• TABLE OF CONTENTS
HIDE
 Title Page
 Credits
 Table of Contents
 Introduction
 Equipment
 Methods
 Experimental results
 Summary and discussion
 Conclusion
 Acknowledgement
 Literature cited














Group Title: Bulletin - University of Florida. Agricultural Experiment Station ; no. 363
Title: Fusarium wilt of watermelons
CITATION THUMBNAILS PAGE IMAGE ZOOMABLE
Full Citation
STANDARD VIEW MARC VIEW
Permanent Link: http://ufdc.ufl.edu/UF00027266/00001
 Material Information
Title: Fusarium wilt of watermelons
Series Title: Bulletin University of Florida. Agricultural Experiment Station
Alternate Title: Effect of soil temperature on the wilt disease and the growth of watermelon seedlings
Physical Description: 29 p. : ill., charts ; 23 cm.
Language: English
Creator: Walker, M. N ( Marion Newman ), 1900-
Publisher: University of Florida Agricultural Experiment Station
Place of Publication: Gainesville Fla
Publication Date: 1941
 Subjects
Subject: Wilt diseases -- Florida   ( lcsh )
Watermelons -- Diseases and pests -- Florida   ( lcsh )
Soil temperature -- Florida   ( lcsh )
Fusarium   ( lcsh )
Watermelons -- Seedlings   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Bibliography: Bibliography: p. 29.
Statement of Responsibility: by M.N. Walker.
General Note: Cover title.
Funding: Bulletin (University of Florida. Agricultural Experiment Station) ;
 Record Information
Bibliographic ID: UF00027266
Volume ID: VID00001
Source Institution: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: aleph - 000924596
oclc - 18229862
notis - AEN5223

Table of Contents
    Title Page
        Page 1
    Credits
        Page 2
    Table of Contents
        Page 3
    Introduction
        Page 3
    Equipment
        Page 4
    Methods
        Page 5
        Page 6
        Page 7
        Page 8
    Experimental results
        Page 9
        Page 10
        Page 11
        Page 12
        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
    Summary and discussion
        Page 26
        Page 27
    Conclusion
        Page 28
    Acknowledgement
        Page 29
    Literature cited
        Page 29
Full Text


October, 1941


UNIVERSITY OF FLORIDA
AGRICULTURAL EXPERIMENT STATION
WILMON NEWELL, Director
GAINESVILLE, FLORIDA







FUSARIUM WILT OF WATERMELONS

I. Effect of Soil Temperature on the Wilt
Disease and the Growth of Watermelon
Seedlings.

BY Ml. N. WALKER



















Single copies free to Florida residents upon request to
AGRICULTURAL EXPERIMENT STATION
GAINESVILLE, FLORIDA


Bulletin 363








EXECUTIVE STAFF
John J. Tigert, M. A., LL. D., President
of the University3
Wilmon Newell, D.Sc., Director3
Harold Mowry, M. S. A., Asst. Dir.,
Research
W. M. Fifield, M. S., Asst. Dir., Admin.
J. Francis Cooper, M. S. A., Editor3
Clyde Beale, A.B.J., Assistant Editors
Jefferson Thomas, Assistant Editors
Ida Keeling Cresap, Librarian
Ruby Newhall, Administrative Manager3
K. H. Graham, Business Manager3
Rachel McQuarrie, Accountant3
MAIN STATION, GAINESVILLE
AGRONOMY
W. E. Stokes, M.S., Agronomisti
W. A. Leukel, Ph.D., Agronomists
Fred H. Hull, Ph.D., Agronomist
G. E. Ritchey, M.S., Associate2
W. A. Carver, Ph. D., Associate
Roy E. Blaser, M.S., Associate
John P. Camp, M.S., Assistant
Fred A. Clark, B.S.A., Assistant
ANIMAL INDUSTRY
A. L. Shealy, D.V.M., An. Industrialist'
R. B. Becker, Ph.D., Dairy Husbandmans
E. L. Fouts, Ph.D., Dairy Technologist3
D. A. Sanders, D.V.M., Veterinarian
M. W. Emmel, D.V.M., Veterinariana
L. E. Swanson, D.V.M., Parasitologist
N. R. Mehrhof, M.Agr., Poultry Husb.S
W. M. Neal. Ph.D., Asso. in An. Nutrition
T. R. Freeman, Ph.D., Associate in Dairy
Manufactures
D. J. Smith, B.S.A., Asst An. Husb.3
P. T. Dix Arnold, M.S.A., Asst. Dairy
Husbandman3
L. L. Rusoff, Ph.D., Asst. in An, Nutr.'
L. E. Mull, M.S., Asst. in Dairy Tech.
C, K. Moore, M. S. Asst. Poultry Husb.
ECONOMICS, AGRICULTURE
C. V. Noble, Ph.D., Agr. Economist1 3
Zach Savage, M.S.A., Associate
A. H. Spurlock, M.S.A., Associate
Max E. Brunk, M.S., Assistant
ECONOMICS, HOME
Ouida D. Abbott, Ph.D., Home Econ.1
Ruth Overstreet, R.N., Assistant
R. B. French, Ph.D., Asso. Chemist
ENTOMOLOGY
J. R. Watson, A.M., Entomologist'
A. N. Tissot, Ph.D., Associate
H. E. Bratley, M.S.A., Assistant
HORTICULTURE
G. H. Blackmon, M.S.A., Horticulturist'
A. L. Stahl, Ph.D., Associate
F. S. Jamison, Ph.D., Truck Hort.3
R. J. Wilmot, M.S.A., Asst. Hort.
R. D. Dickey. M.S.A., Asst. Horticulturist
J. Carlton Cain, B.S.A., Asst. Hort.
Victor F. Nettles, M.S.A., Asst. Hort.
Lee B. Nash, Ph.D., Assistant
F. S. Lagasse, Ph.D., Asso. Horticulturist'
H. M. Sell, Ph.D., Asso. Horticulturist2
PLANT PATHOLOGY
W. B. Tisdale, Ph.D., Plant Pathologist 3S
George F. Weber, Ph.D., Plant Path.3
L. O. Gratz, Ph.D., Plant Pathologist
Erdman West, M.S., Mycologist
Lillian E. Arnold, M.S., Asst. Botanist
SOILS
R. V. Allison, Ph.D., Chemist'
Gaylord M. Volk, M.S., Chemist
F. B. Smith, Ph.D., Microbiologists
C. E. Bell, Ph.D., Associate Chemist
H. W. Winsor, B.S.A., Assistant Chemist
J. Russell Henderson, M.S.A., Associates
L. H. Rogers, M.S., Asso. Biochemist
Richard A. Carrigan. B.S., Asst. Chemist
Geo. D. Thornton, M. S., Asst. Chemist


BOARD OF CONTROL
H. P. Adair, Chairman, Jacksonville
W. M. Palmer. Ocala
R. H. Gore, Fort Lauderdale
N. B. Jordan, Quincy
T. T. Scott, Live Oak
J. T. Diamond, Secretary, Tallahassee
BRANCH STATIONS
NORTH FLORIDA STATION, QUINCY
J. D. Warner, M.S. Agron. in Charge
R. R. Kinkaid, Ph.D., Asso. Plant Path.
Elliott Whitehurst, B.S.A., Assistant An.
Husbandman
Jesse Reeves, Asst. Agron., Tobacco
CITRUS STATION, LAKE ALFRED
A. F. Camp, Ph.D., Horticulturist in Chg.
John H. Jeffries, Asst. in Cit. Breeding
Chas. K. Clark, Ph.D., Chemist
V. C. Jamison, Ph.D., Soils Chemist
B. R. Fudge, Ph.D., Associate Chemist
W. L. Thompson, B.S., Associate Ento.
F. F. Cowart, Ph.D., Asso. Horticulturist
W. W. Lawless, B.S., Asst. Horticulturist
R. K. Voorhees, M.S., Asst. Plant Path.
EVERGLADES STA., BELLE GLADE
J. R. Neller, Ph.D., Biochemist in Chg.
J. W. Wilson, Sc.D., Entomologist
F. D. Stevens, B.S., Sugarcane Agron.
Thomas Bregger, Ph.D., Sugarcane
Physiologist
G. R. Townsend, Ph.D., Plant Pathologist
R. W. Kidder, M.S., Asst. An. Husb.
W. T. Forsee, Ph.D., Asso. Chemist
B. S. Clayton, B.S.C.E., Drainage En-
gineer2
F. S. Andrews. Ph.D., Asso. Truck Hort.
Roy P. Bair, Ph.D., Asst. Agron.
J. C. Hoffman, M. S., Asst. Hort.
SUB-TROPICAL STA., HOMESTEAD
Geo. D. Ruehle, Ph.D., Associate Plant
Pathologist in Charge
S. J. Lynch., B.S.A., Asst. Horticulturist
E. M. Andersen, Ph.D., Asst. Hort.
W. CENTRAL FLA. STA.,
BROOKSVILLE
W. F. Ward, M.S., Asst. An. Husband-
man in Charge2
RANGE CATTLE STA., WAUCHULA
W. G. Kirk., Ph.D., Animal Husbandman
in Charge
FIELD STATIONS
Leesburg
M. N. Walker, Ph.D., Plant Pathologist
in Charge
K. W. Loucks, M.S., Assistant Plant
Pathologist
Plant City
A. N. Brooks, Ph.D., Plant Pathologist
Hastings
A. H. Eddins, Ph.D., Plant Pathologist
E. N. McCubbin, Ph.D., Asso. Truck
Horticulturist
Monticello
A. M. Phillips, B.S., Asst. Entomologist2
Bradenton
Jos. R. Beckenbach, Ph.D., Truck Horti-
culturist in Charge
David G. Kelbert, Asst. Plant Pathologist
Sanford
R. W. Ruprecht, Ph.D., Chemist in
Charge. Celery Investigations
W. B. Shippy, Ph.D., Asso. Plant Path.
Lakeland
E. S. Ellison, Meteorologist2
'Head of Department
2In cooperation with U. S.
sCooperative, other divisions, U. of F.








FUSARIUM WILT OF WATERMELONS


I. Effect of Soil Temperature

Disease and the Growth of

Seedlings.

BY M. N. WALKER


CONTENTS


Equipment .......---------..-
Methods ... .----- --. .... .
Soil and Infestation._-.._ ______
Seed Stocks -........- ---.-. .- ----.
Planting ---..
Records ..-_ -
Experimental Results
Effect of temperature on infection ___....
Watermelons .-.....-
Citrons ..-..- --------- ---
Citron-watermelon hybrids
Effect of temperature on growth of plants .-- --_
Effects of temperature on the wilt organism _-_.-....
Summary and Discussion -.. ----. -- .-
Conclusions ...... ----. -------.
Literature Cited _... ---- --


on the Wilt

Watermelon


Page
.. 4
5
---------- 5
7
8
----- 9
9
10
S13
....- -- -. 14
....---..--16
....--.-- .. 21
--.------ -- 26
-.---. --... 28
I---- ------ -- 29


INTRODUCTION
In preliminary field and greenhouse tests with Fusarium
wilt of watermelons during 1930 and 1931 there were indications
that soil temperatures were responsible for some of the incon-
sistencies in the germination of seed and infection of seedlings
that appeared. A detailed study of the soil temperature relation-
ship was started in the spring of 1932 in a small temperature
tank at Gainesville. Results obtained from two tests in this
tank shed some light on the problem, but they were not as defin-
ite nor complete as was desired.
Through the courtesy of the Department of Plant Pathology
in theUniversity of Wisconsin the use of a sufficient number of
soil temperature tanks was obtained for more detailed tests un-
der controlled conditions than had been possible in Florida. Dur-
ing August and September of 1932 four soil temperature series
were conducted at Madison, Wisconsin, and tests were made to
determine the effect of temperature on the causal organism in
culture. It is the purpose of this bulletin to report the results
of temperature studies of the wilt disease made under controlled
conditions.
When the present work was begun very little information on
the temperature relations of the Fusarium wilt disease of water-
melons was available, although there was an extensive literature
on the effects of soil temperature on similar diseases of other








FUSARIUM WILT OF WATERMELONS


I. Effect of Soil Temperature

Disease and the Growth of

Seedlings.

BY M. N. WALKER


CONTENTS


Equipment .......---------..-
Methods ... .----- --. .... .
Soil and Infestation._-.._ ______
Seed Stocks -........- ---.-. .- ----.
Planting ---..
Records ..-_ -
Experimental Results
Effect of temperature on infection ___....
Watermelons .-.....-
Citrons ..-..- --------- ---
Citron-watermelon hybrids
Effect of temperature on growth of plants .-- --_
Effects of temperature on the wilt organism _-_.-....
Summary and Discussion -.. ----. -- .-
Conclusions ...... ----. -------.
Literature Cited _... ---- --


on the Wilt

Watermelon


Page
.. 4
5
---------- 5
7
8
----- 9
9
10
S13
....- -- -. 14
....---..--16
....--.-- .. 21
--.------ -- 26
-.---. --... 28
I---- ------ -- 29


INTRODUCTION
In preliminary field and greenhouse tests with Fusarium
wilt of watermelons during 1930 and 1931 there were indications
that soil temperatures were responsible for some of the incon-
sistencies in the germination of seed and infection of seedlings
that appeared. A detailed study of the soil temperature relation-
ship was started in the spring of 1932 in a small temperature
tank at Gainesville. Results obtained from two tests in this
tank shed some light on the problem, but they were not as defin-
ite nor complete as was desired.
Through the courtesy of the Department of Plant Pathology
in theUniversity of Wisconsin the use of a sufficient number of
soil temperature tanks was obtained for more detailed tests un-
der controlled conditions than had been possible in Florida. Dur-
ing August and September of 1932 four soil temperature series
were conducted at Madison, Wisconsin, and tests were made to
determine the effect of temperature on the causal organism in
culture. It is the purpose of this bulletin to report the results
of temperature studies of the wilt disease made under controlled
conditions.
When the present work was begun very little information on
the temperature relations of the Fusarium wilt disease of water-
melons was available, although there was an extensive literature
on the effects of soil temperature on similar diseases of other







Florida Agricultural Experiment Station


plants. Since that time a few papers dealing with this phase of
the watermelon wilt problem have appeared. Such temperature
work as has been done with watermelons will be referred to in
the discussion of the results of the present work.

EQUIPMENT
The tank used in the tests at Gainesville was of the internal-
ly balanced, single unit type (1)', having a heating unit in one
end and refrigeration in the other, each controlled by a thermos-
tat. The temperatures of the six intermediate compartments
were adjusted by the interchange of heat from the end tanks
through the metal cross walls of the units. An agitator was
operated continuously in each compartment to keep a uniform
temperature. Each compartment of the tank held four soil cahs.
On account of the comparatively wide range of temperatures
attempted, 15 to 35 degrees Centrigrade, there was not a uni-
form gradation of temperatures in the six compartments of the
tank. Difficulty was experienced in maintaining the low tem-
perature compartment at a constant level of 15 degrees. Read-
ings of from 14 to 20 degrees were made in this compartment,
and corresponding, though somewhat smaller, variations occur-
red in readings made in the two tanks at the next higher tem-
peratures. Such fluctuations caused the results from the three
compartments at the lower end of the range to approximate
those that would normally be expected from slightly higher
temperatures than the averages given in the tables, since rises in
temperature for a relatively short period of time might cause
observable effects, particularly on germination and infection.
The tanks used in the work at Wisconsin were of the multiple
unit type, having a heater and thermostat in each tank. Each
tank held eight soil cans and eight tanks were used in each series
to secure a range of temperatures at approximately 3-degree in-
tervals between 18 and 39 degrees Centigrade. Eighteen was
taken as the lower limit because of the difficulty in maintaining
lower temperatures during the summer; and 39 degrees as the
upper limit, as this temperature was assumed to be close to the
maximum for growth of the watermelon plant.
A series of incubators maintained by the Department of
Plant Pathology of the University of Wisconsin was used in the
culture work.

1Italic figures in parentheses refer to Literature cited in the back of this bulletin.







Fusarium Wilt of Watermelons


METHODS
Soil and Infestation.-The soil used in the Florida experi-
ments was a Norfolk sand from the vicinity of Gainesville. This
soil was placed in large pans to a depth of about four inches and
thoroughly moistened. It was then heated for several hours over
a similar pan filled with water in a brick oven and after expos-
ure to the air for several days it was brought to a uniform mois-
ture content and placed in the tank cans. The soil in two cans
of each of the six compartments of the tank was infested with
the wilt organism by mixing with the surface three inches of
soil approximately a teaspoonful of agar cultures macerated with
clean sand. This proved too light an infestation to bring about
infection during the first series, but sufficient growth of the
fungus occurred during the 30 days that the series was main-
tained to produce infection in the second series.
The soil for the first two Wisconsin tests was prepared by
composting for about two months naturally infested soil from
the experimental plots at Leesburg, Florida, with large quan-
tities of plants killed by the wilt disease. A week after planting
the seed in these two series a heavy spore suspension of the
wilt organism from steamed oat cultures was added to the soil.
This was supplemented six days later with two teaspoonfuls
of the infested soil prepared for the third series. A heavy,-black,
virgin soil from the woods in the vicinity of Madison was used in
the control cans. This soil proved too heavy in comparison with
the Florida soil, and for the third Wisconsin series it was mixed
with one-third clean sand.
For this series all cans other than the controls were infested
with the wilt organism by filling the upper third of the cans
with soil mixed with sand-cornmeal cultures of the wilt organ-
ism. The sand-cornmeal mixture was made up of equal volumes
of sand and cornmeal moistened with tap water. The mixture,
after cooking in an Arnold sterilizer for half an hour, was rolled
into small balls about three-fourths inch in diameter and placed
in quart Mason jars to about two-thirds their capacity. After
pressure sterilization the jars were inoculated with small quan-
tities of a spore suspension of the wilt organism, and incubated
for two weeks at room temperatures. The balling of the med-
ium allowed much greater growth to the unit of medium than
is ordinarily possible. The mixture of the soil used in the upper
third of the cans of the third series was made up so that eight







Florida Agricultural Experiment Station


cans received six cultures, and this gave a great amount of in-
fective material to each can. Such an application was much in
excess of what had been considered a heavy application during
earlier work, and it would seem probable that natural field in-
festation never approaches this mixture in the amount of in-
fective material present. The experience of workers in the Plant
Pathology Department at Madison, however, had shown this type
of soil inoculation the best in work with soil-borne diseases of the
wilt type. After the completion of the third series the same
soil was used without further treatment for the fourth series.
The soil for each series was moistened to the proper degree
before the cans were filled, and all cans for a particular series
were filled to a standard weight. The standard weights were
maintained by the addition of water when necessary. The cans
at the higher temperatures in the Wisconsin series had to be
brought up to weight almost every day, but the cans at the lower
temperatures were weighed and watered at longer intervals.
No water at all was needed in the cans at the lowest temperatures
in each series. In fact, some of these cans gained a little weight
during the time the series were in progress. It is possible that
some of the variations in germination at the higher temperatures
in the Wisconsin tests were due to insufficient moisture, and two
weighing a day perhaps should have been made.
Seed Stocks.-Seed of two commercial varieties of water-
melons, Tom Watson and Kleckley Sweet, were used as suscep-
tible stocks in the tests, and three strains of citrons selfed one
year, numbers 5-01, 6-01 and 90-02, were used as resistant
stocks. Strains 5-01 and 90-02 were derived from the stock
citron, while strain 6-01 was an unnamed citron selection from
California. Three corresponding Fi strains of citron crossed
with watermelon, number 5-x2, 6-x2 and 90-xl, were used as
strains showing an intermediate degree of resistance. The male
watermelon parents were Tom Watson for number 5-x2 and
Wondermelon, a Kleckley type, for numbers 6-x2 and 90-xl. The
selfed and crossed lines of citrons were used in an effort to de-
termine if possible something of the nature of inheritance of
such resistance as appeared. Citrons were used for this purpose
in preference to some of the breeding stocks of watermelons be-
cause such resistance as had been observed appeared to be more
definite in the citron lines. With the exception of Kleckley
Sweet all strains were tested in at least two tank series, and the






Fusarium Wilt of Watermelons


Tom Watson variety and citron strain number 5-01 were planted
in three series. After the seed were planted approximately a
half inch of ground cork was placed on the surface of the
soil in each can as an insulation against air temperatures.
Planting.-The first Florida series was planted on April 24.
Ten seed of the commercial variety, Tom Watson, were planted
in each of the two cans of infested soil at each temperature, and
five seed were planted in each of the two cans of non-infested
soil at each temperature. No infection occurred in this series
within a month and on May 24 the plants were removed and the
second series was planted immediately without further treat-
ment of the soil. This series was planted in the same manner as
the first. Sufficient growth of the organism in the soil had oc-
curred during the course of the first series to bring about in-
fection in the second. This series was terminated June 21, 28
days after planting.
In the first Wisconsin series, planted on August 5, two lots of
seed were used, commercial Tom Watson and a selfed strain of
citron, number 5-01. Four of the five cans of infested soil in
each tank were planted with Watson, the other with citron
seed. Of the three control cans, two were planted with Watson
and the third with citron seed. Ten seed were planted in each
can. The series was continued for 32 days until September 6
when the plants were removed.
The second Wisconsin series was planted on August 11
under conditions similar to those described for the first series.
Seed of three selfed strains, numbers 5-01, 6-01 and 90-02, of
citron were planted in this series of tests. As only two control
cans were kept, four seed of each of the selfed strains were
planted in one control can, and the same number of seed of
each of the hybrid strains was planted in the other. One can
of infested soil at each temperature was planted with 10 seed
of each of the six seed-lots. The series was discontinued on
September 13 after 33 days.
The planting schedule for the third Wisconsin series in-
cluded a repetition of the trials of citron strains 90-02 and 90-xl
used in the second series of tests, and of the commercial water-
melon variety. Tom Watson, used in the first series. Plantings
were made on August 23 according to the following schedule:
one control can in each tank was planted with 10 Watson seed;
one control can with five seed of each of the two citrons; each







Florida Agricultural Experiment Station


of four cans of infested soil with 10 Watson seed; one can of in-
fested soil with 10 seed of citron strain, 90-02; and with 10 seed
of citron strain 90-xl. This series was continued until Septem-
ber 14, 22 days after planting, when final notes were takeri.
The fourth series was planted on September 14. Ten seed
of the watermelon variety Kleckley Sweet were planted in each
of two cans of infested soil in each tank. One can in each tank
was planted with 10 seed of the citron strains 5-01, 5-x2, 6-01
and 6-x2. The two remaining control cans in each tank were
planted with five seed of each of the four citron strains, cor-
responding selfed and crossed lines being planted in the same
can. No control cans were planted with watermelon seed. Be-
cause of extremely rapid killing in this series it was possible to
terminate it on October 1, 16 days after planting.
Records.-Temperature readings were made two to three
times a day from thermometers kept in one of the cans in each
tank or tank compartment and weighted averages of these read-
ings to the nearest one-half degree are used in the tables. More
nearly constant temperatures were maintained during all of the
Wisconsin tests than had been possible during the preliminary
tests in Florida.
As plants died they were removed and the roots were split
in order to make a more definite diagnosis of wilt. At the end
of the various series all remaining plants were removed and
observations were made on the cut stems. The plants showing
browning of the vascular tissue when cut at the end of the ex-
periment plus the plants killed by the disease while the experi-
ment was in progress gave the total wilt infection.
Incident to the observation made on wilt infection, records
were made in both of the Florida series of the number of days
necessary for 90% germination to take place; of the daily growth
increments; and of the total growth during the course of the ex-
periment. Germination was considered as complete when the
cotyledons appeared above the cork covering in the soil cans.
The measurements of daily and total growth were made from
the soil level to the terminal buds. The Tom Watson variety
was used in both of these tests. At the end of the first Wiscon-
sin series measurements from the soil level to cotyledons and to
terminal buds were made on the plants surviving at the differ-
ent temperatures in both infested and non-infested soil. Counts









Fusarium Wilt of Watermelons


were also made of the true leaves on these plants. The Tom
Watson variety of watermelons and a citron strain, number 5-01,
were used.
EXPERIMENTAL RESULTS
EFFECT OF TEMPERATURE ON INFECTION
Because of variations in the soil used and differences in the
degree of soil infestation in the different series, results from dif-

TABLE 1.-EFFECT OF SoL. TEMPERATURE ON THE INFECTION OF WATERMELON SEED-
LLNGS BY Fusarium Bulbigenum VAR. i'iveunm.


Germination in Killing in Infested Soil After"
Non-inf. Inf. 10 days 15 days 20 days Total
Aver. SoilP SoiP Killing'
Temp. No. % No. % No. % No. % No. % No. %


Total
Infection'
No. %


Tom Watson -
90 17 85 0
80 16 80 0
90 14 70 0
90 18 90 0
80 17 85 0
80 14 70 0
Tom Watson -
80 36 90 0
80 36 87 0
100 36 87 0
85 34 85 0
80 33 82 2
90 24 60 1
70 31 77 0
80 19 47 0
Tom Watson -
70 36 90 0
100 37 93 0
70 34 85 0
80 38 95 2
100 33 83 12
90 36 90 3
70 34 85 0
60 25 63 0
Kleckley Sweet -
13 65 0
18 90 0
17 85 0
17 85 2
14 70 3
16 80 0
15 75 0
4 20 0


2nd Florida Series (28 days)'
0 0 0 0 0
0 0 0 0 0
0 5 36 8 57 1
0 3 17 7 39 1
0 0 0 1 6
0 0 0 0 0
1st Wisconsin Series (32 days)
0 0 0 0 0
0 0 0 0 0
0 1 3 1 3
0 5 15 5 15
6 10 30 15 45 2
4 1 4 5 21 1
0 0 0 1 3 1
0 0 0 1 5
3rd Wisconsin Series (22 days)
0 0 0 0 0
0 0 0 0 0
0 0 0 1 3
5 28 74 32 84 3
36 30 97 33 100 3
8 34 94 35 97
0 32 94 34 100
0 0 0 11 44 1
4th Wisconsin Series (16 day
0 0 0
0 0 0
0 3 18
12 13 76
21 13 93
0 15 94
0 6 40
0 0 0


0 0
0 0
1 3
7 21
24 73
.6 67
6 52
9 47

0 0
0 0
4 12
17 97
13 100
35 97
14 100
13 52
s)
0 0
0 0
3 18
13 76
14 100
16 100
9 60
0 0


110 seed planted in non-infested soil, 2nd. Fla. Series; 20 in 1st. Wis. Series; 10 in
3rd. Wis. Series.
20 seed planted in infested soil 2nd. Fla. Series; 40 in 1st. Wis. Series; 40 in 3rd
Wis. Series; and 20 in 4th. Wis. Series.
'No killing occurred in non-infested soil in any of the tests.
"At the end of test.
"Includes all plants killed as well as those showing infection when stems were cut.
'Indicates numbers of days test was continued.


35 C.
31
29
23.5
19.5
16

390C.
36
330
31
27
24.5
21.5"
18.50

390C.
36
33
300
27.50
24.5"
21.5"
18.50

390C.
36
33
30.5'
27'
240
21
18







Florida Agricultural Experiment Station


ferent series have not been combined, except in the case of the
citron and citron-watermelon hybrid strains where smaller num-
bers and generally lower infection rates made combination ad-
visable. The results from the different watermelon series,
though not combined, are grouped together in the same table
in order to make comparisons easier.
Watermelons.-A summary of the rate and degree of infec-
tion that occurred in plantings of the Tom Watsons and Kleckley
Sweet varieties in the second Florida and the first, third and
fourth Wisconsin series is presented in Table 1.
Germination in the second Florida series was good, and
though a somewhat greater variation took place in the infested
soil than in the uninfested soil, the variations were such that they
could not be definitely attributed to the wilt organism. The
summary of the infection records for this series is presented
in Table 1. In this experiment 290C. was the most favorable
temperature for wilt infection. Killing appeared at 29 and 23.5
at about the same time, but the rate of killing and the percent-

100


5
75 4



50



U 25




15 20 25 30 35 40
DEGREES C.
Fig. 1.-Percent of wilt infection 10 (curve 1), 15 (2) and 20 (3)
days after painting at different temperatures. Curve 4 shows the percent
of total killing and curve 5 the percent of total infection during the test.







Fusarium Wilt of Watermelons


age of plants killed was highest at 29'C. Killing at both of these
temperatures occurred much earlier than at the other tempera-
tures, and was practically complete before plants at the other
temperatures showed infection. The table also shows that in-
fection declined rapidly between 310 and 350, and that at the
lowest temperature, 16', no infection developed though good
germination occurred at this temperature. None of the plants in
the control cans became infected during the course of the experi-
ment.
Results of the first Wisconsin series, tabulated in Table 1,
are charted in Fig. 1. Photographs of a representative series of
plants in cans of infested and non-infested soil are shown in
Figures 2 and 3. In this series the percentages of germination
were fairly uniform in all cans planted with Watson seed, except
the cans of infested soil at 18.50, and the reduction in the infested
soil could not be positively attributed to wilt. Killing was ob-
served first after six days at 270C., and the plants in certain of
the cans of infested soil at this temperature showed 100% killing
by the end of the test. The rapidity of infection and the greater
kill indicated that 270C. was the optimum temperature for de-
velopment of the disease. The percentages of total infection at
the lower temperatures were almost as high as that at 27, but
the difference in the percent of killing indicated a slight de-
crease in the degree of injury. The reduced germination in the
infected soil at 18.5 C. was possibly due to rotting of the seed by
the wilt organism, though in heavier infested soil in the third
series a similar condition was not noted, and in the second Flori-
da series where sterilized soil was used similar injury was not
noted at 16'. The fact that field soil repeatedly planted to wat-
ermelons was used in the first Wisconsin series, whereas virgin
woods soil was used in the third series, is probably significant,
since there have been indications that other organisms than the
wilt Fusarium have multiplied in the field soil.
At temperatures above 27 the percentages of killing and of
total infection of watermelon seedlings declined rapidly. At 330
only one plant out of 35 died and no other plants showed evi-
dences of the disease when the stems were cut. Above this tem-
perature no killing or injury was observed. No infection ap-
peared in any of the control cans of this series of tests.
The germination of the watermelon seed in the third Wis-
consin series was generally good, but was more uniform in the















Fig. 2.-A representative series of plants in cans of infested soil
from the first Wisconsin series. The temperatures at which the plants
were grown are from left to right: 39, 360, 33, 31, 270, 24.50, 21.5 and
18.5 C. Note that greatest killing occurred at 27' and 24.50. The photo-
graph was taken 32 days after planting.
cans of infested soil than in the cans of non-infested soil, prob-
ably because of the larger number of plants involved. Killing of
Tom Watson plants by wilt in this test was much more rapid than
in the two previous tests in which seed of the same strain were
planted. The increased infection in this series is attributed to the
large amounts of infective material added to the soil. Because of
the extremely rapid infection, this series was continued only 22
days. Photographs of plants grown in infested and non-infested
soil at different temperatures are shown in Figures 4 and 5.
Injury by wilt was much more severe in the watermelons in
this test than in the previous one, and approximately 100%
killing occurred between 21.50 and 300C. The fact that killing
occurred first in the tank at 27.5', however, again indicated that
this temperature was the most favorable for infection and killing
of watermelon seedlings. However, within a period of approxi-
mately three weeks all plants were killed in heavily infested
soil in the temperature range of 21.5 to 300C. Above
300 infection and killing declined rapidly, and it will be noted
that only 12 percent killing occurred at 330 against 97 percent
at 300.
It was not possible to continue the fourth series longer than
17 days, but the general severity of infection was so great that
more injury was observed in this period than had occurred dur-
Fig. 3.-A representative series of plants in cans of non-infested
soil from the first Wisconsin series. Order of arrangement is the same
as in Fig. 2. The photograph was taken 32 days after planting.



'I,-. ..- "' .. ; i ':,
%Ile *^ M '
0t- J~pG~~I







Fusarium Wilt of Watermelons


ing the 30 days of the first series. Germination of the watermel-
on seed was fairly uniform except at 180 C. The reduction at
this temperature was probably due merely to lack of sufficient
time for complete germination. Though not so clear cut as in
previous tests, 27'C. again appeared as the most favorable tem-
perature for infection, since infection appeared first at this tem-
perature. Under the conditions of extremely heavy soil infesta-
tion of this test infection was practically the same within the
range of 240 to 30.50C., but the data show a marked reduction in
injury at temperatures outside of this range.
It is interesting to note in Table 1 the progressively greater
killing in the three tests of the Tom Watson variety of water-
melons. The Florida test was in sterilized soil that had been
lightly infested with the wilt organism, while the second test
was made in field soil heavily infested with the wilt organism
and to which additions of infective material had been made.
The soil for the third series was prepared by adding very large
quantities of infective material.
Citrons.-Although there were some variations in the de-
gree of soil infestation and the kind of soil used in the different
tests, the records of infection of citron seedlings were combined
because of the relatively small numbers of plants in the tests and
the low rate of killing in certain of the strains. The combined
records of infection of the three citron strains are presented in
Table 2. Strain 5-01 was tested in the first, second and fourth
Wisconsin series; strain 6-01 was tested in the second and fourth
Wisconsin series; and strain 90-02 was tested in the second and
third Wisconsin series.
As may be seen by comparing the data of Tables 1 and 2
there was less killing and infection of citron plants than of wat-
Fig. 4.-A representative series of plants in cans of infested soil
from the third Wisconsin series. The temperatures at which the
plants were grown are from left to right: 39, 36, 33, 30, 27.50, 24.5,
21.50 and 18.5'C. Note almost complete killing of plants at all tempera-
tures below 30C. This photograph was taken 22 days after planting.





I;, Ip .

SC. -

















Fig. 5.-A representative series of plants in cans of non-infested soil
from the third Wisconsin series. Order of arrangement is the same as
in Fig. 4. This photograph was taken 22 days after planting.

ermelon plants. However, the infection that did develop occurred
in the range of temperatures optimum for infection of watermel-
on plants, and the same sharp decline in killing and infection
above 300C. was also evident. The results with citron also show-
ed a difference in the degree of resistance of the strains tested.

Citron-Watermelon Hybrids.-The combined results of the

TABLE 2.--EFECTS or SOIL TEMPERATURE ON INFECTION OF CITRON SEEDLINGS BY
Fusarium Bulbigenum VAR. Niveum.
Germination in Killing in Infested Soil After'
Non-Inf. Inf. 10 days 15 days 20 days Total Total
Aver. Soil' Soil2 Killing' Infection'
Temp. No % No % No % No % No % No % No %


9 45
14 70
19 95
18 90
20 100
14 70
14 70
1 5

12 60
12 60
20 100
17 85
20 100
14 70
12 60
2 10


5-01
0 0 0 0
0 0 0 0
0 0 0 0
1 4 1 4
0 0 1 5
0 0 2 12
0 0 0 0
0 0 0 0
6-01
0 0 0 0
0 0 0 0
0 0 2 11
1 6 5 28
0 0 3 15
0 0 1 7
0 0 0 0
0 0 0 0
9-02
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
0 0 1 5
0 0 1 7
0 0 1 8
0 0 0 0


119 seed of 5-01 were planted in non-infested soil at each temperature; 9 of 6-01;
and 9 of 90-02.
230 seed of 5-01 were planted in infested soil at each temperature; 20 of 6-01;
and 20 of 90-02.
'No killing occurred in non-infested soil.
'At end of test.
'Includes all plants killed as well as those showing infection when stems were cut.


390C.
36
33
30.50
270
240
210
18.50

39C.
360
33*
30
270
240
21
18"

390C.
360
330
30
27.5
24.5
21
18.5S


~I ~tCai~dB~e~T~ r-r pr$C?;








Fusarium Wilt of Watermelons


TABLE 3.-EFFECT or SOIL TEMPERATURE ON THE INFECTION OF SEEDLINGS OF CITRON-
WATERMELON HYBRIDS BY Fusarium Bulbigenum VAR. Niveum.
Germination in Killing in Infested Soil After3


Non-Inf. Inf.
Aver. Soil' Soil2
Temp. No % No %


39'C.
36
33
30
27.5
24
21
18.50

390C.
36'
330
30
27.5
24
21
18.5'

39C.
36
33
30
27.5"
24
21
18.5'


19 95
19 95
20 100
20 100
20 100
18 90
8 40
4 20


10 days 15 days

No % No %
5-x2
0 0 0 0
0 0 0 0
0 0 4 20
4 20 10 50
1 5 11 55
0 0 8 44
0 0 7 87
0 0 0 0
6-x2
0 0 0 0
0 0 0 0
0 0 1 5
5 26 12 63
6 30 14 70
5 29 10 59
0 0 9 50
0 0 0 0
90-xl
0 0 0 0
0 0 0 0
0 0 0 0
0 0 2 10
0 0 4 21
0 0 2 22
0 0 1 11
0 0 0 0


'9 seed of each of these strains were planted in non-infected soil at each temper-
ature.
"20 seed of each of these strains were planted in infested soil at each temperature.
'No killing occurred in non-infested soil.
4At end of test.
'Includes all plants killed as well as those showing infection when stems were cut.

tests of citron-watermelon hybrids are presented in Table 3.
Strain 5-x2 was tested in the second and fourth Wisconsin series;
strain 6-x2 was tested in the same two series; and strain 90-xl
was tested in the second and third Wisconsin series.
As shown in Table 3, the rapidity of infection and the total
killing in these tests would indicate that 27.5C. was the most
favorable temperature for infection and killing of the citron-
watermelon seedlings by wilt, but all temperatures between 21o
and 300C. are favorable for attack by the wilt organism. Reduc-
tion in killing and infection was marked at temperatures above
300C.
It is especially interesting to compare the extent of killing
and total wilt infection of the citron-watermelon crosses with


20 days

No %


Total
Killing'
No %


Total
No %
Infection"

0 0
0 0
9 45
16 60
19 95
17 94
8 100
1 25






Florida Agricultural Experiment Station


the corresponding selfed lines of citrons. The crossed line 5-x2
showed much heavier percentages of killing and infection than
did the corresponding selfed line 5-01. The same thing is true
but to a less marked extent with the strains 6-x2 and 6-01,
whereas slightly less wilt and killing occurred in the hybrid
strain 90-xl than took place in the corresponding selfed strain
90-02. The difference between the latter two strains is probably
not significant, as the second test of this strain showed consid-
erably more killing than the first. These tests show that the
resistance of the citrons practically disappears in the F1 genera-
tion of citrons crossed with susceptible watermelon stocks, as the
percentages of infection and killing in F, hybrids 5-x2 and 6-x2
were as high as occurred in the tests of a susceptible watermel-
on. In other words, resistance appears to be recessive. Although
the percentages of total wilt and killing in the crossed lines
were greater than in the selfed strains, the temperatures at which
infection and injury were greatest were practically the same.

EFFECT OF TEMPERATURE ON GROWTH OF PLANTS
To gain some information on the influence of soil tempera-
ture upon the growth of the watermelon plant, records were
maintained on the rate of germination, the daily and total stem
elongation, and the number of leaves at different temperatures
during the two Florida and the first Wisconsin infection series.
The records are based on but small populations, and the experi-
ments were not properly designed to give a true picture of the
relationship of soil temperature to growth. A measurement of
green or dry weight would perhaps have been better than that
of stem elongation. It is believed, however, that the results are
suggestive of the effect of soil temperature on the early growth
of the watermelon plant.
Summaries of the records of the measurements made during
the two Florida series are given in Table 4. A photograph of
the control plants in the second series is presented in Figure 6
to show the appearance of the plants at the different tempera-
tures. This photograph was taken 16 days after planting. The
records show that a general range of soil temperature between
290 and 35C. gave the most rapid germination of seed of the
Tom Watson variety. There were indications in the first trial
that 350 was beginning to show a retarding effect on germination
but definite direct injury was apparent only at the lowest tem-






J- /
















Fig. 6.--Plants in non-infested soil showing comparative growth
at different temperatures. Temperatures at which the plants were grown,
from left to right were: 35, 31', 29', 23.5, 19.5o, and 16oC. The photo-
graph was taken 16 days after planting.

perature. It would appear from the records of this test and the
general condition of the seedlings that 31 to 330 was the most
favorable temperature range for the germination of watermelon
seed.

TABLE 4.-EFFECTS OF SOIL TEMPERATURES ON GERMINATION AND GROWTr or
WATERMELON SEEDLINGS.1
Plant Growth in mm.
Germination Aver. No. Total Daily
Aver. Days Re- No. Average Average
Temp. quired for Plants per per
No.' %C Germination Measured' Plant Plant
First Tank Series, Florida
35.0'C. 24 80 4.4 6 369 23.6
31.50 29 97 3.6 8 354 24.0
29.0 25 83 3.3 8 294 15.7
23.5 22 73 5.7 10 217 12.7
19.5 17 57 8.3 7 178 10.6
15.5 8 27 13.4 6 92 4.3
Second Tank Series, Florida
35.0C. 26 87 3.8 8 531 19.8
31.00 24 80 4.0 8 487 18.3
29.0 23 77 4.4 9 342 11.8
23.5 27 90 5.8 9 251 9.4
19.5' 25 83 8.2 8 187 8.9
16.0 22 73 12.2 9 119 5.8
'Tom Watson variety.
230 seed planted at each temperature.
'Only plants in non-infested soil were measured.

The most rapid stem elongation occurred between 310 and
350C. but the plants at the highest temperatures showed a ten-
dency to be spindling and pale in color. It would appear that
the optimum range is probably the same as for germination.
It should be noted that in spite of the greater total growth in









18 Florida Agricultural Experiment Station


the second series the daily growth increments in the first series
are somewhat larger. This is accounted for by an irregular but
constant increase in the growth increments for the plants of
both series as they become older, and the fact that measurements
were started later in the development of the plants of the first
series and were continued over a shorter period of time.

At the conclusion of the first Wisconsin series all of the
surviving plants, both citron and watermelon, in both infested
and non-infested soil were measured and a record was made of
the number of true leaves. Measurements were made from the
soil level to the cotyledons and to the terminal buds. The aver-


TABLE 5.-EFFECT OF SOIL TEMPERATURE ON THE GROWTH OF WATERMELON AND
CITRON SEEDLINGS.


Average
Temperature
Watermelon (Watson)

39.0C.
36.0
33.0
31.00
27.00
24.50
21.50
18.50

39.0 C.
36.0
33.00
31.00
27.0
24.50
21.50
18.50
Citron (5-02)

39.0C.
36.0
33.00
31.00
27.0
24.5
21.50

39.0oC.
36.00
33.00
31.0
27.0
24.5
21.50
18.50


No. o
Plant
Measu:


Aver. Heights in mm.: Aver. No.
etsd of Leaves per
red To Cotyledons To Bud Plant


Non-infested Soil (Checks)
14 142
14 138
19 143
14 138
15 120
17 133
13 112
14 67
Infested Soil
31 143
32 130
32 130
26 121
9 115
8 108
12 91
9 59

Non-infested Soil (Checks)
7 109
7 110
4 102
5 105
3 109
2 113
5 98
Infested Soil
8 104
9 110
5 102
6 90
4 92
5 90
6 80
3 64







Fusarium Wilt of Watermelons


ages of these measurements and of the numbers of leaves are
given in Table 5. The photograph of control cans from the first
Wisconsin series given in Fig. 3 shows the comparative growth of
watermelons at different soil temperatures.
Length of the stems from the soil level to the cotyledons
would indicate that soil temperatures above 22 to 230C. have
little effect in determining the height of the cotyledons above
the soil level, as the averages for both watermelon and citron
plants in infested and non-infested soil were fairly uniform above
these temperatures. The difference between the averages for
both strains in infested and non-infested soil is attributed to the
greater fertility of the non-infested soil, which was virgin woods
soil, whereas the infested soil was the light, sandy soil from
Florida. The infestation of the soil was not considered as being
significant, since the curves for average heights of cotyledons in
both infested and control cans showed flattening above the
temperatures mentioned. It is believed that the innate character
of the strain of seed is probably the chief limiting factor for the
height of cotyledons where fertility of the soil and other possible
factors are favorable.
The average length of stems from the soil to the terminal
bud presents a somewhat different picture, although the matter
of soil fertility is again evident in a comparison of the plants in
infested and non-infested soil. In the case of watermelons it
will be noted that the greatest stem elongation occurred at soil
temperatures above 30C., and that a sharp increase was re-
corded at 390C. The plants at the latter temperature, however,
were spindling and did not show the thriftiness of plants at
360C. and below. Below 30 the decrease in total elongation of
stems was rapid, but at temperatures above 240 growth and
general appearance of the plants were satisfactory. Below this
temperature, growth, as indicated by stem elongation, was slow
and the plants were of a very dark green color, and appeared
hard and without the succulence characteristic of watermelon
plants when growing vigorously.
Good elongation of citron stems occurred over a wider range
of soil temperatures than was noted with watermelons, and at soil
temperatures at 240 and above no marked nor consistent varia-
tions were noted. Even at 390 the plants showed less evidence
of weakening than was observed with watermelons. This was
true in both infested and non-infested soil, although the differ-
ence in the plants in the two kinds of soil was more marked than







Florida Agricultural Experiment Station


in the case of watermelons. The irregular and comparatively
small variations in the citron plants in both infested and non-
infested soil would indicate the possibility of the presence of
some limiting factor other than soil temperature in the range
above 240. Consequently, no conclusions can be drawn from
this experiment as to the most favorable soil temperature for
stem elongation of this strain.
The average number of leaves of the watermelon plants on
both kinds of soil showed the same general trend as the measure-
ments to the terminal buds, but only the counts of the watermel-
on leaves in non-infested soil furnished a relatively smooth
curve. No retarding effect was noted at the highest tempera-
ture, 390. The texture and size of leaves were adversely affect-
ed at this temperature, however. Due to the small number of
plants at.each temperature the variations in the number of the
leaves of citron plants at different temperatures were not con-
sistent or large enough to allow of interpretation.
Although the increase in length of stems of both citron and
watermelon plants and the number of leaves in themselves gave
no clear cut indication of the most favorable soil temperatures
for the best all-round growth of the plants, observations on the
general condition of the plants indicated that 330C. was probably
the most favorable soil temperature for the germination and
continued growth of the two strains under test. These observa-
tions were borne out in subsequent tests where no measurements
were made. It would also appear from a comparison of the
control plants of the watermelons in the first Wisconsin series
(Fig. 3) with those of the third Wisconsin series (Fig. 5) which
were considerably younger, that the effects of soil temperatures
are most marked during germination and the earlier growth
stages of the seedlings, and that after plants have attained some
size, growth occurs at a uniform rate over a fairly wide range
of soil temperatures. This would suggest that air temperatures
are of increasing importance as the plants become older, and
that only extreme soil temperatures are limiting to any marked
degree.
Results of these tests point toward a disparity in the most
favorable temperatures for the development of the wilt disease
and the growth of the host. A comparison of the relationship
of soil temperature to the wilt disease with the growth of the
host brings out this disparity more clearly. This comparison
shows that the temperature at which infection begins to decline







Fusarium Wilt of Watermelons


sharply falls in the lower part of the range of temperature most
favorable to the host, and that higher temperatures affect the
disease much more than they do the host. This would suggest
the reason for reduced wilt injury observed in field tests in 1931
after average soil temperatures at a depth of six inches exceeded
850F., or roughly 30C.; and why in 1932, which was a much
cooler and wetter season, injury was more severe throughout the
season in spite of the fact that total infection was approximately
the same in the two years.

EFFECTS OF TEMPERATURE ON THE WILT ORGANISM
Some work was done to determine the effects of different
temperatures on Fusarium bulbigenum, var. niveum Woll., in culture,
and to compare strains of the fungus from different states as to
their relation to temperatures. Cultures of various strains of
the fungus on petri plates were exposed for different periods of
time to temperatures ranging from 7.50 to 370C. The diameter
of the mycelial weft was the basis of comparison. The agar used
throughout the tests was Leonian's malt extract agar, and though
no attempt was made to control the hydrogen ion concentra-
tion, colorimetric readings showed the initial pH to be about 6.5.
Approximately equal amounts of mycelium from the edges of
actively growing plate cultures of the wilt organism were trans-
ferred to the centers of the test plates. The plates were allowed
to remain at room temperatures for 24 hours, after which time
cultures showing no growth or contamination were discarded.
The others were transferred to incubators adjusted to the follow-
ing temperatures 8', 12 16, 20, 240, 280, 320 and 360C. In prac-
tically all tests two plates of each strain were placed at each
temperature, though three plates were used in most of the
tests at 28 and 320. Contaminations appearing in some incuba-
tors reduced the number of plates to one. Wide fluctuations in'
temperatures of certain of the incubators occurred during the
first series and the results had to be discarded. Some variation
occurred at the higher temperatures in the other tests and the
average to the nearest half degree of temperature readings in
the incubators are given for each test. Only two incubators dur-
ing the period of the tests showed as much as 3 degrees dif-
ference in the readings.
A summary of the results from the incubator series is pre-
sented in Table 6. Curves showing the growth in millimeters
of a typical strain at different temperatures and at different time










TABLE 6.-EFFECT OF TEMPERATURE ON THE GROWTH OF Fusarium Bulbigenum VAR. Niveum IN CULTURE.

Culture Time Average Diameters in Millimeters of Mycelial Wefts at:
and in
Source Hours 7.5C 12.0C 16.00C 21.D0C 24.0'C 26.5C 32.50C 37.5C
Second Series
4 (Fla.) 48 Slight' 10.0 19.0 27.5 30.0(3) 33.0 28.0 Slight
72 Slight 15.5 29.0 40.0 43.0(3) 45.0 37.5 Slight
96 Slight 19.0 34.5 49.5 55.0(3) 59.5 49.0 Slight
6 (Fla.) 48 Slight 11.5 20.5 28.0 30.0 32.5 28.0 Slight
72 Slight 17.0 29.0 39.0 43.0(1) 46.0 39.0 Slight
96 Slight 19.0 34.5 48.0 54.0(1) 60.0 49.0 Slight
8 (Ga.) 48 Slight(3) 9.0(3) 17.0 25.0 29.5 32.5(3) 28.5 Slight
72 Slight(3) 13.0(3) 24.5 37.5 43.0 47.0(3) 40.0 Slight
96 Slight(3) 15.5(3) 31.0 47.0 56.0 61.0(3) 52.5 Slight


Third, Fourth and Fifth Series
12.0C 16.0C 20.5C
18.0 26.5 33.5
22.5 33.5 44.5
11.5 21.5 28.0
16.5 30.0 40.0
11.0 20.0 27.0
14.5 27.0 37.0
12.5 19.0 26.0
17.5 27.5 37.5
15.0 24.0 32.0
21.0 30.5 42.0
10.0 17.5 24.0
12.5 25.0 35.0
10.5 18.5 24.5
12.5 24.0 32.0
13.0 19.5 27.0
16.0 25.5 35.0
9.5 16.0 23.0
10.5 21.0 30.0
10.0 17.5 24.5
11.5 21.0 31.5


24.5C 27.5C 33.0C 37.0C
39.0(3) 38.5(3) 36.0(3) 14.0
51.5(3) 52.5(3) 47.0(3) 15.0
34.0(3) 35.0(3) 30.5 12.5
47.5(3) 50.0(3) 42.5 16.0
34.5(3) 34.0(3) 31.5 9.0
47.5(3) 48.5(3) 43.0 12.0
32.5(3) 32.5(3) 26.0 10.0
45.5 45.5(3) 37.0 18.0
36.0(3) 39.5(3) 33.5(3) 16.0
50.5(3) 54.5(3) 45.5(3) 21.0
29.5(3) 30.5(3) 25.0 7.0
43.0(3) 45.5(3) 36.0 12.5
30.0(3) 33.0(3) 26.0 14.5
40.5(3) 44.5(3) 34.5 16.5
31.5(3) 36.5(3) 31.0 7.5
41.5(3) 47.5(3) 51.0 9.0
29.0(3) 31.5(3) 21.5 8.5
38.0(3) 41.5(3) 30.0 10.0
30.0(3) 31.5(3) 24.0 9.0
39.5(3) 41.5(3) 31.0 11.5


4 (Fla.)

5 (Fla.)

7 (Fla.)

9 (Ga.)

10 (Iowa)

11 (Iowa)

2 (Ark.)

16 (N. C.)

17 (S. C.)

18 (S. C.)


7.5C
Slight
Slight
Slight
Slight
Slight
Slight
Slight
Slight
Slight
Slight
Slight
Slight
Slight
Slight
Slight
Slight
Slight
Slight
Slight
Slight






Slight
Slight
Slight
Slight
Slight
Slight


19.5 27.0
25.5 35.0
16.0 23.0
21.0 30.0
17.5 24.5
21.0 31.5


(N. C.)

(S. C.)

(S. C.)




(Ark.)
(La.)
(La.)
(Miss.)
(N.C.)
(W. Va.)
(W. Va.)


31.5(3)
41.5(3)
29.0(3)
38.0(3)
30.0(3)
39.5(3)


Slow
Slow
Slow
Slow
Slow
Slow


28.50C 33.50C 36.0C
28.0(1) 28.0(1) 17.0(1)
29.0 25.0 14.0
27.0 20.5 17.5
31.0(1) 26.0(1) 7.0(1)
29.5 24.5 8.5
26.0 18.0 10.0
28.5 23.5 13.5


'Two plates were used at each temperature. Where more or less than this number was used the number is given in parentheses.


36.5(3)
47.5(3)
31.5(3)
41.5(3)
31.5(3)
41.5(3)


Sixth Series
7.5C 11.5'C 16.0C 20.0C 24.0C
Slight 10.5 17.0 22.5 31.0(1)
Slight 7.0 15.5 23.5 31.0
Slight 8.0 18.5 23.5 31.5
Slight 10.5(1) 19.0(1) 22.0(1) 23.0(1)
Slight 8.5 17.0 24.0 31.5
Slight 12.5 18.5 23.0 31.0
Slight 8.0 17.5 23.0 30.5


__






24 Florida Agricultural Experiment Station
intervals are shown in Fig. 7. In these experiments good growth
of all the strains tested occurred at temperatures between 200
and 330C., but the most favorable range of temperatures for the
rapid growth of the wilt fungus in culture appears to be between
260 and 280C. Though variations in relative vigor and appear-
ance of the mycelial wefts occurred, all of the strains tested
showed the greatest growth within this range. Fair growth oc-
curred at temperatures as low as 120C., but at 7.5C. growth was
sparse, consisting for the most part of isolated hyphal strands,
indicating that the minimum for growth was not much below this
temperature. The character of the growth at this temperature
did not permit exact measurements. Above 330C. the decline
in the growth of the fungus was very rapid, and only slight evi-
dences of growth were noted between 36 and 380C., indicating
that the maximum temperature for growth is but slightly high-
er. Growth at the highest temperature was irregular and often
tufted, which made exact measurements difficult. A photograph
of a series of culture plates at different temperatures is shown
in Fig. 8, and is typical for the group of strains as a whole.
601 1 I I I ^ 1 I


5 10 15 20 25 30 35 40
DEGREES C,
Fig. 7.-Growth in millimeters of a ,typical culture of Fusarium
bulbigenum var. niveum at different temperatures. Curves 1, 2 and 3 rep-
resent growth after 38, 72 and 96 hours, respectively. These curves are
typical of the growth of the other cultures tested.


























cC











Fig. 8.-Photograph of a temperature series with culture 8 of Fusarium bulbigenum var. nive2um after 96 hours ex-
posure to different temperatures. This series is typical for the cultures tested. Plates from different temperatures
alternate from bottom to top, beginning at the lower left. Temperatures represented are 37.5, 32.5, 26.50, 24', 21',
16, 12', and 7.5"C. respectively






Florida Agricultural Experiment Station


SUMMARY AND DISCUSSION
From the results of the soil temperature studies as a whole
it can be concluded that temperatures around 27 are probably
most favorable for rapid and severe injury to watermelons by
Fusarium wilt; that infection declines rapidly below and above
the range of 200 to 30C.; that practically no infection occurs
above 33.; and that under conditions of extreme soil infestation
little difference in total injury can be observed following varia-
tions in soil temperatures within the range 20 to 30 where ex-
posure of plants is for periods as long as 30 days. These find-
ings are in accord with the work of Porter and Melhus (6) who
found that, though seedling rot is more severe at 160 to 180C.
than at 220 to 250C. or 25" to 280C., the three temperature ranges
used, seedling wilt was most severe at 25 to 28'C.
Demonstrations of the importance of soil temperature in re-
lation to soil-borne Fusarium diseases of other crops have for the
most part dealt with parasites growing best at or above the
optimum range of temperatures for the host, whereas in the
present work the condition is reversed and the optimum for the
host appears to be several degrees higher than that of the para-
site. For instance, in the case of the yellows disease of cabbage
(8), it has been found that the host develops best at above 200C.,
while the casual fungus grows best at around 280C., being in this
respect similar to the organism causing the wilt diseases of
watermelons. The attacks of the yellows organism are most se-
vere as the optimum temperature for the fungus is approached
and as the temperature becomes less favorable to the cabbage
plant itself. This is typical of a number of other similar diseases
in which the host grows most satisfactorily at or below the tem-
peratures most suitable to the causal fungus (2, 3). As a result
of such studies the term "high" temperatures, based on the tem-
perature relations of the host, has come to signify in the lit-
erature the range between 260 and 300C., because under the field
conditions where most of these crops are grown 300C. is practi-
cally an upper limit, although there are occasional and brief
periods during which this temperature may be exceeded, usually
at the shallower depths. In Florida with greater solar radiation
and lighter soils, temperatures above 300C. are relatively com-
mon at a depth of six inches. Consequently, in the present work
temperatures above 30 are those considered as high.
An approximation of the conditions encountered during the
present study is found in the work of the writer with the "sore-







Fusarium Wilt of Watermelons


shin" disease of cotton (1), caused by Rhizoctonia solani... The op-
timal range of temperatures for infection by this fungus is lower
than that of Fusarium bulbigenum var. niveum, but the most favor-
able development of the host occurred at practically the same tem-
perature as that indicated for watermelons. The reduction in
injury from Rhizoctonia was consequently more clear-cut than
in the present case where higher temperatures were needed to
bring about a reduction in infection and injury. "High" temper-
atures in the case of the "sore-shin" disease were more nearly
the same as those given in the literature for other soil-borne
diseases, though based on the optimum of the fungus rather than
that of the host. These "high" temperatures always occurred
during a particular year, as they were constantly within the
normal seasonal range of temperatures for the crop at a fairly
early date, whereas the "high" temperatures of the present work
were more intermittent and of generally shorter duration.
The behavior of the citrons tested indicated some degree of
resistance in the selfed lines, but crossing with susceptible water-
melons served to induce infections in the F1 generation approxi-
mately equal to those occurring in the watermelon parent. This
would indicate the dominance of susceptibility over resistance. It
also appeared that such resistance as occurred in the citrons was
one of degree and, though the disease appeared in citrons,
both infection and progress of the disease were slow. Certain of
the tests with citron strains indicated that they were more
susceptible to low soil temperatures than watermelons, particu-
larly those below 210C., and also the possibility of a somewhat
higher optimum temperature for most satisfactory growth.
From the observations on the growth of watermelons inci-
dental to the more detailed study of the relation of soil tempera-
ture to the wilt disease, it would appear that temperatures in the
neighborhood of 32 to 33C. are probably the most favorable
for germination and growth of watermelons. Growth was still
vigorous and rapid at 390C., but there was a tendency for the
plants to become spindling. From the general vigor of the plants
at this high temperature it would appear that the maximum
temperature for growth of watermelons is somewhat higher.
There was a sharp increase in the growth between 27 and 300,
and temperatures above the latter temperature were much more
favorable to growth of watermelons than those below, until the
maximum was approached. Although no evidence of definite
injury to watermelon plants appeared until a temperature of 180






Florida Agricultural Experiment Station


was reached, germination and growth below 240 degrees were
very slow. Consequently, it would appear inadvisable to plant
watermelons when sustained average soil temperatures much
below this temperature may be expected.
Results of the experiments on the growth of the watermelon
wilt organism at different temperatures are in essential accord
with the findings of other workers with similar organisms at-
tacking other plants. Porter '(5) reported that the greatest
growth of the watermelon wilt organism in plate culture took
place between 240 and 320C., and that very slow growth occurred
at 12 and 350C. In a later paper Porter and Melhus (6) noted
the occurrence of rather pronounced variations in cultures from
different states when grown at the same temperature. Sleeth
(7) found marked variations in the rates of growth of different
strains of the watermelon wilt organism, but greatest growth of
all strains occurred between 240 and 28'C. Maximum tempera-
ture for growth was slightly above 350 and the minimum above
5C. It would thus appear that the temperature relations of
Fusarium bulbigenum var. niveum Woll. are in all respects similar to
those of other vascular-invading Fusaria, such as F. conglutinans
on cabbage (8), F. vasinfectum on cotton (4), F. lini on flax (2),
F. lycopersici on tomato (2).
CONCLUSIONS
From the series of tests reported herein the following con-
clusions can be drawn:
1. The optimum temperature for infection and injury of
watermelon seedlings of the Tom Watson and Kleckley Sweet
varieties by Fusarium bulbigenum var. niveum Woll. is 270C.
2. The optimum temperature for early infection is not
particularly important, since under conditions of heavy soil in-
festation killing is practically the same throughout the range of
20 to 300C. where exposure to infection is as long as 30 days.
3. Infection declines rapidly above 30'C. and practically
ceases at 330C.
4. Duration of the tests was too short to determine definite-
ly the minimum temperature for infection. In field soil it is
likely that other soil organisms are complicating factors.
5. Certain citron strains are resistant to infection and de-
velopment of wilt.
6. The resistance of the citrons appears to be relative, and
infection and progress of the disease appear merely to be slower
than in watermelons.







Fusarium Wilt of Watermelons


7. Tests of crosses of resistant citron with susceptible wat-
ermelons indicate that the resistance is recessive, as the hybrids
showed practically as high a rate of infection and injury as
susceptible watermelons.
8. Germination of watermelons and citrons is optimum at
temperatures between 31 and 33C. Optimum temperatures for
the early growth of watermelon and citron seedlings is also be-
tween 31 and 330C.
9. Soil temperature effects are most marked during the
early stages of growth of watermelon and citron plants. Later on
air temperatures or other factors appear to be more important
within the range of 240 to 35'C.
10. There are some indications that citron seedlings are
somewhat more vigorous than watermelon seedlings and are
somewhat more tolerant of temperatures above 35C. than wat-
ermelon seedlings.
11. Growth of the wilt organism, Fusarium bulbigenum var.
niveum Woll., appears greatest at 270C. The minimum tempera-
ture for growth appears to be slightly below 7.5 C. and the
maximum around 380C.

ACKNOWLEDGEMENT

All photographs except that in Fig. 6 were made by Mr. Gene
Herrling of the Department of Plant Pathology, University of Wis-
consin.
LITERATURE CITED

1. Camp, A. F., and M. N. Walker. Soil temperature studies with cot-
ton. Florida Agr. Exp. Sta. Bul. 189. 1927.
2. Jones, L. R., et al. Wisconsin studies upon the relation of soil tem-
perature to plant disease. Wis. Agr. Exp. Sta. Res. Bul. 71. 1926.
3. Linford, M. B. A Fusarium wilt of peas in Wisconsin. Wis. Agr.
Exp. Sta. Res. Bul. 85. 1928.
4. Neal, D. C. Cotton Wilt: A pathological and physiological investi-
gation. Miss. Agr. Exp. Sta. Bul. 16. 1938.
5. Porter, D. R. Infection studies with watermelon wilt caused by
F. niveum E.F.S. Iowa Agr. Exp. Sta. Res. Bul. 112. 1928.
6. Porter, D. R., and I. E. Melhus. The Pathogenicity of F. niveum. E.F.S.
and the development of wilt resistant strains of Citrullus vulgaris
Schrad. Iowa Agr. Exp. Sta. Res. Bul. 149. 1932.
7. Sleeth, Bailey. F. niveum, the cause of watermelon wilt. W. Va. Agr.
Exp. Sta. Bul 257. 1934.
8. Tisdale, W. B. The influence of soil temperature and soil moisture
upon the Fusarium disease in cabbage seedlings. Jour. Agr. Res. 24:
55 86. 1923.







Fusarium Wilt of Watermelons


7. Tests of crosses of resistant citron with susceptible wat-
ermelons indicate that the resistance is recessive, as the hybrids
showed practically as high a rate of infection and injury as
susceptible watermelons.
8. Germination of watermelons and citrons is optimum at
temperatures between 31 and 33C. Optimum temperatures for
the early growth of watermelon and citron seedlings is also be-
tween 31 and 330C.
9. Soil temperature effects are most marked during the
early stages of growth of watermelon and citron plants. Later on
air temperatures or other factors appear to be more important
within the range of 240 to 35'C.
10. There are some indications that citron seedlings are
somewhat more vigorous than watermelon seedlings and are
somewhat more tolerant of temperatures above 35C. than wat-
ermelon seedlings.
11. Growth of the wilt organism, Fusarium bulbigenum var.
niveum Woll., appears greatest at 270C. The minimum tempera-
ture for growth appears to be slightly below 7.5 C. and the
maximum around 380C.

ACKNOWLEDGEMENT

All photographs except that in Fig. 6 were made by Mr. Gene
Herrling of the Department of Plant Pathology, University of Wis-
consin.
LITERATURE CITED

1. Camp, A. F., and M. N. Walker. Soil temperature studies with cot-
ton. Florida Agr. Exp. Sta. Bul. 189. 1927.
2. Jones, L. R., et al. Wisconsin studies upon the relation of soil tem-
perature to plant disease. Wis. Agr. Exp. Sta. Res. Bul. 71. 1926.
3. Linford, M. B. A Fusarium wilt of peas in Wisconsin. Wis. Agr.
Exp. Sta. Res. Bul. 85. 1928.
4. Neal, D. C. Cotton Wilt: A pathological and physiological investi-
gation. Miss. Agr. Exp. Sta. Bul. 16. 1938.
5. Porter, D. R. Infection studies with watermelon wilt caused by
F. niveum E.F.S. Iowa Agr. Exp. Sta. Res. Bul. 112. 1928.
6. Porter, D. R., and I. E. Melhus. The Pathogenicity of F. niveum. E.F.S.
and the development of wilt resistant strains of Citrullus vulgaris
Schrad. Iowa Agr. Exp. Sta. Res. Bul. 149. 1932.
7. Sleeth, Bailey. F. niveum, the cause of watermelon wilt. W. Va. Agr.
Exp. Sta. Bul 257. 1934.
8. Tisdale, W. B. The influence of soil temperature and soil moisture
upon the Fusarium disease in cabbage seedlings. Jour. Agr. Res. 24:
55 86. 1923.




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