Bulletin 299 July, 1936
UNIVERSITY OF FLORIDA
AGRICULTURAL EXPERIMENT STATION
WILMON NEWELL, Director
BROWN ROT OF IRISH POTATOES
AND ITS CONTROL
By A. H. EDDINS
Fig. 1.-Healthy potato plant (left) and a plant affected with brown rot.
Bulletins will be sent free to Florida residents upon application to
AGRICULTURAL EXPERIMENT STATION
EXECUTIVE STAFF BOARD OF CONTROL
John J. Tigert, M.A., LL.D., President of Geo. H. Baldwin, Chairman, Jacksonville
the University Oliver J. Semmes, Pensacola
Wilmon Newell, D.Sc., Director Harry C. Duncan, Tavares
H. Harold Hume, M.S., Asst. Dir., Research Thomas W. Bryant, Lakeland
Harold Mowry, M.S.A., Asst. Dir., Adm. J. T. Diamond, Secretary, Tallahassee
J. Francis Cooper, M.S.A., Editor
Jefferson Thomas, Assistant Editor
Clyde Beale, A.B.J., Assistant Editor BRANCH STATIONS
Ida Keeling Cresap, Librarian
Ruby Newhall, Administrative Manager NORTH FLORIDA STATION, QUINCY
K. H. Graham, Business Manager
Rachel McQuarrie, Accountant L. O. Gratz, Ph.D., Plant Pathologist in
AIN STATRN, GAINEV R. Kincaid, Ph.D., Asso. Plant Pathologist
MAIN STATION, GAINESVILLE J. D. Warner, M.S., Agronomist
AGRONOMY Jesse Reeves, Farm Superintendent
W. E. Stokes, M.S., Agronomist* CITRUS STATION, LAKE ALFRED
W. A. Leukel, Ph.D., Agronomist A. F. Camp, PhD., Horticulturist in Charge
G. E. Ritchey, M.S.A., Associate* John H. Jefferies, Superintendent
Fred H. Hull, Ph.D., Associate Michael Peech, Ph.D., Soil Chemist
W. A. Carver, Ph.D., Associate W. A. Kuntz, ALM., Assoc. Plant Pathologist
John P. Camp, M.S., Assistant B. R. Fudge, Ph.D., Associate Chemist
ANIMAL HUBANDRY W. L. Thompson, B.S., Asst. Entomologist
EVERGLADES STATION, BELLE GLADE
A. L. Shealy, D.V.M., Animal Husbandman* EVERGLADES STATION, BELLE GLAD
R. B. Becker, Ph.D., Dairy Husbandman A. Daane, Ph.D., Agronomist in Charge
L. M. Thurston, Ph.D., Dairy Technician R. N. Lobdell, M.S., Entomologist
W. M. Neal, Ph.D., Asso. in An. Nutrition F. D. Stevens, B.S., Sugarcane Agronomist
D. A. Sanders. D.V.M., Veterinarian Thomas Bregger, Ph.D.. Sugarcane Physiologist
M. W. Emmel, D.V.M., Veterinarian G. R. Townsend, Ph.D., Assistant Plant
N. R. Mehrhof, M.Agr., Poultry Husbandman Pathologist
W. W. Henley, B.S.A., Asst. An. Hush.* J. R. Neller, Ph.D., Biochemist
W. G. Kirk, Ph.D., Asst. An. Husbandman R. W. Kidder, BS., Assistant Animal
R. M. Crown, B.S.A., Asst. An. Husbandman Husbandman*
P. T. Dix Arnold, B.S.A., Assistant Dairy Ross E. Robertson, B.S., Assistant Chemist
Husbandman B. S. Clayton, B.S.C.E., Drainage Engineer*
L. L. Rusoff, M.S., Laboratory Assistant
Jeanette Shaw, M.S., Laboratory Technician SUB-TROPICAL STATION, HOMESTEAD
H. S. Wolfe, Ph.D., Horticulturist in Charge
CHEMISTRY AND SOILS W. M. Fifield, M.S., Asst. Horticulturist
R. W. Ruprecht, Ph.D., Chemist** Geo. D. Ruehle, Ph.D., Associate Plant
R. M. Barnette, Ph.D., Chemist Pathologist
R. B. French, Ph.D., Associate
H. W. Winsor, B.S.A., Assistant W. F. Ward, M.S., Asst. An. Husbandman
C. V. Noble, Ph.D., Agricultural Economist** FIELD STATIONS
Bruce McKinley, A.B., B.S.A., Associate FIEL I
Zach Savage, M.S.A., Associate Leesburg
A. H. Spurlock, M.S.A., Assistant Lsh
M. N. Walker, Ph.D., Plant Pathologist in
ECONOMICS, HOME Charge
W. B. Shippy, Ph.D., Asso. Plant Pathologist
Ouida Davis Abbott, Ph.D., Specialist** K. W. Loucks, M.S., Asst. Plant Pathologist
C. F. Ahmann, Ph.D., Physiologist *. W. Wilson, Ph.D., Associate Entomologist
ENTOMOLOGY Plant City
J. R. Watson, A.M., Entomologist** A. N. Brooks, Ph.D., Plant Pathologist
A. N. Tissot, Ph.D., Associate
H. E. Bratley, M.S.A., Assistant Cocoa
HORTICULTURE A'. S. Rhoads, Ph.D., Plant Pathologist
G. H. Blackmon, M.S.A., Horticulturist and Hastings
"Acting Head of Department A. H. Eddins, Ph.D., Plant Pathologist
A. L. Stahl, Ph.D., Associate Monticello
F. S. Jamison, Ph.D., Truck Horticulturist
R. J. Wilmot, M.S.A., Specialist, Fumigation Asst. Entomologist
R. D. Dickey, B.S.A., Assistant Horticulturist D d G. K ert, a ant ath st
David G. Kelbert, Asst. Plant Pathologist
PLANT PATHOLOGY C. C. Goff, M.S., Assistant Entomologist
W. B. Tisdale, Ph.D., Plant Pathologist** Sanford
George F. Weber, Ph.D., Plant Pathologist E. R. Purvis, Ph.D., Assistant Chemist,
R. K. Voorhees, M.S., Assistant*** Celery Investigations
Erdman West, M.S., Mycologist
Lillian E. Arnold, M.S., Assistant Botanist Lakeland
Stacy O. Hawkins, M.A., Assistant Plant E. S. Ellison, Ph.D., Meteorologist*
Pathologist B. H. Moore, A.B., Asst. Meteorologist*
SPECTROGRAPHIC LABORATORY In cooperation with U.S.D.A.
L. W. Gaddum, Ph.D., Biochemist ** Head of Department.
L. H. Rogers, M.A., Spectroscopic Analyst *** On leave.
INTRODUCTION ....... ......... ......-.......... ........... ........-.. 5
THE DISEASE .................... .... .....- .......- ............. ..... ............ 6
Names, Distribution and Economic Importance ............................ 6 .
H costs ... .....- .......... ..- ...... .... ......... ....... ................... 6
Symptoms ..... ...... .... ..................-- ....................---.......-... 9
Factors Affecting the Occurrence and Severity of Brown Rot ............ 11
Temperature and Rainfall ............................ ..........- ........... 11
Soils and Soil Reactions ........................................ ........ 13
Growth of Organism on Media of Different pH Reactions .................... 15
FIELD EXPERIMENTS ..... ....................................................- 15
Materials and Methods ..............-...... ................ ..................... 15
Effect of the Soil Treatments on the Soil Reaction, Brown Rot
Infection and Yield of Potatoes .............................-.......-.. 17
LaCrossq Experiments ..............-............................ 17
West Tocoi Experiments _................... .................. ... 20
Results of treatments made in 1932 ................................----........ 20
Results of treatments made between potato crops in 1934 .... 24
Effect of Cowpeas Used as a Green Manure in Controlling Brown Rot 31
Effect of Wounding Potato Roots on Brown Rot Infection ................ 31
Susceptibility of Different Varieties of Potatoes to Brown Rot ........ 33
Tuber Infection Studies ................... ................................ 36
Ratio of Tubers Showing External Symptoms of Brown Rot to
Those Showing Internal Symptoms Only .............................. 36
Decay of Infected Tubers after Digging ..................................... 36
DISCUSSION -...................... ..................... 37
CONCLUSIONS ......................... .... ..-........................................ 39
SUMMARY ...................... .................. .............. ....................... 40
LITERATURE CITED .................. .. .. .. .................. 42
Y-1 WRATS--TREATED- --3-NOT TlZATzIjr-.-
Fig. 2.-Three rows of potatoes (center) grown in 1935 in soil which had been treated with sulfur in 1932, were
almost free of brown rot, while plants grown in untreated soil in five adjacent rows (right and left) were severely
affected and many were killed by the disease.
BROWN ROT OF IRISH POTATOES
AND ITS CONTROL1
By A. H. EDDINS
Since brown rot (Bacterium solanacearum E. F. S.) of solana-
ceous crops and other susceptible plants was first noted by
Burrill (1)2 in 1890 and described by Smith (27) in 1896, at-
tempts have been made by workers in various countries to
control it. Smith (27, 28) reported that plant infection could
be reduced by controlling insect carriers of the disease, particu-
larly the Colorado potato beetle, and that cultivation practices
which would minimize root injury would help to prevent infec-
tion. He further suggested that losses could be reduced in
potato crops grown in severely infested soil if the tubers were
dug early and stored in a cool dry place until consumed (27).
Maruda (18) reported that the causal organism in seed potatoes
could be killed by dry heat at 530 C. without injuring the germi-
nation of the tubers. Jochems (12) got absolute control of bac-
terial wilt of young tobacco plants in seedbeds in which the
soil had been sterilized at 850 C. for 15 minutes, the bacteria
being destroyed to a depth of 30 to 40 cm. Nakata (20) reported
that B. solanacearum grew only in soils in Japan with a reaction
between pH 6.0 and 8.1 and that its growth was inhibited in the
presence of certain soil fungi and bacteria. He also found that
there was a "detrimental water content" of the soil which would
kill the organism. Kuyper and Jochems (15) found that infec-
tion of tobacco by B. solanacearum in Sumatra was reduced from
50 percent or more to 5 percent where Mimosa invisa had been
grown for upwards of six years.
Studies of brown rot of Irish potatoes reported herein were
begun at Hastings, Florida, in 1929 and completed in 1935 and
deal mainly with the development of a soil treatment method
for controlling the disease, factors affecting its occurrence and
tests of the reaction of different varieties of potatoes to the
IThese investigations were conducted from 1929 to 1930 by Dr. L. O.
Gratz, now Plant Pathologist in Charge, North Florida Experiment Station,
Quincy, Florida; from 1930 to 1931 by Dr. C. M. Tucker, now Plant Patholo-
gist, Department of Botany, University of Missouri, Columbia, Mo.; from
1931 to date by the author.
2Italic figures in parentheses refer to "Literature Cited" in the back of
6 Florida Agricultural Experiment Station
NAMES, DISTRIBUTION AND ECONOMIC IMPORTANCE
The disease caused by B. solanacearum is known throughout
the world by a number of different names, the more common ones
being bacterial wilt, brown rot, brown rot of Solanaceae, bac-
terial ring disease, slime disease, and Granville wilt (5, 27, 29).
In this publication it will be referred to either as brown rot or
bacterial wilt, depending upon whether the tubers or plants are
Smith (29, 30), Elliott (5) and Meier (19) studied reports
of the distribution of bacterial wilt and found that it occurred
throughout the world in most of the countries located in the
torrid and temperate zones. At present the disease is one of
the chief pathological problems of plants in the islands of the
Dutch East Indies (9), Japan (20), Philippine Islands (40),
Ceylon (22) and Puerto Rico (35).
In the United States, bacterial wilt of such important crops
as Irish potatoes, tomatoes, eggplants, peppers, tobacco and pea-
nuts occurs in certain localities in the South Atlantic and Gulf
Coast States from Maryland to Texas (2, 4, 14, 19, 29, 34), and
in Pennsylvania (21), Ohio (10), Illinois (36), West Virginia
(8) and Kentucky (7).
In Florida the disease has caused considerable damage to
potatoes, tomatoes, peppers and eggplants when they have been
grown in infested soils in the central, northern and western
parts of the State (25, 38, 39). In the Hastings section losses
due to brown rot of potatoes have been estimated at $25,000,
$50,000 and $117,000 for 1933, 1934, and 1935, respectively,
and the disease has been so severe in some fields and parts of
fields in recent years that it has been necessary to abandon them
for growing potatoes, eggplants and tomatoes. In 1935 the
damage from brown rot in the Hastings area was greater than
that caused by all other potato diseases, including late blight
and rhizoctonia which, in most years, cause the greatest losses.
Of a number of different plants grown in infested soil at
West Tocoi, Florida, in 1934, the following became infected with
B. solanacearum: Capsicum annuum L. (red pepper), Lycoper-
sicon lycopersicon (L.). Karst (tomato), Ricinus communis L.
(castor bean), Solanum melongena L. (eggplant), Tagetes erecta
Brown Rot of Irish Potatoes 7
L. (marigold), Zinnia elegans Jacq. (zinnia), Solanum citri-
folium A. Br., S. pyracanthum Jacq., S. sisymbrii Sam., and
Datura metel L. The last four are new hosts, Solanum pyracan-
thum Jacq. and S. sisymbrii Sam., being possibly synonymous.
Plants which grow in Florida and other regions in the south-
eastern United States (26) that have been reported as hosts of
B. solanacearum (5, 6, 11, 14, 31, 32, 41) are listed in Table 1
according to family, genera and species with their common
names, if any. Since these hosts include some very common
truck and field crop plants which are frequently grown on
potato land, together with some flowering plants and weeds
which are common in or near potato fields, it is evident that
it would be impracticable to attempt to eliminate the brown
rot organism from infested land by crop rotation or weed
TABLE 1.-HosT PLANTS OF B. solanacearum THAT GROW IN FLORIDA AND
OTHER REGIONS IN THE SOUTHEASTERN UNITED STATES
Convmelina longicaulis Jacq. ................................*Baby dewflower
Musa cavendishii Lamb. ..................................... Dwarf banana
Musa sapientum L. .................................................. Common banana
Canna indica L. ........................................ Wild canna
Vanilla vanilla (L.) Britton ................................ Vanilla-vine
Rheum rhaponticum L. ....................................... Rhubarb
Beta vulgaris L. .......... --........... ..........-...............*Beet
Leucaena glauca (L.) Benth ................................ Lead tree
Arachis hypogaea L. .... ...................................*Peanut
Phaseolus vulgaris L. .......................................*Kidney bean
Phaseolus lunatus L. ................................................*Lima bean
Pisum sativum L. ....................................................*Pea
Soja max (L.) Piper ................................................ Soybean
Stizolobium niveum Kuntze ....................................*Velvet bean
Crotalaria striata Roth .......................................*Crotalaria
Vigna sinensis (L.) Endl. .....................................*Cowpea
Linum usitatissimum L ...................................... Common flax
Tropaeolum majus L. ........................................ *Nasturtium
Impatiens balsamina L ........................................ Garden balsam
8 Florida Agricultural Experiment Station
TABLE 1.-HosT PLANTS OF B. solanacearum THAT GROW IN FLORIDA AND
OTHER REGIONS IN THE SOUTHEASTERN UNITED STATES.-Continued.
Croton glandulosus septentrionalis L. ............. Wild croton
Phyllanthus niruri L.
Ricinus communis L. ..................--- .....................*Castor bean
Corchorus acutangulus Lam. .---.............................. Jew's mallow
Gossypium sp ........................... .................... Cotton
Hibiscus cannabinus L ....----.................................... Brown Indian hemp
Hibiscus sabdariffa L. ................................-......... Jamaica sorrell
Melochia corchorifolia L.
Daucus carota L. .................-. ............................ *Carrot
Ipomoea batatis (L.) Lam. ....................................*Sweet potato
Ipomoea triloba L. ........................................------*Morning glory
Datura metel L.
Datura stramonium L. ..........................................*Jimson weed
Lycopersicon lycopersicon (L.) Karst. ................*Tomato
Lycopersicon cerasiforme Dunal. .......................... Cherry tomato
Capsicum annuum L. ...............................................*Red Pepper
Nicotiana tabacum L. ............................................. *Common tobacco
Nicotiana rustica L.
Nicotiana glauca Graham
Petunia sp. ----.------------............-...--- *Petunia
Physalis angulata L...-.......... --..-..........................--*Ground Cherry
Solanum carolinense L. .....................................- *Horse nettle
Solanum torvum Sw ............................................... Turkey berry
Solanum nigrum L. ....------.............----- .. ..---- ...... Common nightshade
Solanum melongena L. .................................... *Eggplant
Lantana camera L. .-.........-.....--..............-...--....... Lantana
Lantana aculeata L .....-......--- ............. ................. Lantana
Scoparia dulcis L. -.....---- ---...... ----.....-..............-- -*Goat-weed
Martynia louisiana Mill. ........................................ Unicorn plant
Sesamum indicum L. ....................................---- ... Benne
Ambrosia elatior L. .........-........... ............-*Ragweed
Ageratwm conyzoides L .......................---............---*Ageratum
Bidens bipinnata L. .............---... ....-- ............. *Spanish needle
Chrysanthemum coronarium L. ...........................*Chrysanthemum
Cosmos bipinnatus Cav ..................---................*Cosmos
Dahlia pinnata Cav. .------.......--....----... .....*Dahlia
Erechtites hieracifolia (L.) Raf. ...............-........--*Fireweed
Helianthus annuus L.-.........-- ..........---- ..............*--- Sunflower
Leptilon canadense (L.) Britton .....--............. Horseweed
Tagetes erecta L. ... ........ .................. *Marigold
Zinnia elegans Jacq. ............................... .......-..- *Zinnia
"*Plants which have been seen growing in or near potato fields at
Brown Rot of Irish Potatoes 9
Symptoms of bacterial wilt of various economic plants have
been described by other workers (27, 29, 33), and orily the out-
standing characteristics of this disease of potatoes as it appears
in Florida are reviewed in this paper.
The first symptom of the disease to appear in the potato plant
is a slghtwiltinA of the leaves at the ends of the branches dur-
ing the hottest period of the day. The plant recovrds during the
night but the wilting becomes more pronounced each day until
finally, there is no further recovery and the plaftl-ies (Fig. 1).
All of the top of the plant usually wilts or dies but in some in-
stances only a few of the branches may beaffected and the re-
mainder will appear healthy. The vascular bundles in the stem,
Fig. 3.-A, Healthy tuber; B, Tuber showing external symptoms of
brown rot; C, Cross-section of tuber showing vascular and surrounding
tissues invaded and partially destroyed by B. solanaeearunm.
10 Florida Agricultural Experiment Station
root, and stolons turn browvn when they become clogged with
the bacteria which cause wilting and death by cutting off the
plant's water supply. The brown color is finally evident on the
outer surface of these parts and may be seen on the stem of
severely affected plants one or more inches above the soil line.
When the vascular bundles of the affected parts are cut or
broken, the bacteria ooze from them as a white slimy mass.
The affected tubers may or rAay not show external signs
of the disease, depending upon the stage of development of
brown rot when they are dug. The vascular arteg is first in-
vaded by the bacteria and, when the disease has developed to
a certain stage, the brown color of the affected tissues near the
surface can be seen, particularly around the stem 'and eyes
(Fig. 3). The bacterial ooze exudes from the eyes and stem
end of the severely diseased tuber and when it is mixed with
dirt and dried it sticks to the surface of the tuber. If left in
the ground the tubers continue to decay; the bacteria destroy
tissue which surrounds the vascular ring and finally break
through the skin. Secondary organisms enter the tuber at this
stage and assist in making it a slimy mass with an offensive
Fig. 4.-A potato plant in an early stage of growth with its stem, stolons,
tubers and roots infected with B. solanacearum (left); and a healthy plant
of the same age from an adjoining hill.
Brown Rot of Irish Potatoes 11
Plants with their tops killed by brown rot may bear healthy
as well as diseased tubers. Others showing no signs of the
disease in their tops may produce diseased tubers, and plants
with both wilted and healthy stems may have tubers affected
with brown rot on the wilted stems and sound tubers on the
healthy stems. The disease may cause a great reduction in
yield if the plants are infected at an early stage of growth
.and are killed before tuber formation or before the tubers have
grown to a marketable size (Fig. 4).
FACTORS AFFECTING THE OCCURRENCE AND SEVERITY OF
TEMPERATURE AND RAINFALL
Meier and Link (19) found that temperatures between 770 F.
and 970 F. were optimum for the development of brown rot
and that the disease was inhibited at temperatures below 550 F.
It was also reported by Smith (28) that young potato plants
were more susceptible to the disease than older plants.
Since brown rot was more severe at Hastings in 1935 than
in any of the preceding nine years, a study was made of the
weather records for March and April for the 10-year period
1926-1935 to determine whether there was any correlation be-
tween the temperatures and rainfall of 1935 and the epidemic
of brown rot in that year. The records (Table 2) show that
the rainfall was less and the mean temperature was higher in
March 1935 than in March of any other year and that there
were more days in March 1935 when the minimum temperature
was favorable and the maximum was optimum for the develop-
ment of brown rot. There were also more days with tempera-
tures favorable for the disease in April 1935 than in April of
other years, although there were two years when the mean
temperature of this month was the same or slightly higher
and three years when the rainfall was less than in 1935.
In experimental plots near Hastings, 47.6 percent of the
potato plants was affected with brown rot on April 1, 1935,
while only 1.6 percent of a like number of plants growing in
the same plots was affected on the same date in 1934. Since
the increase in the percentage of diseased plants in these plots
from April 1 to April 26 was approximately the same in both
years, as 47.6 percent was affected with brown rot in 1934 and
91.7 percent in 1935, it was concluded that the greater severity
of the disease in 1935 was due to the high March temperatures
which enabled the causal organism to infect a high percentage
TABLE 2.-RELATION OF THE TEMPERATURE AND RAINFALL AT HASTINGS, FLA., FOR MARCH AND APRIL TO THE SEVERITY OF
BROWN ROT IN EACH YEAR FROM 1926 TO 1935 INCLUSIVE.
March | April _
Number of Days Whe Number of Days When
Temperatures Were Temperatures Were
Mean Favorable for Mean Favorable for Severity
Year Temper- Brown Rot Rainfall Temper- Brown Rot Rainfall of
nature Inches nature Inches Brown
Minimum Maximum Minimum Maximum Rot
(55" F. or (77" F. or (55 F. or (77" F. or
Above) Above) _Above) Above)
1926 59.6 9 8 3.61 66.0 16 18 4.20 Light to
1927 63.7 11 20 2.03 70.2 9 28 0.44 "
1928 64.4 10 18 2.45 65.5 12 20 9.08 "
1929 66.7 18 17 1.62 70.0 10 25 6.10 "
1930 58.7 9 3 8.68 66.8 15 24 1.62 "
1931 57.6 2 5 5.45 65.0 14 20 4.78 "
1932 58.8 8 14 5.75 67.1 13 22 0.50 "
1933 62.0 7 15 1.76 66.9 19 21 8.31 "
1934 61.2 10 16 2.84 67.7 15 23 5.92
1935 68.1 21 23 0.29 70.0 18 26 2.21 Very
Brown Rot of Irish Potatoes 13
of young plants, while low March temperatures in 1934 were
unfavorable for early plant infection.
SOILS AND SOIL REACTIONS
Brown rot occurs on all types of sandy soil in which potatoes
are grown in the Hastings section. Growers generally have
found the disease less troublesome in old cultivated fields than
in newly cleared land (19). However, the causal organism
does not disappear completely from soil which has been cul-
tivated, for in 1935 the disease was severe on many farms where
it had caused no damage for a number of years. On one farm
at LaCrosse, Florida, a potato crop on newly cleared land was
almost completely destroyed by brown rot in 1929 but since
then the disease has almost disappeared and in 1935 less than
10 percent of the tubers were affected. On the other hand the
disease has continued to be very severe in some soils. At West
Tocoi, Florida, in a field which has been cultivated for 10 years
over 90 percent of the potato plants was killed by brown rot
in 1935; the loss was as great as that in an adjacent field which
had been cleared in the summer of 1934 and planted to the first
crop of potatoes in 1935. Low, moist locations in fields appear
to be more suitable for the survival of the causal organism and
the development of the disease than high, dry land, for in 1935
plant infection on most farms was greatest in moist or wet soil
located near water furrows and ditches.
It is not known why the disease usually becomes less severe
in fields the longer they are cultivated, but some of the condi-
tions which might cause this are as follows: (1) Increase in
the acidity of the soil, which may be created by natural agencies
or by the addition of fertilizers, causing the death of most of
the population of B. solanacearum; (2) death of the causal bac-
teria caused by the activities of competing organisms of the soil
flora; (3) death of the bacteria caused by exposing them to
strong sunlight when the field is cultivated; and (4) absence
of host plants which do not ordinarily grow in cultivated fields.
Since 1931 pH determinations have been made of numerous
samples of soil in which brown rot of potatoes occurred. These
samples were taken during the potato-growing season from
areas in fields where the disease was killing plants and rotting
tubers. The results of this survey showed that the disease
occurred in old ground where the soil tested pH 4.42 to 6.20
and in new ground where the reaction ranged from pH 4.32
TABLE 3.-GROWTH OF B. solanacearum IN POTATO BROTH AND POTATO-DEXTROSE AGAR ADJUSTED TO DIFFERENT PH RE-
ACTIONS WITH THE REAGENTS INDICATED. -A
Duration No. I Growth i No Growth
of Test Culture 1 | 2 | 3 | 4 I 5 1 6 I 7 8 I 1 | 2 | 3 4 5 6 7
Initial pH 5.31 4.87 4.67 4.44 4.38 4.23 4.02 3.72 3.00 3.00
30 days I
Final pH 6.88 6.68 6.77 6.35 6.68 4.53 3.93 3.42 2.96 2.96 (I
POTATO DEXTROSE AGAR
Initial pH 4.32 4.52 4.70 3.43 3.79 4.02 4.15
23 days I
____Final pH 4.57 4.74 4.74 4.23 4.23 4.23 4.06
Initial pH 4.32 4.52 4.52 4.70 4.70 4.70 3.43 3.43 3.79 3.79 3.79 4.02 4.15
102 days I
Final pH 5.84 4.65 5.08 7.19 7.53 7.95 4.49 4.40 4.32 4.32 4.32 3.98 4.06 k
Initial pH 4.49 4.65 4.74 4.94 3.56 3.81 4.15
Final pH 6.94 7.36 7.45 7.02 4.15 4.15 4.40 "
Initial pH 4.49 4.49 4.65 4.65 4.74 4.74 4.94 4.94 3.56 3.56 3.81 4.15 4.15 I
102 days I
SFinal pH 7.11 7.45 7.70 7.87 7.87 7.95 7.87 7.87 3.72 3.72 3.98 4.40 4.40 I
23 days Initial pH 5.75 7.02 7.70 8.71 1 1
Final pH 7.95 7.95 7.95 8.12
Initial pH 5.75 5.75 7.02 7.02 7.70 7.70 8.71 8.71
102 dayspH 7.9 8.04 8.
Final pH 7.70 7.87 7.95 8.04 8.04 8.04 8.46 8.38 _
Brown Rot of Irish Potatoes 15
GROWTH OF ORGANISM ON MEDIA OF DIFFERENT pH REACTIONS
The pH reactions at which B. solanacearum would grow and
would not grow when planted in potato broth and on hard potato-
dextrose agar were determined. The media were adjusted to
different pH reactions with sulfuric acid, hydrochloric acid and
sodium hydroxide. The quinhydrone method of pH determina-
tion was used and the duration of the tests ranged from 23 to
102 days. At the end of each test, the viability of the organism
in each culture was determined by transferring portions of it
to potato-dextrose agar, which had a pH reaction favorable
The results, as reported in Table 3, show that B. solanacearum
caused the media on which it grew to become more alkaline and
that the greatest changes in reaction were produced in cultures
with initial reactions of pH 5.75 and lower. This production
of alkali by B. solanacearum agrees with the findings of Smith
(27), who stated that it was an intense alkali producer.
In potato broth acidified with sulfuric acid, B. solanacearum
grew and remained viable when the initial pH was 4.23 or above
and was killed when the reaction was pH 4.02 or lower. On
potato-dextrose agar acidified with sulfuric acid, it grew at
initial pH reactions of 4.32 and higher and was killed at initial
reactions of pH 4.15 and lower. When hydrochloric acid was used
as the acidifying agent, the minimum pH reaction for growth
was 4.49 and the organism died when it was exposed to reactions
of pH 4.15 and lower. On media made alkaline with sodium
hydroxide, the organism grew and remained viable in the pH
range 5.75 to 8.71. Its growth was not tested on media more
alkaline than pH 8.71.
The field experiments discussed herein were conducted in
soil infested with B. solanacearum at LaCrosse from 1929 to
1935 and at West Tocoi from 1932 to 1935.
MATERIALS AND METHODS
Plots:-The soil treatment plots at LaCrosse measured 28
feet by 62.2 feet and all data on the soil reaction, yield and brown
rot were obtained from three rows located in the center of each
plot. The rows were 50 hills in length, except in 1930 when
they measured 62.2 feet. At West Tocoi the plots were 10/2
and 17 feet wide and 50 feet long and the data were obtained
16 Florida Agricultural Experiment Station
from the middle row of each plot. The rows were 50 hills in
length unless otherwise indicated in the tables. Ten-foot borders
were left between the ends of the plots in both places. Varieties
tested for reaction to brown rot were grown in single-row plots
50 hills in length.
Treatments:-The agents used in treating the soil consisted
of hydrated lime, calcium and dolomitic limestone and sulfur,
known commercially as inoculated sulfur, which had been inoc-
ulated with bacteria to cause a rapid conversion of the sulfur
into sulfuric acid after it was incorporated with the soil (18).
The treatments were made in the fall, winter and summer and
the agents were broadcasted on plowed soil and mixed with it
Soil Samples and pH Determinations:-The soil sample for
each pH determination consisted of a well-mixed composite of
three samples taken from the upper 6 to 8 inches of soil near
the ends and middle of each plot. Each determination was made
in duplicate by the quinhydrone method and the reaction of the
soil was determined three or more times during each growing
Data:-Individual plot records were kept on plant and tuber
infection and yields in pounds of U. S. D. A. grades Nos. 1 and
2 tubers produced. The infected tubers comprised those show-
ing both external and internal symptoms of brown rot. The
weights of those showing internal symptoms of the disease in
each plot were calculated from the percentage infection found
by cutting five pounds (or less if this amount was not produced)
of apparently healthy No. 2 tubers. The weight of healthy
tubers was determined by subtracting the weight of those with
internal symptoms of the disease from the total weight of the
apparently healthy ones. The percentage of tuber infection
was found by dividing the weight of the tubers showing both
external and internal symptoms of the disease by the sum of
the weights of the healthy and infected tubers. The yields
were calculated in barrels3 per acre and are so reported in the
tables. Odds of probability for evaluating the significance of
the differences in yields and brown rot infection were calculated
by Love's modification of Student's method (16) in all tests
except the varietal tests in which the probable errors were cal-
culated and used to determine the odds (23).
3The weight of a barrel of potatoes is 165 pounds and is therefore
equivalent to 2.75 bushels.
Brown Rot of Irish Potatoes 17
EFFECT OF SOIL TREATMENTS ON SOIL REACTION, BROWN ROT
INFECTION AND YIELD OF POTATOES
The first tests to determine the effect of different soil treat-
ments on the soil reaction, yield and brown rot infection of
potatoes were conducted at LaCrosse, where the soil, which is
a Scranton fine sand, was treated in December 1929 with inoc-
ulated sulfur (17) and hydrated lime at the rates per acre in-
dicated in Table 4. The soil which had been treated with sulfur
at the rate of 1,200 pounds per acre in 1929 was treated in
1933 with limestone at the rate of 2,000 pounds per acre. Each
treatment was replicated five times in plots distributed at ran-
dom throughout the field.
The reaction of the soil before the treatments varied from
pH 5.79 to 6.02 in different sections of the field and the un-
treated areas gradually became more acid until in 1935 they
tested from pH 4.74 to 4.83. The sulfur treatments caused
increases in the acidity of the soil and the lowest reaction, pH
4.45, was produced by the 1,200-pound treatment in the second
growing season after it was made. Increases in the alkalinity
of the soil followed the use of lime and the highest reaction, pH
6.56, was created by the 3,000-pound treatment in the first grow-
ing season after it was made. The changes in the reaction
caused by the sulfur, hydrated lime and limestone treatments
were not maintained and in 1935 the treated and untreated soil
had approximately the same reaction in all plots except those
treated with 3,000 pounds of lime per acre in 1929 and those
treated with 2,000 pounds of limestone in 1933 in which the
soil was slightly more alkaline than the soil of the check plots.
Unfortunately, the soil varied greatly in its productiveness
throughout the field and the data obtained from some of the
plots had to be discarded because of damage from excessive water
in some seasons and drought in others. Furthermore, frost
destroyed the crop the third year, and the disease began to
disappear from the field after the first year and was very mild
during the last three years. Owing to these conditions, the
data could not be evaluated properly by using the usual biometri-
The data indicated, however, that sulfur applied at the various
rates was controlling brown rot in each of the five years. The
800-pound and 1,200-pound treatments caused the greatest de-
creases in plant and tuber infection. Sulfur applied at the rate
TABLE 4.-EFFECT OF SOIL TREATMENTS WITH DIFFERENT AMOUNTS OF INOCULATED SULFUR, HYDRATED LIME AND CALCIUM
LIMESTONE FOLLOWING SULFUR ON THE SOIL REACTION, BROWN ROT INFECTION AND YIELD OF POTATOES AT LACROSSE,
FLA., IN 1930, 1931, 1933, 1934 AND 1935.
SBrown Rot Yield in Barrels per Acre
Number of Soil Reaction* Percent Plants
Replications pH Infected Percent Tubers Infected Total Healthy
Year | | Decrease | Increase
ICheck Treated Check Treated Check Treated Check Treated Due to Check Treated Check Treated Due to -
III Tr'tment Trtment
1,200 lbs. SULFUR
1930 5 5 5.95 4.88 ** .... 44.1 19.4 24.7 80.2 54.1 44.8 43.6 -1.2
1931 4 4 5.30 4.45 28.9 8.4 25.0 9.2 15.8 87.6 59.6 65.7 54.1 -11.6 .
1933 2 2 4.80 4.50 6.4 0.4 3.5 0.3 3.2 69.5 64.4 67.1 64.2 -2.9
1,200 lbs. SULFUR plus 2,000 lbs. LIMESTONE
1934 5 5 5.00 5.21 10.5 0.7 8.8 0.2 3.6 50.5 59.6 48.6 59.5 10.9
1935 5 5 4.75 4.98 44.1 14.3 7.7 1.3 6.4 41.6 54.0 38.4 53.3 14.9
800 lbs. SULFUR
1930- 5 5 5.91 5.18 43.9 13.9 30.0 74.8 66.5 42.0 57.3 15.3 M
1931 3 3 5.38 4.75 18.8 1.3 19.4 3.9 15.5 88.6 86.6 71.4 83.2 11.8
1933 4 4 4.80 4.55 4.4 0.0 3.1 0.4 2.7 74.8 69.8 72.5 69.5 -3.0
1934 5 5 5.00 4.76 9.1 1.3 4.3 0.0 4.3 60.2 58.7 57.6 58.7 1.1
1935 5 5 4.75 4.69 48.4 9.3 6.8 0.2 6.6 41.0 47.7 38.2 47.6 9.4 C
400 lbs. SULFUR
1930 4 4 5.79 5.57 55.3 49.2 6.1 77.7 81.1 34.7 412 6.5
1931 5 5 5.34 4.93 39.3 27.0 29.8 19.7 I 10.1 90.3 91.4 63.4 73.4 10.0
1933 4 4 4.74 4.74 5.2 0.8 4.6 1.0 3.6 75.0 73.3 71.5 72.6 1.1
1934 5 5 4.87 4.83 12.3 5.6 5.7 3.8 1.9 53.0 52.5 50.0 50.5 0.5
1935 5 5 4.78 4.73 52.2 32.5 7.5 4.5 3.0 39.8 35.6 36.8 34.0 -2.8
TABLE 4.-EFFECT OF SOIL TREATMENTS WITH DIFFERENT AMOUNTS OF INOCULATED SULFUR, HYDRATED LIME AND CALCIUM
LIMESTONE FOLLOWING SULFUR ON THE SOIL REACTION, BROWN ROT INFECTION AND YIELD OF POTATOES AT LACROSSE,
FLA., IN 1930, 1931, 1933, 1934 AND 1935.-Continued.
Brown Rot Yield in Barrels per Acre
Number of Soil Reaction* Percent Plants I
Replications pH Infected Percent Tubers Infected | Total Healthy
Year I I Decrease Increase
Check TreatedTreated Check Treated I Due to heck Treated Check eatt C Treated Due to
ITreatment C ITr'tment
1.000 lbs. HYDRATED LIME o
1930 5 5 6.02 6.42 ...... ...... 46.9 36.8 10.1 81.2 72.8 43.1 46.0 2.9
1931 5 5 5.35 5.48 81.0 23.0 22.7 18.2 4.5 91.0 92.3 70.3 75.5 5.2
1933 4 4 4.75 4.95 4.4 4.1 3.1 3.4 -0.3 77.6 64.1 75.2 61.9 -13.3
1934 5 5 4.94 5.12 10.0 7.9 4.7 2.4 2.3 59.2 58.6 56.4 57.2 0.8 .
1935 5 5 4.83 4.90 49.8 35.4 6.8 1.8 5.0 42.6 48.5 39.7 42.7 3.0
2,000 lbs. HYDRATED LIME
1930 5 5 5.93 6.49 ...... ...... 47.7 20.2 27.5 75.7 74.8 39.6 59.7 20.1 Z_
1931 3 3 5.34 5.75 36.5 10.8 24.6 6.1 18.5 93.4 82.5 70.4 77.5 7.1
1933 3 3 4.67 5.11 6.4 3.0 4.6 2.4 2.2 74.6 65.8 71.2 64.2 -7.0
1984 5 5 5.00 5.23 12.0 7.0 4.7 2.2 2.5 61.6 62.5 58.7 61.1 2.4
1935 5 5 4.80 4.96 61.6 38.8 9.6 10.3 -0.7 41.8 45.4 37.8 40.7 2.9
8,000 lbs. HYDRATED LIME
1930 5 5 5.88 6.56 ...... 44.5 42.2 2.3 84.1 73.9 46.7 42.7 -4.0
1931 4 4 5.88 6.18 24.4 20.5 19.5 15.5 4.0 92.1 91.4 I 74.1 77.2 3.1
1933 4 4 4.69 5.16 83.1 2.0 3.4 2.3 1.1 78.6 74.9 75.9 78.2 -2.7
1934 5 5 4.94 5.43 7.2 2.9 2.6 1.2 1.4 50.3 56.6 49.0 55.9 6.9
1935 5 5 4.76 5.05 28.7 37.2 3.7 4.1 -0.4 46.4 53 3 44.7 51.1 6.4
*Average of three determinations which were made in February, March and April of each year.
"**The plants were killed by late blight and wet weather before final notes could be taken.
20 Florida Agricultural Experiment Station
of 1,200 pounds per acre depressed the yields the first three
years, but after this heavily sulfured soil was treated with lime-
stone at the rate of 2,000 pounds per acre in 1933, the yields
produced in the treated plots exceeded those produced in the
untreated. There were marked increases in yields of healthy
tubers in the 800-pound sulfur plots in 1930, 1931 and 1935,
and in the 400-pound plots in 1930 and 1931.
There was also less plant and tuber infection in most seasons
in the plots treated with lime at the different rates as compared
with the checks. Greatest reductions in percentage of plant
and tuber infection and greatest increases in yields of healthy
tubers resulting from lime treatments occurred in plots treated
with lime at the rate of 2,000 pounds per acre. The reduction
in percentage of tuber infection in the plots treated with 1,000
pounds of lime per acre was outstanding in 1930 only, and the
3,000-pound treatment had little effect on infection or yield in
WEST TOCOI EXPERIMENTS
The soil in the field at West Tocoi is Scranton fine sand with
little variability in its productiveness, except on one end which
was too high and dry for the best growth of potatoes. Potato
crops grown in this field prior to 1933 had been generally in-
fected with brown rot but the infection had varied with the
slope of the land, the disease having been more severe in the
highest section. However, it was possible to locate the treated
and check plots in uniformly infested soil by placing them ad-
jacent to each other with their length parallel with the slope of
the field. Very satisfactory results were obtained from the
experiments conducted in this field, as the disease continued
to be severe and good yields of potatoes were produced.
RESULTS OF TREATMENTS MADE IN 1932
The treatments made at West Tocoi in October 1932 consisted
of applications of sulfur at the rates of 200 pounds, 400 pounds,
600 pounds and 800 pounds per acre, and calcium limestone which
was applied to the heavily sulfured plots in November 1933 at
the rate of 2,000 pounds per acre. The results of the treat-
ments are given in Table 5.
In this field little change occurred in the pH reaction of the
untreated soil during the three-year period, as the average re-
action was pH 5.00 in 1932, while in 1935 the reaction varied
from pH 4.70 to 4.85 in the different check plots. Sulfur applied at
TABLE 5.-EFFECT OF SOIL TREATMENTS WITH SULFUR AND LIMESTONE FOLLOWING SULFUR UPON THE SOIL REACTION,
YIELD AND BROWN ROT INFECTION OF POTATOES; WEST TOCOI, 1933-1935.
SPAULDING ROSE. 1933
Check .............................. 5.00 4.70 4.44 35.9 26.3 58.2 42.9
800 Ibs. Sulfur .............. 4.62 3,94 4.04 18.7 9.5 16.8 > 9999:1 35.8 32.4 -10.5 182:1
Check .............................. 5.21 4.77 4.42 40.3 29.8 55.4 38.9 R
600 lbs. Sulfur ............. 4.63 3.96 4.0 1.4 11.4 18.4 9999:1 37.7 33.4 -5.5 13.6:1
Check .............................. 4.99 4.76 4.47 44.5 30.2 55.6 38.8 I
400 lbs. Sulfur ............. 4.79 4.15 4.17 18.8 16.8 18.4 61.1:1 50.6 42.1 3.3 2.55:1
Check .............................. 5.04 4.72 4.50 38.9 26.6 62.7 46.0
200 Ibs. Sulfur .............. 4.79 4.38 4.26 82.9 27.3 -0.7 < 1.59:1 57.9 42.1 -3.9 2.55:1
SPAULDING ROSE, 1934
----------- ------------- ----- --- - --------- __ ---- --- --------- 0
Check ...... ............ ..... 4.85 4.82 4.63 41.2 22.5 70.2 54.4
800 Ibs. Sulfur
2,000 lbs. Limestone .... 5.14 5.40 4.72 3.0 0.6 21.9 1999:1 72.0 71.6 17.2 216:1
Check .............................. 4.86 4.83 4.61 47.7 28.4 73.5 52.6
600 lbs. Sulfur .......... 4.63 4.77 4.28 4.0 1.2 27.2 3832:1 683.0 62.2 9.6 29.8:1
Check ..... ............... 4.88 4.93 4.62 46.2 26.6 71.0 52.1
400 lbs. Sulfur ............ 4.61 4.93 4.85 8.1 4.5 22.1 81.0:1 68.2 65.1 18.0 22.5:1
Check ..................... 4.97 4.83 4.66 S9.9 21.5 74.0 58.1
200 lbs. Sulfur .............. 4.76 4.97 4.48 22.8 11 0. 100:1 74.7 66.5 8.4 22.5:1
Chec..............4.8 4.2 468 4.2 2.570.2 54. ci
800 ls. Sufur
TABLE 5.-EFFECT OF SOIL TREATMENTS WITH SULFUR AND LIMESTONE FOLLOWING SULFUR UPON THE SOIL REACTION,
YIELD AND BROWN ROT INFECTION OF POTATOES; WEST TOCOI, 1933-1935.-Continued.
|Brown Rot | Yield in Barrels per Acre
Soil Reaction pH 1 Percent Tubers Infected I Healthy
Treatment* Percent Decrease Increase
Feb. Mar. Apr. Plants Total Due to Odds Total Total Due to Odds
Infected Treatment Treatment
SPAULDING BOSE, 1935
Cheek ............................ 4.79 4.90 4.46 90.6 40.9 17.1 10.1
800 Ibs. Sulfur t1
2,000 lbs. Limestone .... 4.93 5.03 4.55 24.0 4.7 36.2 1999:1 55.6 53.0 42.9 4999:1
Check ..................... 4.76 4.86 4.53 90.3 31.0 16.8 11.6
600 lbs. Sulfur ....... 4.64 4.91 4.42 5.9 1.1 29.9 768:1 36.7 36.3 24.7 138:1
Check 4.77 4.91 4.51 94.2 42.2 12.8 7.4 I
400 lbs. Sulfur .............. 4.67 4.93 4.47 24.0 5.8 36.4 832:1 38.1 35.9 28.5 1666:1
Check .................. 4.85 4.95 4.49 89.0 31.3 23.3 16.0 I
200 lbs. Sulfur .............. 4.77 4.91 4.47 62.9 18.5 12.8 34.0:1 1 36.8 30.0 14.0 369:1
GREEN MOUNTAIN, 1935
Cheek ......................... 4.71 4.94 4.52 75.7 7.6 42.2 39.0 M
800 lbs. Sulfur
2.000 lbs. Limestone .... 4.95 4.97 4.58 5.3 0.3 7.3 42.5:1 64.4 64.2 25.2 102:1
Check ................ 4.80 4.95 4.52 86.1 17.3 40.4 33.4
600 lbs. Sulfur .............. 4.48 4.88 4.44 30.8 2.4 14.9 73.1:1 41.9 40.9 7.5 1249:1
Check ............................. 4.70 5.00 4.50 74.5 18.1 43.1 35.3
400 lbs. Sulfur ........ 4.71 4.76 4.52 80.0 3.4 14.7 18.6:1 46.9 45.3 10.0 8.22:1
Cheek .............................. 4.83 4.93 4.52 65.6 10.2 51.2 46.0
200 lbs. Sulfur .............. 4.70 4.83 4.51 60.1 10.7 -0.5 1.46:1 52.4 46.8 0.8 1.29:1
*Each treatment and check was replicated 10 times in 50 hill rows in 1933 and 1934, and 6 times for the Spaulding Rose and 4 times for the
Green Mountain in 40 hill rows in 1935, except for the sulfur-limestone treatment which was replicated 3 times in the Green Mountain plots.
Brown Rot of Irish Potatoes 23
the rates of 800 pounds and 600 pounds per acre reduced the reac-
tion from pH 5.00 to a pH below 4.00 b; March 1933. The 400-
pound and 200-pound sulfur treatments caused the soil to become
quite acid but the reaction remained.above pH 4.00 during the first
growing season in the plots receiving these treatments. After
limestone was applied to the heavily sulfured soil in November
1933 at the rate of 2,000 pounds per acre, the reaction became
pH 5.40 in March 1934. The effect of the sulfur and limestone
treatments on the soil reaction at West Tocoi was similar to
that observed at LaCrosse, for the greatest changes in the
reaction occurred during the first year after the treatments
were made, and the reactions of the treated soils finally became
approximately the same as the untreated at both places. How-
ever, it took only three years for the reactions of the treated
soils to revert to that of the untreated at West Tocoi, while
six years were required for soils receiving similar treatments
at LaCrosse for the reaction to become readjusted to that of
the untreated soil.
The 200-pound and 400-pound sulfur treatments did not give
as good control of brown rot as the 800-pound and 600-pound
treatments, as indicated by the significant decreases in the
percentages of tuber infection and in the increases in the yields
of healthy potatoes produced by these treatments (3). How-
ever, the 800-pound treatment caused a significant reduction in
yield of healthy tubers in 1933, but after this treatment was
followed by an application of limestone at the rate of 2,000
pounds per acre, yields produced in treated soil were signifi-
cantly greater than those produced in untreated soil in adjacent
check plots. The 600-pound treatment was as effective as the
800-pound treatment in reducing the rates of plant and tuber
infection, but yields of healthy tubers produced in the 800-pound
sulfur plots after the limestone was applied exceeded those pro-
duced in the 600-pound plots (Fig. 5).
It is interesting to note that all treatments, except the 200-
pound one, gave excellent control of the disease in 1935 when
conditions were ideal for the development of brown rot which
appeared early in the season and killed many plants in the un-
treated soil prior to the production of marketable tubers (Fig.
2). In 1935 the superiority of the sulfur-limestone treatment
over the others was outstanding, as much better yields were
produced in the plots treated with both sulfur and limestone.
Although the decreases in the percentages of tuber infection
24 Florida Agricultural Experiment Station
and the increases in yield in the treated plots where the Green
Mountain was grown were not as great as in similarly treated
plots of'the Spaulding Rose, the higher yields of the former
variety and its greater resistance to brown rot are sufficient
to warrant its substitution for the latter for planting in treated
H lALTHT TU31BE
10 53.0 10.1 36.3 11.6
6. 0. 20.1 0.4
Fig. 5.-Histograms showing a comparison of the yields of healthy and
infected tubers produced during three years at West Tocoi, Fla., in un-
treated soil and in soil which had been given the sulfur-limestone treatment
(left) and the 600 pound sulfur treatment (right).
RESULTS OF TREATMENTS MADE BETWEEN POTATO CROPS IN 1934
The sulfur-limestone treatment was made between potato
crops in the West Tocoi field in 1934 to determine if it were
possible to control brown rot without decreasing the yield of
potatoes by applying the sulfur in June and the limestone in
November. Other treatments made in November included appli-
Brown Rot of Irish Potatoes 25
TABLE 6.-CHANGES PRODUCED IN SOIL REACTION BY THE SULFUR-LIME-
STONE, SULFUR AND DOLOMITIC LIMESTONE TREATMENTS WHICH WERE
MADE BETWEEN POTATO CROPS IN 1934.
pH Reactions Produced by-
800 lbs. Sul-
fur Applied I
Date June 21 1 800 lbs. 1,000 lbs. 2,000 lbs. 3,000 lbs.
3,000 lbs. Check Sulfur Limestone Limestone Limestone
Limestone Applied Applied Applied Applied
Applied Nov. 16 Nov. 16 Nov. 16 Nov. 16
Nov. 16 _
June 20 5.00 5.06
July 19 3.86 5.15
Aug. 29 3.58 5.05
Sept. 26 3.48 5.28
Oct. 24 3.86 4.90
Nov. 12 4.00 4.97 5.00 5.00 5.00 5.00
Dec. 17 4.68 5.04 4.49
Feb. 16 5.23 5.02 4.34 5.51 5.45 5.71
Mar. 4 5.34 4.95 4.20 5.30 5.37 5.75
Apr. 16 4.81 4.54 3.95 4.87 5.02 5.40
May 16 ...... 4.60 3.82
June 20 4.76 4.30 3.74
Aug. 8 4.54 4.85 3.50
cations of sulfur alone and dolomitic limestone at different rates.
Plots receiving the different treatments were located in the
highest section of the field where the soil was not very pro-
ductive but where the disease had been very severe. Tomatoes
and eggplants were also grown in the sulfur-limestone plots to
test the effectiveness of the combination treatment in controlling
the disease in the three crops.
Effect of Treatments on Soil Reaction:-When 800 pounds of
sulfur were applied in June, only one month was required for
the reaction of the soil to change from pH 5.07 to 3.86, while
four months elapsed before the reaction was reduced from pH
26 Florida Agricultural Experiment Station
5.00 to 3.95 by a similar treatment which was made in November
(Table 6). In soil receiving the June treatment, the reaction
remained at pH 4.00 and below until limestone was applied in
November. The highest reaction, pH 5.34, resulted from the
limestone treatment and was recorded in March 1935, after
which the soil became more acid and tested pH 4.54 in August
1935. Soil treated with sulfur in November 1934 became very
acid during the hot summer months of 1935, and tested pH 3.50
The dolomitic limestone treatments caused the soil to become
less acid and the highest reaction, pH 5.75, resulting from the
3,000-pound treatment was recorded in March 1935.
The reaction of the untreated soil did not remain constant
during the 131/2-month period. The -highest reaction, pH 5.28,
was recorded in September 1934, and the lowest, pH 4.30, in
Changes in Reaction of the Top Foot of Soil Treated with Sul-
fur in June:-The top foot of soil was made very acid by treating
it with 800 pounds of sulfur per acre in June, as is shown in
TABLE 7.-HYDROGEN-ION CONCENTRATION OF SOIL SAMPLES TAKEN SEP-
TEMBER 26, 1934, AT INTERVALS OF TWO INCHES FROM THE TOP FOOT OF
SOIL WHICH WAS TREATED WITH 800 POUNDS OF SULFUR PER ACRE ON
JUNE 21, 1934.
pH Reaction of Samples at Depths of-
Treatment Sur- 2 4 6 8 10 12
faceI inches inches inches inches inches inches
Treated .......... 3.17 2.89 3.10 3.44 3.78 4.05 4.19
Not Treated .... 5.48 5.36 5.26 5.31 5.32 5.28 5.21
Within three months after this treatment was made, the upper
8 inches had a reaction below pH 4.00. The 2-inch sample, test-
ing pH 2.89, was most acid and the 12-inch sample, testing pH
4.19, was least acid. There was little variation in the reaction
of the untreated soil which tested pH 5.48 at the surface and
pH 5.21 at 12 inches from the surface.
Effect of Treatments on Brown Rot Infection and Yield of
Potatoes:-Table 8 shows that treatment of the soil with 800
pounds of sulfur per acre in June 1934 followed by an applica-
tion of 3,000 pounds of dolomitic limestone per acre in Novem-
ber 1934 gave outstanding control of brown rot and significant
TABLE 8.-EFFECT OF TREATING THE SOIL BETWEEN POTATO CROPS IN 1934 WITH DOLOMITIC LIMESTONE, SULFUR ALONE
AND SULFUR PLUS DOLOMITIC LIMESTONE ON THE BROWN 'ROT INFECTION AND YIELD OF POTATOES AT WEST TOCOI,
FLA., IN 1935.
Num- Brown Rot Yield in Barrels per Acre
Treatment Repli- Percent Percent Tubers Infected Healthy and Infected I Healthy
cations Plants Decrease I Increase I Increase
Infected Total Due to Odds Total Due to Odds Totall Due to Odds
Treatment I Treatment I TreatmentI
Check ........................ 6 93.4 54.1 11.1 5.1
1,000 lbs. Limestone
Applied Nov. 16 ...... 6 89.2 38.9 15.2 6.25:1 22.1 11.0 29.1:1 13.5 8.4 11.8:1
Check .................. 6 91.7 47.6 12.6 6.6
2,000 lbs. Limestone
Applied Nov. 16 ...... 6 90.4 50.7 -3.1 < 1.40:1 22.5 9.9 75.9:1 11.1 4.5 21.7:1 '
Check ......... ........ 6 83.7 28.8 19.1 13.6
3,000 lbs. Limestone
Applied Nov. 16 ...... 6 88.7 33.7 -4.9 1.40:1 20.3 1.2 1.40:1 13.5 -0.1 1.40:1
800 lbs. Sulfur ....... ..
Applied Nov. 16 ...... 6 84.6 35.5 42.8 11.8:1 5.7 3.4 7.42:1 3.7 3.2 11.8:1
Check ............... 8 98.5 70.4 2.7 0.8
800 lbs. Sulfur
Applied June 21
3,000 lbs. Limestone
Applied Nov. 16 ...... 8 4.3 0.8 69.6 1666:1 24.1 21.4 > 9999:1 23.9 23.1 > 9999:1
Check .............. 7 99.7 55.2 14.5 6.5
800 lbs. Sulfur
Applied June 21
3.000 lbs. Limestone
Applied Nov. 16 ...... 7 5.4 0.6 54.6 > 9999:1 30.3 15.8 285:1 30.1 23.6 3332:1
28 Florida Agricultural Experiment Station
increases in yields in plots planted to the Spaulding Rose and
Green Mountain varieties. This treatment caused a great re-
duction in the percentage of plant infection and death, as is
illustrated in Figure 6. Apparently the low soil reactions, pH
4.00 and below, which were created by the sulfur in the summer
of 1934, killed the brown rot organism in this soil, and the lime-
stone treatment brought the soil back to a more normal reaction
and thus induced normal plant growth and yields without caus-
ing a return of brown rot.
Fig. 6.-Brown rot plant infection was approximately 5 percent in
potatoes which were grown in the soil that had received the sulfur-limestone
treatment in 1934, staked rows (left); while plant infection in untreated
soil was over 99 percent, staked rows (right).
The sulfur treatment, which was made in November, caused
a reduction in the percentage of tuber infection but very little
reduction in the percentage of plant infection which was prob-
ably due to the fact that reactions lethal to the brown rot or-
ganism were not created in this soil until near the end of the
1935 growing season (Table 8).
The limestone treatments alone did not cause any marked
reductions in the percentages of plant and tuber infection or
produce significant increases in the yields of potatoes.
Effect of Sulfur -Limestone Treatment in Controlling Bacterial
Wilt of Tomatoes and Eggplants:-Table 9 shows that at the
beginning of the fruit-picking season, most of the tomato and
eggplant plants in the check plots were affected with bacterial
wilt, while only a few plants in the plots treated with sulfur
Brown Rot of Irish Potatoes 29
and limestone were diseased. In the untreated soil, the dis-
ease killed 98.8 percent of the tomato plants and reduced the
yield of fruit to 0.1 pounds per plant, while only 17.9 percent
of the plants in the treated soil was killed and the yield per
plant was 1.1 pounds. In the treated soil, only 5.9 percent of
the eggplant plants was killed by the disease and the plants
produced an average yield of 5.2 pounds of fruit, and in the
untreated soil, 92.4 percent of the plants was infected, 70.6
percent was killed and the average yield was reduced to 0.4
TABLE 9.-EFFECT OF SOIL TREATMENT WITH SULFUR PLUS DOLOMTIC
LIMESTONE ON THE BACTERIAL WILT INFECTION AND YIELD OF TOMATOES
I Percent Plants Infected J Pounds Fruit
S Num- I Produced
Treatment ber IFirst Last Fruit I
ofj Fruit Picking Date Per
Plants Picking L J I Total Plant
Date I Total I Dead I
Check ............................... 83 73.5 1 98.8 98.8 5.8 0.1
800 Ibs. Sulfur
3,000 lbs. Limestone ...... 78 2.6 30.8 17.9 87.5 1.1
Check ................................ 289 88.6 92.4 70.6 122.9 0.4
800 lbs. Sulfur
3,000 lbs. Limestone ..... 237 4.2 5.9 5.9 1234.0 5.2
It is probable that the sulfur-limestone treatment would have
given better control of bacterial wilt of tomatoes and eggplants
in these tests if the disease had been prevented from spreading
from the check plots to the treated. Since treated plots were
adjacent to the check plots, some of the infection in the treated
soil was caused by the organism which entered the treated plots
when rains washed infested soil into them and when the land
was cultivated. The disease was also probably spread to some
extent by the Colorado potato beetle which was not controlled
during the latter part of the growing season (27).
TABLE 10.-EFFECT OF COWPEAS USED AS A GREEN MANURE ON THE BROWN ROT INFECTION AND YIELD OF POTATOES GROWN
IN SOIL INFESTED WITH B. solanacearu'm AT WEST TOCOI, FLA., IN 1934 AND 1935.
Brown Rot Yield in Barrels per Acre
Treat- Percent Plants Infected at Percent Tubers Healthy
ment* Date Indicated Infected Total ...
4-1 4-8 4-15 4-24 5-2 Total Due to Odds Total Due to Odds
S______________Cowpeas _Cowpeas F
Check .....-- .** 22.1 45.8 67.2 ...... 43.2 57.2 32.5
Cowpeas ... 14.5 32.7 50.2 ...... 32.9 10.3 14.2:1 67.2 45.1 12.6 40.0:1
Check 61.6 86.9 94.5 97.6 99.7 55.0 2.0 0.9
Cowpeas 13.8 48.9 55.5 66.8 82.1 22.7 32.3 13.6:1 27.7 21.4 20.5 216:1
*Each replicated 8 times in 1934 and 7 times in 1935 in rows 50 hills long.
**No notes taken.
Brown Rot of Irish Potatoes 31
EFFECT OF COWPEAS USED AS A GREEN MANURE IN
CONTROLLING BROWN ROT
Cowpeas were grown as a green manuring crop in the summers
of 1933 and 1934 in soil infested with B. solanacearum to de-
termine if they would give any control of brown rot. In 1934
the disease did not become severe until late in the growing
season and few potato plants were killed before they produced
marketable tubers. Although the yield was greater and tuber
infection less in the cowpea plots than in the checks in 1934,
the differences were not statistically significant (Table 10). In
1935 the disease became very severe during the latter part of
March and on April 1 plant infection was approximately five
times greater in the check plots than in the cowpea plots. Most
of the plants in the check plots were killed by the disease before
they produced tubers, while in the cowpea plots a large per-
centage escaped early infection and death and outyielded the
plants in the checks 20.5 barrels of healthy tubers per acre.
The control obtained was evident only in the increase in yield,
as the rate of tuber infection in the cowpea plots was not signifi-
cantly less than in the checks.
EFFECT OF WOUNDING POTATO ROOTS ON BROWN ROT
Since it is a common practice to plant corn in potato fields
at Hastings several weeks before the potatoes are dug, a test
was conducted in 1934 to determine the effect of this practice in
increasing the damage from brown rot in soil infested with B.
solanacearum and the results are given in Table 11. The plant-
ing was done three weeks before the potatoes were dug by run-
ning a V-shaped, spike-tooth harrow between the rows followed
by the corn planter and this in turn by a middle-buster with
wings, which threw more dirt on the corn and against the sides
of the rows. This operation wounded many potato roots and
plant infection increased more rapidly in the planted rows than
in the unplanted. There was no significant difference in the
total yield of both healthy and infected tubers in the planted
and unplanted rows, but there was a significant increase in
the percentage of tuber infection and a significant reduction
in yield of 21.2 barrels of healthy tubers per acre caused by
planting the corn on the sides of the rows (28).
TABLE 11.-EFFECT OF PLANTING CORN ON THE SIDES OF POTATO ROWS THREE WEEKS PRIOR TO DIGGING ON THE BROWN
ROT INFECTION AND YIELD OF POTATOES AT WEST TOCOI, FLA., IN 1934.
Brown Rot Yield in Barrels per Acre
Treatment* Percent Plants Infected Percent Tuber Healthy and Infected Healthy
at Date Indicated Infection_ _
Increase Decrease Decrease -
Due to Due to Due to
4-4 4-11 4-17 4-25 Total Planting Odds Total I Planting Odds Total Planting Odds
Corn Corn Corn
Corn Not Planted ..0.5 10.0 29.0 40.1 24.2 91.2 26.8:1 69.1
Corn Planted**........ 0.5 15.8 43.1 68.8 42.1 17.9 302:1 82.8 8.4 47.9 21.2 184:1
*Each replicated 8 times in rows which were 50 hills long.
**April 4, 1934.
Brown Rot of Irish Potatoes 33
SUSCEPTIBILITY OF DIFFERENT VARIETIES OF POTATOES
TO BROWN ROT
Since it had been observed for several years that the Spauld-
ing Rose, which is the only variety grown to any extent in the
Hastings section, appeared to be more seriously affected by
brown rot than the Green Mountain or Bliss Triumph grown
in infested soil near it, varietal tests were conducted in 1934
and 1935 at LaCrosse and West Tocoi, Fla. The susceptibility
of a number of different varieties was compared with that of
the Spaulding Rose by growing them in replicated plots in soil
severely, moderately and mildly infested with B. solanacearum.
Results of these tests are given in Table 12 and a comparison
of the percentages of plant and tuber infection present in the
varieties grown in severely and moderately infested soil is shown
in Figure 7.
I I PRONTAGO PLATS IIECOTD
SPIROMITAGE TUBBR I PECTID
t3 a S|60g
Q 00n 0 .38 W W X1
Fig. 7.-Histograms showing the percentages of plant and tuber in-
fection in different varieties of potatoes which were grown in soil moder-
ately infested with B. solanacearum in 1934 and severely infested in 1935.
TABLE 12.-COMPARISON OF THE BROWN ROT INFECTION AND YIELD OF DIFFERENT VARIETIES OF POTATOES GROWN IN SOIL
INFESTED WITH B. solanacearum AT WEST TOCOI AND LACROSSE, FLA.
Brown Rot Yield in Barrels Per Acre
Percent Tubers Infected Healthy
Variety* Percent Decrease I Increase
Plants Over Total Over I
Infected Total Spaulding Odds** Total Spaulding Odds**
I _Rose i ____ Rose_
MODERATELY INFESTED SOIL, WEST TOCOI, 1934
Spaulding Rose ...... 58.9 31.8 ................ 72.7 49.6 ...........
Chippewa ................ 61.9 42.8 -11.0 9.89:1 51.9 29.7 -19.9 19230:1
Seedling 41914 ........ 62.6 28.7 3.1 ............ 84.9 60.5 10.9 6.26:1
Irish Cobbler ......... 71.1 29.5 2.3 ........... 76.0 53.6 4.0 1.63:1
Bliss Triumph........ 64.2 21.4 10.4 8.48:1 88.5 69.6 20.0 1350:1
Katahdin .................. 41.2 20.4 11.4 15.95:1 77.1 61.4 11.8 18.80:1
Green Mountain .... 55.3 16.1 15.7 44.87:1 72.6 60.9 11.3 22.26:1
SEVERELY INFESTED SOIL, WEST TOCOI, 1935
Spaulding Rose ...... 99.0 62.0 ............ 7.9 3.0 ...
Irish Cobbler............ 91.8 55.6 6.4 4.64:1 19.6 8.7 5.7 519:1 .
Bliss Triumph ........ 99.0 41.2 20.8 434782:1 21.1 12.4 9.4 ***M:1
Katahdin .................. 77.2 19.1 42.9 M:l 48.3 39.1 36.1 M:l
Green Mountain ...... 87.5 18.7 43.3 M:l 47.7 38.8 35.8 M:1 *
MILDLY INFESTED SOIL, LaCROSSE, 1985
Spaulding Rose ...... 26.4 6.1......... 48.9 45.9
Irish Cobbler ...... 15.5 4.4 1.7 ............ 49.3 47.1 1.2 ............
Bliss Triumph ........ .. 3.7 2.4 1.63:1 48.3 46.5 0.6 ............
Katahdin .............. 10.9 2.2 3.9 7.28:1 67.8 66.3 20.4 15.95:1
Green Mountain ...... 16.6 2.3 3.8 ,5.38:1 69.6 68.0 22.1 22.26:1
*Each replicated 8 times in moderately infested soil, 6 times in severely infested soil and 10 times in mildly in-
**Odds of over 22.26:1 are significant.
***Million or more to one.
Brown Rot of Irish Potatoes 35
Although the differences in tuber infection and yields of the
different varieties when compared with the Spaulding Rose
were not always significant, each had a lower percentage of
tuber infection and produced a greater yield when grown in
the three kinds of infested soil, with the exception of the Chip-
pewa, which was grown in moderately infested soil in 1934.
Plants of the Chippewa were younger when they were attacked
by the disease than the plants of the Spaulding Rose. Conse-
quently, a greater number were killed before they produced
tubers, tuber infection became greater and the reduction in
yield was significant.
When grown in moderately infested soil in 1934 the Green
Mountain was the only variety which had a significantly lower
percentage of tuber infection than the Spaulding Rose, the in-
fection being approximately half that present in the latter. The
Bliss Triumph was the only variety that produced a significantly
greater yield of healthy tubers than the Spaulding Rose, which
probably was due in part to the greater natural yielding ability
-of the former and in part to its greater resistance to brown rot.
When grown in severely infested soil, the plants of the Green
Mountain and Katahdin showed greater resistance to the dis-
ease than did those of the other varieties (Fig. 8). The plants
2, [7 -BN -
SOOBBL R: I ROB MOUNT TAIN TR IU P
Fig. 8.-View of potato trial plots showing the effect of brown rot in
killing potato plants of each of five varieties which were grown in soil
severely infested with B. solanacearum.
36 Florida Agricultural Experiment Station
of the Spaulding Rose variety were attacked early and many
were killed before they produced marketable tubers and the
plants which survived bore a high percentage of diseased tubers.
The decreases in the percentages of tuber infection of the Green
Mountain, Katahdin, and Bliss Triumph and their increases in
yield of healthy tubers over the Spaulding Rose were significant;
the increases of 35.8 barrels and 36.1 barrels per acre for the
Green Mountain and Katahdin, respectively, were outstanding.
The increase in yield of the Irish Cobbler over the Spaulding-
Rose was significant but the decrease in percentage of tuber
infection was not.
When grown in mildly infested soil the percentages of tuber
infection and yields of healthy tubers of the other varieties
did not differ significantly from those of Spaulding Rose.
The percentage of plants wilted and killed by brown rot was
not a reliable criterion for judging the susceptibility of a vari--
ety, for some of the varieties which were grown in moderately
and severely infested soil had significantly lower percentages
of tuber infection and produced greater yields of healthy tubers.
than the Spaulding Rose but their percentages of plant infection
were almost the same.
TUBER INFECTION STUDIES
RATIO OF TUBERS SHOWING EXTERNAL SYMPTOMS OF BROWN
ROT TO THOSE SHOWING INTERNAL SYMPTOMS ONLY
Tubers of each of five varieties of potatoes which were
grown in 1935 in soil severely and mildly infested with B. solana-
cearum were examined for symptoms of brown rot which were
evident on the surface of the tubers and symptoms which could
be seen only in the vascular and adjacent tissues when the tubers
were cut. The results, as given in Table 13, show that most of
the infected tubers exhibited external symptoms of the disease.
In both severely and mildly infested soil a greater proportion of
the infected tubers of the Green Mountain showed external
symptoms of brown rot than did those of any other variety.
DECAY OF INFECTED TUBERS AFTER DIGGING
In 1933 a lot of tubers produced in soil infested with B. solana-
cearum was examined and all tubers showing external symptoms
of brown rot were discarded and the remainder consisting of
1,614 pounds were placed in barrels and stored in an open shed.
When the entire lot was examined eight days later, 1.2 percent
Brown Rot of Irish Potatoes 37
showed external symptoms of brown rot and, when 90 pounds
of the tubers which appeared healthy were cut and examined
for internal symptoms of the disease, 3.1 percent were found
TABLE 13.-RELATION OF INFECTED TUBERS SHOWING EXTERNAL SYMPTOMS
OF BROWN ROT TO THOSE SHOWING INTERNAL SYMPTOMS ONLY IN EACH
OF FIVE VARIETIES OF POTATOES GROWN IN SOIL INFESTED WITH B.
Percentage of Tubers with Brown Rot
Variety* In Severely Infested Soil In Mildly Infested Soil
with Symptoms with Symptoms
External InternalI Ratio IExternal Internal I Ratio
(a) only (b) I (a): (b) (a) only (b) I (a): (b)
Spaulding Rose .. 55.7 6.3 8.8:1 5.1 1.0 5.1:1
Irish Cobbler ...... 48.9 6.1 8.0:1 4.0 0.4 10.0:1
Bliss Triumph .... 36.0 5.2 6.9:1 2.5 1.2 2.1:1
Katahdin ............ 16.8 2.3 7.3:1 1.9 0.3 6.3:1
"Green Mountain 17.2 1.5 11.4:1 2.2 0.1 22.0:1
*Each grown in 50-hill rows replicated 6 times in severely infested soil
and 10 times in mildly infested soil.
Potatoes produced in the Hastings section are shipped to
Northern markets as soon as they are dug. Digging usually
starts about March 20 and is completed by May 7. Shippers
have reported that maximum damage from brown rot occurs
-during the latter part of the digging season when soil tempera-
tures are optimum for the disease, and that the decay which
*develops in transit is greatest in those shipments of potatoes
that have been dug during hot weather from severely infested
soil and packed without having been carefully sorted for brown
At West Tocoi, Florida, when soil infested with B. solana-
-cearum was treated with a sufficient amount of sulfur to reduce
the pH reaction to 4.00 and lower, bacterial wilt was almost
,completely eliminated from susceptible crops grown in this soil,
thus indicating that the organism was killed by the acid con-
dition produced. This conclusion was confirmed by the fact
that the organism was killed when transferred to artificial media
with pH reactions of 4.15 and lower which were produced by
both sulfuric acid and hydrochloric acid. These conclusions are
38 Florida Agricultural Experiment Station
further verified by the findings of van der Poel (37) working
in Medan, Sumatra. Van der Poel acidified black dust soil, red
volcanic ash soil and a sandy loam alluvial soil infested with B.
solanacearum with "flour of sulfur" and made these soils free
of bacterial wilt as determined by absence of the disease in
tobacco, tomatoes and castor beans grown in the soils. Van der
Poel stated that his tests were limited in scope and he did not
establish any definite pH reactions lethal to B. solanacearum in
three kinds of soil. He also found that the disease did not re-
appear in tomato plants grown in infested soil which had been
acidified and then made alkaline with tobacco ash.
Apparently the pH reactions required to control brown rot
are not the same for all soils. At LaCrosse good control of the
disease was obtained in 1930 in sulfured soil which tested pH
4.88 to 5.18 (Table 4), while at West Tocoi the disease remained
very severe in untreated soil which tested pH 4.46 to 5.00 (Table
5). The great decrease in severity of the disease in untreated
soil in the LaCrosse field from 1930 to 1933 might have been
due to the increase in the acidity of this soil, which tested pH
5.79 to 6.02 in 1930 and pH 4.67 to 4.80 in 1933. Van der Poel
(37) also observed that brown rot disappeared from soils which
were brought to different degrees of acidity.
If the heavy applications of sulfur which were made at La-
Crosse in December 1929 and at West Tocoi in October 1932
had been made in the summer of these years, it is possible that
the soil would have been made acid enough to give as good con-
trol of brown rot the first year as the second at these places.
This conclusion is supported by the fact that excellent control
of the disease was obtained at West Tocoi in 1935 when pH re-
actions lethal to B. solanacearum were created in the soil during
the summer of 1934 by applying 800 pounds of inoculated sulfur
in June of that year.
In addition to creating pH reactions lethal to B. solanacearum
in soil of the Scranton fine sand type at LaCrosse and West Tocoi
in the experimental plots by the addition of 800 pounds of sul-
fur per acre, lethal reactions have been secured also in Bladen
fine sand and Bladen fine sandy loam soils, which were given
the same treatment on three farms located near Hastings. The
reactions of these boils, which tested pH 4.51 to 5.50 before they
were treated, were reduced to pH 4.00 and below in the follow-
ing summer after the soil was treated in October on two of the
farms and in July on the other. Tests conducted in June 1936
Brown Rot of Irish Potatoes 39
proved that commercial flour sulfur was as effective as inoculated
sulfur in producing reactions lethal to B. solanacearum in Bladen
fine sand, Bladen fine sandy loam, and Scranton fine sand. The
soils treated with sulfur represented the common types used
for growing potatoes in the Hastings section, but the results
obtained probably would not apply to other types of soils with
more alkaline reactions in Florida and other parts of the country.
Van der Poel (37) discovered that he could eliminate bacterial
wilt from soil by making it strongly alkaline and that the disease
did not reappear when the soil was readjusted to a pH reaction
favorable to the disease. He used bicarbonate of soda and
tobacco ash as the alkaline agents and grew tomatoes in the
soil; in one series the disease did not appear at pH 8.4 and in
another at pH 7.7.
In the experiments conducted in Florida some control of
brown rot was obtained from hydrated lime but not from dolo-
mitic limestone. However, these alkaline agents were not used
in sufficient amounts to bring the reaction to those established
by van der Poel; consequently, it is not known how effectively
they might control the disease. It is possible that the organism
in the United States represents a different biologic strain (24)
and would not react the same to alkaline treatment as did van
der Poel's organism. There is some indication that this might
be true, for B. solanacearum from Florida grew on media ad-
justed to the pH reaction 8.71 with sodium hydroxide.
The cost of the inoculated sulfur-limestone treatment is $30.00
per acre, based on present prices of materials and labor required
to make the treatment. When commercial flour sulfur is used,
the cost is $22.80 per acre. If the marketable grades of potatoes
are valued at $3.00 per barrel, the treatment gave a net yearly
return of $39.70 per acre in plots at West Tocoi, where it was
used from 1933 to 1935. In the plots which were given the
treatment between potato crops in 1934, the net return in 1935
was $33.90 per acre after deducting cost of treatment.
Experimental results secured warrant the following recom-
mendations for controlling brown rot in the Hastings potato
section, where the soils have reactions generally ranging be-
tween pH 4.50 and 5.50:
1. The sulfur-limestone treatment should be used in fields
and parts of fields where the disease is usually severe. Inoculated
40 Florida Agricultural Experiment Station
or commercial flour sulfur should be applied to freshly plowed
soil in May or June at the rate of 800 pounds per acre, and cal-
cium or dolomitic limestone should be applied in November or
December at the rate of 2,000 pounds per acre to soil testing
pH 4.00 to 4.50, and at the rate of 3,000 pounds if the soil tests
pH 3.50 to 4.00. Both agents should be mixed thoroughly with
the soil by disking the land at least three times. During the
summer, the sulfured soil becomes too acid for the growth of
a normal weed and grass cover crop. However, cowpeas will
grow in very acid soils and this crop should be planted to pro-
vide cover during the summer months and green material for
plowing under in the fall.
2. In fields where losses from brown rot do not justify the
expense of the sulfur-limestone treatment, the control measures
recommended are as follows: (1) Grow the Green Mountain
or Katahdin variety; either of which is more resistant to the
disease than the Spaulding Rose. (2) Grow cowpeas as a green
manuring crop preceding potatoes. (3) Do not plant corn on
the sides of the potato rows before the crop is dug. (4) Dig
the tubers as soon as they are in a marketable condition, for,
if they are left in the infested soil, they, will become infected
with brown rot.
A review of the host plants of B. solanacearum shows that it
would be impracticable to attempt to eliminate brown rot from
Potato land by crop rotation or weed eradication. New hosts
listed are Solanum citrifolium A. Br., S. pyracanthum Jacq., S.
sisymbrii Sam. and Datura metel L.
Epidemics of brown rot occur infrequently at Hastings, Flor-
ida, and the 1935 epidemic was attributed to the extremely high
temperatures which prevailed in March of that year.
The disease has occurred at Hastings in all types of sandy
soil; in new land testing pH 4.32 to 5.42, and in old land with
reactions ranging between pH 4.42 and 6.20.
B. solanacearum was killed by sulfuric acid in potato broth
cultures with initial reactions of pH 4.02 and lower and by
both sulfuric acid and hydrochloric acid in potato-dextrose agar
at initial reactions of pH 4.15 and lower. It grew in media of
higher initial concentrations and survived at pH 8.71 when
sodium hydroxide was used as the alkaline agent.
Brown rot of potatoes was controlled in experimental plots
at LaCrosse and West Tocoi by treating the soil with single
Brown Rot of Irish Potatoes 41
applications of inoculated sulfur at rates ranging from 400 to
1,200 pounds per acre. Potato yields were reduced by the
treatments, the heaviest applications causing the greatest de-
creases, but the yields were recovered when the soil naturally
adjusted itself to a more alkaline condition and when this con-
dition was created by the addition of calcium or dolomitic lime-
stone one to three years after the soil was treated with sulfur.
Th6 return to a normal pH reaction was not accompanied by a
return of brown rot in potatoes which were grown in treated
soil three years at West Tocoi and five years at LaCrosse.
When soil was treated between potato crops at West Tocoi
with 800 pounds of inoculated sulfur per acre in June and 3,000
pounds of dolomitic limestone per acre in November, excellent
control of bacterial wilt of potatoes, eggplants and tomatoes was
obtained. During the summer the sulfur created soil reactions
which were lethal to the causal organism, and during the winter
and spring months the limestone made the soil less acid and
thus induced normal plant growth and yields of the three crops
without causing a return of bacterial wilt.
fHydrated lime gave some control of brown rot at LaCrosse
when it was used at the rates of 1,000 pounds and 2,000 pounds
per acre, but dolomitic limestone, which was applied to the soil
at the rates of 1,000 pounds to 3,000 pounds per acre at West
Tocoi, did not control the disease.
Inoculated sulfur applied to the soil at the rate of 800 pounds
per acre created pH reactions lethal to B. solanacearum in the
upper 8 inches of soil more rapidly when the treatment was made
in June than when it was made in the fall or winter months.
In 1934 when bacterial wilt was moderate in severity, cow-
peas used as a green manure crop did not reduce the rate of tuber
infection, but in 1935 when the disease was very severe, this
green manure crop caused a significant increase in yield of
healthy potatoes by preventing early infection and death of the
When corn was planted in a potato field severely infested with
B. solanacearum three weeks prior to digging, injury to the
roots increased the percentage of plant and tuber infection and
caused a significant reduction in yield of healthy tubers.
When different varieties of potatoes were grown at LaCrosse
and West Tocoi in soil severely, moderately and mildly infested
with B. solanacearum, the Green Mountain proved to be the most
resistant to brown rot, Katahdin ranking second; Chippewa was
42 Florida Agricultural Experiment Station
the most susceptible, followed in order by the Spaulding Rose,
Irish Cobbler, Seedling 41914 and Bliss Triumph.
Most tubers affected with brown rot in each of five varieties
showed the presence of the disease by external symptoms, the
Green Mountain variety ranking above other varieties in this
respect. Brown rot infected tubers continued to decay while
in transit to market and when stored in a field shed eight days
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Brown Rot of Irish Potatoes 43
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