• TABLE OF CONTENTS
HIDE
 Title Page
 Credits
 Table of Contents
 Fig. 2
 Black rot
 Scurf
 Pox
 Stem rot
 Southern blight
 Foot rot
 Mosaic
 Leaf blight
 White rust
 Alternaria leaf spot
 Mold
 Cercospora leaf spot
 Leaf spot
 Minor diseases
 Soft rot
 Java black rot
 Charcoal rot
 Dry rot
 Surface rot
 Other rots
 Growth cracks
 Seed selection and treatment
 Bedding seed sweet potatoes














Group Title: Bulletin - University of Florida. Agricultural Experiment Station ; no. 212
Title: Diseases of sweet potatoes in Florida
CITATION THUMBNAILS PAGE IMAGE ZOOMABLE
Full Citation
STANDARD VIEW MARC VIEW
Permanent Link: http://ufdc.ufl.edu/UF00027375/00001
 Material Information
Title: Diseases of sweet potatoes in Florida
Series Title: Bulletin University of Florida. Agricultural Experiment Station
Physical Description: 40 p. : ill. ; 23 cm.
Language: English
Creator: Weber, George F ( George Frederick ), b. 1894
West, Erdman, 1894-
Publisher: University of Florida Agricultural Experiment Station
Place of Publication: Gainesville Fla
Publication Date: 1930
 Subjects
Subject: Sweet potatoes -- Diseases and pests -- Florida   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
non-fiction   ( marcgt )
 Notes
Statement of Responsibility: by George F. Weber and Erdman West.
General Note: Cover title.
Funding: Bulletin (University of Florida. Agricultural Experiment Station)
 Record Information
Bibliographic ID: UF00027375
Volume ID: VID00001
Source Institution: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: aleph - 000923520
oclc - 18173363
notis - AEN4071

Table of Contents
    Title Page
        Page 1
    Credits
        Page 2
    Table of Contents
        Page 3
    Fig. 2
        Page 4
    Black rot
        Page 5
        Page 6
    Scurf
        Page 7
        Page 8
    Pox
        Page 9
        Page 10
        Page 11
    Stem rot
        Page 12
        Page 13
        Page 14
    Southern blight
        Page 15
        Page 16
    Foot rot
        Page 17
    Mosaic
        Page 18
        Page 19
        Page 20
        Page 21
        Page 22
        Page 23
    Leaf blight
        Page 24
    White rust
        Page 25
        Page 26
    Alternaria leaf spot
        Page 27
    Mold
        Page 28
        Page 29
    Cercospora leaf spot
        Page 30
    Leaf spot
        Page 31
    Minor diseases
        Page 31
    Soft rot
        Page 32
    Java black rot
        Page 33
        Page 34
    Charcoal rot
        Page 35
    Dry rot
        Page 36
    Surface rot
        Page 36
    Other rots
        Page 37
    Growth cracks
        Page 38
    Seed selection and treatment
        Page 39
    Bedding seed sweet potatoes
        Page 39
        Page 40
Full Text


March, 1930


UNIVERSITY OF FLORIDA
AGRICULTURAL EXPERIMENT STATION
Wilmon Newell, Director





DISEASES OF SWEET POTATOES
IN FLORIDA

By
GEORGE F. WEBER AND ERDMAN WEST



-" ;7 .


Fig. 1.-Sweet potato infected with soft rot, showing the fungus causing
the disease.




Bulletins will be sent free upon application to the
Agricultural Experiment Station,
GAINESVILLE. FLORIDA.


Bulletin 212








BOARD OF CONTROL


P. K. YONGE, Chairman, Pensacola
A. H. BLENDING, Leesburg
W. B. DAVIS, Perry


RAYMER F. MAGUIRE, Orlando
FRANK J. WIDEMAN, West Palm Beach
J. T. DIAMOND. Secretary, Tallahassee


STATION EXECUTIVE STAFF


JOHN J. TIGERT, M.A., LL.D., President
WILMON NEWELL, D. Sc., Director
S. T. FLEMING, A.B., Asst. Director
J. FRANCIS COOPER, M.S.A., Editor
R. M. FULGHUM, B.S.A., Asst. Editor


InA KEELING CRESAP. Librarian
RUBY NEWHALL, Secretary**
K. H. GRAHAM, Business Manager
RACHEL McQUARRIE, Accountant


MAIN STATION-DEPARTMENTS AND INVESTIGATORS


AGRONOMY
W. E. STOKES. M.S., Agronomist
W. A. LEUKEL, Ph.D., Associate
G. E. RITCHEY, M.S.A., Assistant*
FRED H. HULL, M.S.A., Assistant
J. D. WARNER, M.S., Assistant

ANIMAL HUSBANDRY
A. L. SHEALY, D.V.M., Veterinarian in
Charge
E. F. THOMAS, D.V.M., Asst. Veterinarian
R. B. BECKER, Ph.D., Associate in Dairy
Husbandry
W. M. NEAL, Ph.D., Assistant in Animal
Nutrition
C. R. DAWSON, B.S.A., Assistant Dairy
Investigations

CHEMISTRY
R. W. RUPRECHT, Ph.D., Chemist
R. M. BARNETTE, Ph.D., Associate
C. E. BELL, M.S., Assistant
H. L. MARSHALL, M.S., Assistant
J. M. COLEMAN, B.S., Assistant
J. B. HESTER, B.S., Assistant

COTTON INVESTIGATIONS
W. A. CARVER, Ph.D., Assistant
E. F. GROSSMAN, M.A., Assistant**
RAYMOND CROWN, B.S.A., Field Assistant


ECONOMICS, AGRICULTURAL
C. V. NOBLE, Ph.D., Agricultural Economist
BRUCE McKINLEY, A.B., B.S.A., Associate
M. A. BROOKER, M.S.A., Assistant**
JOHN L. WANN, B.S.A., Assistant
ECONOMICS, HOME
OUIDA DAVIS ABBOTT, Ph.D., Head
L. W. GADDUM, Ph.D., Biochemist
C. F. AHMANN, Ph.D., Physiologist
ENTOMOLOGY
J. R. WATSON, A.M., Entomologist
A. N. TISSOT. M.S., Assistant
H. E. BRATLEY, M.S.A., Assistant
HORTICULTURE
A. F. CAMP, Ph.D., Horticulturist**
M. R. ENSIGN, M.S., Assistant
HAROLD MOWRY, B.S.A., Assistant'
A. L. STAHL, Ph.D., Assistant
G. H. BLACKMON, M.S.A., Pecan Culturist

PLANT PATHOLOGY
W. B. TISDALE, Ph.D., Plant Pathologist
G. F. WEBER, Ph.D., Associate
A. H. EDDINS, Ph.D., Assistant
K. W. LOUCKS, M.S., Assistant
ERDMAN WEST, B.S., Mycologist


BRANCH STATION AND FIELD WORKERS
R. R. KINCAID, M.S., Assistant Plant Pathologist (Quincy)
JESSE REEVES, Foreman, Tobacco Experiment Station (Quincy)
J. H. JEFFERIES, Superintendent, Citrus Experiment Station (Lake Alfred)
W. A. KUNTZ, A.M., Assistant Plant Pathologist (Lake Alfred)**
B. R. FUDGE, Ph.D., Assistant Chemist (Lake Alfred)
W. L. THOMPSON, B.S., Assistant Entomologist (Lake Alfred)
GEO. E. TEDDER, Foreman, Everglades Experiment Station (Belle Glade)
R. V. ALLISON, Ph.D., Soils Specialist in charge Everglades Experiment Station (Belle Glade)
FRED YOUNT, Office Assistant (Belle Glade)
E. R. PURVIS, B.S., Laboratory Assistant in Soils (Belle Glade)
L. O. GRATZ, Ph.D., Associate Plant Pathologist (Hastings)
A. N. BROOKS, Ph.D., Associate Plant Pathologist (Plant City)
A. S. RHOADS, Ph.D., Associate Plant Pathologist (Cocoa)
STACY O. HAWKINS, M.A., Field Assistant in Plant Pathology (Homestead)
L. R. TOY, B.S.A., Assistant Horticulturist (Homestead)
D. G. A. KELBERT, Field Assistant in Plant Pathology (Bradenton)
R. E. NOLEN, M.S.A., Field Assistant in Plant Pathology (Monticello)
FRED W. WALKER, Assistant Entomologist (Monticello)**
D. A. SANDERS, D.V.M., Associate Veterinarian (West Palm Beach)
M. N. WALKER. Ph.D., Associate Plant Pathologist (Leesburg)
W. B. SHIPPY, Ph.D., Assistant Plant Pathologist (Leesburg)
C. C. GOFF, M.S., Assistant Entomologist (Leesburg)
J. W. WILSON, Ph.D., Assistant Entomologist (Pierson)

*In cooperation with U. S. Department of Agriculture.
**On leave of absence.











CONTENTS

PAGE

BLACK ROT ................................ ....... ...... 5

SCURF ................................... ......... ................. 7

P ox ...................................... ....... ...... ........ 9

STEM ROT .......... ............................................ 12

SOUTHERN BLIGHT ................................................ 15

F OOT R OT .......... .................................... ......... 17

MOSAIC ............. ...... ........................ ............ 18

LEAF BLIGHT ........... ............................... .......... 24

WHITE RUST ........... ...................................... 25

ALTERNARIA LEAF SPOT ..................... ..................... 27

MOLD ............................................................ 28

CERCOSPORA LEAF SPOT ........................................... 30

SEPTORIA LEAF SPOT .............. ........................... 31

MINOR DISEASES ............ ..................... ................ 31

SOFT ROT ............ ................................ ............ 32

JAVA BLACK ROT ................. ................... ................ 33

CHARCOAL ROT ........... ....... ............................... 35

DRY ROT ................ ................ ............... 36

SURFACE ROT .................................... ................ 36

OTHER ROTS ............. ...... ................. ................. 37

GROWTH CRACKS ..... ........ ................................... 38

SEED SELECTION AND TREATMENT ............... ............... 39

BEDDING SEED SWEET POTATOES ............. .. .................. 39














'V5~


Fig. 2.-Above, yield of nine mosaic sweet potato plants; below,
yield of one healthy adjacent plant.









DISEASES OF SWEET POTATOES IN FLORIDA
By GEORGE F. WEBER AND ERDMAN WEST'
Sweet potatoes are one of the important truck crops in Florida.
During the past 10 years Florida has ranked among the first
dozen states in the United States2 in the production of this impor-
tant crop. The importance is not limited to commercial production,
for sweet potatoes are important in every garden and draws are
shipped every spring for early planting in the states farther north.
Of the millions of bushels of potatoes raised in the state every
year, from 10 to 20 percent are lost by the attacks of various
diseases. It is the purpose of this bulletin to discuss briefly the
nature and causes of these diseases and to give control measures.
BLACK ROT
Black rot of sweet potatoes is caused by the fungus Ceratosto-
mella fimbriata (Ell. & Hals.) Ell. This disease is distributed
over the entire sweet potato-growing section of the United States,
including Florida. The greatest loss suffered from it, however,
is in the central south rather than in the coastal plains. In Flor-
ida the disease is of considerable importance and has caused
losses in specific fields in the northern and western portions of
the state. In the central part of the state it is seldom found. It
causes losses by attacking the roots of the plant in the field, by
killing sprouts growing from diseased potatoes in the seedbed,
and by producing an important storage rot. The latter spreads
from diseased potatoes to healthy ones in contact with it. The
fungus may live from season to season in the field on plant refuse.
This is not as important in the development of the disease in the
succeeding crop of potatoes as infected draws which are carried
to the field from diseased seedbeds. All parts of the plant that
grow below ground are attacked. After the potatoes are dug,
numerous secondary organisms may enter the black rot lesions
and cause more rapid decay.
Symptoms: The first indication of the presence of the disease
is a yellowing of the leaves of growing plants, which, however, is
not a specific symptom for this disease alone. The yellowing
progresses rapidly, the leaves become brown, and the plant may
wither and die or, when young, the sprout may wilt before it has
become entirely yellow. Such plants die quickly because the lower
1Photographs made by David G. A. Kelbert.
2Year Books of the U. S. Department of Agriculture.






Florida Agricultural Experiment Station


portion of the stem becomes girdled by the fungus near or below
the soil surface. The lesion is dark brown or black. All of the
roots and underground stems are susceptible to the disease, yet
the most important parts attacked are the potatoes where circular
dark colored spots appear, either scattered or entirely covering











Fig. 3.-Sweet Potato severely infected with black rot.

the potato surface. The disease develops gradually and at dig-
ging time infections are often entirely overlooked. When infected
potatoes are placed in storage the spots become sunken, and occa-
sionally the skin cracks across them. As the lesions age they
become dark in their center, and this coloration gradually spreads
as the lesions enlarge. Upon close examination of the dark center
one can distinguish short, glossy, black, stiff, bristle-like struc-
tures often with barely distinguishable whitish tips. These are
the spore bearing parts of the fungus causing the decay and the
whitish tips are clusters of spores ready for dissemination. These
fruiting structures definitely characterize the disease and its
cause. The decay of the inner portions often involves a large part
of the potato. Its extent can be determined by slicing the potato.
The decayed areas are dark brown in color with a distinct line of
demarcation between the diseased and the healthy parts. The
disease produces a disagreeably bitter taste found throughout the
whole potato. Completely rotted potatoes usually dry, shrink and
mummify. However, infection by secondary organisms often
takes place and the potato exhibits symptoms of soft rot.
Control: Black rot is combatted by various means, chief among
which are proper and careful seed selection, care in making the
seedbed, clean storage, and crop rotation. In selecting seed, all
potatoes should be rejected that show any bruises, injuries or any
sign of disease. This clean seed should be stored where there is






Bulletin 212, Diseases of Sweet Potatoes in Florida


no danger of bin infection, or infection from contact with dis-
eased potatoes. Such seed should receive every care indicated
later under the heading, "The Seedbed."
Bins that have previously held black rot potatoes should be
disinfected with bluestone or formaldehyde solution before being
used for storage again. Black rot also spreads faster where the
tubers are stored in bulk, dumped in bins, because there is less
ventilation and more potatoes are in contact. Storing the crop


'I'


Fig. 4.-Section of sweet potato through black rot lesions, showing depth
of decay.

in hampers or barrels is a much safer method. The potatoes
should be disturbed as little as possible from the time of storage
until marketed or bedded.
Since the parasite may live over in the soil for a year or more,
it is a poor practice to plant sweet potatoes on infested land two
years in succession. It is also well to use little or no manure on
land infected with black rot as this favors the growth of the
fungus.
SCURF
Scurf of sweet potato is caused by the fungus Monilochaetes
infuscans E. & H. The disease is widely distributed, occurring
on this host plant wherever it is generally grown, but the actual






Florida Agricultural Experiment Station


damage done is slight since the fungus is quite superficial and
does not penetrate the root to any extent. However, the discol-
ored areas on the surface of the underground parts are of consid-
erable consequence, since they spoil the attractiveness of the


Fig. 5.-Sweet potato affected with scurf, showing early symptoms of
discolored blotches on lower half and scurfy epidermis on upper portion.







Bulletin 212, Diseases of Sweet Potatoes in Florida


potatoes on the market and scurfy potatoes cannot be sold as first-
class products. There may also be some shrinkage in storage in
badly affected potatoes because moisture evaporates more readily
from these infected, discolored areas than from potatoes that are
normal in this respect.
In Florida this disease is important and may be found in any
field at digging time, being more prevalent on the heavier types
of soil. Most Florida soils planted to sweet potatoes are relatively
light and probably for this reason scurf is not a very serious
disease. The disease does not readily spread from potato to
potato in storage. The fungus lives in the soil for sometime and,
if very prevalent one season, will infect new plantings when made
the following year. However, it is more often spread at planting
time by removing and transplanting infected draws from diseased
seedbeds to the fields.
Symptoms: Scurf attacks the various parts of the sweet potato
plant below the surface of the ground but produces no symptoms
upon the vine. This makes it absolutely impossible for the grower
to estimate the extent of damage that his growing crop is suffer-
ing. The fungus grows on the roots of the plant, forming brown
or black discolored areas on the skin. In light infections these
may occur only at one end of the potato or as scattered blotches
of various sizes. In severe infections these blotches coalesce and
frequently cause the whole surface to become brown or black. On
light skinned varieties this discoloration is very conspicuous, even
in the case of mild infections. Moderate infection on red skinned
varieties may pass almost unnoticed, since the lesions are similar
in color to the skin. However, the spots are superficial, rarely
extending deeper than the epidermis, and impair the food value
of the potato very little. Potatoes left in the ground after the
vines are cut for harvesting are likely to develop a high percent-
age of scurf.
Control: Rotation is essential for growing potatoes free from
scurf. All seed should be treated with 1:1,000 corrosive sublimate
for 10 minutes before being placed in seedbeds. Proper bedding
is also important.
POX
Pox of sweet potatoes is caused by a fungus, Actinomyces sp.
Until recently it was thought to be caused by Cystospora batata
Ell. The disease was first recognized about 1890. Since that
time it has been found in widely distributed areas and is more or







Florida Agricultural Experiment Station


less common wherever sweet potatoes are grown. The disease
is classed as one of the important diseases of sweet potatoes. It
has been observed in Florida for a number of years. Its occur-
rence and severity seem to be associated with certain areas of the
sweet potato sections, probably being associated with certain soil
types. In Florida, however, total damage attributed to this dis-
ease, i of minor importance, although it is severe and destructive
local. It occurs in most sections but few fields are more than
slightly infected. This organism also attacks Irish potatoes, tur-


Fig. 6.-Pox of sweet potato, showing characteristic markings.







Bulletin 212, Diseases of Sweet Potatoes in Florida


nips and beets. It lives in the soil from season to season and is
spread principally by transported soil particles, either through
cultivation or by wind and rain. The disease is not common in
the seedbed nor in storage. It develops on all parts of the plant
below ground and is most characteristic on the developing pota-
toes.
Symptoms: Sometimes the yellowing, stunting or lack of dense
growth may indicate infected plants. Otherwise, no distinguish-
able characteristic symptoms appear above ground on growing
plants that definitely point to this disease and none other. The
parasite attacks the root system and causes brownish spots to
appear on the potatoes. These spots are at first rather small,
circular, may become more or less sunken and, in many instances,
especially the larger spots, show cracks. They may enlarge and


Fig. 7.-Large pox lesions on sweet potatoes.


coalesce (run together). The potatoes may be stunted or killed
altogether. Often growth is retarded in certain places on a devel-
oping root, causing it to develop knotty or serially bulbous. As
the season progresses the tissue within these spots becomes corky
and cracks and often falls out, leaving the rough pits that are
characteristic of the disease.







Florida Agricultural Experiment Station


In addition to marring the appearance of the potatoes, pox
renders them susceptible to severe rotting. The organism itself
does not cause extensive rotting nor is it known to spread in
storage. The pits afford ideal entry points for organisms causing
storage rots and the resulting loss is often severe.
Control: Pox is carried in the soil. It is found most frequently
in alkaline soils. The application of sulphur to badly infected
fields has given satisfactory control. Rotation is recommended
for fields consistently showing the disease.
Use only clean, sound seed, since the organism causing the
trouble may be transported in soil particles and affected potatoes
to seedbeads and thence to the fields.
STEM ROT
Stem rot is one of the most important diseases of sweet potatoes
and is caused by either or both of two fungi, namely, Fusarium
batatatis Woll. and Fusarium hyperoxysporum Woll. It is found
in all sweet potato growing sections of the United States,1 but
varies somewhat in importance in different localities. In the
central and northern limits of the sweet potato area of the United
States, the disease appears to be of more importance than in the
extreme southern limits where the optimal temperature for the
development of the host is higher than the optimal for the best
growth of the fungus. The potato plant will grow very well at
90' F. at which temperature the growth of the fungus is almost
inhibited. The-disease is distributed generally over the sweet
potato-growing area of Florida but the losses resulting from it
are relatively light.
Under normal conditions the fungi causing this disease may
survive from year to year in the soil. The important means of
dissemination is the transportation of the potatoes for planting
seedbeds and on draws and vines for planting in the field. If
infected potatoes are placed in the seedbeds, the developing draws
become infected. The draws are carried to the fields and planted.
Thus the disease is spread, unknowingly by most growers. The
disease develops under all moisture conditions in which the host
plant will grow. The occurrence and spread of the disease are
not primarily affected by kinds and applications of fertilizer. It
is not possible to control the disease economically in the field by
the application of fungicides. The fungus lives within the plant
itself, in the vascular tissue. No varieties of sweet potatoes have
IPlant Disease Reporter, U.S.D.A., 1920-1928.






Bulletin 212, Diseases of Sweet Potatoes in Florida 13
been found that are immune to this disease though some show
partial resistance.
Symptoms: In the seedbed stem rot usually originates from
diseased tubers infected the year before, although bedding the
seed in soil in which diseased plants have grown also may cause
infection. The diseased sprouts become yellowish in color and
the lower leaves drop off. The base of the stem becomes dark and
on splitting it, a brown or black layer just under the bark is
found. If the infection comes from the soil there is practically no
indication of the presence of the disease by the time the sprouts
are ready for pulling. However, if the stem be split, the vascular
ring or fibers will be found to have a brownish to black color.
On plants in the field the symptoms vary according to the time
at which infection occurred. Plants infected at or previous to
the time of planting make little or no growth after setting in the
field. The leaves of such plants gradually turn yellow and die.
The stem becomes dark, sometimes a bluish color, and dies soon
after the leaves are shed. The part below ground is black and
rotted at this time. The black layer evident just under the bark
is responsible for the dark appearance of the steam of the dis-
eased plant.
Plants contracting the disease after setting live longer and
make more growth than plants that were diseased when set. The
first symptoms of disease in such plants are the yellowing and
dying of the leaves near the base of the vine at the center of the
hill. Gradually the vine loses the leaves, progressively outward
until 2 to 4 feet of the vines have been defoliated and a darkening
of the stems follows. Frequently, the stem will split near the
base in severely infected hills and expose diseased tissue. After
this period of defoliation, the vine wilts, sometimes recovering at
night, and gradually dies. If infection took place early and
potatoes had formed, these will sprout upon the death of the vine
and produce a cluster of young shoots on the site of the old hill.
The hills with the yellowing leaves and those with bunches of
young sprouts are characteristic of fields heavily infected with
stem rot. The bunches of sprouts never produce any marketable
potatoes and add much diseased material to the soil.
In the case of later infections, death of the vine does not occur
before digging time. The potatoes may be apparently healthy
and are suitable for market but they are carriers of the disease
and are unsafe for seed purposes. These later infected hills can






14 Florida Agricultural Experiment Station
be detected only by splitting the stem near the base and searching
for the dark layer beneath the bark.
The disease is carried over to the next season in the soil on
parts of the dead sweet potato plants, such as roots, small potatoes
and portions of the vine. Plowing and other processes of cultiva-
tion spread the diseased plant parts over a large area and a field
becomes generally infested. The disease may be carried to clean
fields by soil on the feet of animals, tools, by drainage water and
wind-carried dust. Debris and manure from farmyards where
diseased potatoes have been fed to cattle also will carry the dis-
ease. The most important means of spread over long distances
and into new territory, is by the use of diseased seed potatoes or
sprouts from diseased seed.
In storage stem rot is not easily detected and does not develop
on the potatoes. However, potatoes from infected hills are much
more susceptible to attacks by the ordinary storage rots, and
losses in this manner from infected fields are likely to be large.
Control: Since the stem rot parasite attacks the internal por-
tions of the plant and survives in the soil, it cannot be controlled
by ordinary fungicides. Neither do liming or special fertilizers
seem to check the progress of the disease. Among the principal
means of control are seed selection, care in seedbed preparation
and the use of resistant varieties.
Seed selection should be made at digging time and not at bed-
ding time. When the hills are dug, those having a good shape,
color, yield and any other desirable qualities, should be examined
for stem rot. This is easily accomplished by splitting the main
stem of the hill and examining it for any discoloration.' Healthy
vines should show a pure white or greenish white color inside
without any brown or dark streaks and only tubers from hills
showing these characteristics should be considered for seed. This
selected seed should be stored and should not come in contact with
any diseased potatoes or infested soil and in the spring, it should
be treated and bedded, according to the directions given later.
A long rotation will aid in keeping the disease in check. If soil
has become badly infested, sweet potatoes should not be planted
in it again for a number of years, since the organism causing the
disease persists in the soil from year to year.
Varietal resistance as a means of combatting the disease is
limited in its application by the adaptability of the resistant vari-
eties to the soil and climatic conditions as well as to the market






Bulletin 212, Diseases of Sweet Potatoes in Florida 15
requirements. As a whole the "Jersey" group is most susceptible
to the disease and the "Spanish" group, including especially
Dahomey, Red Brazilian, Pierson, Southern Queen, Porto Rico
and Yellow Strassburg are considered more resistant to the dis-
ease.
SOUTHERN BLIGHT
Southern blight of sweet potatoes is caused by a fungus, Scle-
rotium rolfsii Sacc. This disease is typically southern in its range,
being almost wholly confined to regions south of the Ohio River
and east of the Rocky Mountains. Its occurrence outside of this
area is infrequent and of little consequence. From these northern
and western borders south it becomes progressively prominent
and important. It is most destructive in. the coastal plains regions.
In Florida the disease caused by this fungus is generally distrib-
uted over the whole state on 50 or more host plants including
practically all of the annual truck and flowering plants, and often
under favorable conditions for its development, will attack and
girdle semi-hardy perennials. This disease is one of the most
destructive in the state but fortunately there are fewer acres
planted during the summer months when the fungus is most
favored by temperature and moisture and when it makes its
greatest development.
The disease has been found on all types of soil from light sand
to heavy muck and peat, and has caused considerable damage to
January truck crops as well as July plantings. The fungus lives
from season to season in the soil and on decaying plant parts.
The brown sclerotia about the size and shape of cabbage seed are
produced in abundance on parasitized plants. These sclerotia are
masses of fungus hyphae closely matted together, and are capable
of enduring long periods of dry and other unfavorable conditions.
They have remained viable in the laboratory in vials for four
years. When seasonal conditions are favorable, the sclerotia'ger-
minate and the mycelium attacks succulent plants growing in its
vicinity, rapidly kills them and produces a large number of
sclerotia which in turn repeat the life history. The sclerotia are
scattered by cultivation, wind and water. Because of the infre-
quency of its occurrence in the seedbed, the danger of transport-
ing the disease from the seedbed to the field is not great.
Symptoms: Southern blight is prominent in three distinctive
stages of the development of sweet potatoes, namely in the seed-
bed, on transplanted sprouts and on old established vines. It






16 Florida Agricultural Experiment Station

occurs in the seedbed, but is seldom of serious consequence.
Usually the seed potatoes are planted deep enough to escape the
fungus, since it inhabits and grows best in the top half inch of
soil. Potatoes planted shallow occasionally become infected and
the sprouts are attacked at their base, girdled and killed. When
sprouts and vines are transplanted to the field during early sum-
mer for late crops, this disease is often destructive. The fungus
already inhabiting the soil attacks the sprouts and vines at the
soil line, girdles
and kills them be-
fore they have fully
recovered from
transplanting. The
white mycelium
over-runs the stem
to the extent of an
inch or two above
the soil and usually
a half inch below it.
The lesion is at first
water-soaked and
later the bark or
cortex sloughs off,
leaving only the
vessels and liquefied
tissue. The plant
falls over, bending
or breaking at the
lesion. After a few
days the sclerotia
begin to form in
abundance and usu-
ally a week later
the white mycelium
has wholly disap-
peared and only the
brown sclerotia re-
main. They are
Readily dissemi-
nated and repro-
duce the disease.
Fig. 8.-Summer planted vines which show the de- The runner of the
structiveness of Southern blight.he runners of the






Bulletin 212, Diseases of Sweet Potatoes in Florida


vines also are occasionally attacked but the injury produced sel-
dom causes losses of economic importance. The disease on the
runners is practically the same as just described on the trans-
planted runners and vines.
Control: Seed potatoes should be treated and bedded in clean
soil. Plants killed immediately following transplanting should
be removed along with a double handful of adjacent soil; resets
should be put in their places. Wherever Southern blight is found
in the field the affected parts and immediately adjacent soil should
be removed or buried 6 to 8 inches deep. Measures other than
sanitation as mentioned have not been found profitable.

FOOT ROT
Foot rot of sweet potatoes is caused by the fungus Plenadomus
destruens Hart. This disease, although recently discovered and
described, has a wide geographical distribution and is of consid-
erable importance. It is believed to occur wherever the host plant
is grown commercially in the United States. In Florida its dis-
tribution is general, though losses caused by it have never been
great. The fungus causes stem lesions and potato decay, the
latter taking place both in the field and in storage. It is more
serious as a storage rot, however. The fungus survives from one
season to the next on stored potatoes and on plant refuse left in
the field. The disease develops slowly during its early stages, but
is aggressive after becoming firmly established.
Symptoms: Primary symptoms of foot rot are recognized as
premature yellowing and dying of the leaves. Closer examination
of such plants will reveal a brownish area of the stem, usually
close to the soil line. This lesion may occur on any of the runners,
however, but such is the exception. Leaf yellowing is often fol-
lowed by wilting and premature death of the part or plant thus
affected. The brown area develops slowly at first, but later it
rapidly girdles the stem and the fruiting bodies appear in the
form of small black pycnidia scattered over the discolored area.
The decay produced on the potatoes is brown and firm and seldom
involves the entire potato, except under most favorable conditions,
at which times the small black fruiting bodies are produced on
the decayed area. The spores may be spread to other potatoes
and, if used for seed, they decay in the seedbed. The rotting takes
place slowly, however, but draws or sprouts may become infected
before they attain suitable size for transplanting. If infected






Florida Agricultural Experiment Station


draws are transplanted to the field, they may die within a short
time and leave a poor stand.
Control: Seed selection and seed treatment are the most impor-
tant methods for the control of foot rot. The careful selection
of seed will obviously eliminate the potatoes that show evidence
of this trouble. This operation followed by seed treatment will
free otherwise healthy potatoes of microscopic, contaminating
organisms clinging to their surfaces.
MOSAIC
Mosaic of sweet potatoes is a disease which manifests itself on
the parts of the plant above ground, principally on the leaves and
petioles. It belongs to the group of "virus" diseases, the exact
cause of which is unknown. The disease has been found in widely
scattered localities but is seldom of considerable importance.
Apparently it is more common in Florida than in other sweet
potato growing states from which it has been reported. Undoubt-
edly the chief source of diseased plants is the seedbed from which
affected sprouts are transplanted to the field. Often sprouts
growing in the seedbed show various types and shapes of leaves
and large numbers of healthy sprouts could easily pass as mosaic
infected or vice versa. Actual field counts made in Florida have
shown as much as 6 percent infected plants, the maximum per-
centage ever encountered in this state, but the average is probably
less than 1 percent. The loss attributed to the disease corre-
sponds with the number of infected plants, since affected plants
do not produce any marketable potatoes. Potatoes formed on
mosaic plants are almost without exception only slightly enlarged
roots. Occasionally one of these may be large enough to be gath-
ered; after the largest potatoes in the hill are marketed the
smaller ones, including the mosaic potatoes, will be used for seed
for the next year. In this way the disease is perpetuated.
Symptoms: Symptoms of mosaic are not manifested on the
potatoes and the trouble is recognized with difficulty in the seed.-
bed. After being transplanted the healthy plants grow normally
while the diseased plants present a stunted appearance. The
internodes of the affected plants are decidedly shortened, bring-
ing the nodes from which the leaves originate close together. This
gives the plant a somewhat rosetted appearance. The stems of
the leaves are shorter and thicker than those of healthy plants.
Often the petiole is larger at the place of attachment to the leaf
blade than where attached to the stem; at the point of union with







Bulletin 212, Diseases of Sweet Potatoes in Florida 19

the blade, it is frequently flattened and reddish rose, rather than
natural green in color. The larger veins branch out into the leaf
abnormally, suggesting fasciation, and appear much wider and
more prominent than the veins in normal leaves. The edges of
these widened veins are often quite transparent when held up to
the light for examination. The tissue between the veins is usually


Fig. 9.-Sweet potato plant showing extreme symptoms of mosaic.







Florida Agricultural Experiment Station


of a uniform green color, decidedly raised and sunken, forming
pouch-like areas. In extreme cases, however, the petiole may
flatten out and be-
come practically
the whole blade,
green to creamy
yellow in color
and supporting
merely a narrow
undulating fringe
or ruffle-like band
of green leaf tis-
sue that normally
is the blade. The
leaves in this case
are extremely
malformed, being
very narrow, long
and pointed.
There is no mot-
tling in the sense
that leaves of cu-
cumber and to-
bacco plants are
mottled by mosaic.
The blades are
crinkly along the
margins and often
curled backward.
A second type of
mosaic has been
observed on sweet
potatoes that
closely resembles
the transmissible
mosaic of tobacco,
cucumber, tomato
and other plants.
It differs from the
first type in that
Fig. 10.-Portion of sweet potato vine showing char- there may be less
acteristic mosaic symptoms.






Bulletin 212, Diseases of Sweet Potatoes in Florida 21

shortening of the internodes, little or no dwarfing in leaf sizes or
malformation and that the mottling may follow the larger veins
of the leaf.


Fig. 11.-Sweet potato plants of same age growing side by side. Upper
left, plant affected with mosaic; right, healthy plant.






Florida Agricultural Experiment Station


The type of mosaic previously described on sweet potatoes is
distinct. The described type is characterized by extreme dwarfing
and rosetting of affected plants, a definite malformation of peti-
oles and leaves and very little characteristic mottling on the
blade comparable to mosaic mottling as observed on tobacco or
tomato.
A description of the newly observed mottling is as follows:
The first leaves showing this disease were growing on plants
about four weeks after the vine cuttings were planted. The leaves
were of normal size or slightly smaller. In marginal outline they
closely resembled leaves of unaffected plants. The mottling, con-
sisting of light and dark green areas on the blade, was readily
distinguishable. The shades of green showed abrupt margins,
giving a patchwork impression rather than a gradual blend. The
lighter green areas often followed the larger primary veins of
the leaf, whereas the darker green areas were often associated
with and followed the secondary connecting veins. Portions of
the blades of certain leaves showed none of this mottling and
appeared to be of a healthy, even green color. Leaves thus par-
tially affected showed a slight stunting of the portion containing
the mottling. The petioles were long, smooth and indistinguish-
able from healthy ones. The midrib of mottled leaves was usually
as straight as the midrib of healthy leaves, except where the
ruffle-like pouches were present. In such instances it was rough,
undulating and kinky. The runners bearing these leaves appeared
normal in size, length and internode distances. The mottled leaves
were usually interspersed with leaves showing no mottling what-
soever and were usually located several nodes back from the
growing tip. Observations several weeks later, following a cool
rainy period, showed an apparent increase in the percent of mot-
tled leaves. The earlier observed mottling in which the lighter
green areas followed the principal veins was less prominent than
the more irregular type of mottling. Some stunting also was
evident, accompanied by the mottling of leaves up to the growing
tip. Certain runners showing an occasional old mottled leaf now
showed distinct mottling on all new growth at the growing tip
as well as on newly developed branches in the leaf axils along the
runner from the tip to the hill.
Control: Plants showing the first type of mosaic should be
removed and replants should be put in their places, provided the
season is not too far advanced. The second type is not known to
cause serious damage and control methods are not recommended.






Bulletin 212, Diseases of Sweet Potatoes in Florida 23


Fig. 12.-Two distinct types of mosaic mottling of sweet potato leaves.






Florida Agricultural Experiment Station


Fig. 13.-Sweet potato leaf showing Phyllosticta leaf spot lesions.

LEAF BLIGHT
This disease of the foliage, probably the most common of the
various sweet potato leaf diseases, is caused by the fungus, Phyl-
losticta batatas (Thun.) Cke. Leaf blight can be found in almost
any sweet potato field in the sweet potato area of the United
States. It is common and widespread in Florida, being the most
prevalent leaf disease in the state. It is found only on the leaf
blades and may appear in any stage of their development. It
occurs from early spring to late fall but is most abundant during
the summer rainy period. At times it causes some damage by
defoliation. The fungus lives from season to season on dead
leaves and other plant material and is readily spread by wind,
rain and cultivation.
Symptoms: The first infections of the leaf blight fungus cause
yellowish spots to appear on the leaf blades. These spots enlarge
to approximately one-fourth to three-eighths inch in diameter
and are angular to circular in outline. When the spots are about






Bulletin 212, Diseases of Sweet Potatoes in Florida 25

full size the central tissue becomes whitish to brown within and
falls away. Just preceding the final drying there appear numer-
ous small black specks irregularly distributed over the spot.
These specks are the fruiting structures of the fungus when the
spores develop. A single leaf may contain one or few of the spots
or under more favorable conditions the spots may be so numerous
as to cover a greater part of the leaf blade. In such instances the
leaf becomes yellow rapidly and slowly withers and dies. The
spots are slightly sunken on both surfaces of the leaf and when
old they often fall out, leaving holes. This fungus has not been
found attacking any parts of the sweet potato except the leaf
blade.
Control: Even though leaf blight is common and widespread it
has not been considered of economic importance and no control
methods have been worked out.
WHITE RUST
White rust of sweet potatoes is caused by a fungus. Albugo
ivomoeae-vanduranae (Schw.) Sw. The disease is more or less
general over the potato-growing area of the United States. In
Florida it has been collected but not found common on this plant.
However, it is common on various species of wild morning glory
which are closely related to sweet potatoes. The disease is found
only on the plant parts above ground, usually the leaves. The
actual loss in this state caused by this disease is negligible and
reports of its recurrence are not numerous. It lives from season
to season on decaying plant parts, by hibernation and on wild
plants. The spores are disseminated by wind and water.
Symptoms: White rust is first noticeable on the foliage where
light green or yellowish spots appear. Closer examination on the
reverse side of such spots reveals whitish slightly raised blister-
like, pustular spots. Later these pustules break open, revealing a
white powdery granular mass of spores of the fungus. Often no
further damage is done. However, in severe cases the infected
area becomes brown, dies and may drop out, leaving holes in the
leaf. Occasionally the infection takes place on the stem or petiole,
which results in swellings or even distortion of the affected
parts. The disease may appear at any time during the develop-
ment of, and usually remains more or less virulent during the
life of, the plant. Often the spots are very numerous and cover
almost the entire lower surface of the leaf. Seldom, however, is






6 Florida Agricultural Experiment Station


Fig. 14.-White rust on sweet potato leaf. Upper, top side of leaf;
lower, bottom side of leaf.






Bulletin 212, Diseases of Sweet Potatoes in Florida


Fig. 15.-Enlarged view of portion of sweet potato leaf, showing white
rust pustules.
this disease the direct cause of defoliation. The potatoes, while
not directly affected, may nevertheless be indirectly affected by
the reduction of leaf area.
Control: Because of the unimportance of the disease, control
methods are not considered worth while.

ALTERNARIA LEAF SPOT
This leaf spot of sweet potato is caused by Alternaria sp. The
disease is widespread over the United States and is most severe
in the South. In Florida the disease usually can be found in
sweet potato fields after mid-summer. The leaf blades are the
only part of the plant attacked by the fungus. The disease ap-
pears most often on the older leaves in the hill and observations
indicate that the fungus is probably a weak parasite, since the
mature leaves are more often spotted and that plants are seldom
defoliated. The losses caused by this disease are not important.
Symptoms: Alternaria leaf spot is readily identified by the con-
spicuous spots on the leaf blade. These spots are irregular in






Florida Agricultural Experiment Station


outline, black and shiny above, dull brownish black beneath and
definitely outlined. The young spots are small, more or less cir-
cular and brown while the old spots are dried, black and often
cracked.
Control: This disease is of minor importance and does not war-
rant control measures.


Fig. 16.-Alternaria leaf spot of sweet potato.

MOLD
Mold is caused by the fungus Choanephora cucurbitarum (B. &
Br.) Thax. The disease has been collected on sweet potatoes
during several past seasons, but is not common except during
rainy summer months. The mold is often found on the declining
flowers of various plants and on certain cucurbit fruits. The
fungus is considered to be one of a class of weak-parasites and is
capable of causing damage only under the most favorable en-
vironmental conditions, consisting of warm temperatures accom-
panied by excessive moisture. The fungus attacks the leaves of
sweet potatoes, causing portions to die. The parasitized areas
become black and dry and they are usually overgrown with a







Bulletin 212, Diseases of Sweet Potatoes in Florida


Fig. 17.-Sweet potato leaf, showing lesion caused by mold, Choanephora
cucurbitarum. Below is shown an enlarged view of the lesion, in which
the fungus is visible.

coarse mold easily detected in the morning before the dew disap-
pears. Close examination of this mold reveals small black bead-
like heads supported by bronze or steely colored hairs. The dis-
ease is of little consequence and control methods are not necessary.







Florida Agricultural Experiment Station


CERCOSPORA LEAF SPOT
This leaf spot, less common than any of the others found in
Florida, is caused by a fungus, Cercospora sp.1 The disease has
been collected in Florida but has not been reported as occurring
elsewhere in the United States. It has never caused any damage


Fig. 18.-Cercospora leaf spot of sweet potato.


by defoliation, neither has it been observed in areas other than
the East Coast section of the state. The spots are angular to
roughly circular in shape, inconspicuous at first, gradually becom-
ing yellowish and finally a smoky brown color. The conidia are
produced in abundance over the surface of the yellow spots.
Because the disease is not important as a parasite, further
description and control methods are unnecessary.
1Probably not Cercospora batatae Zimm. U.S.D.A. Tech. Bul. 99:60. 1929.






Bulletin 212, Diseases of Sweet Potatoes in Florida


LEAF SPOT
Leaf spot of sweet potatoes is caused by a fungus Septoria
bataticola E.M. This disease of sweet potatoes is widely distrib-
uted over the sweet potato-growing areas of the country but no-
where is it of more than very minor importance. It is not com-
mon in Florida and has not been collected often.
. The disease is found only on the leaves of the plant and is recog-
nized as small dull, white spots. It is not known to attack other
plants nor other parts of the sweet potato plant.
Symptoms: The fungus attacks the leaves and produces small
spots which show on both surfaces of the leaf. These spots vary
from one-eighth to one-fourth inch in diameter, with the majority
of them nearer the smaller size. They are white in the central
area with a brown border which blends into the healthy green
portion of the leaf. In the central portion of the mature spots
small black specks appear. These small bodies, called pycnidia,
are barely visible to the unaided eye, and are the fruiting bodies
of the fungus. The spores are borne inside of these pycnidia, ooze
out in moist weather and are blown by the wind and carried by
the rain or insects to other plants. It lives from season to season
on portions of the dead potato leaves and vines, left in the field.
Control measures for this disease have never been necessary.
MINOR DISEASES
There are several field diseases of sweet potato that have been
collected in Florida but they are of very little importance. Some
of them are caused by virulent parasites and deserve mention here.
Rhizoctonia-This trouble is caused by the fungus Corticiznt
ivagum B. & C. Under most favorable conditions the fungus may
cause damping-off, stem cankers or potato lesions. These, how-
ever, are very seldom observed.
Pythium-The fungi, Pythium spp., found associated with cer-
tain diseased conditions of sweet potatoes such as mottle necrosis,
rootlet decay and certain types of damping-off are not considered
of any importance in Florida. Diseased specimens of sweet pota-
toes caused by infection by Pythium spp. seldom have been ob-
served and as far as known are rare in Florida, although they
have been observed in other parts of the United States. Because
of the scarcity of diseases caused by Pythium spp., further discus-
sion is not warranted.
Slime molds-There are occasional reports of certain slime
molds, Fuligo spp. and Physarum sp. found on sweet potato stems






Bulletin 212, Diseases of Sweet Potatoes in Florida


LEAF SPOT
Leaf spot of sweet potatoes is caused by a fungus Septoria
bataticola E.M. This disease of sweet potatoes is widely distrib-
uted over the sweet potato-growing areas of the country but no-
where is it of more than very minor importance. It is not com-
mon in Florida and has not been collected often.
. The disease is found only on the leaves of the plant and is recog-
nized as small dull, white spots. It is not known to attack other
plants nor other parts of the sweet potato plant.
Symptoms: The fungus attacks the leaves and produces small
spots which show on both surfaces of the leaf. These spots vary
from one-eighth to one-fourth inch in diameter, with the majority
of them nearer the smaller size. They are white in the central
area with a brown border which blends into the healthy green
portion of the leaf. In the central portion of the mature spots
small black specks appear. These small bodies, called pycnidia,
are barely visible to the unaided eye, and are the fruiting bodies
of the fungus. The spores are borne inside of these pycnidia, ooze
out in moist weather and are blown by the wind and carried by
the rain or insects to other plants. It lives from season to season
on portions of the dead potato leaves and vines, left in the field.
Control measures for this disease have never been necessary.
MINOR DISEASES
There are several field diseases of sweet potato that have been
collected in Florida but they are of very little importance. Some
of them are caused by virulent parasites and deserve mention here.
Rhizoctonia-This trouble is caused by the fungus Corticiznt
ivagum B. & C. Under most favorable conditions the fungus may
cause damping-off, stem cankers or potato lesions. These, how-
ever, are very seldom observed.
Pythium-The fungi, Pythium spp., found associated with cer-
tain diseased conditions of sweet potatoes such as mottle necrosis,
rootlet decay and certain types of damping-off are not considered
of any importance in Florida. Diseased specimens of sweet pota-
toes caused by infection by Pythium spp. seldom have been ob-
served and as far as known are rare in Florida, although they
have been observed in other parts of the United States. Because
of the scarcity of diseases caused by Pythium spp., further discus-
sion is not warranted.
Slime molds-There are occasional reports of certain slime
molds, Fuligo spp. and Physarum sp. found on sweet potato stems






Florida Agricultural Experiment Station


and leaves. These fungi are not parasitic on this plant and con-
sequently cause very little damage other than growing over leaf
blades and partially smothering them. No control is recom-
mended.
Texas root rot, caused by Ozonium omnivorum (Pam.) Shear
and Trichoderma rot, caused by Trichoderma konigi Oud., have
not been authentically recorded from Florida.

SOFT ROT
Soft rot, or ring rot as this disease is sometimes called, is
typically a(storage disease and is the result of attacks by the
fungus Rhizopus nigricans Ehr. This disease isqfpund univer-
sally in the United States where sweet potatoes are stored. In
Florida it is common and causes considerable losses. The fungus
spores are omnipresent and consequently infection and later
decay are liable to occur in stored sweet potatoes at any time. The
fungus will grow well within a wide range of temperatures and
different degrees of dampness. The disease is held in check by
extremely low storage temperatures and dry air. The losses, at
times 100 percent, caused by this disease are probably as great
as those from all other storage rots combined.
The disease is seldom found in the field and the fungus does not
attack any part of the plant except the potato. Infection may
take place not only at both ends but at any point where the skin
of the potato is broken. Because of rough handling during dig-
ging and storing, a large percentage of the potatoes become
bruised. These skin abrasions are openings for infection by the
fungous spores, which are always found in storage houses where
it is very difficult to clean and disinfect thoroughly enough to
kill all of them.
Symptoms: The fungus causing soft rot usually enters the
potatoes at the ends but may cause infection at any place where
the skin is broken. When infection takes place at the ends the
decay rapidly involves the whole potato and in a very few days
the potato becomes soft, mushy, of a brown color, thring\v and
soggy in appearance. It gives off pleasant, characteristic, per-
fiue-like odor that is readily recognized upon entering storage
sheds where soft rot is present. The fungus produces its fruiting
bodies in the form of numerousblack headed hyphae accompanied
by more or less superficial mold. This mold is the same as that
usually found on old, damp bread with which everyone is more or






Bulletin 212, Diseases of Sweet Potatoes in Florida


less familiar. Air currents readily carry the spores away in
visible clouds when the sporulating fungus is disturbed. The
fruiting structures may be produced both at the ends of the













Fig. 19.-Soft rot of sweet potato, showing fungus on the surface of
the potato.
potato and at any place on the surface where the skin is broken.
Infections may take place in wounds on the surface of the
potatoes and in these instances a type of ring rot may develop.
The potato decays slowly and often only a portion of it may be-
come involved. The surface of the infected area becomes sunken,
slightly darker in color and usually remains quite firm. Seldom
is it soggy and wet, except in later stages when the whole potato
is involved and even then it may dry out very rapidly and become
mummified. The ring rot is not usually more than one-fourth to
one-half inch deep.
Control: The keynote in the control of soft rot of sweet potatoes
is the prevention of injury. Handling the potatoes carefully after
digging, and moving them as little as possible between digging
alD7Tnarketing will go far towards preventing severe losses from
this cause. The various species of Rhizopus causing soft rot
thrive best at various temperatures above 600 F. with the rela-
tive humidity above 75 percent. Keeping the temperature of the
storage house at about 55' F. with good ventilation after a grad-
ual lowering from about 800 F. and observing sanitary measures
will materially assist in preventing the spread of soft rot.

JAVA BLACK ROT
Java black rot of sweet potatoes is caused by a fungus, Diplodia
tubericola (E. & E.) Taub. This disease is common in the United
States wherever sweet potatoes are stored. It is widely distrib-






Florida Agricultural Experiment Station


uted and is the most important of the dry storage decays in
Florida. In importance and prevalence in storage it is exceeded
only by soft rot. Probably one reason for relatively high losses


Fig. 20.-Sweet potato completely involved, dry and mummified by
Java black rot.
from this disease is that after potatoes are stored a few weeks
they are sorted and most rots, especially soft rot, are detected and
culled out. At this time Java black rot can hardly be detected and
consequently is placed in storage for longer periods. The fungus
is probably well distributed in most storage houses and thus in-
fection is easily made possible. Decay takes place slowly but
completely.
Symptoms: The decay cannot be detected easily until the for-
mation of the black fruiting bodies or until the potato becomes
shrunken from loss of moisture. By this time usually the potato
is entirely involved and worthless. The fungus enters the potato












Fig. 21.-Longitudinal section of sweet potato affected with Java black
rot. Infection took place at each end.






Bulletin 212, Diseases of Sweet Potatoes in Florida


from the ends or from any surface abrasions, but the early pene-
tration is not apparent from the outer surface. The fruiting
bodies develop on the surface of the potato and are black in color
and slightly raised.
Control: Sanitation in the field and storage house, and careful
handling will help minimize losses from Java black rot.
CHARCOAL ROT
This disease of sweet potatoes is caused by the fungus, Sclero-
tium bataticola Taub. The rot has been found generally distrib-
uted in the United States. In Florida it is not uncommon and
can usually be found in almost any storage house. Even though
it is quite prevalent it has not been considered a serious trouble.
It usually, though not always, occurs following injury.


Fig. 22.-Cross-section of sweet potato showing interior discoloration
caused by the charcoal rot organism.






Florida Agricultural Experiment Station


Symptoms: Charcoal rot gets its name from the appearance of
affected potatoes in the later stages of development of the disease.
In such cases the whole interior of the potato is charcoal black
and at this stage, dry and more or less powdery.
The fungus causes a decay that is first brown in color and may
involve varying portions of the potato. With age small black
sclerotia form throughout the brown area in such numbers that
the general color becomes black. The sclerotia are exceedingly
small and can be distinguished only upon close examination.
There is little or no evidence of early infection on the exterior
portion of the potato. Later, however, dessication causes the
potato to become wrinkled, lighter in weight, dry and brittle.
Control: No control is to be suggested other than storage house
sanitation and the elimination of all decaying potatoes.

DRY ROT
Dry rot of sweet potatoes is caused by the fungus Diaporthe
batatatis (E. & Hals.) Harter & Field. This disease may appear
in the seedbed and field, but is most serious as a storage decay. It
is not of much importance in Florida or in the United States.
Symptoms: Field symptoms of the disease are noticeable only
when the plant is weakened from other causes. The roots are
attacked usually at the ends and the decay progresses slowly until
the whole potato is involved. If dry rot tubers are bedded, the
young sprouts are likely to have the disease. The plant will make
considerable growth and then die slowly, turning yellow first and
later becoming brown and dry. After the death of the plant, the
fruiting bodies will appear on the stem end and upper surfaces
of the leaves.
The potatoes show the first indication of infection on the stem
end. The root shrivels and the skin becomes brown and hard.
These symptoms advance until the whole root has become mummi-
fied and discolored.
Control: Dry rot is controlled by the usual good sanitary meas-
ures and proper care in seed selection and bedding.

SURFACE ROT
Surface rot of sweet potatoes is caused by the fungus, Fusarium
oxysporum Schlect. This disease is not common and causes very
little damage in Florida.
Symptoms: Wet weather just prior to digging favors infection,
which usually takes place through small rootlets. After the pota-






Florida Agricultural Experiment Station


Symptoms: Charcoal rot gets its name from the appearance of
affected potatoes in the later stages of development of the disease.
In such cases the whole interior of the potato is charcoal black
and at this stage, dry and more or less powdery.
The fungus causes a decay that is first brown in color and may
involve varying portions of the potato. With age small black
sclerotia form throughout the brown area in such numbers that
the general color becomes black. The sclerotia are exceedingly
small and can be distinguished only upon close examination.
There is little or no evidence of early infection on the exterior
portion of the potato. Later, however, dessication causes the
potato to become wrinkled, lighter in weight, dry and brittle.
Control: No control is to be suggested other than storage house
sanitation and the elimination of all decaying potatoes.

DRY ROT
Dry rot of sweet potatoes is caused by the fungus Diaporthe
batatatis (E. & Hals.) Harter & Field. This disease may appear
in the seedbed and field, but is most serious as a storage decay. It
is not of much importance in Florida or in the United States.
Symptoms: Field symptoms of the disease are noticeable only
when the plant is weakened from other causes. The roots are
attacked usually at the ends and the decay progresses slowly until
the whole potato is involved. If dry rot tubers are bedded, the
young sprouts are likely to have the disease. The plant will make
considerable growth and then die slowly, turning yellow first and
later becoming brown and dry. After the death of the plant, the
fruiting bodies will appear on the stem end and upper surfaces
of the leaves.
The potatoes show the first indication of infection on the stem
end. The root shrivels and the skin becomes brown and hard.
These symptoms advance until the whole root has become mummi-
fied and discolored.
Control: Dry rot is controlled by the usual good sanitary meas-
ures and proper care in seed selection and bedding.

SURFACE ROT
Surface rot of sweet potatoes is caused by the fungus, Fusarium
oxysporum Schlect. This disease is not common and causes very
little damage in Florida.
Symptoms: Wet weather just prior to digging favors infection,
which usually takes place through small rootlets. After the pota-







Bulletin 212, Diseases of Sweet Potatoes in Florida


toes have been placed in storage, the characteristic spots develop.
These are circular, seldom over three-fourths inch in diameter
and grayish brown or brown in color. In the early stages the
rot does not penetrate below the vascular bundles, but later the
root dries or shrivels until all of it becomes hard and dry.
Control: Good cultural practices.


Fig. 23.-Surface rot of sweet potato. Below, early development on
potato; above, cross-section of mummied potato.
OTHER ROTS
Following are a number of fungi which produce different, but
chiefly dry, rots: Botrytis cinerea Pers., Mucor racemosus Fes.,
Alternaria sp., Penicillizum sp., Epicoccum sp., Gibberella saubi-
netii (Mont.) Sacc., Fusarium culmorum Woll., and F. acumina-
tum E. & E. Individual methods of control are unnecessary. Care
in handling and proper storage conditions will keep any or all of
them from becoming a menace to the crop.






Florida Agricultural Experiment Station


GROWTH CRACKS
Sweet potato growers suffer considerable losses during certain
seasons because of the cracking or rupturing of the potatoes.
This trouble is known as growth cracks and is the result of certain
physiological and environmental conditions of the potatoes. It is
not caused by parasites of any kind, although they may enter such
cracks and cause decay. Neither is it contagious. This trouble
may be found in sweet potato fields at any time during the grow-
ing season, with considerable variation in severity.
It is thought that this trouble is the result of too rapid growth
of the potatoes. The potato expands in its growth and, when the
pressure from within is too great to be confined by the strength
of the outer layers, the rupture takes place. These cracks usually
















Fig. 24.-Growth cracks of sweet potato.

are associated with rich soils and heavy applications of nitro-
genous fertilizer, followed by alternating wet and dry periods
after midseason. There may be other influencing factors un-
known at the present time.
These cracks may occur from end to end or they may appear
at right angles to the long axis of the potato or both may be found
on individual potatoes. There may be but a single fissure or
there may be so many on the surface of the potato that it is hardly
recognizable. The cracks may be barely through the skin or they
may be a third of the way through the potato. Cracked potatoes
are total losses to the grower, as far as marketing is concerned.






Bulletin 212, Diseases of Sweet Potatoes in Florida


Control: Control measures for growth cracks consist of good
cultural methods and well balanced fertilizer formulae. Water
should be controlled, if possible.
SEED SELECTION AND TREATMENT
Inasmuch as the majority of sweet potato diseases are the
result of parasites carried on the seed potatoes, selection of the
seed stock is one of the most important steps in the production of
a maximum marketable crop.
The proper time to select for bedding is at digging time and
not in the spring from the storage bins or pits. Hills should be
selected for high yield and of the type which the grower desires
to produce. All hills carrying discolored or diseased potatoes
should be rejected. The stems of the remaining hills should be
split and examined. Potatoes which show any discoloration are
likely to carry stem-rot and the entire hill should be cast aside.
In short, only those potatoes that are apparently free from all
diseases should be picked for seed.
Seed carefully selected in this manner deserves good care in
storage. The bins and hampers into which it is placed, should
be new or sterilized with a disinfectant. The bins should be
washed thoroughly with formaldehyde solution (2 quarts to 50
gallons of water) or bluestone solution (2 pounds to 50 gallons
of water) and allowed to dry before the potatoes are put in, or
they may also be sprayed with Bordeaux mixture or whitewashed.
The potatoes should not be disturbed again until time for bedding.
Before bedding, all seed sweet potatoes should be dipped or
treated. This will kill the spores of the various disease producing
organisms that may be present on the skin. The best solution is
bichloride of mercury (corrosive sublimate) 1 part to 1,000 of
water, which is made by dissolving 1 ounce of the crystals in 8
gallons of water, in a wooden barrel. Metal containers should
not be used. The tubers are immersed in this solution for 10
minutes and then spread out to dry without being rinsed. They
should be bedded as soon as dry.
CAUTION: Bichloride of mercury (corrosive sublimate) solu-
tion is a deadly poison to man and beast and must be kept away
from humans and animals.
BEDDING SEED SWEET POTATOES
The beds should be made on clean soil, in a sheltered location
and of convenient size, so that the middle may be reached without






Bulletin 212, Diseases of Sweet Potatoes in Florida


Control: Control measures for growth cracks consist of good
cultural methods and well balanced fertilizer formulae. Water
should be controlled, if possible.
SEED SELECTION AND TREATMENT
Inasmuch as the majority of sweet potato diseases are the
result of parasites carried on the seed potatoes, selection of the
seed stock is one of the most important steps in the production of
a maximum marketable crop.
The proper time to select for bedding is at digging time and
not in the spring from the storage bins or pits. Hills should be
selected for high yield and of the type which the grower desires
to produce. All hills carrying discolored or diseased potatoes
should be rejected. The stems of the remaining hills should be
split and examined. Potatoes which show any discoloration are
likely to carry stem-rot and the entire hill should be cast aside.
In short, only those potatoes that are apparently free from all
diseases should be picked for seed.
Seed carefully selected in this manner deserves good care in
storage. The bins and hampers into which it is placed, should
be new or sterilized with a disinfectant. The bins should be
washed thoroughly with formaldehyde solution (2 quarts to 50
gallons of water) or bluestone solution (2 pounds to 50 gallons
of water) and allowed to dry before the potatoes are put in, or
they may also be sprayed with Bordeaux mixture or whitewashed.
The potatoes should not be disturbed again until time for bedding.
Before bedding, all seed sweet potatoes should be dipped or
treated. This will kill the spores of the various disease producing
organisms that may be present on the skin. The best solution is
bichloride of mercury (corrosive sublimate) 1 part to 1,000 of
water, which is made by dissolving 1 ounce of the crystals in 8
gallons of water, in a wooden barrel. Metal containers should
not be used. The tubers are immersed in this solution for 10
minutes and then spread out to dry without being rinsed. They
should be bedded as soon as dry.
CAUTION: Bichloride of mercury (corrosive sublimate) solu-
tion is a deadly poison to man and beast and must be kept away
from humans and animals.
BEDDING SEED SWEET POTATOES
The beds should be made on clean soil, in a sheltered location
and of convenient size, so that the middle may be reached without






40 Florida Agricultural Experiment Station

stepping on the bed. The soil in which the potatoes are set and
covered should not have come in contact with sweet potatoes
before or it should be sterilized. Clean soil from the woods is
usually entirely satisfactory. If the board sides are used from
year to year, they must be washed with the disinfectant recom-
mended above for treating the bins.
In bedding the potatoes, they should be placed one-half to
one inch apart to check possible infection spreading rapidly in
the bed. Water should be applied sparingly and only in sufficient
amounts to insure good growth. Early morning is the best time
to water. Covers should be removed in the daytime, especially in
moist weather, to allow proper ventilation.
When pulling the plants or draws, discard all spindly or sickly
looking plants. These will likely die in the field, spread diseases
and give an uneven stand.




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