• TABLE OF CONTENTS
HIDE
 Title Page
 Credits
 Main
 Table of Contents
 Introduction
 Anthracnose
 Wilt
 Downy mildew
 Gummy-stew blight
 Stem-end rot
 Southern blight
 Blossom-end rot
 Powdery mildew
 Cercospora leaf spot
 Macrosporium leaf spot
 Soil rot
 Speckle
 Mosaic
 Minor diseases not found in...
 Cold, wind and sand injury
 Handling
 Summary














Group Title: Bulletin - University of Florida. Agricultural Experiment Station ; no. 491
Title: Diseases of watermelons
CITATION THUMBNAILS PAGE IMAGE ZOOMABLE
Full Citation
STANDARD VIEW MARC VIEW
Permanent Link: http://ufdc.ufl.edu/UF00027378/00001
 Material Information
Title: Diseases of watermelons
Series Title: Bulletin University of Florida. Agricultural Experiment Station
Physical Description: 48 p. : ill. ; 23 cm.
Language: English
Creator: Parris, G. K ( George Keith ), 1908-
Publisher: University of Florida Agricultural Experiment Station
Place of Publication: Gainesville Fla
Publication Date: 1952
 Subjects
Subject: Watermelons -- Diseases and pests -- Florida   ( lcsh )
Watermelons -- Diseases and pests -- Control -- Florida   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
non-fiction   ( marcgt )
 Notes
Statement of Responsibility: by G.K. Parris.
General Note: Cover title.
General Note: "A revision of Bulletins 225 and 459"--T.p.
Funding: Bulletin (University of Florida. Agricultural Experiment Station)
 Record Information
Bibliographic ID: UF00027378
Volume ID: VID00001
Source Institution: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: aleph - 000925756
oclc - 18266296
notis - AEN6412

Table of Contents
    Title Page
        Page 1
    Credits
        Page 2
    Main
        Page 3
    Table of Contents
        Page 4
    Introduction
        Page 5
    Anthracnose
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
        Page 10
        Page 11
        Page 12
        Page 13
    Wilt
        Page 14
        Page 15
        Page 16
        Page 17
    Downy mildew
        Page 18
        Page 19
    Gummy-stew blight
        Page 20
        Page 21
        Page 22
        Page 23
        Page 24
        Page 25
    Stem-end rot
        Page 26
        Page 27
        Page 28
        Page 29
        Page 30
        Page 31
    Southern blight
        Page 32
        Page 33
    Blossom-end rot
        Page 34
        Page 35
    Powdery mildew
        Page 36
    Cercospora leaf spot
        Page 37
        Page 38
    Macrosporium leaf spot
        Page 39
    Soil rot
        Page 39
    Speckle
        Page 40
    Mosaic
        Page 40
        Page 41
        Page 42
    Minor diseases not found in Florida
        Page 43
    Cold, wind and sand injury
        Page 43
        Page 44
    Handling
        Page 45
    Summary
        Page 46
        Page 47
        Page 48
Full Text


February 1952
February 1952


Bulletin 491


(A Revision of Bulletins 225 and 4591


UNIVERSITY OF FLORIDA
AGRICULTURAL EXPERIMENT STATIONS
WILLARD M. FIFIELD, Director
GAINESVILLE, FLORIDA







Diseases of Watermelons


By G. K. PARRIS
Formerly Plant Pathologist in Charge Watermelon and Grape Investigations
Laboratory, Leesburg


Fig. 1.-Watermelon showing anthracnose.


'41




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









BOARD OF CONTROL

Frank M. Harris, Chairman, St. Petersburg
Hollis Rinehart, Miami
Eli H. Fink, Jacksonville
George J. White, Sr., Mount Dora
Mrs. Alfred I. duPont, Jacksonville
George W. English, Jr., Ft. Lauderdale
W. Glenn Miller, Monticello
W. F. Powers, Secretary, Tallahassee

EXECUTIVE STAFF
J. Hillis Miller, Ph.D., President3
J. Wayne Reitz, Ph.D., Provost for Agr.3
Willard M. Fifield, M.S., Director
J. R. Beckenbach, Ph.D., Asso. Director
L. O. Gratz, Ph.D., Asst. Dir.,
Rogers L. Bartley, B.S., Admin. Mgr.3
Geo. R. Freeman, B.S., Farm Superintendent


MAIN STATION, GAINESVILLE

AGRICULTURAL ECONOMICS
H. G. Hamilton, Ph.D., Agr. Economist13
R. E. L. Greene, Ph.D., Agr. Economist 3
M. A. Brooker, Ph.D., Agr. Economist 3
Zach Savage. M.S.A., Associate
A. H. Spurlock, M.S.A., Associate
D. E. Alleger, M.S., Associate
D. L. Brooke, M.S.A., Associate4
M. R. Godwin, Ph.D., Associate 3
H. W. Little, M.S., Assistant
Tallmadge Bergen, B.S., Assistant
D. C. Kimmel, Ph.D.. Assistant
W. K. McPherson, M.S., Economist
Eric Thor, M.S., Agr. Economist
J. L. Tennant, Ph.D., Agr. Economist

Orlando, Florida (Cooperative USDA)
G. Norman Rose, B.S., Asso. Agr. Economist
J. C. Townsend, Jr., B.S.A., Agr.
Statistician
J. B. Owens, B.S.A., Agr. Statistician "
J. K. Lankford, B.S., Agr. Statistician

AGRICULTURAL ENGINEERING
Frazier Rogers, M.S.A.. Agr. Engineer'3
J. M. Johnson, B.S.A.E., Agr. Eng.'
J. M. Myers, B.S., Asso. Agr. Engineer
R. E. Ch-ate, P % E Asso. Agr. Eng.3
A. M. Pettis, P \ E Asst. Agr. Eng.1 3
J. S. Norton, M.S., Asst. Agr. Eng.
AGRONOMY
Fred H. Hull, Ph.D., Agronomist'
G. B. Killinger. Ph.D., Agronomist
H. C. Harris, Ph.D., Agronomist
R. W. Bledsoe, Ph.D., Agronomist
W. A. Carver, Ph.D., Associate
Darrel D. Morey, Ph.D., Associate
Fred A. Clark, B.S., Assistant
Myron C. Grennell, B.S.A.E., Assistant 4
E. S. Horner, Ph.D., Assistant
A. T. Wallace, Ph.D., Assistant
D. E. McCloud, Ph.D., Assistant 3
H. E. Buckley, B.S.A., Assistant
E. C. Nutter, Ph.D., Asst. Agronomist

ANIMAL HUSBANDRY AND NUTRITION
T. J. Cunha, Ph.D., An. Hush.13
G. K. Davis, Ph.D., Animal Nutritionist3
J. E. Pace. M.S., Asst. An. Hush.3
S. John Folks, Jr., M.S., Asst. An. Hush. 4
Katherine Boney, B.S., Asst. Chem.
A. M. Pearson, Ph.D., Asso. An. Husb.'
John P. Feaster, Ph.D., Asst. An. Nutri.
H. D. Wallace, Ph.D., Asst. An. Husb.s
M. Koger, Ph.D., An. Husbandman 3

DAIRY SCIENCE
E. L, Fouts, Ph.D., Dairy Tech.13
R. B. Becker, Ph.D., Dairy Husb.3
S. P. Marshall, Ph.D., Asso. Dairy Husb.3
W. A. Krienke, M.S., Asso. Dairy Tech. 3


P. T. Dix Arnold, M.S.A., Asst. Dairy Hush.2
Leon Mull, Ph.D., Asso. Dairy Tech.
H. H. Wilkowske, Ph.D., Asst. Dairy Tech.
James M. Wing, M.S., Asst. Dairy Hush.
EDITORIAL
J. Francis Cooper, M.S.A., Editor
Clyde Beale, A.B.J., Associate Editor
L. Odell Griffith, B.A.J., Asst. Editor3
J. N. Joiner, B.S.A., Assistant Editor a

ENTOMOLOGY
A. N. Tissot, Ph.D., Entomologist1
L. C. Kuitert, Ph.D., Associate
H. E. Bratley, M.S.A., Assistant
F. A. Robinson, M.S., Asst. Apiculturist
R. E. Waites, Ph.D., Asst. Entomologist

HOME ECONOMICS
Ouida D. Abbott, Ph.D., Home Econ.1
R. B. French, Ph.D., Biochemist
HORTICULTURE
G. H. Blackmon, M.S.A., Horticulturist
F. S. Jamison, Ph.D., Horticulturist
Albert P. Lorz, Ph.D., Horticulturist
R. K. Showalter, M.S., Asso. Hort.
R. A. Dennison, Ph.D., Asso. Hort.
R. H. Sharpe, M.S., Asso. Horticulturist
V. F. Nettles, Ph.D., Asso. Horticulturist
F. S. Lagasse, Ph.D., Asso. Hort.2
R. D. Dickey, M.S.A., Asso. Hort.
L. H. Halsey, M.S.A., Asst. Hort.
C. D. Hall, Ph.D., Asst. Horticulturist
Austin Griffiths, Jr., B.S., Asst. Hort.
S. E. McFadden, Jr., Ph.D., Asst. Hort.
C. H. VanMiddelem, Ph.D., Asst. Biochemist

LIBRARY
Ida Keeling Cresap, Librarian
PLANT PATHOLOGY
W. B. Tisdale, Ph.D., Plant Pathologist'
Phares Decker, Ph.D., Plant Pathologist
Erdman West, M.S., Mycologist and Botanist
Robert W. Earhart, Ph.D., Plant Path.2
Howard N. Miller, Ph.D., Asso. Plant Path.
Lillian E. Arnold, M.S., Asst. Botanist
C. W. Anderson, Ph.D., Asst. Plant Path.

POULTRY HUSBANDRY
N. R. Mehrhof, M.Agr., Poultry Hush.1
J. C. Driggers, Ph.D., Asso. Poultry Hush.
SOILS
F. B. Smith, Ph.D., Microbiologist 1
Gaylord M. Volk, Ph.D., Soils Chemist
J. R. Henderson, M.S.A., Soil Technologist 3
J. R. Neller, Ph.D., Soils Chemist
Nathan Gammon, Jr., Ph.D., Soils Chemist
Ralph G. Leighty, B.S., Asst. Soil Surveyors
G. D. Thornton, Ph.D., Asso. Microbiologist'
Charles F. Eno, Ph.D., Asst. Soils Micro-
biologist 4
H. W. Winsor, B.S.A., Assistant Chemist
R. E. Caldwell, M.S.A., Asst. Chemist34
V. W. Carlisle, B.S., Asst. Soil Surveyor
James H. Walker, M.S.A., Asst. Soil
Surveyor
S. N. Edson, M.S., Asst. Soil Surveyor 3
William K. Robertson, Ph.D., Asst. Chemist
0. E. Cruz, B.S.A., Asst. Soil Surveyor
W. G. Blue, Ph.D., Asst. Biochemist
J. G. A. Fiskel, Ph.D., Asst. Biochemist
H. F. Ross, B.S., Soils Microbiologist
L. C. Hammond, Ph.D., Asst. Soil Physicist 3

VETERINARY SCIENCE
D. A. Sanders, D.V.M., Veterinarian 1
M. W. Emmel, D.V.M., Veterinarian 3
C. F. Simpson, D.V.M., Asso. Veterinarian
L. E. Swanson, D.V.M., Parasitologist
Glenn Van Ness, D.V.M., Asso. Poultry
Pathologist
W. R. Dennis, D.V.M., Asst. Parasitologist









BRANCH STATIONS

NORTH FLORIDA STATION, QUINCY

R. R. Kincaid, Ph.D., Plant Pathologist
L. G. Thompson, Jr., Ph.D., Soils Chemist
W. C. Rhoads, Jr., M.S., Entomologist
W. H. Chapman, M.S., Asso. Agronomist
Frank S. Baker, Jr., B.S., Asst. An. Husb.

Mobile Unit, Monticello
R. W. Wallace, B.S., Associate Agronomist

Mobile Unit, Marianna
R. W. Lipscomb, M.S., Associate Agronomist

Mobile Unit, Pensacola
R. L. Smith, M.S., Associate Agronomist

Mobile Unit, Chipley
J. B. White, B.S.A., Associate Agronomist


CITRUS STATION, LAKE ALFRED
A. F. Camp, Ph.D., Vice-Director in Charge
W. L. Thompson, B.S., Entomologist
R. F. Suit, Ph.D., Plant Pathologist
E. P. Ducharme, Ph.D., Asso. Plant Path.
C. R. Stearns, Jr., B.S.A., Asso. Chemist
J. W. Sites, Ph.D., Horticulturist
H. O. Sterling, B.S., Asst. Horticulturist
H. J. Reitz, Ph.D., Horticulturist
Francine Fisher, M.S., Asst. Plant Path.
I. W. Wander, Ph.D., Soils Chemist
J. W. Kesterson, M.S., Asso. Chemist
R. Hendrickson, B.S., Asst. Chemist
Ivan Stewart, Ph.D., Asst. Biochemist
D. S. Prosser, Jr., B.S., Asst. Horticulturist
R. W. Olsen, B.S., Biochemist
F. W. Wenzel, Jr., Ph.D., Chemist
Alvin H. Rouse, M.S., Asso. Chemist
H. W. Ford, Ph.D., Asst. Horticulturist
L. C. Knorr, Ph.D., Asso. Histologist
R. M. Pratt, Ph.D., Asso. Ent.-Pathologist
J. W. Davis, B.S.A., Asst. in Ent.-Path.
W. A. Simanton, Ph.D., Entomologist
E. J. Deszyck, Ph.D., Asso. Horticulturist
C. D. Leonard, Ph.D., Asso. Horticulturist
I. Stewart, M.S., Asst. Biochemist
W. T. Long, M.S., Asst. Horticulturist
M. H. Muma, Ph.D., Asst. Entomologist
F. J. Reynolds, Ph.D., Asso. Hort.
E. J. Elvin, B.S., Asst. Hort.
W. F. Spencer, Ph.D., Asst. Chem.
V. L. Guzman, Ph.D., Asst. Hort.

EVERGLADES STATION, BELLE GLADE
R. V. Allison, Ph.D., Vice-Director in Charge
Thomas Bregger, Ph.D., Physiologist
J. W. Randolph, M.S., Agricultural Engr.
W. T. Forsee, Jr., Ph.D., Chemist
R. W. Kidder, M.S., Asso. Animal Husb.
C. C. Seale, Asso. Agronomist
N. C. Hayslip, B.S.A., Asso. Entomologist
E. A. Wolf, M.S., Asst. Horticulturist
W. H. Thames, M.S., Asst. Entomologist
W. N. Stoner, Ph.D., Asst. Plant Path.
W. A. Hills, M.S., Asso. Horticulturist
W. G. Genung, B.S.A., Asst. Entomologist
Frank V. Stevenson, M.S., Asso. Plant Path.
R. H. Webster, Ph.D., Asst. Agronomist
Robert J. Allen, Ph.D., Asst. Agronomist
V. E. Green, Ph.D., Asst. Agronomist
J. F. Darby, Ph.D., Asst. Plant Path.
H. L. Chapman, M.S.A., Asst. An. Hush.
Thos. G. Bowery, Ph.D., Asst. Entomologist


SUB-TROPICAL STATION, HOMESTEAD
Geo. D. Ruehle, Ph.D., Vice-Dir. in Charge
D. O. Wolfenbarger, Ph.D., Entomologist
Francis B. Lincoln, Ph.D., Horticulturist
Robert A. Conover, Ph.D., Plant Path.
John L Malcolm, Ph.D., Asso. Soils Chemist
R. W. Harkness, Ph.D., Asst. Chemist
R. Bruce Ledin, Ph.D., Asst. Hort.
J. C. Noonan, M.S., Asst. Hort.
M. H. Gallatin, B.S., Soil Conservationist

WEST CENTRAL FLORIDA STATION,
BROOKSVILLE
William Jackson, B.S.A., Animal Husband-
man in Charge 2

RANGE CATTLE STATION, ONA
W. G. Kirk, Ph.D., Vice-Director in Charge
E. M. Hodges, Ph.D., Agronomist
D. W. Jones, M.S., Asst. Soil Technologist

CENTRAL FLORIDA STATION, SANFORD
R. W. Ruprecht, Ph.D., Vice-Dir. in Charge
J. W. Wilson, Sc.D., Entomologist
P. J. Westgate, Ph.D., Asso. Hort.
Ben. F. Whitner, Jr., B.S.A., Asst. Hort.
Geo. Swank, Jr., Ph.D., Asst. Plant Path.

WEST FLORIDA STATION, JAY
C. E. Hutton, Ph.D., Vice-Director in Charge
H. W. Lundy, B.S.A., Associate Agronomist
W. R. Langford, Ph.D., Asst. Agron.

SUWANNEE VALLEY STATION,
LIVE OAK
G. E. Ritchey, M.S., Agronomist in Charge

GULF COAST STATION, BRADENTON
E. L. Spencer, Ph.D., Soils Chemist in Charge
E. G. Kelsheimer, Ph.D., Entomologist
David G. A. Kelbert, Asso. Horticulturist
Robert O. Magie, Ph.D., Plant Pathologist
J. M. Walter, Ph.D., Plant Pathologist
Donald S. Burgis, M.S.A., Asst. Hort.
C. M. Geraldson, Ph.D., Asst. Hort.
W. G. Cowperthwaite, Ph.D., Asst. Hort.


FIELD LABORATORIES
Watermelon, Grape, Pasture--Leesburg
C. C. Helms, Jr., B.S.. Asst. Agronomist 4
L. H. Stover, Asst. in Hort.

Strawberry-Plant City
A. N. Brooks, Ph.D., Plant Pathologist

Vegetables-Hastings
A. H. Eddins, Ph.D., Plant Path. in Charge
E. N. McCubbin, Ph.D., Horticulturist

Pecans-Monticello
A. M. Phillips, B.S., Asso. Entomologist2
John R. Large, M.S., Asso. Plant Path.

Frost Forecasting-Lakeland
Warren O. Johnson, B.S., Meteorologist 2

1 Head of Department
2 In cooperation with U. S.
3 Cooperative, other divisions, U. of F.
On leave.
























CONTENTS


Page


ANTHRACNOSE .....-... ......-......-- ....


W ILT .................-...............-- ....-


DOWNY MILDEW --......... ...- .....-.. -


GUMMY-STEM BLIGHT ...........-...----


STEM-END ROT ... ....... ... ......


SOUTHERN BLIGHT .......---...- ...---


BLOSSOM-END ROT ...... ..-. ........


POWDERY MILDEW ............-..-...... ..-.........


CERCOSPORA LEAF SPOT ....-....-.. ......


MACROSPORIUM LEAF SPOT ....-......----


SOIL ROT ..-----.............. -...----


SPECKLE ........... .-.. .- ...... .. ....


MOSAIC ----- ...--- ---- --------- .


MINOR DISEASES NOT FOUND IN FLORIDA....


COLD, WIND AND SAND INJURY ................


HANDLING ......


SUMMARY ......


.--------. ---. ------ ..... ...... 5


. ---..-.-...-. .....- ..... 14


.. ...-.........--........ 18


.................... .... ... 20


..- ..-....-. ........ 26


.......... 32


............................. 34


-..-.......... .. ..--- ....- ... 3 6


.-.-.....--.- .... 37


.................... .... 39


-..-......- -.......-...... 39


..-- .-....- ... -- 40


........... ....... --.... -- 40


-.--.--.... -- 43


.......-.......-.........- 43


.... 45


...........- ...- 46









Diseases of Watermelons

By G. K. PARRIS

INTRODUCTION
Watermelons are grown in commercial quantities in almost
every county in Florida but the bulk of the crop is produced in
comparatively few of them. Between 25,000 and 45,000 acres
are planted to the crop every year and approximately 9,000 to
12,000 carloads, about 15 percent of the watermelon crop of the
United States, are shipped. Returns to growers amount to from
$2,000,000 to 86,000,000 a year.
It is impossible to give an accurate estimate of the financial
loss incident to watermelon diseases, but they are often an im-
portant factor in determining the success or failure of the crop.
Under conditions favorable for their development certain of the
diseases, if not controlled, may destroy the crop entirely, and
few are the melon fields in which one or more diseases of water-
melons are not doing some damage.
The area planted to watermelons each year is dependent largely
on the financial success of the preceding melon crop. Although
there are a number of important factors besides diseases which
determine the success of the crop, it is the purpose of this bul-
letin to discuss only the diseases, to give descriptions of them,
and to outline means for their prevention and control.

ANTHRACNOSE
Anthracnose (Fig. 1), caused by the fungus Colletotrichum
lagenarium (Pass.) Ell. & Hals., is the most destructive disease
of watermelons in the United States and is found everywhere
that melons are grown, with the possible exception of California.
The seriousness of anthracnose on watermelons may be realized
from the annual losses of from 20 to 30 percent of the crop in
some states. It is especially severe in the Southern states. In
Florida it once caused more loss than all the other diseases of
watermelons combined. With the almost universal adoption of
pre-planting seed treatments and application of such spray ma-
terials as nabam and zineb, anthracnose causes appreciably less
loss today in Florida than five years ago. Melon anthracnose
attacks only members of the Cucurbitaceae, namely watermelons,
cucumber, cantaloupe, squash, pumpkin, etc.









Diseases of Watermelons

By G. K. PARRIS

INTRODUCTION
Watermelons are grown in commercial quantities in almost
every county in Florida but the bulk of the crop is produced in
comparatively few of them. Between 25,000 and 45,000 acres
are planted to the crop every year and approximately 9,000 to
12,000 carloads, about 15 percent of the watermelon crop of the
United States, are shipped. Returns to growers amount to from
$2,000,000 to 86,000,000 a year.
It is impossible to give an accurate estimate of the financial
loss incident to watermelon diseases, but they are often an im-
portant factor in determining the success or failure of the crop.
Under conditions favorable for their development certain of the
diseases, if not controlled, may destroy the crop entirely, and
few are the melon fields in which one or more diseases of water-
melons are not doing some damage.
The area planted to watermelons each year is dependent largely
on the financial success of the preceding melon crop. Although
there are a number of important factors besides diseases which
determine the success of the crop, it is the purpose of this bul-
letin to discuss only the diseases, to give descriptions of them,
and to outline means for their prevention and control.

ANTHRACNOSE
Anthracnose (Fig. 1), caused by the fungus Colletotrichum
lagenarium (Pass.) Ell. & Hals., is the most destructive disease
of watermelons in the United States and is found everywhere
that melons are grown, with the possible exception of California.
The seriousness of anthracnose on watermelons may be realized
from the annual losses of from 20 to 30 percent of the crop in
some states. It is especially severe in the Southern states. In
Florida it once caused more loss than all the other diseases of
watermelons combined. With the almost universal adoption of
pre-planting seed treatments and application of such spray ma-
terials as nabam and zineb, anthracnose causes appreciably less
loss today in Florida than five years ago. Melon anthracnose
attacks only members of the Cucurbitaceae, namely watermelons,
cucumber, cantaloupe, squash, pumpkin, etc.







Florida Agricultural Experiment Stations


The losses caused by this disease can be divided into those
occurring in the field and those occurring in transit. If it is
controlled in the field it will not occur in transit. Anthracnose
can cause defoliation of the plants, which is a direct cause of
melons failing to mature. Infected melons may be refused by
inspectors at loading time or rot while in transit to Northern
markets (Fig. 17). The spots or lesions on the melons caused
by the parasite are a common and severe form of the disease.
These lesions vary considerably in size, depth and number, and
a few small spots on a melon missed by inspectors may cause
decay and loss in transit. Anthracnose causes more transporta-
tion losses than any other disease.

DESCRIPTION
The anthracnose fungus can attack all parts of the water-
melon plant above ground in all stages of their development.
Usually the disease is first noticed on the oldest leaves of plants
that have attained some size (Fig. 2). The leaves show irregular
black spots (Fig. 3) which may enlarge until the whole leaf be-
comes black and shriveled. During periods of weather favor-
able to the disease an entire field may show these blackened
leaves in a few days, and is described by the grower as having


Fig. 2.-Watermelon leaves around the center of the hill killed by anthrac-
nose. Arrow indicates base of the plant.







Diseases of Watermelons


been "burned over" (Fig. 4). Heavy defoliation caused by at-
tacks of anthracnose results in the occurrence of many immature
melons that usually sunburn. Often, under less favorable con-
ditions for the disease, only the leaves at the center of the plant
are killed, leaving the stem and a portion of the runners bare.
During wet weather the spots on the leaves show small,


Fig. 3.-Anthracnose of watermelon showing typical lesions on leaf blade.







Florida Agricultural Experiment Stations


orange-pink, oily, "heaped up" masses of the spores of the
fungus; in dry weather the spore masses are greyish and less
conspicuous. These spores spread the fungus in wet weather. The
stems and runners also may be attacked and killed. The lesions
on the stems are rather long and narrow in the direction of the
stem, and in the early stages of infection appear as small, water-
soaked areas which later become sunken, reddish brown, tan,
and finally black in color. The spore masses also are found in
these lesions. Both leaf and stem lesions serve as sources of
infection for the melons that develop later.


















Fig. 4.-Watermelon plant showing most of the leaves killed by
anthracnose (Courtesy USDA).

When young melons are infected black spots appear on the
young fruits similar to those on the leaves and the melons can
become malformed. On older melons the disease causes small,
water-soaked spots with greasy yellowish center which can be
somewhat elevated, giving a bumpy appearance to the surface
of the melon. These raised areas later become sunken (Fig. 1)
and pitted and of a creamy to black color. They are covered
with the pink spore masses of the fungus (Fig. 5). The lesions
enlarge slowly, but finally may attain an inch or more in diameter
and 1/8 to 1/4 inch in depth, and by coalescence a large part of
the surface of the melon may be involved.
Melons that show this pimply, bumpy surface when loaded
rot in transit, both from the continued growth of the anthrac-







Diseases of Watermelons


nose fungus and from other rot-producing fungi that enter
through the anthracnose lesions.
The anthracnose fungus is carried from crop to crop as spores
on the surface of melon seeds and on other cultivated host plants
such as cantaloupe and cucumber. The ability of the fungus
to survive in Florida soil through the hot summer is not known.
The spores are easily and widely spread by wind, rain, cultural
implements, animals and laborers. It is often observed that an
outbreak of anthracnose occurs soon after the first picking, due
to laborers walking among wet and diseased vines.











0X. 4











Fig. 5.-Anthracnose of watermelon, slightly enlarged to show characteristic
lesions and masses of spores.

The cotyledons, or seed leaves, become infected from the spores
on the seed coat, and from there the anthracnose spreads to the
true leaves; these are the oldest leaves when the vines are run-
ning. Symptoms of anthracnose on the cotyledons are very
similar to those due to gummy-stem blight (Fig. 11) and the
grower often will confuse the two. Both diseases are seed-
borne. Under average conditions it takes 10 days from the
germination of the fungus spore and its infection of the plant
to the production of mature spores on the diseased spots. Dur-







Florida Agricultural Experiment Stations


Fig. 6.-Watermelon affected with "measles," cause unknown. This can be
confused with anthracnose.

ing dry, cool periods (below 60F.) this time may be longer but,
on the other hand, several wet, warm days may reduce it to six or
seven days. From these facts it can be understood why the
disease early in the season develops slowly and why, with the
advent of warmer weather, whole fields may become diseased in
less than a week if conditions are right.







Diseases of Watermelons


A disorder of melons outwardly resembling anthracnose but
not caused by a fungus has been found in Florida. It goes by
various names, such as "sand blister," "gum pocket," "pumpkin
bug injury," "measles," etc. The symptoms of the disorder seem
to be confined to the fruit and may appear on small as well as
on mature melons. The surface of the rind shows round,
slightly raised discolored spots or pimples (Fig. 6). In the
centers of these pimples may be a pale yellowish area; this area
resembles the spore masses of the anthracnose fungus. How-
ever, microscopic examination has shown that this yellowish
deposit is the waxy cuticle of the melon fruit. Almost every
melon in a field may show this trouble, which would not be likely
if anthracnose were the cause. Another feature in which this
disorder resembles anthracnose is that the spots may extend
into the rind of the melon 1/8 to 1 inch and appear as brownish
areas.
Melons showing "measles," which is used to describe it for
want of a better word, do not break down when held in storage.
If anthracnose were the cause then breakdown and rot would
be inevitable. The cause of the disorder is unknown. An in-
sufficiency of boron in the soil has been mentioned but no re-
search has been performed, so far, to substantiate such an idea.
The variety Black Diamond or Cannon Ball seems more affected
than others.
CONTROL OF ANTHRACNOSE
To control anthracnose it is imperative that the seed be
treated before planting. Treatment is simple and cheap. Use
one of the newer organic fungicides, semesan, spergon or arasan,
all of which under average conditions seem to do the job of
destroying the spores of the fungus present on the seed coat.
If the seeds are very heavily infested these chemicals are not
able to eradicate all spores. Research is under way to produce
better seed-treatment materials than these three.
A can with tight-fitting cover is half-filled with seed, the
fungicide added to the seed at the rate of 0.3 percent by weight,
that is, one-third of a pound of fungicide to every 100 pounds
of seed, the cover is placed on the can, and can and contents are
vigorously shaken until all seeds are well coated with the chemi-
cal. Shaking for two to five minutes will suffice. Treated seed
can be planted immediately or stored without harm to germina-
tion. If melon seeds are to be stored in Florida and their vitality







Florida Agricultural Experiment Stations


preserved they should be placed in cold storage. The combina-
tion of high temperature and high humidity quickly destroys the
power of germination. The presence of a fungicide on the seed
will have no effect, by itself, on the vitality of watermelon seed.
It is not recommended that growers sprout melon seeds that
have been treated with semesan, spergon or arasan, as possible
chemical injury may follow, due to an accumulation of the chemi-
cal at a toxic concentration. Therefore, growers who wish to
rid their melon seed of the anthracnose fungus and also to sprout
the seed should dip the seed before sprouting in 1: 1,000 cor-
rosive sublimate mercuricc chloride) solution for 10 minutes in
a wooden or crock container and then very thoroughly wash
them in running water or several changes of water. Corrosive
sublimate is a deadly poison to man, to animals and also to
melon seed, so be sure to use it with caution and make sure that
none of the chemical remains on the seed after washing. Do
not store the treated seed in old bags unless they, too, have been
dipped in corrosive sublimate solution and then washed. It has
been found that corrosive sublimate hardens the melon seed
coat, retards germination, and causes slower growth than nor-
mal. The longer the time that seeds treated with this material
are stored after treatment the greater seem to be the detri-
mental effects.
Of the three dry (dust) treatments semesan is the most poison-
ous, for its active ingredient is mercury. Arasan is the least
poisonous and spergon, while not called "non-poisonous," is
practically so. Where labor is employed to plant melon seed
and there is danger of transferring the chemical from their
hands to their mouth it might be best to avoid semesan and use
either spergon or arasan. From the standpoint of actual con-
trol of anthracnose all three give good results, but semesan-
treated melon seed has been found to germinate slightly better
than when other materials are used, which it is doubtful a grower
ever considers, since he plants five to eight seeds per hill.
Later development of anthracnose, originating from fields of
untreated seed or possibly from the soil of former melon plant-
ings, can be checked by applying the fungicides nabam or zineb
as a spray or zineb as a dust, at rates recommended by the
manufacturers. A spray sticks better than a dust, particularly
if a sticker is added to the fungicide. Several good stickers are
available on the market. Dusting of large plants by plane, fixed






Diseases of Watermelons


wing or helicopter, is rapid and efficient. Dusting young, and
therefore small, vines by plane is probably more costly than
necessary.
The time of first application of fungicide varies with the crop
and its growing conditions, presence or absence of disease, etc.
If a grower can watch his fields closely it is believed that the
fungicide need not be put on until the anthracnose has made
its initial appearance on the cotyledons or young leaves. With
the adoption of seed treatment by most Florida growers during
the last six years anthracnose does not appear in melon plant-
ings on new land as early as it did in former years. On old land
it will appear on the melon vines as soon as the soil, if infested,
is splashed onto the leaves. Subsequent fungicide applications
can be made at 7- to 10-day intervals as deemed necessary. A
disease forecasting service, similar to services available to apple,
cucumber, tomato and potato growers'in other parts of the nation,
lends itself to watermelon disease control in Florida. Such a
service would tell growers what diseases threatened the melon
crop and whether application of a fungicide is advised. The
cost to spray or dust one acre of watermelons ranges from $3.00
to $4.00. Most growers apply a fungicide at least once, some
twice, a few three or more times. Dusting or spraying for
aphids is always practiced, and the fungicide could be applied
simultaneously. The insecticide is applied when it is known
that the insect is present; this cannot be said of the fungicide.
The savings to growers in fungicide costs by intelligent timing
should pay for employing personnel to staff and direct such a
forecasting service.
Some growers still spray their melon vines with 4-4-50 bor-
deaux, but research has shown that this material, while an ex-
cellent fungicide, injuries the melon vine through hardening the
foliage and without doubt reduces the set. All of the copper
fungicides tried at Leesburg, copper compound A, COGS, copper-
zinc-lime, copper-lime and tri-basic copper sulfate, cause copper
burning of the melon leaf, which is expressed as a loss of green
color of the margin of the leaf. The green color is replaced by,
or transformed to, a bright yellGw. Neither nabam or zineb
causes this injury and, since they are good fungicides against
anthracnose, they are recommended over anything else hereto-
fore mentioned.
Much anthracnose infection can be avoided by keeping persons






Florida Agricultural Experiment Stations


out of the melon field when plants are wet, for the spores of the
fungus are easily spread by people walking through the vines.
The varieties Congo and Black Kleckley are resistant to an-
thracnose. The Congo is a fine shipping melon, while the Black
Kleckley, also resistant to wilt, is a good melon for home use.

WILT
The wilt disease of watermelons, caused by the soil-inhabiting
fungus Fusarium oxysporum (E.F.S.) f. niveum Snyder and
Hansen, has been recognized as a serious disease for many years.
Growers have found by experience that it is inadvisable to grow
susceptible varieties on the same ground without an 8- to 10-
year interval between crops. The disease is known in every
melon-growing area of the United States but does most damage
on the lighter, sandier soils. When a field is infested heavily
a total loss of the crop may result. Ordinarily, in commercial
practice where the same ground is rarely planted to watermelons
two years in succession the disease is not especially serious, but
in escaping the disease by rotation there is the additional expense
of changing to new land every year. In small home plantings, or
in new fields located near to older fields that have shown the
presence of the disease, wilt can cause heavy loss.
The fungus causing the wilt disease can live in the soil in the
absence of a watermelon crop for as long as 16 years on decaying
organic matter, though it gradually becomes less abundant if
melons are not planted. It is spread in a number of ways and
may be introduced into new areas on seed, the feet of men or
animals, by cultivation implements, drainage water, infested
manure, wind-blown spores, soil, or plant debris. It is a com-
mon practice to turn cattle into an abandoned field and this
usually spreads the disease to the surrounding territory. The
disease apparently is limited in its attack to watermelon, not
attacking the closely related cucumber, squash, cantaloupe and
muskmelon.

DESCRIPTION
The name wilt accurately describes the appearance of the
diseased plants, as wilting is the most noticeable symptom of
the disease (Fig. 7). Wilt can attack watermelon plants in any
stage of development, though the symptoms are most spectacular
after the plants have attained some size. The disease may occur






Diseases of Watermelons


Fig. 7.-Watermelon plants in the field killed by Fusarium wilt.

as a rot of germinating seed or as a damping-off of seedlings
after they are well out of the ground. This latter type of in-
jury is preceded by a loss of green color of the cotyledons and
small leaves, which soon droop and become completely dry and
withered after a day or so. The stems of such plants when
pulled are found to be girdled by a soft rot just at or below
the soil level. Often the stems appear to be water-soaked for
some distance above the rotted area. After the plants are killed
the fungus grows out over the surface and produces spores in
great numbers. Stunting is another type of injury caused by
the wilt organism on seedling plants not immediately killed.
Wilt is observed most commonly in older plants. Their leaves
wilt and shrivel, starting at the tips of the runners. Plants
may appear healthy in the morning, wilted by noon and healthy
next morning. This alternate wilting and recovery may con-
tinue for several days, but plants never recover permanently
after they are attacked and eventually they die. Melon plants
in the later stages of the disease often show a light colored
cottony growth, indicating the presence of the fungus which
bears large numbers of spores. The water-carrying vessels of
wilted plants are yellow or brown, sometimes reddish brown,
or a large part of the stem may be brown (Fig. 8).
The wilt fungus grows best at temperatures around 800F.,
which explains the appearance of the disease in the field late


AW&r~~Y-"~







Florida Agricultural Experiment Stations


Fig. 8.-Longitudinal sections of watermelon stems
showing darkened areas caused by Fusarium wilt. A
healthy plant is shown on right.







Diseases of Watermelons


in the season. Even under favorable conditions there appears
to be considerable variation in the susceptibility of different
strains and varieties of watermelons to the disease.

CONTROL OF WILT
Where wilt occurs, about the only sure way to avoid the dis-
ease is not to plant watermelons on land that has produced a
watermelon crop before. It is known that the fungus can live
for 16 years in the soil, but in general eight to 10 years between
melon crops will reduce the amount of the fungus in the soil
to a point where land can be used again for planting susceptible
varieties. There is always danger of wilt occurring on "new"
land where drainage water from a diseased field has flowed over
the new field, or where cattle have had access to both fields.
The most hopeful means of combating wilt is by using re-
sistant varieties. However, it pays to wait at least two years
between crops of watermelons on the same land when wilt-
resistant varieties are grown. Other organisms, mainly fungi
of the genera Rhizoctonia and Pythiuim, attack all varieties of
melons on this type of land and cause severe losses as damping-
off of young seedlings. The Watermelon and Grape Investiga-
tions Laboratory, Leesburg, developed the wilt-resistant varieties
Leesburg (1936) and Blacklee (1944), but neither found favor
with the Florida melon grower, who wishes an early melon such
as the wilt-susceptible Cannon Ball, Florida Giant, Texas Giant,
Black Diamond or Clara Lee, as it is variously called. In other
parts of the national the high quality Blacklee has been tried and
is well liked. A more recent development (1950) is the wilt-
resistant Ironsides variety, which is a joint product of the
Watermelon and Grape Investigations Laboratory, Leesburg, and
the USDA Regional Vegetable Breeding Laboratory, Charleston,
South Carolina. The Ironsides is much earlier than the Blacklee.
Other wilt-resistant varieties available are the Klondike, Miles,
Black Kleckley, Kleckley No. 6, Dixie Queen, Georgia Wilt-Re-
sistant, Wilt Resistant Garrison, Missouri Queen, and Wilt-
Resistant Dixie Queen (several strains of this variety exist, and
not all are equally resistant to wilt). Brownlee and Improved
Leesburg, developed at Leesburg but not released, are also wilt-
resistant. In 1948, 1949, and 1951 much wilt appeared in the
Florida plantings of Cannon Ball. Unless a wilt-resistant strain
of this variety can be obtained by breeding the variety is on its
way out or will not be grown to any extent until the wilt fungus is







Florida Agricultural Experiment Stations


reduced by rotation. Work to develop a wilt-resistant Cannon
Ball is the object of research at the Watermelon Laboratory and
elsewhere in the United States.

DOWNY MILDEW
Downy mildew, caused by the parasitic fungus Peronoplasmo-
para cubensis (B. & C.) Rostow, has been found in most Gulf and
Atlantic states and in a few of the interior ones that produce
watermelons. It is common year after year, is widespread in
Florida and now is a greater disease threat than anthracnose.
This is because anthracnose has been lessened by use of seed
treatments. The mildew fungus has been collected also on most
of the cucurbits, such as cucumber, cantaloupe, squash, pumpkin
and gourd. It is the worst disease of cucumbers in the State
and, since cucumbers are widely grown during most of the year,
there is an abundant source of inoculum for watermelons.
The disease usually does not become of serious consequence
until after the middle of the growing season. It attacks only
the leaf blades of the watermelon, where it causes lesions which
eventually kill the foliage. The spores are produced on the dis-
eased spots, most plentfully on the lower surfaces, and are easily
detached and spread by rain and wind. These fruiting struc.-
tures are difficult to detect except in the early morning before
the dew has disappeared. The spots caused by the downy mildew
fungus generally are mistaken for anthracnose.
DESCRIPTION
The first symptom of infection by the downy mildew fungus is
the appearance of a slightly yellowish spot on the blade of the
leaf. The yellowing becomes more pronounced and about two
days later the fruiting fungus can be observed. These yellow
spots are circular or oval in outline and blend gradually into the
green color of the leaf (Fig. 9). Later, however, the tissue is
killed and turns brown or blackish. The spots now are angular
in outline and are easy to confuse with anthracnose (Fig. 3)
and gummy-stem blight. They may be few or many on a single
leaf, depending on the abundance of primary infection, and may
coalesce when in close proximity and thus rapidly involve the
entire leaf blade which then turns black, dries and disintegrates.
They vary in size from 1/8 to 12 inch or more in diameter.
On severely attacked leaves the fungus has a characteristic
effect. The leaves curl inward towards the midrib, so that what







Diseases of Watermelons


was formerly the lower surface of the leaf is now almost com-
pletely uppermost as it curves. Such leaves tend to stand erect
rather than be semi-procumbent or flattened. In general appear-
ance they resemble a human hand and wrist, whose fingers are
bent inwards, but not closed, as if holding a large ball. No other
melon leaf disease produces these symptoms.


V.'


Fig. 9.-Spotting of watermelon leaf caused by downy mildew.







Florida Agricultural Experiment Stations


Downy mildew can produce the same effect in a field of almost
mature watermelons as can anthracnose, except that it takes
place late in April through May. Almost overnight the foliage
is blasted and dy-
ing; by micro-
scopic examina-
tion the causal
organism is de-
termined by its
spores and coni-
diophores (Fig.
\ 10).
Much leaf de-
struction thought
Sto be caused by
anthracnose in
Florida is due to
downy mildew. If
lesions are found
on the stem of the
melon plants at
the same time as
they appear on
the leaves, then it
Fig. 10.-Branched conidiophore of downy mildew m a y safely be
fungus and spores that grow upon its branches said that downy
(highly magnified). mildew is not im-
plicated; not always does anthracnose attack the stems of the
melon vine, however, and it is easy to confuse with gummy-stem
blight.
CONTROL OF DOWNY MILDEW
Downy mildew can be controlled by using the same materials
recommended to control anthracnose, namely nabam or zineb,
but tri-basic copper sulfate also is excellent. Generally, both
diseases have to be controlled, so it seems wise to use materials
which will control both, and these materials are nabam and
zineb. As with anthracnose, a disease forecasting service for
melon growers would be desirable for downy mildew control.
GUMMY-STEM BLIGHT
Gummy-stem blight, caused by the fungus Mycosphaerella
melonis (Pass.) Chiu and J. C. Walker, is widely distributed







Diseases of Watermelons


over the eastern and central United States and has been reported
as far west as Arizona. The disease can cause considerable
losses by killing the emerging seedling and through defoliation
and killing of runners when weather conditions are favorable.
Though prevalent almost every year in Florida, the disease is
ordinarily of secondary importance. Its wide distribution can
be accounted for by its ability to live for several years on seeds
and on decaying plant material in the soil from which it attacks
cotyledons (Fig. 11), leaves and runners as they develop. Later,
on the attacked parts, the fungus forms great quantities of spores
which serve for its further spread by wind and water or other
agencies such as cultural implements and animals.

DESCRIPTION OF GUMMY-STEM BLIGHT
The circular, sunken, blackish and wrinkled spots on the
cotyledons (Fig. 11) and primary stems of young plants are




r F












Fig. 11.-Spots of gummy-stem blight on cotyledons of watermelon plants.

often the first symptoms of gummy-stem blight. These infec-
tions usually originate from seed-borne spores. As mentioned
previously, symptoms of gummy-stem on the cotyledons are
identical with those of anthracnose, except for the pycnidia or
fruiting structures containing spores which are produced in
abundance on spots caused by gummy-stem. If these spots are
located on the portion of the cotyledon close to the petiole the
whole blade can be killed (Fig. 12), and the axillary bud or the
whole growing tip may be destroyed by the enlargement of the









Florida Agricultural Experiment Stations


Fig. 12,-Gummy-stem disease on blade and petiole of watermelon leaf.


.e~4~FR


rJIB..
4
i~ FY~






Diseases of Watermelons 23


FV
W1-
''^'***o r
i1Ffl*LY. .)- _


Fig. 13.-Watermelon leaf showing inter-mixed spots caused by Cerco-
spora citrullina, Peronoplasmopara cubensis, Macrosporium cucumerinum,
Colletotrichum lagenarium, and Mycosphaerella melonis.







Florida Agricultural Experiment Stations


primary lesion. The first true leaves are infected by spores from
the spots on the cotyledons and girdling and death of the grow-
ing tip can take place in the same manner as described for the
cotyledon infections. The disease can also attack and girdle
the stem below the cotyledons and on old plants can produce a
wilt not unlike that caused by Fusarium wilt. Gummy-stem
blight spreads from the center of the hill outward just as an-
thracnose does (Figs. 2 and 4) as the season advances, attack-
ing leaves and stems. Sometimes more than one disease can
occur simultaneously and it is not uncommon to find gummy-
stem blight, anthracnose and downy mildew on the same leaf
(Fig. 13), or gummy-stem and anthracnose on the same stem.


Fig. 14.-Gummy-stem blight affecting crown of melon plant. The point of
attack is indicated by point of knife.







Diseases of Watermelons


On the leaves the disease is characterized by reddish brown
spots, in the centers of which the minute, at first straw colored,
later brown and finally black fruiting bodies, the pycnidia of
the fungus, develop. On the stems the disease appears as
elongated, water-soaked areas that later become light brown to
whitish (Fig. 14). The fruiting bodies, in which are formed
great quantities of spores, appear abundantly in these cankered
areas, sometimes in numbers sufficient to give the lesion a black
appearance. The cankers often crack open and exude a reddish-
amber gum and this characteristic has given the disease its
name. But anthracnose likewise will cause exudation of a simi-
lar gum. A hand lens or microscope is needed to distinguish
between the two. If small brown to black dots, the pycnidia,
are present on the lesions it can be said with sureness that the
disease is gummy-stem blight.
The following procedure is recommended for positive micro-
scopic identification of gummy-stem blight. Cut small pieces of
diseased tissue from affected leaves or stems. Place the pieces
in a drop of water on a glass slide; do not cover with a cover
slip and leave for a few minutes, then place under the low power
of the microscope. If the disease is gummy-stem blight the
spores of the fungus will be seen issuing from the small-necked
ostioles or openings of the flask-shaped pycnidia. The spores
adhere together and are forced out of the necks of the pycnidia
as these absorb water, like toothpaste from a tube. As this
mass, five to eight spores "thick," is squeezed out it forms shapes
that resemble coils, tendrils, or spirals. Later, the adhering
spores separate and float free in the water.
The anthracnose fungus produces its spores in flat, open, dish-
like structures called acervuli. The spores may appear in water
in clumps but never in these coils or tendrils or spiral shapes.
The spores of the gummy-stem blight fungus may be one- or
two-celled, mostly one-celled, and are very similar to the one-
celled spores of the anthracnose fungus. The lesions on the
stems or runners persist until the end of the season if the plant
is not killed and late in the season they may become continuous
and the stem and all runners appear bleached, often cracked
(Fig. 15) and sometimes blackened with the dried gum and the
masses of fruiting bodies of the fungus. In some instances the
cankered stem enlarges to two or three times its normal size,
as shown in Fig. 15.






Florida Agricultural Experiment Stations


Fig. 15.-Typical lesions on watermelon stem caused by gummy-stem blight.

The gummy-stem blight fungus also occurs on the melon
fruit, though to no great extent.

CONTROL OF GUMMY-STEM BLIGHT
Seed treatment, as given for anthracnose control, will do much
to reduce losses from this disease. But control is imperfect, as
1948, 1949 and 1951 field observations showed. Semesan, sper-
gon and arasan reduce gummy-stem blight considerably and are
recommended until something better is developed.
Spraying with nabam or zineb, or dusting with zineb, controls
this disease on the growing plants. Copper fungicides will not
control the disease as well as nabam or zineb.

STEM-END ROT
Stem-end rot of watermelons is primarily a transit disease.
It is caused by the fungus Physalospora rhodina (Berk. & Curt.)
Cooke. This organism also causes diseases of sweet potato, ear
and stalk rot of corn, boll rot of cotton and gumming of citrus.
It is widely distributed throughout the tropics, growing as a
wound parasite or secondary organism. The imperfect stage
lies in the genus Diplodia, members of which produce spores called
pycnidiospores because they are borne in a pycnidium. These
spores often are indistinguishable from one another (Fig. 16).
The disease is common and destructive in most of the water-







Diseases of Watermelons


Fig. 16.-Photomicrographs of pycnidiospores of Diplodia spp., the im-
perfect stage of Physalospora rhodina that causes stem-end rot of water-
melon. A, Diplodia frummenti; B, Diplodia gossypina; C, Diplodia tubericola.

melon-growing regions of the United States and is especially
prevalent in melons from the Southern states, because of the long
time the fruit is in transit to Northern markets. In Florida
the disease is almost wholly a transportation problem. The
melons are loaded at the local shipping points in a healthy, sound
condition as far as can be determined and when they arrive at
their destination, from four to eight days later, decay is evident,
usually about 10 percent (Fig. 17).
The exact source of the inoculum is not definitely known but
since it has been proved that the organisms affecting citrus,
sweet potatoes, corn and cotton will also cause the disease, it
can be readily understood where some of the fungus spores
originate. Records of collections in Florida show that Diplodia
spp., producing spores very much like those found on diseased
watermelons, have been found infecting more than 40 different
kinds of plants.
The disease can cause serious loss to watermelon shippers in
all parts of this State during the shipping season. Reports of
inspectors at destination have shown as high as 40 percent stem-
end rot. In every instance where more than 12 to 15 percent
decay occurred the stems had not been treated. The average of
several hundred reports was 8 to 10 percent. Such losses are
entirely too high when the disease can be so easily controlled.
The uninjured melon on the vine is resistant to the disease.
When it is injured or cut from the vine the unprotected areas






Florida Agricultural Experiment Stations


are exposed to infection. Where the skin is broken or the stem
is cut a watery exudate appears and persists for considerable
time. This makes a very suitable medium for the germination
of the spores and growth of the fungus, which later advances
into the melon and causes decay. The percent of decayed melons
increases with the length of exposure of these injured places.
Melons should be handled carefully and removed from the field
to loading points as soon as possible. The spores germinate
quickly and in less than 12 hours may infect the cut stem and


Fig 17.-Interior of railroad car at destination after sound melons were
removed. Loss caused principally by stem-end rot and anthracnose.







Diseases of Watermelons


later grow into the fruit, causing the rot typical of the disease.
Development of the disease in transit is favored by high temper-
atures, high humidity and lack of ventilation in the cars. All
commercial varieties of melons are subject to the disease to
about the same extent.








V








Fig. 18.-Stem-end rot of watermelon caused by Physalospora rhodina.


DESCRIPTION OF STEM-END ROT
The first symptom of the disease is the browning and shrivel-
ing of the stem, which may be observed before the melon itself
shows infection. The first decay of the fruit takes place around
the stem, where the rind becomes slightly darkened and then
water-soaked, later followed by a light tan or brown color (Fig.
18). After the decay first shows on the rind the advance of the
line of demarcation between invaded and sound tissue is rapid
and can be observed readily from day to day. After 1/ or more
of the melon has been invaded the stem-end portion becomes
dark colored and wrinkled and gradually shrinks as evaporation
continues (Fig. 19). The outer rind becomes overgrown with
a dense greenish or grayish-black mold, or under certain condi-
tions the mold may be light gray whitish, speckled with black.
The spores of the fungus are produced in abundance on the
older decayed areas. An indication of decay often can be de-
tected before any discoloration appears by the softness of the
rind around the stem end as determined by light pressure of
the thumb.







Florida Agricultural Experiment Stations


CONTROL OF STEM-END ROT
Control of this disease should begin in the field at harvest time,
and instructions should be given labor to guard against injury
to the fruit by rough and careless handling. The stems should
be clipped close to the vine rather than close to the melon, so
as to permit a second clipping after the melons are loaded in
the railroad car. The truck bodies used to collect melons in
the field should be free from gravel, sand, splinters, nails, etc.,
and should be lined with burlap. Laborers should not be per-
mitted to ride on top of a load of melons. The melons should
be loaded carefully a tier at a time in alternate ends of the car
so that the stems can be clipped a second time and the cut ends
disinfected with copper sulfate paste in one end of the car while
untreated melons
are being loaded in
the opposite end.
The stems can be
cut off with an or-
dinary knife (Fig.
20) ,but thin-bladed,
sharp clippers are
preferred. Imme-
diately following
clipping the stem
should be painted.
The first cutting in
the field should have
left the stems about
two to three inches
long and when the
melons are loaded
the stems should be
cut to about an inch
long. A reliable
workman should do
Fig. 19.-Stem-end view of watermelon showing t h i s job, making
development of stem-end rot.
certain that there
are no bruises on the stem when it is recut. If bruised, the
stem should be recut closer to the melon. The clipped off stems
should be collected and removed from the car. An additional
precaution would be an occasional disinfection of the clippers.







Diseases of Watermelons 31

The paste used to paint the clipped stems should be about as
thick as ordinary paint. It is easily applied with a long-handled
paint brush (Fig. 21) from a container that will not easily tip.
The disinfecting paste can be bought ready made or can be
made up according to the following formula: Place 31/2 quarts
of water in an enamelware kettle and add 8 ounces of copper
sulfate or bluestone and bring to a boil. As soon as the blue-
stone is dissolved and while boiling, add a starch mixture made
by mixing 4 ounces of either laundry or corn starch with a
pint of cold water, stirring until a milky mixture is obtained.
Continue the boiling and stirring until the mixture thickens
to an even paste, but the boiling should last only one or two
minutes after the starch is added, as the paste must not be too
thick to be easily applied with a brush. This paste serves as a

Fig. 20.-Clipping stems of tier of watermelons after loading in railroad car
and preparatory to treating to prevent stem-end rot.







32 Florida Agricultural Experiment Stations

disinfectant and kills the germinating spores, if they are present,
and treated melons arrive in the market without stem-end decay.

SOUTHERN BLIGHT
Southern blight, caused by the fungus Corticium rolfsii (Sacc.)
Curzi, is common in the southeastern and south central United
States, the northern limit approximating the Ohio River. This
area also coincides with the great watermelon producing area of
this country. This fungus has been found on more than 180
species of plants. Attacks of C. rolfsii on various hosts result in
wilting and dying of the plant, because of the girdling of the
main stem at or near the soil line. On watermelons this disease
has not been of much importance except in occasional fields
where melons followed a crop of sweet potatoes or peanuts
Fig. 21.-Painting the clipped stem ends with copper sulfate paste for the
prevention of stem-end rot.






Diseases of Watermelons


which are highly susceptible. The fungus lives in the soil from
season to season as sclerotia, which are hard, brown, round
structures of fungus growth about the size of mustard seec
(Fig. 22). These sclerotia de-
velop in large numbers on dy-
ing plant parts, from which
they are scattered by wind,
water and cultivation, and re-
main in the soil until the next
crop is developing. When
warm, wet periods prevail the Fi
sclerotia germinate and pro-
duce the fungus mycelia which "
attack the stems of the plants ''
in their immediate vicinity.
After the stems are girdled and ,.
killed numerous sclerotia are
again produced.

DESCRIPTION OF SOUTHERN
BLIGHT
When the fungus has attack-
ed the main stem of the melon
plant at the soil line the first
indication of disease to the
casual observer is a slight lack m r i o
Fig. 22.-Corticium rolfsii on a
of turgidity or wilting of some girdled and killed watermelon plant
of the younger leaves near the (after 72 hours in moist chamber).
growing tips of the runners. The disease seldom has been found
attacking melon vines before runners have started. This wilt-
ing gradually becomes more pronounced and involves more and
more of the plant until complete collapse takes place and the
plant quickly dies. Often the fungus will attack and kill only a
single runner at some distance from the main root.
The fungus also attacks the melons themselves (Fig. 23)
where the lower surface is in contact with the soil. The fungus
hyphae penetrate the rind, causing it to turn brown. The watery
contents of the cells in this area escape through the rind and
collect on the surface in colorless droplets. The fungus rapidly
overgrows the attacked areas, producing an abundance of white
mycelium until the melon collapses, shrivels and dries, leaving
the innumerable newly-developed sclerotia.






Florida Agricultural Experiment Stations


CONTROL OF SOUTHERN BLIGHT
Rotation and sanitation are of most importance. All infected
melons should be removed from the field or buried at least a
foot deep.


Fig. 23.-Young melon killed by Southern Blight. The fungus is fruiting
on one side of it.

BLOSSOM-END ROT

Blossom-end rot is the name applied to a group of abnormali-
ties, either parasitic or physiological, which affect the end of
the melon opposite the stem. All the causes have not been
definitely decided upon by investigators. Some attribute the
cause to deranged nutrition, closely associated with moisture
supply and high temperatures. Others contend that parasitic
organisms, principally Pythium debaryanum Hesse and P.
aphanidermatum (Eds.) Fitz., are the cause of the decay, or at
least start the infection (Fig. 24). A large number of organ-
isms, both fungi and bacteria, either saprophytic or parasitic,
have been isolated from diseased melons. Pythium debaryanum
has been definitely determined to be the cause of blossom-end rot
of citrons. Other organisms found associated with blossom-end
rot are Mycosphaerella melonis (Pass.) Chiu and J. C. Walker






Diseases of Watermelons


(gummy-stem blight), Colletotrichum lagenarium (Pass.) E. &
H. anthracnosee), Cladosporium sp., Fusarium sp., Alternaria
sp. and Diplodia spp.
Blossom-end rot is common in melon fields throughout the
country, especially in the South. In Florida it can be found in
almost any field after the first fruits set. The disease has been


Fig. 24.-Blossom-end rot of melon caused by the gummy-stem blight
organism.

found on fruits of all sizes and ages, but appears to be most
common on melons approximately 1/4 grown, or melons measur-
ing 6 to 10 inches by 4 to 6 inches. The affected ones are con-
spicuous at this stage of development because of the contrasting
black color of the diseased area and because a large percent of
the melons are also crook-necked or malformed. Whether the
"crooks" are a result of invasion by a parasite or of mechanical
injury which permits the entrance of fungi is a matter of opinion.

DESCRIPTION
The first indication of blossom-end rot is the water-soaked-
appearing tissue at the blossom end, which gradually becomes
darker in color. As the spot enlarges and involves more tissue
the center changes from green to a tannish brown and becomes






Florida Agricultural Experiment Stations


soft. It is at this stage of development of the spot, which may
be one to three inches in diameter, that secondary invasion is
most common and the spots become almost black (Fig. 24).
Often there is no secondary invasion and the whole melon be-
comes involved in the decay. Other instances occur where the
progress of the rot is arrested and only a portion of the fruit
decays, shrinks and becomes wrinkled and black, while the re-
mainder of the melon is of normal green color.

CONTROL OF BLOSSOM-END ROT
No control methods are known, although the "crooks" and
malformed fruits should be removed from the vines and field to
prevent the development of parasitic organisms on them which
may possibly infect other fruits and portions of the vine. These
malformed melons have no sales value and are a drain on the
vine which could be producing normal fruits.

POWDERY MILDEW
The fungus Erysiphe cichoracearum DC. has been found at-
tacking watermelon leaves and fruits, causing the disease known
as powdery mildew. This disease has been reported from vari-
ous foreign countries and the watermelon-growing sections of
the United States. In Florida it is common on melon leaves but
seldom is serious, as it develops most rapidly in warm, moist
weather which occurs usually toward the end of the growing
season. Powdery mildew is serious in Florida on cantaloupe,
cucumber (Fig. 25), squash, gourd and pumpkin. The early
symptoms appear on both leaf surfaces as small, superficial,
circular spots of a white, powdery nature which is the vegetative
growth of the fungus. The spots enlarge and the white coating
becomes more dense as spores are produced in increasingly large
numbers on both surfaces of the leaves, but are more plentiful
on the upper surface; they serve to spread the mildew. The
fungus seldom kills the leave quickly but overgrows them with-
out causing much loss of turgidity until they appear grey or
whitish. When this stage is reached brownish spots begin to
develop, which gradually involve the whole leaf, causing it to
dry and die.
CONTROL
Sulfur dust is specific for control of the powdery mildews
but sulfur can injure cucurbits. If the spray schedule recom-







Diseases of Watermelons


mended for the
control of other
melon diseases
will not control
the powdery mil-
dew, and losses
are certain if it
is not controlled,
then sulfur prob-
ably would be
beneficial, e v e n
though foliar in-
jury might fol-
low.

CERCOSPORA
LEAF SPOT
This charac-
teristic spot of
watermelon
leaves is caused
by Cer)cospora
citrullina Cke.
The disease has
been reported
from most of
the watermelon-
growing sections
of the United
States. In Flor-
ida it has been
found widespread
but is not com-
mon or of eco-
nomic importance
until toward the
Fig. 25.-Powdery mildew on cucumbers; early
end of the season, state in upper photo; lower photo shows production
The disease us- cf large numbers of spores. Both pictures are of
ually is first ob- upper surfaces of leaf.
served on the leaves around the center of the hill but as the
season advances it involves all of the leaves toward the growing
tip of the runners. The spotted leaves turn yellow, wilt and die.






Florida Agricultural Experiment Stations


110,
V
j* <. --..^




C-" .' *








*:; Ib. .*












DESCRIPTION OF CERCOSPORA LEAF SPOT
The first symptom of the disease is the appearance of small,
water-soaked spots scattered over the surface of the leaf. These
spots enlarge up to several millimeters in diameter, become
sunken and dry out in the center (Fig. 26). The tissue in the
center of the spots changes from green to light tan and finally
to a dull white. The whitish centers are separated from the
green tissue by a dark brown to black ring which is very char-
Fig. 26.--Cercospora leaf spot of watermelon.

DESCRIPTION OF CERCOSPORA LEAF SPOT
The first symptom of the disease is the appearance of small,
water-soaked spots scattered over the surface of the leaf. These
spots enlarge up to several millimeters in diameter, become
sunken and dry out in the center (Fig. 26). The tissue in the
center of the spots changes from green to light tan and finally
to a dull white. The whitish centers are separated from the
green tissue by a dark brown to black ring which is very char-







Diseases of Watermelons


acteristic. Often these spots are so close together that they
coalesce, forming larger brown areas that may include several
of the individual spots. The spots are often invaded by other
parasitic fungi, such as Colletotrichum and Alternaria, which
hasten the death of the leaf. The fungus may be detected with
a hand lens on either surface of the leaf, but is usually more con-
spicuous on the lower surface where the spores occur in the form
of dark, hairy specks. They are long and thin and are easily
detached and distributed by wind or water.

CONTROL
Cercospora leaf spot is not important enough to warrant
specific control measures. The fungicides recommended for the
control of other leaf diseases will control this spot.

MACROSPORIUM LEAF SPOT
A leaf spot caused by Macrosporium cucumerinum E. & E.
is a very widely distributed disease of watermelons and related
plants, especially cantaloupe. It usually causes little damage
and is seldom observed, except during warm, wet seasons which
favor development of the fungus. The disease attacks the leaves
around the center of the hill and causes brown, concentrically
ringed spots to develop, on which the spores are borne in black
tufts. These spots may enlarge and, when numerous, they
coalesce to involve the whole leaf and cause it to shrivel and die.
Occasionally in warm, moist weather the disease may cause con-
siderable defoliation. The disease is often confused with an-
thracnose but the lighter colored and concentrically zoned spots
serve to distinguish it from anthracnose spots, which are black.
The application of fungicides recommended for anthracnose
also serves to reduce the amount of leaf spot present.

SOIL ROT
Soil rot, caused by Corticium vagum B. & C., has not been of
much importance in Florida or the United States as a whole
up to the present. Occasionally a watermelon will show lesions
on the side in contact with the soil but these usually are small
and insignificant. The spots at first appear almost superficial
and are slightly tan colored in contrast to the surrounding green.
In the more advanced stages the spots are sunken and the brown
color may cover an area several inches in diameter. This dis-







Diseases of Watermelons


acteristic. Often these spots are so close together that they
coalesce, forming larger brown areas that may include several
of the individual spots. The spots are often invaded by other
parasitic fungi, such as Colletotrichum and Alternaria, which
hasten the death of the leaf. The fungus may be detected with
a hand lens on either surface of the leaf, but is usually more con-
spicuous on the lower surface where the spores occur in the form
of dark, hairy specks. They are long and thin and are easily
detached and distributed by wind or water.

CONTROL
Cercospora leaf spot is not important enough to warrant
specific control measures. The fungicides recommended for the
control of other leaf diseases will control this spot.

MACROSPORIUM LEAF SPOT
A leaf spot caused by Macrosporium cucumerinum E. & E.
is a very widely distributed disease of watermelons and related
plants, especially cantaloupe. It usually causes little damage
and is seldom observed, except during warm, wet seasons which
favor development of the fungus. The disease attacks the leaves
around the center of the hill and causes brown, concentrically
ringed spots to develop, on which the spores are borne in black
tufts. These spots may enlarge and, when numerous, they
coalesce to involve the whole leaf and cause it to shrivel and die.
Occasionally in warm, moist weather the disease may cause con-
siderable defoliation. The disease is often confused with an-
thracnose but the lighter colored and concentrically zoned spots
serve to distinguish it from anthracnose spots, which are black.
The application of fungicides recommended for anthracnose
also serves to reduce the amount of leaf spot present.

SOIL ROT
Soil rot, caused by Corticium vagum B. & C., has not been of
much importance in Florida or the United States as a whole
up to the present. Occasionally a watermelon will show lesions
on the side in contact with the soil but these usually are small
and insignificant. The spots at first appear almost superficial
and are slightly tan colored in contrast to the surrounding green.
In the more advanced stages the spots are sunken and the brown
color may cover an area several inches in diameter. This dis-






Florida Agricultural Experiment Stations


ease, most troublesome during wet seasons or when the melons
rest on wet soil, may occur in any field. The causal organism
is present in practically all Florida soils and is capable of caus-
ing infection whenever favorable conditions develop.


Fig. 27.-Speckle of watermelon (cause unknown).

SPECKLE
Speckle is an abnormal condition of the rind of watermelons,
the cause of which is not known. The disease is characterized
by the appearance of numerous yellow or whitish spots scattered
over the surface of the melon, principally on the upper half
(Fig. 27). The spots usually are more or less circular in shape
and vary in size from those barely visible to those almost a
centimeter in diameter. The surface of the melon is smooth
and the epidermis appears to be normal except in color. The
speckled melons show no effects other than their poor appear-
ance. The condition apparently is not parasitic in nature and
is due to some abnormality in the plant or its environment.

MOSAIC
Watermelon mosaic was first observed in Florida in Polk
County, near Fort Meade, in April 1932. This is the first record
of its natural occurrence in the United States. The disease,
while serious on other cucurbits, causes little damage to water-






Florida Agricultural Experiment Stations


ease, most troublesome during wet seasons or when the melons
rest on wet soil, may occur in any field. The causal organism
is present in practically all Florida soils and is capable of caus-
ing infection whenever favorable conditions develop.


Fig. 27.-Speckle of watermelon (cause unknown).

SPECKLE
Speckle is an abnormal condition of the rind of watermelons,
the cause of which is not known. The disease is characterized
by the appearance of numerous yellow or whitish spots scattered
over the surface of the melon, principally on the upper half
(Fig. 27). The spots usually are more or less circular in shape
and vary in size from those barely visible to those almost a
centimeter in diameter. The surface of the melon is smooth
and the epidermis appears to be normal except in color. The
speckled melons show no effects other than their poor appear-
ance. The condition apparently is not parasitic in nature and
is due to some abnormality in the plant or its environment.

MOSAIC
Watermelon mosaic was first observed in Florida in Polk
County, near Fort Meade, in April 1932. This is the first record
of its natural occurrence in the United States. The disease,
while serious on other cucurbits, causes little damage to water-







Diseases of Watermelons


Fig. 28.-Effect of mosaic on watermelon leaves.

melon or citron when they are artificially inoculated. In 1950
a mosaic caused appreciable losses to watermelons in the Im-
mokalee, Felda and Webster areas of the state.

DESCRIPTION
In the 1932 outbreak diseased vines were conspicuous because
the tips of the runners and a proliferation of shoots from around


Fig. 29.-Mottling of surface of watermelon due to mosaic.


~~rC"'~ '1
""""""""""""""""""""
,,
9~


ori






Florida Agricultural Experiment Stations


Fig. 30.-Misshapen melon diseased with mosaic.


the crowns protruded stiffly above the general level of the vines,
giving a "petunia-like" appearance. The tips and shoots had
shorter than normal internodes which produced a crowding of
the young leaves which were stunted and rolled. Mottling some-
times was present, marked or diffuse (Fig. 28), and consisted
of irregular yellow areas only slightly lighter than the normal
green of the young melon leaves. Flowers on severely infected
plants showed abnormalities of petals which were of unequal
size and green. Stamens were distorted and unevenly developed
and the flowers dropped prematurely. Melons produced on dis-
eased vines were mottled (Fig. 29) and also sometimes mis-
shapen (Fig. 30). Diseased vines bore few melons.
In the 1950 outbreak certain differences from the preceding
symptoms were noted. There was no "petunia-like" effect pro-
duced on mosaiced plants and the leaves and stems on diseased
plants were excessively hairy or pubescent. Further, the flowers
on these diseased plants did not appear abnormal. When small
watermelon plants became diseased further growth was checked
and the plants bore no fruits. Fruits almost mature, on recently
diseased plants, showed no symptoms but young fruits were
mottled and even distorted.
The 1932 disease was suspected of being caused by a virus and
the 1950 outbreak was proved to be caused by this type of organ-
ism. There probably are several forms of virus or mosaics that
attack watermelons in Florida. The 1950 form was transmitted
by Aphis gossypii Glover, a common aphid in melon fields.






Diseases of Watermelons


Control of the aphid carrier is important if the disease ap-
pears. This insect is well known for its ability to stunt and
injure watermelons, even in the absence of a virus. Nicotine
sulfate (Black Leaf 40), lindane, parathion, or TEPP (tetra-
ethyl pyrophosphate) will control it; use at strengths recom-
mended by the manufacturers.

MINOR DISEASES NOT FOUND IN FLORIDA
Bacterial wilt, caused by Erwinia tracheiphila (Erw. Smith)
Holland, has been reported on watermelons but few times and
watermelons are considered as practically immune to the dis-
ease, although other cucurbits are susceptible in the following
order: squash, cantaloupe and cucumber.
Bacterial leaf spot, caused by Xanthomonas cucurbitae (Bryan)
Dowson, has been found on squash in South Carolina and Geor-
gia but there are no authentic reports of its natural occurrence
on watermelons, although artificial inoculations have shown
them to be susceptible.
Black root rot, caused by Thielavia basicola (B. & D.) Zopf.,
has been reported on watermelons in the northwestern part of
the United States on only two occasions. Rather serious losses
were reported in one of these cases, however.
Watery soft rot, caused by Sclerotinia sclerotiorum,, (Lib.)
Mass., has been reported as causing damage to melons in transit.
Other parasites reported on watermelons are Phytophthora
citrophthora (Sm. and Sm.) Leonian, Corticium centrifugum
(Lev.) Bres., and Pestalotia torulosa B. & C., and the virus-
produced curly top disease of sugar beets.

COLD, WIND AND SAND INJURY
During the early part of the watermelon-growing st-ason in
Florida, January through February, the young plants often are
exposed to temperatures considerably below the optimum for
growth and development of the crop. Freezing results in im-
mediate death of plants but temperatures below 40F. which
do not kill can cause severe damage by stunting young plants to
such an extent that further growth is slow. Plants turn yellow,
oftentimes with the tips and margins of the cotyledons browned
and shriveled. This browning, however, is of a lighter color
than that caused by the attacks of various fungi and can be
easily distinguished on most specimens. The older leaves, which






Diseases of Watermelons


Control of the aphid carrier is important if the disease ap-
pears. This insect is well known for its ability to stunt and
injure watermelons, even in the absence of a virus. Nicotine
sulfate (Black Leaf 40), lindane, parathion, or TEPP (tetra-
ethyl pyrophosphate) will control it; use at strengths recom-
mended by the manufacturers.

MINOR DISEASES NOT FOUND IN FLORIDA
Bacterial wilt, caused by Erwinia tracheiphila (Erw. Smith)
Holland, has been reported on watermelons but few times and
watermelons are considered as practically immune to the dis-
ease, although other cucurbits are susceptible in the following
order: squash, cantaloupe and cucumber.
Bacterial leaf spot, caused by Xanthomonas cucurbitae (Bryan)
Dowson, has been found on squash in South Carolina and Geor-
gia but there are no authentic reports of its natural occurrence
on watermelons, although artificial inoculations have shown
them to be susceptible.
Black root rot, caused by Thielavia basicola (B. & D.) Zopf.,
has been reported on watermelons in the northwestern part of
the United States on only two occasions. Rather serious losses
were reported in one of these cases, however.
Watery soft rot, caused by Sclerotinia sclerotiorum,, (Lib.)
Mass., has been reported as causing damage to melons in transit.
Other parasites reported on watermelons are Phytophthora
citrophthora (Sm. and Sm.) Leonian, Corticium centrifugum
(Lev.) Bres., and Pestalotia torulosa B. & C., and the virus-
produced curly top disease of sugar beets.

COLD, WIND AND SAND INJURY
During the early part of the watermelon-growing st-ason in
Florida, January through February, the young plants often are
exposed to temperatures considerably below the optimum for
growth and development of the crop. Freezing results in im-
mediate death of plants but temperatures below 40F. which
do not kill can cause severe damage by stunting young plants to
such an extent that further growth is slow. Plants turn yellow,
oftentimes with the tips and margins of the cotyledons browned
and shriveled. This browning, however, is of a lighter color
than that caused by the attacks of various fungi and can be
easily distinguished on most specimens. The older leaves, which







Florida Agricultural Experiment Stations


Fig. 31.-Cold injury to watermelon leaf.


are even more sensitive than the cotyledons, assume a bronzed,
glazed appearance when subjected to very cool weather, particu-
larly on the higher portions of the leaf surface (Fig. 31), and
feel hard and brittle. Growth is almost entirely stopped and


f







Diseases of Watermelons


when further growth occurs it is often in an irregular manner,
causing a curling of the leaf. Plants or leaves showing cold
injury are more susceptible to the attacks of various fungi than
those in a more normal condition of growth and often are killed
by them.
Strong winds, particularly in March, often whip and tear the
leaves severely. Such leaves dry out on the torn edges, become
hard and brittle, and are susceptible to the attacks of fungi such
as Macrosporium and Colletotrichum. The most serious type of
wind injury, however, occurs during fruit setting by whipping
off blossoms and young melons.
High wind, particularly during dry weather, often causes sand
burn, which is characterized by the almost white, rough and
brittle areas that appear on leaves and stems. Wind-blown sand
injures the epidermis and cortex of the main stems and runners,
causing the formation of thick calluses over the injured areas.

HANDLING
The profits in watermelon growing may be greatly reduced,
if not lost, by growers who consider their work finished when
the melon is cut from the vine. Melons, although they have
thick rinds, have thin skins and are tender. They should be
handled with this in mind so as to prevent abrasions, bruises,
cuts, gouges or cracks. Any blemish of a melon that breaks the
skin opens a way for the invasion by any of a large number of
organisms that cause it to rot while in transit. Laborers used to
harvest melons should be carefully and thoroughly instructed
before they are sent into the fields. The cutting and windrowing
should be done with care.
As mentioned previously in the section on stem-end rot, the
rack or box of the wagon or truck used for transporting melons
from the field should be lined with several layers of burlap or
some similar material. Too much speed in handling and hauling
is often a detriment.
Strict attention should be given the railroad car into which
the melons are loaded, being sure first that it is in good order.
See that a good, solid false lining is present. Clean the car care-
fully, removing all traces of foreign material, especially fertil-
izers, which have been found to burn the melons and thus allow
rot fungi to enter (Fig. 32). Smooth out all obstructions such
as splinters and nails. After the car has been cleaned a wash-
ing with formaldehyde is beneficial, but the car should be aired






Florida Agricultural Experiment Stations


Fig. 32.-Injury of watermelon caused by contact with chemicals or
fertilizer on floor of uncleaned railroad cars.

thoroughly afterward. Unless diseased melons have been in the
car this treatment may be omitted.
The material used for bedding should be according to law (in
certain counties local materials such as hay, straw, moss and
pine straw cannot be used on account of tick eradication regu-
lations). Any bedding used should be perfectly dry, as moist
conditions are especially favorable to diseases. Excelsior is
being used more and more and is very satisfactory, as it is clean,
soft and cheap. It should be carefully pulled apart as it is put
into place.
The melons should not be walked upon, even with bare feet,
during any of the loading operations; the weight causes the
melons to crack internally and deteriorate much faster than
sound melons. They should be handed from the truck to helpers
in the car who place them gently in the tiers. While melons may
be pitched from one worker to another they should not be tossed
or too roughly forced into place in packing, though no spaces
should be left. The melons should be placed with stems toward
the middle of the car, in rows and tiers, alternating between
the two ends of the car, in order that the stems can be cut and
treated in one end while the other is being loaded.

SUMMARY
Diseases of watermelons are responsible for losses to Florida
growers amounting to many thousands of dollars annually. The
area in Florida planted to watermelons each year is dependent
largely on the financial success of the preceding melon crop.






Diseases of Watermelons


Although there are a number of strictly horticultural factors
besides diseases which determine crop success or failure, melon
diseases and their control loom large in the mind of the average
grower. This bulletin contains the latest and best information
available on this subject.
Anthracnose, the worst disease of melons in the United States,
caused by Colletotrichum lagenarium (Pass.) Ell. & Hals., is
best controlled by preplanting seed treatment with semesan,
spergon or arasan (0.3 percent by weight), by staying out of
melon fields while the foliage is wet with rain or dew, and by
timely applications of a fungicide such as nabam or zineb. The
time of first fungicide application is uncertain; to save money
and at the same time protect melon vines from defoliation and
melon fruits from spotting, the plants should be watched care-
fully and the fungicide applied as soon as the disease makes its
first appearance. Subsequent fungicide applications, at 7- to
10-day intervals, may be needed, but their application will be
determined by the weather and by disease conditions in the field.
A watermelon disease forecasting service would tell growers
when to apply, and more important, when not to apply, fungi-
cides. Copper fungicides as a rule burn the watermelon foliage
and may injure the young fruits, thereby reducing the set. The
varieties Congo and Black Kleckley are resistant to anthracnose.
The best way to avoid losses from Fusaritm wilt, caused by
Fusarium oxysporum (E.F.S.) f. niveum Snyder and Hansen,
is to plant wilt-resistant varieties on land that has been out of
melons for two to three years. Such varieties are Leesburg,
Blacklee, Black Kleckley, Ironsides, Wilt-Resistant Dixie Queen,
Missouri Queen, Miles, Wilt-Resistant Garrison, Kleckley No. 6,
and Georgia Wilt-Resistant, but none, so far, has found accept-
ance with Florida growers. In general, the wilt-resistant va-
rieties are later to mature than the wilt-susceptible variety
Cannon Ball. The Watermelon Laboratory is undertaking the
development of wilt-resistant strains of this variety. In the
absence of desirable wilt-resistant varieties, an 8- to 10-year
interval between watermelon crops is a necessary rotation;
even so, wilt may be serious enough to reduce yields and a 15-
year rotation would be less risky.
Downy mildew, caused by Peronoplasmopara cubensis (B. &
C.) Rost., often is confused with anthracnose. Only by using a
microscope can the observer report with accuracy which fungus
is concerned. No varieties are resistant to downy mildew but






Florida Agricultural Experiment Stations


certain Santo Domingo wild watermelons possess this character
and are being used at Leesburg in the watermelon breeding pro-
gram. This disease is controlled with the same materials recom-
mended to control anthracnose, but tri-basic copper sulfate also
will give excellent control of downy mildew. The disease fore-
casting service mentioned above could include downy mildew in
its coverage.
Gummy-stem blight, caused by the fungus Mycosphaerella
melonis (Pass.) Chiu and J. C. Walker, can be easily confused
with both anthracnose and downy mildew. It can be controlled
in part by seed treatment with the same materials recommended
for anthracnose control, but better materials are needed to give
complete protection against this organism. Fungicides recom-
mended for control of anthracnose by dusting or spraying will
also control gummy-stem blight; the copper fungicides will not
control gummy-stem blight. No varieties are known to possess
useful resistance to gummy-stem blight.
The stem-end rot disease, due to the fungus Physalospora
rhodina (Berk. & Curt.) Cooke, can be controlled with little
difficulty by proper clipping and disinfecting of the stems of
harvested watermelons. Improper usage of proven methods and
materials are all too often the cause of unnecessary losses from
this disease.
Other diseases of less importance, which are discussed and
pictured, are Southern blight (Corticium rolfsii (Sacc.) Curzi),
blossom-end rot, powdery mildew (Erysiphe cichoracearum DC.),
Cercospora leaf spot (Cercospora citrullina Cke.), Macrosporium
leaf spot (Macrosporium cucumerinum E. &. E.), soil rot (Corti-
cium vagum B. & C.), and mosaic. A list of watermelon diseases
not known to occur in Florida is included.
The effects of cold, wind and sand injuries on melon foliage
are discussed; often, these are mistaken for disease by growers.
Proper handling of harvested melons is discussed and em-
phasized.




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