar. The original cul-
tures were purified by the dilution plate method.
The organism also was isolated from a soft rot following late
blight tuber rot, although considerable difficulty was experienced
in obtaining a pure culture, the isolation plates bearing a large
proportion of saprophytic colonies.
Pathogenicity.-The strains isolated from tubers and from po-
tato stems produced typical rot when inoculated from pure cul-
tures into sound tubers. Some difficulty was experienced at first
in causing active rot to develop from the artificial inoculations.
It was found necessary to incubate the inoculated tubers in an
atmosphere practically saturated with moisture before rapid
decay would take place.
Cultures 48 hours old were used for inoculations. Tubers were
washed thoroughly, immersed for several minutes in 1-1,000
mercuric chloride, rinsed in several changes of sterile water, and
allowed to dry. They were inoculated by dipping a sterile needle
into the culture and puncturing each potato to a depth of about
one-half inch. Checks were prepared by puncturing potatoes
with a sterile needle.
The inoculated and check tubers were incubated at room
temperatures in several situations, which included the top of a
laboratory table, large covered culture dishes, and tightly closed
sterilized canisters. No soft rot developed in the check tubers
and inoculated tubers incubated on a laboratory table failed to
develop soft rot. A slight amount of growth took place at the
needle punctures, but stopped completely after 48 hours and the
wounds dried and healed in the latter case.
SA number of inoculated tubers held in the large culture dishes
developed typical soft rot. Decay started in every instance, but
in some cases stopped development after 72 hours. The affected
tissues dried out to form funnel-shaped cavities up to 4 inch in
diameter at the needle punctures. Best results were obtained

10 Florida Agricultural Experiment Station
in the tightly closed canisters, in which filter paper moistened
with sterile water had been introduced with the potatoes to
increase humidity. Without exception the inoculated needle
punctures were exuding a clear amber-colored liquid and a soft
decay was advancing into the flesh after 48 hours. The decayed
areas ranged from 4 to 3 inch in diameter on the surface of
the tubers at the end of 72 hours (Fig. 4). The rot had progress-

? .<' ,

Fig. 4.-Inoculated and check tubers after 72 hours. Tubers at the
top punctured with sterile needle; at the bottom, punctured with needle
dipped in culture of E. carotovora.

ed laterally in the pith at a faster rate than in the cortex (Fig. 5).
There was very little discoloration of the affected tissues and
little odor at this stage. After several hours of exposure to the
air, the cut surface of the decay at the advancing edge changed
in color to a dark brown. Re-isolations were made from several
tubers inoculated with the different cultures, resulting in colonies
on nutrient agar similar to the original isolation cultures.
One series of inoculated tubers was held in the canisters for 48
hours, until the rot was well started at the needle punctures,
and the containers were opened for about 30 minutes daily
thereafter. The filter papers were not re-moistened with water
after the first day of incubation. The decay was checked in most
of the tubers of this series, as a result of this exposure to fresh
air and lowered humidity. In another series the tubers were
moistened with sterile water each day after opening the canisters.

Bacterial Soft Rot of Potatoes 11

Fig. 5.-Cross section of tubers shown in Fig. 4. Note that the decay
is developing more rapidly in the pith than in the cortex.

The decay continued unchecked in this series and at the end of
seven days some tubers were completely destroyed, while in
others a portion of the cortex remained firm with the pith com-
pletely broken down. The importance of humid conditions for
the rapid development of the soft rot was demonstrated by this
simple experiment.
Morphology and Cultural Characters.-The procedures general-
ly used in the pure culture studies of the isolates were those
recommended by the Society of American Bacteriologists (3).
Purified cultures, all originating from single-colony isolations,
were maintained on nutrient agar and nutrient broth until need-
ed. Growth reactions on various media and staining reactions
were tested a second, and in some cases a third, time before final
conclusions were reached. Studies of nitrate reduction, indole
production, and of sugar fermentation were not made and the
serological reactions were not determined.
The organism is a small rod, usually occurring singly but oc- r
casionally in short chains, and is actively motile by means of two
to many peritrichic flagella which are much longer than the rod.
The rods are gram-negative, non-sporiferous, non-capsulate, and
are not acid fast.
Colonies on nutrient agar were round, raised, smooth, glisten-
ing, grayish-white or somewhat opalescent in transmitted light,
with entire margins if thickly sown. The growth was similar
but slightly more rapid on potato dextrose agar, with the mar-
gins entire at first but becoming finely crenate with age. Colo-

12 Florida Agricultural Experiment Station
nies appeared granular on this medium under low-power magni-
fications. Nutrient gelatin was liquefied fairly rapidly. The
liquefaction was infundibuliform and was complete in 14 days
at room temperatures, with a yellowish-brown precipitate form-
ed at the bottom of the tubes. Growth was rapid in nutrient
broth and after several days the medium became turbid. Litmus
milk was coagulated without clearing and the blue color was
reduced completely in seven days. Growth on both potato and
carrot plugs was very rapid, slightly raised, smooth, glistening,
and creamy white. Cultures on raw potato grew fairly rapidly,
forming a white slimy growth surrounded by a dark brown bor-
Growth of the various isolates was identical on these media, and
since they produced a similar decay of potato tubers they were
considered cultures of the same species.
Identity of the Causal Organism.-The literature dealing with
bacterial soft rots of the potato shows that the trouble is world-
wide in distribution, probably occurring wherever potatoes are
grown. The identification of the bacteria causing the decay has
long been a problem and has been studied by many workers. The
early workers on blackleg and soft rots described, for each of
these diseases, a number of bacteria that were considered to be
distinct species. The similarity in essential characters of these
forms has resulted in much confusion and considerable contra-
dictory literature pertaining to their identify (2, 4, 6, 7, 9) The
literature dealing with the identification of these organisms has
been summarized by Leach (8).
The type of decay, aside from blackleg, commonly found in
early potatoes produced in the South is attributed by Shapovalov
and Edson (12) to bacteria of the Bacillus carotovorus group. Link
and Ramsey (9) state that two species, B. aroideae and B. carotov-
orus, cause a decay of the stems of growing plants as well as a
soft rot of the tubers. They state, in addition, that B. phytophthor-
us, the causal organism of blackleg, and B. solanisaprous often in-
duce serious and abundant soft rot of tubers, which cannot al-
ways be readily differentiated from the other bacterial soft rots.
Decay of potato stems by B. solanisaprous is said to be distinguish-
able by the absence of black pigment in affected tissues. Accord-
ing to Link and Ramsey (9), tissues decayed by this species often
exude a clear liquid which serves to distinguish it from the decay
produced by B. carotovorus and other species. While B. carotovorus
was known to be pathogenic to potatoes in laboratory tests for

Bacterial Soft Rot of Potatoes 13
many years, it was not until rather recently that Lacey (6) iden-
tified it as the cause of soft rot of tubers occurring under nat-
ural conditions.
Leach (8) recently had occasion to compare authentic cultures
of B. carotovorus and B. phytophthorus with many strains of patho-
genic bacteria isolated from different vegetables, from potatoes
affected with blackleg, from the soil, and from various stages of
the seed-corn maggot. He found slight variations in minor char-
acters among the cultures isolated, but was unable to distinguish
B. carotovorus from the blackleg pathogen on the basis of mor-
phological, physiological, and pathological characteristics. He
concluded (8) "that blackleg is nothing more than soft rot of
potato," and that bacteria previously designated as B. phytoph&th-
orus, B. atrosepticus, B. solonisaprous and B. melanogenes are "merely
strains of the earlier-described species, B. carotovorus Jones."
The organism causing the serious soft rot of potatoes in south-
ern Florida in 1936, and causing more or less decay in washed
potatoes annually since 1936, agrees in essential characters with
those described by Leach for B. carotovorus, and is therefore
tentatively assigned to that species. The proper name for
the organism is Erwinia carotovora (Jones) S. A. B., following the
latest system of classification recommended by the Society of
American Bacteriologists (1).

Experiments with washed potatoes were conducted at the Sub-
Tropical Experiment Station, to determine the effects of drying,
chilling, or of treating with a chemical disinfectant on the sub-
sequent development of bacterial soft rot. After treatment the
tubers were placed in an incubation chamber which was especial-
ly constructed to maintain a high relative humidity and a fairly
constant temperature during the tests.
The capacity of the incubation chamber was approximately 53
,cubic feet, which readily accommodated six crates of potatoes
and accessory equipment without crowding. The top and sides
were constructed of Celotex board over a wood frame, and the
floor was constructed of wood. A double window sash was built
into the front so that access to either end could be accomplished
by sliding the movable sash either to the right or to the left. The
relative humidity of the atmosphere within the chamber was
regulated by means of a humidity device constructed after spe-

14 Florida Agricultural Experiment Station
cifications described by Hervey and Horsfall (5) for a similar
device. This device, when operated continuously, maintained
relative humidities that were fairly constant when fluctuations
in temperature were slight. The incubator was placed in a small
room which was constructed with double walls with a dead air
space between. Variations in temperature were seldom greater
than 100 F. and in relative humidity was regulated to vary
not more than plus or minus 5 percent within the chamber
during the tests.
The chamber was equipped with a Friez hygro-thermograph
which recorded continuously the temperature and relative humid-
ity. A Mason hygrometer was also installed to check the hygro-
thermograph readings.
Preliminary experiments were made with lots of 25 to 50
tubers. The potatoes were placed in 4-quart baskets after treat-
ment and incubated in the humidity chamber for from three to
six days. The treatment included washing in tap water and
incubating while the tubers were still wet, washing followed
by drying before an electric fan, and washing followed by im-
mersion in a solution containing 50 p.p.m. available chlorine be-
fore incubation. The latter was made from a proprietary prepar-
ation which contained sodium hypochlorite, sodium chloride, and
sodium phosphate. Soft rot developed in some of the lots treat-
ed with the salt solution which kept the surface of the tubers
wet for a much longer time than plain water and apparently
favored soft rot development. No decay developed in the other
lots and the 4-quart baskets were discarded in favor of standard
bushel crates for incubating the treated tubers in all later ex-
periments, unless otherwise noted.
Potatoes for the experiment with crated tubers were, in most
cases, washed, graded, and created in a commercial packinghouse
They were taken directly from the lidding machine and trans-
ported at once to the laboratory where the tests were conducted.
Some experiments were also made with unwashed potatoes pro-
duced on the Sub-Tropical Experiment Station farm. The crates
were lidded in all cases before they were placed in the humidity
Experiments with Sodium Hypochlorite Solutions.-There are
few chemical disinfectants suitable for commercial treating of
potatoes that are to be used as table stock. Sodium hypochlorite
was the only one tested and it proved to be of no value for the
prevention of bacterial soft rot.

Bacterial Soft Rot of Potatoes 15
Dilutions of 1-200 and 1-100 were made from a bleaching solu-
tion containing 5.25 percent by weight of sodium hypochlorite.
Potatoes of U. S. No. 1 "B" size (1Y2 to 2 inches in diameter) were
washed in the usual commercial manner and were then immersed
for five minutes in the treating solutions. In one test the tubers
were drained for several minutes after removal from the treat-
ing bath, crated, and incubated while still wet. In another test
tubers were dried before an electric fan after removal from the
treating bath, and then crated and incubated. Checks were pre-
pared in each case from the same lots of potatoes, by handling
them in the same manner except for immersion in the hypoch-
lorite solution.
The potatoes were left in the humidity chamber for 72 hours
and were then removed and examined for soft rot. The treat-
ments and the results are summarized in Table 1.
It may be noted from Table 1 that the hypochlorite bath failed
to affect markedly the development of soft rot in either test. It
is probable that the traces of soil and organic matter left on
potatoes, in spite of washing in the usual commercial manner, are
sufficient to prevent effective germicidal action by chemical
Effect of Chilling Washed Potatoes.-Several lots of potatoes
were washed in the usual manner in a commercial packinghouse,
and were chilled in a compartment of an electric refrigerator be-
fore incubation in the humidity chamber. The chilling was com-
pared to drying by means of electric fans as a method of "con-
ditioning" the potatoes to increase their resistance to bacterial
The air temperature of the refrigerator compartment ranged
from 41 to 46 F. during the chilling period. Its capacity would
accommodate the contents of only one crate of potatoes at a
time. Tubers of U. S. No. 1 "B" size were used for the first test.
They were left in the refrigerator for 17 hours, and upon removal,
were crated and placed at once in the humidity chamber together
with a crate of potatoes from the same lot, which had been fanned
for 6 hours and then allowed to dry in a well ventilated room 12
additional hours before incubation.
The chilling period was shortened to 6 hours, and "A" size
potatoes (over 2 inches in diameter) were used in later tests. The
chilled potatoes, in one instance, were crated and placed at once
in the refrigerator. In another lot the condensation moisture
was allowed to dry off for several hours at room conditions be-
fore crating and incubation. Checks were prepared in each case


February 22-25, 1936.
Mean temp. Mean rel.
Treatment of U. S. No. 1 "B" Number Percentage showing soft rot after of air in humidity
Lot size potatoes after being of incubation for 72 hours in incuba- chamber of air in
No. washed in a commercial tubers tion chamber chamber

1 Crated wet and incubated 528 16.1 69" F. 88%
while tubers were still wet

Immersed for 5 minutes in
2 1-200 sodium hypochlorite 509 20.6 69" F. 88%
solution; crated and incu-
bated while still wet

February 26-29, 1936

Dried before fan 6 hours; 0r
3 crated and allowed to dry 545 1.5 70* F. 90%
12 hours in ventilated
room before incubating

Immersed for 5 minutes in
1-100 sodium hypochlorite
4 solution; dried before fan 517 1.2 70 F. 90%
6 hours; crated and allowed
to dry 12 hrs. in ventilated
room before incubating.

Bacterial Soft Rot of Potatoes 17
from the same lots of potatoes by drying the crated tubers before
a fan for six hours prior to their incubation in the humidity
chamber. The treatments and the results after 72 to 76 hours of
incubation are summarized in Table 2.
Chilling the tubers for 6 hours in Lots 4 and 6 (Table 2) was
much less effective for prevention of decay, subsequent to treat-
ment, than drying before a fan for the same length of time. Con-
densation moisture began to form on the surface of the tubers
immediately when they were removed from the cold air of the
refrigerator to the warmer atmosphere outside, and in 30 minutes
they were practically as wet as when they came from the lidding
machine. Chilling for a long period in Lot 2 (Table 2) was fairly
effective, but did not entirely prevent decay development.
It is worthy of note, in this connection, that commercial pre-
cooling methods in use for washed potatoes during 1936 in south-
ern Florida were fairly effective for prevention of decay in tran-
sit. It is probable, however, that the results obtained from com-
mercial pre-cooling were due in larger part to the drying rather
than the cooling of the tubers, since the methods in use involved
the withdrawal of moisture from the air and the rapid movement
of the cold dry air through the load. Pre-cooling methods will
be discussed later in more detail.
Drying of Washed Potatoes Under Ordinary Atmospheric Con-
ditions.-Potatoes washed in the usual manner were dried under
ordinary atmospheric conditions in various ways before they
were placed in a humid atmosphere where they were incubated
together with potatoes from the same lots with the tubers still
wet with the wash water when incubation started. Potatoes of
both A and B sizes from several farms were included in the ex-
periment. Drying the wet potatoes by means of large fans before
and after crating, and, in other lots, allowing the crated tubers
to dry in an airy situation without the aid of fans, were methods
The treatments and the soft rot development in the various lots
after 72 to 76 hours' incubation in the humidity chamber are sum-
marized in Table 3.
The experiments demonstrated the value of drying washed
potatoes for the prevention of bacterial soft rot, and the danger
of introducing crated wet tubers into a humid environment. The
importance of rapid and complete drying was indicated. Drying
with the aid of a large fan in Lot 4 (Table 3) gave better results


February 26-29, 1936 __

Treatment of U. S. No. 1 Percentage showing soft Mean rel.
Lot potatoes after being wash- Number rot after incubation for Mean temp. humidity
No. Size ed in a commercial pack- of 72-76 hours in incubation of air in of air in
inghouse tubers chamber chamber chamber

Dried before fan 6 hours;
1 B crated and allowed to dry 545 1.5 75 F. 89%
12 hours in ventilated
room before incubating
Chilled in refrigerator
2 B 17 hours at 41-46 F.; 515 0.6 75" F. 89%
crated and incubated

March 2-5, 1936

Crated wet; dried in crate
3 Large 6 hours before a fan; 178 0.6 79.6 F. 84.5%
Chilled in refrigerator
4 Large 6 hours at 41-46 F.; 164 8.5 79.6 F. 84.5%
crated and incubated
March 13-16, 1936

Crated wet; dried in crate
5 Large 6 hours before a fan; 187 1.1 78.1 F. 76.4%
Chilled in refrigerator
6 hours at 41-46* F.;
6 Large Condensation moisture 187 8.0 78.1 F. 76.4%
allowed to dry before
crating and incubating

Bacterial Soft Rot of Potatoes 19
than the slow drying in an airy room in Lot 3. Crating the po-
tatoes while wet and drying before a fan was effective with
tubers of large size in Lot 8, but considerable decay developed
in "B" size potatoes in Lots 6 and 10 that were treated in this
manner. Smaller tubers are more difficult to dry thoroughly
than the larger sizes, due probably to the proportionately greater
area of surface to be dried in the case of small tubers. It has
been the experience in packinghouses that small tubers are more
liable to develop bacterial soft rot in transit than the larger sizes,
other things being equal.
In another experiment, sound U. S. No. 1 tubers of about the
same size and of the same lot were selected from unwashed po-
tatoes produced on the Sub-Tropical Experiment Station farm.
One crate was packed with the dry and powdery soil left on the
tubers, and was placed in the humidity chamber. Two bushels
were washed with tap water and the potatoes were crated while
still wet. One of these was placed in the humidity chamber at
once, but the other was placed in front of a 16-inch electric fan
for 6 hours before incubation in the humidity chamber. Two
bushels were washed as before and then rinsed in water con-
taining decayed tissue from several tubers infected with bacterial
soft rot. The rotten pulp had been thoroughly broken up and
stirred about in the water to furnish an abundance of inoculum.
The potatoes were crated while still wet. One crate was placed
at once in the humidity chamber and the other was placed in
front of the fan for 6 hours before incubation. The lots were left
for 72 hours in the incubation chamber where a mean relative
humidity of 91 percent was maintained with a temperature range
of 80 to 850 F. The bacterial soft rot development in the different
lots at removal from the humidity chamber is recorded in Table 4.
The experiment demonstrated clearly that washing the potatoes
increased their liability to decay and that infection was increased
further by the presence of decayed tissue in the wash water. The
effectiveness of drying the tubers after washing was again dem-
onstrated. Apparently the causal organism may still remain
viable in dry soil adhering to unwashed potatoes, since the tap
water used on Lot 2 (Table 4) could scarcely carry sufficient soft
rot bacteria to account for the infection which developed.
One shipper attempted to dry potatoes after they were loaded
in the refrigerator cars by forcing air through the load by means
of large high-speed fans placed in the ice bunkers. The air blast
from the fans was passed over calcium chloride, in some instances,


February 18-21, 1936 o

Treatment of U. S. No. 1 Percentage showing soft Mean rel.
Lot Size potatoes after being wash- Number rot after incubation for 72- Mean temp. humidity
No. ed in a commercial pack- of 76 hours in incubation of air in of air in
inghouse tubers chamber chamber chamber

Crated wet and incubated
1 B while tubers were 547 28.0 73" F. 91% .
still wet
Dried spread out on floor
2 B before fan 2 hours; crated 525 7.8 73 F. 91%
and incubated

February 26-29, 1936

Crated wet and allowed to
3 Large dry 18 hours in ventilated 225 2.2 72" F. 90%
room before incubating A
Dried before fan 6 hours;
4 Large crated and allowed to dry 212 None 720 F. 90%
12 hours in ventilated
room before incubating
Crated wet and allowed
5 B to dry 18 hours in ventil- 561 2.5 72* F. 90%
ated room before
Dried before fan 6 hours;
6 B crated and allowed to dry 545 1.5 72" F. 90%
12 hours in ventilated room
before incubating

March 2-5, 1936

7 Large Crated wet and incubated 186 9.1 79.6 F. 84.5%
while tubers were still wet
Crated wet and dried in
8 Large crate before fan 6 hours; 178 0.6 79.6 F. 84.5%

9 B Crated wet and incubated 530 21.9 79.6 F. 84.5%
while tubers were still wet
Crated wet and dried in
10 B crate before fan 6 hours; 510 3.1 79.6 F. 84.5%
Crated wet and allowed to
11 B dry in well ventilated shed 551 3.4 79.6 F. 84.5%
24 hours; incubated

March 13-16, 1936

12 Large Crated wet and incubated 206 25.2 78.1 F. 76.4%
while tubers were still wet
Crated wet and dried in
13 Large crate before fan 6 hours; 187 None* 78.1" F. 76.4%

*Two tubers with small decay spots, but lesions had dried and rot development had ceased.

22 Florida Agricultural Experiment Station
in an attempt to increase its ability to take up moisture from wet
surfaces. These attempts failed to prevent soft rot development,
probably because of the impossibility of circulating the air around
the tubers in the center of the crates. It was found that during
rainy periods tubers near the center of a packed crate would
remain moist after six hours in front of a 16-inch fan running
at high speed. Most of the decayed tubers in crates removed
from the humidity chamber in the experiments described above
were found near the center of the packages.
It is concluded that the surface of washed potatoes must be
dried rapidly and thoroughly in order to prevent the develop-
ment of bacterial soft rot in transit. It is apparent that drying
with air at the ordinary atmospheric conditions usually encoun-
tered during the shipping season in southern Florida has definite
limitations as a practice for use in commercial packinghouses. Its
effectiveness is modified by various factors beyond the control
of the shippers, particularly temperature, relative humidity, and
natural movement of the atmosphere in the packinghouses dur-
ing and following packaging. Size of tubers also is a factor
modifying the rapidity and thoroughness of drying. During per-
iods of high atmospheric humidity drying of the tuber surfaces
is greatly retarded and even circulation of air by fans fails in some
instances to remove the moisture rapidly enough to prevent soft
rot development.
Drying Potatoes With Heated Air.-The possibility of facilitat-
ing the drying of washed potatoes by heating the air before pass-
ing it over the tubers had not been tried commercially prior to
1936, in so far as could be determined from a review of the avail-
able literature.
It is a well known fact that raising the temperature of air in-
creases its ability to absorb and hold moisture. It remained to
be determined how much it was necessary to heat the air in order
to accomplish rapid and thorough drying of freshly washed
"tubers and if any deleterious effect would result from exposure
to the heated air.
Drying experiments with heated air were begun in the labora-
tory in 1936. The potatoes used for the first experiments were
gleanings from a farm where the crop had been harvested several
weeks earlier. Tubers free of serious defects were selected in lots
of 25 to 30 and, after washing in tap water, were rolled over large
blotters to remove excess free water. The lots were then weighed
and transferred quickly to a Freas thermo-electric drying oven,


Number Percentage showing soft rot Mean temp. umidn rt
Lot Treatment of after incubation for 72 hours of air in homidiy
No. (U. S. No. 1 large tubers) tubers in incubation chamber, chamber of air
82 F. 91%
1 Crated with field dirt on; 213 None

Washed in tap water; crated
2 wet and incubated while 245 29.8 82" F. 91%
tubers were still wet

Washed in tap water; crated
3 wet and dried before fan 229 None 82 F. 91%
6 hours; incubated

Washed in tap water; rinsed
in water containing decayed
4 potato pulp; crated wet and 248 62.5 82* F. 91%
incubated while tubers were
still wet

Washed in tap water; rinsed
in water containing decayed
5 potato pulp; crated wet and 233 None 82" F. 91%
dried before fan 6 hours;

24 Florida Agricultural Experiment Station
equipped with fans to circulate the hot air and with a thermostat
to regulate the temperature. The lots were dried at 1400 F. or
1500 F. for 1, 2 and 3 minutes, or at 130' F. for 1, 3 and 5 minutes.
At the end of the heating period the tubers were removed from
the oven and re-weighed after 30 minutes to determine the loss
of weight. They were then incubated in the humidity chamber
for four days at temperatures ranging from 84 to 920 F. and rela-
tive humidities above 70 percent.
No soft rot developed in any of the lots and there was no
evidence that the heated air damaged the appearance or quality
of the tubers which were kept under observation for several
weeks. The loss in weight from drying ranged from 0.22 percent
to 0.37 percent in the various lots.
A second series of heating tests was carried out on freshly
harvested Bliss Triumph potatoes which were grown in muck
soil. The skins of the tubers of this lot were quite tender and
easily broken or scuffed in washing. Fifteen lots of 20 to 25
tubers each were washed in tap water and were then rinsed in
water containing the decayed pulp of several potatoes infected
with bacterial soft rot before drying.
The same procedure was followed as in the first experiment
but a wider range of temperatures was used, and when the loss
of weight had been determined after heating, the potatoes were
incubated for six days in the humidity chamber. The temperature
in the latter ranged from 84 to 930 F. and the relative humidity
was maintained above 74 percent. The temperatures, time of
heating, and losses in weight from drying are recorded in Table 5.
Bacterial soft rot failed to develop in any of the lots and no
injury was apparent as a result of using heated air for drying.
The rise in temperature of the tubers from exposure to the heated
air was determined by plunging a pointed thermometer into the
flesh of several potatoes as they were removed from the oven.
The rise was found to be slight, amounting to a maximum of 60
F. in a medium sized tuber heated for 4 minutes at 1500 F. and
less than 20 F. when heated for 2 minutes at 150 F.
It was concluded that wet potatoes may be dried without dam-
age for 4 minutes in air heated to 150 F. The experiments in-
dicated that air heated to 1300 F., if rapidly circulated around
the tubers for 3 minutes, would dry them thoroughly under most
conditions. The desirability of properly balancing the tempera-
ture of the air and time of exposure to the existing atmospheric
conditions affecting the rate of drying, in order to prevent a con-

Bacterial Soft Rot of Potatoes 25

siderable increase in temperature of the tubers during drying,
was also indicated. Apparently the influence of the heated air
on the temperature of the tubers is offset to a considerable ex-
tent by the cooling effect of evaporation.
The amount of free moisture on the surface of freshly washed
potatoes was found by weighing bushel lots before and after
drying, and also by determining the increase in weight after
immersing dry tubers in water for one minute and draining
before re-weighing.


Weight in grams Drying Treatment
of wet potatoes Percent loss
Lot before drying Temp. Time in Minutes in weight
in oven Temp. -e i Roomutes from drying
F. Oven Room

1 2723.0 78* 0 20 0.13

2 3116.0 150 4 15 0.58

3 3144.0 150 3 15 0.57
4 2813.0 150 2 15 0.50
5 2643.5 140 4 15 0.58
6 2502.0 140 3 15 0.56
7 2627.0 140 2 15 0.46
8 2957.0 130 4 15 0.34
9 2606.0 130 3 15 0.36

10 3111.0 130 2 15 0.23
11 2797.5 120 4 15 0.34
12 2858.0 120 3 15 0.31
13 2796.5 120 2 15 0.30

14 2308.5 115 5 15 0.32
15 2299.5 115 4 15 0.24
Room temperature.
Freshly washed small U. S. No. 1 tubers (1Y2 to 2 inches in
diameter) were found to contain on their surfaces approximately
one-half pound of free moisture per bushel. Bushel lots of large

26 Florida Agricultural Experiment Station
U. S. No. 1 tubers showed variations in differences between the
wet and dry weights of from 0.25 to 0.4 pounds. From this, it
appeared likely that heated air would need to absorb from 50 to
100 pounds of water per hour in order to dry 200 bushels of wash-
ed potatoes per hour.

After learning of the results of the laboratory experiments,
local shippers of potatoes became interested in the construction
of hot-air driers to remove the surface moisture from washed
potatoes before packaging.
A drier was constructed and used for the first time on a com-
mercial scale during the 1938 packing season in a packinghouse
at Goulds, Florida. Although the potatoes were not thoroughly
dried by the treatment, no bacterial soft rot developed in carlot
shipments of processed potatoes from this house. The 1938 sea-
son was characterized by good drying weather and little trouble
was experienced from bacterial soft rot by other local shippers
who made no special effort to dry their potatoes before loading.
Improvements in the drier were made and a second unit was
installed before the 1939 shipping season.
Each unit consists of a tunnel (60 feet long in one and 40 feet
long in the other) insulated to retain the heated air, through
which the washed and sorted potatoes are conveyed on a wire
belt. Air that is heated by means of furnaces to 1500 F. is forced
through each unit by means of blowers in the direction opposite
to the movement of the potatoes. The heated air escapes through
vents at the opposite end, and loses approximately 300 of heat
during its passage through the drier. The conveyor is usually
geared to allow the potatoes 4 to 4V minutes exposure to the
heated air, but may be geared down to allow 5 to 51/2 minutes
exposure if necessary for more complete drying. The potatoes are
dried more or less completely when passed through a single
unit when average drying conditions prevail. When conditions
for drying are adverse the potatoes are conveyed through both
units in order to insure complete drying.
The furnaces are thermostatically controlled and burn a low
grade fuel oil. The capacity of each unit is stated to be approxi-
mately 380 bushels per hour and the average cost of drying is
estimated at 11/ cents per crate. When both units must be used

Bacterial Soft Rot of Potatoes 27
for drying the same potatoes, their individual capacity is reduced
accordingly and the cost of drying is increased.
Bacterial soft rot of the carotovora type is reported to have given
practically no trouble in shipments from this house since drying
with heated air was inaugurated. The system is not readily ad-
justable to changing atmospheric conditions, but there has been
no apparent damage to the tubers from even 8 minutes exposure
to air temperatures ranging from 120 to 150 F.
A second drier of larger capacity and different design was
installed in another Goulds packinghouse at the beginning of
the 1939 shipping season and has given highly satisfactory re-
sults to date. This drier is essentially an insulated chamber
containing approximately 100 feet of roller conveyor arranged
on three levels. (Fig. 6). The conveyor is geared to allow the
potatoes approximately 4 minutes exposure to heated air during
their passage through the drier. Air is heated by oil burning
furnaces and supplied to specially designed radiators and thence
through insulated ducts into the drying chamber at the point
where the potatoes are discharged. It is forced over the potatoes
and through the conveyer by means of electrically operated blow-

Fig. 6.-Hot-air drier in use at Goulds, Florida.

s I:A 'Ii a_(

28 Florida Agricultural Experiment Station
er fans. The direction of movement of the air is opposite to that
of the, potatoes. The system is thermostatically controlled
throughout and can be readily adjusted to wide and frequent
variations in relative humidity. When properly adjusted, there
is little or no increase in temperature of the tubers during their
passage through the drier. The system has a capacity of 600 to
800 bushels per hour.
The potatoes are graded after they are dried, which has cer-
tain advantages. Defects are somewhat easier to detect on dried
potatoes, and, as previously reported (11), processed tubers of
No. 2 grade and culls have been shipped to distant markets with-
out trouble developing from soft rot of the carotovora type. If not
processed, potatoes of the lower grades are very susceptible to
decay of this type.
This drying system has been in operation for the past two
years and approximately 900 cars have been processed during
that time. It is reported by this house that there has not been
a single shipment showing decay caused by E. carotovora. During
the spring of 1940 portable units embodying the essential features
of this system were placed in use in other Southern potato sec-
The experience to date indicates that the use of heated air is
the most practical method thus far developed for rapidly and
thoroughly drying washed potatoes and for the prevention of
soft rot of the E. carotovora type in transit. Improvements in hot
air driers doubtless will be made. Packinghouse operators are
experimenting with brass rollers and other drying devices for
removing much of the free water from tuber surfaces before dry-
ing with heated air in an effort to reduce the operating costs of


Two' different methods of pre-cooling washed potatoes were in
commercial use in South Florida during the 1936 shipping season,
and are still being used to a limited extent.
One method utilized a self-contained specially constructed re-
frigerating unit of 15 to 30 tons capacity mounted on a motor

Bacterial Soft Rot of Potatoes 29
truck chassis and operated by the truck motor. The unit was
connected to the loaded car by means of insulated air ducts
placed in the car doorway. The air from the loaded car, after
being drawn out through the lower duct at the floor level, was.
passed through the refrigerating unit, which contained two re-
frigerator condensers and a cold diffuser. The refrigerated air
was "conditioned" by having much of its water vapor condensed
and deposited on the coils, and was then returned to the car by
means of high capacity blowers through a duct in the upper
part of the doorway. An air spreading device at the end of the
return duct was used to distribute the cold air throughout the
car. The time of "conditioning" was usually 2 to 3 hours, and
the cost to the shippers was $25 per car or approximately 4 cents
per crate.
The effect of the pre-cooling unit on the temperature of the air
in the car was checked in a refrigerator car which was left on
the track at Goulds, Florida, for six days after being loaded and
conditioned. A Friez hygro-thermograph, previously tested for
accuracy with a sling psychrometer, was placed on top of the load
about four feet from the ice bunker in one end of the car just
before the pre-cooling unit was attached. The car was condi-
tioned for 3 hours, when the unit was detached, and the car doors
sealed shut. The doors were opened after 40 hours and the in-
strument was removed. The temperature of the air in the car
was 760 F. before pre-cooling, dropped to 38 F. while the unit
was attached, rose sharply to 57' F. at the end of two hours
after the unit was detached, and then rose more gradually to 66"
F. at the end of 24 hours. There was little change in temperature
after the 24-hour period until the car doors were opened.

The hygro-thermograph was replaced in the same car after it
had remained open 7 hours and the load was then re-conditioned
for two hours, when the unit was removed and the doors sealed
for four days. The record of temperature was very similar fol-
lowing the second treatment to that obtained during and after
the first treatment.

Considerable water is removed from the surface of the potatoes
and from the air inside the car by this method of pre-cooling, and
probably the drying it causes rather than the reduction in tem-
perature, is the important service it renders in checking soft rot
development. Wright, Hukill, and Gorman (13) determined the
amount of water condensed on the cooling coils to average 162.4

30 Florida Agricultural Experiment Station
pounds per car, or approximately 0.25 pounds of water per crate.
Some soft rot development was reported in several of the cars
pre-cooled by this method, but considering the volume of potatoes
handled in this manner, and the adverse natural drying condi-
tions encountered in 1936, the results were quite satisfactory.
The second method of pre-cooling in commercial use was older,
having been used with other products for several years. It in-
volved the use of portable fans or blowers at the top of the
bunkers in each end of the car to reverse the natural air circula-
tion by drawing the air upward through the ice bunkers and
distributing it out over the top of the load. The bunkers were
usually loaded with approximately 2,500 pounds of ice and the
length of treatment varied from 2 to 3 hours.
Much of the water vapor of the air is deposited on the ice in
the bunkers when the portable fan method is used. Wright,
Hukill, and Gorman (13) found that the amount of surface water
removed by this method, as determined by weighing crates be-
fore and after pre-cooling, was somewhat greater than when the
mechanical unit was used. They also found that the mechanical
unit caused a greater temperature reduction, averaging approx-
imately 13 F., as against a temperature reduction of approxi-
mately 7 F. by the use of the fan pre-coolers. In cars pre-cooled
with the mechanical refrigeration unit, however, there was a
temperature rise after the unit was removed, whereas following
pre-cooling by the portable fan method, the temperature of the car
was lower 15 hours after pre-cooling than at the end of the treat-
ment because of the ice remaining in the bunkers. Much the
same results were accomplished by either method. The cost of
pre-cooling by the portable fan method amounted to approxi-
mately 6 cents per crate, which probably explains why it was
used less widely than the mechanical units for pre-cooling po-
Pre-cooling washed potatoes has been less reliable than drying
with heated air for the control of bacterial soft rot in transit, but
still performs an important service in the smaller packinghouses,
where the volume of shipments may not warrant the installation
of expensive drying equipment. That pre-cooling will insure
the arrival of early potatoes in terminal markets in much better
condition than where no treatment is given has been amply
demonstrated. In certain packinghouses in southern Florida
heavy decay occurred in shipments earlier in the season but after
pre-cooling was instituted most of this trouble was reported to

Bacterial Soft Rot of Potatoes 31
have ceased. The chief disadvantage of the pre-cooling methods
now in use for early potatoes is that the tubers in the centers of
the crates are not always thoroughly dried by the treatment and
some decay may develop, particularly in the smaller sizes. Pack-
ed wooden crates stacked together in a loaded car do not permit
the ready access of the cold dry air equally to all the tubers. It
seems likely that if all of the tubers could be thoroughly dried
by the treatment bacterial soft rot promoted by the washing
process could be entirely prevented by pre-cooling.


Bacterial soft rot of potatoes in southern Florida, caused by
a strain of E. carotovora, is mainly a transit and storage disease.
Practically all of the soft rot that develops in transit and in
storage, however, has its inception in the field or in the packing-
Erwinia carotovora is a normal inhabitant of the soil where its
relative abundance depends largely on the supply of moisture
and of dead plant material. It will invade dead, senescent, or
injured tissues only when they are wet. Fresh breaks or cracks
in potato tubers and the lesions of other diseases provide the
organism with the water and food to start infection and to carry
on growth and reproduction. Wet, rainy weather favors the dis-
ease directly or indirectly by moistening potato tissues, by dis-
seminating and bringing about an increase in numbers of the
bacteria, by increasing other diseases of the potato, and in some
cases by weakening the lenticels so that they offer an avenue
of entrance for soft rot bacteria.
Spraying experiments for the control of blight diseases car-
ried out in Dade County for several years showed that late blight
distinctly favors the development of bacterial soft rot. In sea-
sons when late blight tuber rot occurred the potatoes harvested
from unsprayed plots invariably showed more bacterial soft rot
than potatoes from plots where late blight was controlled by
spraying or dusting with copper fungicides.
The likelihood of spreading infection during harvesting, grad-
ing, and washing is increased by the presence of tubers infected
in the field. Every precaution should be taken during harvest
and packing to avoid all unnecessary cuts, bruises, or other in-
juries, because they serve as starting places for infection. Rot-
ting tubers should be sorted out in packing, for the decay may
later spread by contact in the packages.

32 Florida Agricultural Experiment Station

Washing the potatoes serves to spread the inoculum and furn-
ishes the moisture necessary for the growth of the bacteria. A
study of packinghouse conditions in 1936 indicated that washing
tubers in tanks was more conducive to the development of decay
than washing by spray jets. The wash water from spray jets is
still a potential source of danger, however, since the bacteria
are not all removed from the tubers and from packing machinery
by this method of washing. Bacterial soft rot is almost certain
to develop unless the washed tubers are dried by either natural
or artificial means before being stored or shipped.
Allowing the tubers to dry under natural conditions is an un-
certain practice, because of the occurrence of periods unfavorable
to rapid drying of crated wet potatoes. Ordinary air blasts from
fans or blowers are not sufficient to give thorough drying of
crated potatoes under all conditions. It is desirable therefore, to
use artificial means to insure thorough drying under the various
conditions encountered during the packing season.
Two basically different methods of hastening the drying of
washed potatoes have proven effective on a commercial scale.
The older method is to pre-cool the crated washed potatoes after
they are loaded in the car. The pre-cooling methods in use in
southern Florida do not thoroughly dry all the potatoes in the
crates, especially the smaller tubers, and some decay has de-
veloped in certain shipments treated in this way. The reduction
in temperature of the tubers doubtless has some effect in check-
ing soft rot development, but the method has proven expensive
and is not being used as extensively as formerly.
The second method employs heated air for drying the potatoes
before they are packaged, and is a recent development. The
feasibility of using heated air for drying was first demonstrated
in the laboratory in 1936 and has been used extensively on a com-
mercial scale in Florida for only the past two seasons. The method
has proved less expensive and surer than pre-cooling for drying
washed potatoes, and results in the control of bacterial soft rot
in transit have been uniformly excellent. Freshly washed po-
tatoes will stand exposure for 4 minutes to air heated to 1500 F.
without apparent damage. From the experience thus far gained,
it seems unnecessary in efficient driers to exceed this tempera-
ture and length of exposure in order to accomplish thorough
drying even under the most adverse conditions, and lower tem-
peratures will give thorough drying under average conditions.
Washed potatoes have been exposed for 8 minutes to air tempera-

Bacterial Soft Rot of Potatoes 33

tures ranging from 1200 to 150 F. in one packinghouse, with no
apparent damage to the potatoes. For greatest efficiency hot
air driers should be so constructed that the temperature of the
air and the time of exposure can be adjusted to the existing
natural atmospheric conditions affecting the rate of drying, in
order to prevent a considerable increase in temperature ot the
tubers during drying. The influence of the heated air on the
temperature of the tubers is offset to a considerable extent by
the cooling effect of evaporation, so that the potatoes go through
a properly adjusted drier with little change in temperature.


Observations and experimental results secured in the field,
laboratory, and packinghouses, warrant the following recom-
mendations for control of bacterial soft rot in southern Florida.
Planting in water-logged soils or in low places where water
may stand for several hours after heavy rain should be avoided.
On low-lying land, shallow planting on high ridges is desirable
to promote aeration.
An adequate spraying or dusting program should be followed
throughout the season to prevent diseases of the growing or ma-
turing crop.
Proper handling of the crop during harvest to avoid all un-
necessary wounds, cuts, cracks, bruises, and sun-scald is of im-
portance. The potatoes should be handled during dry weather
to promote rapid drying and healing of such wounds that can-
not be avoided in handling.
The potatoes should be washed with clean water from spray
jets rather than in soaking tanks.
All potatoes infected with field rot should be sorted out. It is
advisable to store the unwashed potatoes in a well ventilated
situation for 24 to 48 hours, if bacterial soft rot is abundant in
the crop. Aeration causes a color change to take place in tissues
decayed by E. carotovora, thus permitting the ready detection of
infected tubers as they are being sorted.
It is of primary importance to dry washed potatoes rapidly
and thoroughly before or soon after they are loaded to assure
their arrival in satisfactory condition at shipping points. Ordinary
air blasts from fans or blowers are not sufficient to give thorough
drying on a commercial scale under conditions of high atmos-
pheric humidity. The passage of turbulent heated air around

34 Florida Agricultural Experiment Station
the tubers as they pass through a hot-air drier is the best method
of drying thus far devised.
Washed potatoes that are not dried before packaging should
be pre-cooled after the car is loaded, either with mechanical re-
frigeration units located outside the car or with portable fans
and ice in the bunkers.
An investigation was begun in 1936 of the causative factors
responsible for a bacterial soft rot causing heavy losses during
some seasons in shipments of washed potatoes from southern,
The causal organism was isolated, pathogenicity was proven,
and it was identified as a strain of Erwinia carotovora (Jones) S.
A. B.
Development of the disease in the field is closely associated
with rainy weather causing water-logging of the soil, and is
favored by the presence of other diseases, particularly late blight.
Development of the rot in transit or storage is increased by
washing the tubers and crating them while wet when atmos-
pheric conditions are unfavorable to rapid drying of the tuber
surfaces. Infection may be initiated in the presence of free
moisture at cuts, bruises, or cracks incident to handling and may
be spread by contact from decayed to sound tubers during transit
or storage. The bacteria may gain entrance into tubers in wet
soils through lenticels or through lesions produced by other patho-
Laboratory experiments with washed potatoes were carried
out to determine the effect of various methods of drying, of chill-
ing, and of treating with a weak solution of sodium hypochlorite,
on the subsequent development of bacterial soft rot. These stu-
dies were facilitated by a specially constructed apparatus for
maintaining fairly constant temperature and humidity.
Rapid drying of the tuber surfaces before or soon after pack-
ing was found to control the disease in storage or shipment.
Ordinary air blasts from fans or blowers were not sufficient to
give thorough drying on a commercial scale when atmospheric
conditions adverse to drying prevailed. Experiments with heat-
ed air for drying showed that washed potatoes may be dried
without damage for 4 minutes in air heated to 1500 F. Indica-
tions were that air heated to 1300 F., if circulated rapidly around
the tubers for 3 minutes, would dry them thoroughly under most

Bacterial Soft Rot of Potatoes 35
conditions. It was found that the influence of the heated air on
the temperature of the tubers was offset considerably by the
cooling effect of evaporation from the tuber surfaces during
Drying washed potatoes before packaging by means of hot
air driers was begun on a commercial scale in 1938 and was used
extensively in two large packinghouses at Goulds, Florida, during
the 1939 and 1940 seasons. Bacterial soft rot has failed to develop
sufficiently to cause rejection of a single shipment from these
houses since drying with heated air was inaugurated, according
to statements from the packinghouse managers.
Pre-cooling washed potatoes after the car is loaded, either with
mechanical refrigeration units located outside the car or with
portable fans and ice in the bunkers, eliminated most of the
decay in shipments. Pre-cooling by either method removed con-
siderable moisture from the tuber surfaces and it is probable
that the control of soft rot was due to the drying as much or
more than to the reduction of temperature. Pre-cooling methods
were more costly and have been less reliable for the prevention
of bacterial soft rot in transit than drying with heated air.
A study of packinghouse conditions indicated that washing
tubers by spray jets was less conducive to rot development than
washing in tanks. Observations on the disease in the field in-
dicated the desirability of planting seed-pieces in well aerated
soil, of controlling diseases of the growing crop by spraying or
dusting with copper fungicides, and of exercising care in harvest-
ing to reduce mechanical injuries to a minimum.


Grateful acknowledgement is made to the Goulds Growers, Inc., F.
C. Peters, Inc., and Mr. Jeff Baldwin of Goulds, Florida, for furnishing
potatoes for experimental tests and for other helpful cooperation. The
facilities for the investigation were provided in part by the Board of
County Commissioners of Dade County.


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