Sweetpotato diseases

Historic, archived document

Do not assume content reflects current
scientific knowledge, policies, or practices.

I

Farmers' Bulletin No. 1059

U. S. DEPARTMENT OF AGRICULTURE

Contents

Page

Field diseases and their control 3

Stem rot (wilt, blue stem,

yellow blight) 3

Black rot (black shank, black

root) 6

Foot rot (die off) 8

Scurf (soil stain, rust, Jersey

mark) 9

Root rot (Texas root rot) 10

Mottle necrosis 12

Soil rot (pox, ground rot) 13

Phyllosticta leaf blight 14

Septoria leaf spot 15

White rust (leaf mold) 15

Root knot 16

General control measures for field

diseases 17

Seed disinfection 17

Hotbed sanitation 18

Vine or sprout (bed) cuttings- 18

Page

Storage rots and their control 19

Soft rot (ring rot, collar rot) . 19

Black rot 20

Internal cork 21

Surface rot 22

Dry rot 23

Java black rot 24

Charcoal rot 25

Control measures for storage rots. 25
Digging and handling sweet-
potatoes 25

Management of the storage

house 25

List of causal agents of sweetpo-

tato diseases 26

Washington, D. C.

Revised March 1955

For sale by the Superintendent of Documents. IT. S. Government Printing Office
Washington 25, D. C. - Price 15 cents

2

Sweetpotato Diseases

Prepared by Horticultural Crops Research Branch, Agricultural Research Service,
and the Biological Sciences Branch, Agricultural Marketing Service 1

The sweetpotato is one of the
principal food crops of the South.
Field diseases and storage rots 2
each reduced yields by about iy 2
percent for the period 1940-52.
Field diseases reduce yield, affect
the quality, and cause roughness,
poor color, and poor shape. Suc-
cessful storage depends partly on
the control of field diseases, some of
which are also destructive storage
diseases.

Black rot, both a field and a stor-
age disease that gives sweetpotatoes

a bitter taste when cooked, probably
causes as much loss as all the other
diseases combined. If black rot
alone could be eradicated or effec-
tively controlled, the losses in stor-
age would be greatly reduced.

Decay caused by soft and ring
rots and other storage diseases re-
duces the quantity of sweetpotatoes
for market and shortens the period
they can be marketed. Displaying
diseased sweetpotatoes also has a
bad influence on the market.

Field Diseases and Their Control

Stem Rot
(Wilt, Blue Stem, Yellow Blight)

Description

In the field the first indication
of stem rot is a slight change in
the appearance of the youngest
leaves. These become dull, then
yellow between the veins, and
pucker somewhat. Then the vines
wilt and eventually the entire plant
collapses and dies (fig. 1). The
stems of diseased plants darken in-
side and sometimes split open at
about the ground level. This dis-
coloration of the stems sometimes
extends 3 to 5 feet from the hill and
this is a sure sign of stem rot. The
fungus causing stem rot may also
invade the fleshy roots and cause a

blackened ring about a quarter of
an inch below the surface (fig. 2).
Sprouts from such sweetpotatoes
are likely to be diseased.

In the plant bed the symptoms of
stem rot are similar to those in the
field. Diseased plants can gener-
ally be detected by the faint pur-
plish tint that shows through the
white part of the stem and by the
yellow color of the leaves.

Distribution, Prevalence, and Loss

Stem rot probably occurs in every
State where sweetpotatoes are
grown. It is an important field dis-
ease and the most difficult to control.

1 Revised by Harold T. Cook, principal pathologist, Biological Sciences Branch,
Agricultural Marketing Service. The original edition was written by L. L. Harter.

2 Scientific names of causal organisms (fungi and nematodes) are listed on p. 26.

3

Figure 1. — Stem rot symptoms of a sweetpotato plant.

The disease is severe in New Jer-
sey, Delaware, Iowa, in parts of
Kansas, and in southern Illinois.
From 10 to 50 percent of the crop
may be destroyed in those States
each year, and it has killed 95 per-
cent of the plants in some fields. Al-
though the losses in Maryland, Vir-

Fif/nre 2. — A section through a sweet-
potato, showing the blackened ring
caused by the stem rot fungus.

ginia, and Alabama are consider-
able each year, they are relatively
less than in other States.

Means of Distribution

The stem rot fungus can overwin-
ter in the soil on the remains of dead
sweetpotato vines and in the roots
in storage. Therefore, the distri-
bution of the disease from one field
to another in the same locality may
be brought about by (1) insects, (2)
farm animals, (3) farm imple-
ments, (4) drainage water, (5)
wind, and (6) discarded diseased
roots dumped on the fields, either
before or after being fed to stock.

The disease is spread from one lo-
cality to another primarily by the
exchange or sale of seed sweetpota-
toes and plants.

Cause

The fungus that causes stem rot
can live for several years on decayed

4

vegetation in the soil until it again
comes in contact with the sweetpo-
tato.

Infection takes place both in the
plant and in the field. The fungus
in diseased seed stock planted in the
plant bed grows into the plants.
Such infected plants die soon after
they are set in the field. Healthy
plants may become infected after
they are set in the field when the
fungus in the soil grows into the
roots.

The mycelium (threadlike web)
of the fungus develops rapidly and
grows up through the water-carry-
ing vessels of the stem. After the
vines die and turn black, the fungus
lives on the decaying vegetation.
Numerous fruiting bodies, or spores,
develop on the dead vines. Being
very small, the spores are readily
carried by the wind, insects, and
other agencies to other fields.

Control

Fertilizers and Fungicides. — Be-
cause the fungus causing stem rot
invades the plant through its roots,
fungicides applied on the plants or
on the roots will not control the dis-
ease. Applications of lime and
gypsum to the soil are of no control
value.

Immune and Susceptible Vari-
eties. — The following varieties,
none of which are entirely immune,
can be grown with comparative
safety in infested soil : White Yam,
Southern Green, Triumph, Eed
Brazil, Yellow Strasburg, Key
West, and Dahomey. The follow-
ing varieties are very susceptible to
stem rot : Yellow Jersey, Big-Stem
Jersey, Gold Skin, Nancy Hall,
Porto Eico, Red Jersey, Georgia,
Nancy Gold, Kansas 40, and Mary-
land Golden.

Seed Selection. — The stem rot
fungus overwinters in sweetpotatoes
in the storage house and grows from
diseased stock into the plants de-
veloped from them. Slightly dis-

eased plants are hard to detect, and,
in consequence, many of them are
set in the field, where the fungus
continues to grow. It is, therefore,
imperative that you use only healthy
sweetpotatoes for the production of
plants.

Healthy seed stock can be selected
in the fall at digging time while the
sweetpotatoes are still attached to
the vines. Test each hill by split-
ting the stems, and select seed only
from plants with stems that are not
streaked inside with black. Do this
before a killing frost, as a heavy
frost will sometimes darken the
stem. Fall selection of seed stock
is necessary, for it is difficult in the
spring or during the winter to tell
whether the sweetpotatoes are dis-
eased. After a period in storage a
blackened ring a quarter of an inch
below the surface often occurs even
in healthy sweetpotatoes.

Store the sweetpotatoes selected
for seed in a part of the house where
they will not come in contact with
the general stock.

Seed Disinfection and Bedding. —
In the spring just before the roots
are bedded, disinfect them by treat-
ing in a solution of corrosive sub-
limate (mercuric chloride) or of
borax (p. 17). This treatment kills
only the fungus spores that may be
on the surface of the root and will
not kill the fungus within the root.

Bed the sweetpotatoes immedi-
ately after the treatment in a prop-
erly prepared plant bed (p. 18).

Crop Rotation. — The stem rot
fungus will live in the soil indefi-
nitely, even in the absence of sweet-
potatoes. For that reason, do not
plant sweetpotatoes on the same
ground oftener than once in 3 or
4 years. This rotation will not erad-
icate the fungus, but will reduce the
losses. No other crop except tobacco
is known to be attacked by this fun-
gus ; therefore, any crop except to-
bacco commonly grown in the re-
gion may be used in the rotation.

5

Black Rot

(Black Shank, Black Root)

Description

Black rot may occur on any of the
underground parts of the plant. It
produces dark to nearly black some-
what sunken, circular spots on the
surface of sweetpotatoes (fig. 3).

Figure 3. — Black circular spot on sweet-
potato caused by the black rot fungus.

When the plants are young, these
spots are small and nearly round;
under favorable conditions they en-
large and often involve nearly the
whole sweetpotato. Fruiting bod-
ies, or spores, of the fungus may
often be found in circular areas
about one- fourth to one-half inch in
diameter in the center of the spots.
The surface of the diseased spot has
a somewhat metallic luster and the
tissue just beneath is greenish. In-
fection on the plants begins as small
black spots on the lower part of the
stem and enlarges until the whole
stem is rotted off. Frequently the
infection extends up the stem to the
surface of the soil (fig. 4). The
name "black shank" is commonly
applied to this phase of the disease.
If sweetpotatoes affected with black
rot are used for seed, the plants com-

Figure 4. — A small sweetpotato plant
showing the characteristic blackening
of the underground part of the stem
caused by the black rot fungus.

6

ing from them will likely have the
disease.

Sweetpotatoes affected by black
rot have a very disagreeable taste
when cooked, and their sale has a
bad effect on the market.

Distribution, Prevalence, and Loss

Black rot is present in most of the
States where sweetpotatoes are
grown. The disease occurs on the
plants or sprouts in the hotbeds, in
the fields, and on the roots in stor-
age houses in the winter. Heavy
losses are caused by this disease in
storage houses, where it develops
freely under favorable conditions
and renders the sweetpotatoes unfit
for sale.

Means of Distribution

Black rot is spread in about the
same way as stem rot. Unlike stem
rot, however, black rot continues to
develop in the storage house, and
sweetpotatoes that appear sound
when stored may become badly af-
fected within a few weeks. Spread
in the storage house may be brought
about by rodents, by handling the
roots when you are picking them
over and preparing them for mar-
ket, and by the roots settling in the
bins. Washing the sweetpotatoes
also may distribute the germs from
one sweetpotato to another.

Cause

The black rot fungus overwinters
on the dead vines and other decayed
vegetable matter in the soil and on
the sweetpotatoes in storage. No
host plants other than sweetpotatoes
are known. If roots affected by
black rot are used as planting stock,
the fungus usually grows into the
plants while in the plant bed. In-
fection also takes place through the
roots after the plants are set in the
field. Plants that become infected
early soon die, and those that con-
tinue to grow rarely produce any
sweetpotatoes.

Control

None of the present commercial
varieties are resistant to black rot.

Black rot is easily controlled by
using disease-free seed sweetpota-
toes, root disinfection, clean plant
beds, and crop rotation.

The most practical way to obtain
black-rot-free bedding stock is to
grow seed sweetpotatoes from vine
cuttings or sprout (bed) cuttings
(p. 18 ) . Since black rot affects only
the below-ground parts of the plant,
these cuttings will be free of the dis-
ease. They will produce disease-
free sweetpotatoes that can be used
for bedding the next year, if the cut-
tings are planted in fields that are
free of black rot. Sorting out
black - rot - infected sweetpotatoes,
either at harvesttime or before bed-
ding in the spring, is not a satisfac-
tory way to obtain bedding stock.
Many small, recent infections es-
cape detection, and the disease con-
tinues to develop in storage or in the
plant bed.

Disinfect the seed with either cor-
rosive sublimate or borax to destroy
black rot spores that may be on the
surface. Disinfection does not kill
the fungus inside of black-rot-in-
fected sweetpotatoes (p. 17).

Bed the seed stock immediately
after treatment in a properly pre-
pared clean plant bed (p. 18) .

Plant the sprouts on new ground
or on soil that has not been used for
sweetpotatoes for 3 or 4 years. Ro-
tation of sweetpotatoes with other
crops helps control black rot, be-
cause the fungus does not affect
other crops and is only able to live
about 2 or 3 years in the soil.

Treating the soil with sulfur,
lime, gypsum, or various fertilizers
has little effect on the disease. Dip-
ping the plants in a solution of bor-
deaux mixture or in a lime-sulfur
mixture just before they are set in
the field may reduce the incidence
of the disease, but does not prevent
it entirely and has been found to
injure the plants.

7

Foot Rot

(Die Off)

Description

Foot rot appears first as small
brown to black spots on the stem
of the plant near the soil line. The
growth of the foot rot fungus is
very slow at first, but eventually it
girdles the plant and extends up the
stem 4 or 5 inches. Soon the plant
wilts, and rather numerous round,

Figure 5. — The lower part of a sweet-
potato plant killed by the foot rot
fungus.

black specks, just visible to the
naked eye, appear in the diseased
areas (fig. 5) . These specks are the
fruiting bodies of the fungus. This
disease progresses rather slowly,
and the plants do not die off until
about midsummer or later. In
most instances the affected plants
bear no sweetpotatoes, although
long vines may have been produced.
In hills in which the sweetpotato

Figure 6. — A sweetpotato rotted by the
foot rot fungus.

8

develops, the organism causing foot
rot may spread from the infected
stem to the roots and cause a brown
and rather firm rot. Later, fruit-
ing bodies develop close together on
the surface in the form of pimple-
like protuberances (fig. 6). Many
wounds and bruises on sweetpo-
tatoes in storage become infected
with the foot rot fungus.

Distribution, Prevalence, and Loss

Foot rot is distributed in the same
way as stem rot and black rot. It
occurs in Virginia, Maryland, Ohio,
South Carolina, Iowa, California,
and Mississippi. Because it is not
so widely distributed as black rot
and stem rot, the tottJ loss from
foot rot is much less. In certain
parts of Virginia, Ohio, and Iowa,
it causes heavier losses than either
black rot or stem rot.

Cause

Infection from the foot rot fun-
gus takes place primarily through
the roots or underground parts of
the plant. However, during wet
periods, when the growth is very

luxuriant, diseased vines are some-
times found some distance from the
hill. Infection takes place mostly
in the hotbed by spreading from
diseased seed stock to the plants.
Such plants when set in the field
usually die early in the season.

Spores, borne in great numbers,
escape from the pimplelike projec-
tions of the diseased tissue and are
carried by insects or other agencies
to other plants, where new infec-
tions may result. If a diseased
plant produces sweetpotatoes, the
fungus often grows down the stem
and infects them. The fungus
may remain dormant during the
storage period, but it will develop
on the sprouts in the plant bed. As
in the cases of stem and black rots,
therefore, diseased seed stock pro-
duces diseased plants, which in turn
may produce diseased sweetpotatoes
in the field.

Control

For control of foot rot, follow
recommendations for stem rot and
black rot — seed selection, the use of
clean plant beds, seed treatment,
and crop rotation.

Scurf

(Soil Stain, Rust, Jersey Mark)

Description

Scurf produces a brown surface
discoloration of the root (fig. 7).
The discolored areas may take the
form of spots of different sizes and
shapes with no definite outline, or
there may be a uniform rusting of
the surface of the sweetpotato.
Scurf is usually worse at the stem
end. The skin of the sweetpotato
is not broken, and the brown color
is only skin deep and can be scraped
off easily with the fingernail.

Distribution, Prevalence, and Loss

Scurf is commonly found almost
everywhere that sweetpotatoes are

grown and on nearly all varieties.

The losses to the crop caused by
scurf are perhaps small in compari-
son with those caused by some of
the other diseases. Nevertheless,
scurfy sweetpotatoes do not com-
mand so high a market price as
clean ones, although they are just
as good for food.

Scurf, under favorable conditions
of relatively high humidity and
temperature, continues to develop
under storage conditions to a
limited degree. It may damage
the sweetpotato skin, so that when
the storage house is rather dry the
root loses moisture and becomes
shriveled and dried.

324275°— 55 2

9

Figure 7. — A sweetpotato showing dis-
coloration caused by the scurf fungus.

Cause

The scurf fungus overwinters in
storage and on the decayed vines
and other decayed vegetable mat-
ter in, the field. If infected sweet-
potatoes are used for seed, the
fungus grows up on the stem of the
plants and is carried on them to the
field. Later, the organism in the

field grows down onto the roots of
the sweetpotatoes. Scurf is most
severe on heavy soils and on those
containing a large quantity of or-
ganic matter. It is likewise more
severe during a wet season and on
low, wet ground. Such soils should
be avoided.

Control

You can control scurf easily by
practicing crop rotation and using
clean planting stock. Clean plant-
ing stock is most easily obtained by
planting either vine cuttings or
sprout (bed) cuttings in soil that
has not been planted to sweetpota-
toes for 3 or 4 years (p. 18) . Since
scurf affects only the underground
parts, the cuttings will be free of the
fungus. If you plant the cuttings
in scurf- free soil, the sweetpotatoes
produced will be free of that dis-
ease. Bed the scurf- free sweet po-
tatoes produced from the cuttings
the following year, and pull the
sprouts in the usual manner. Se-
lection of scurf-free seed by sorting
is impractical and not very effective,
because many of the scurf spots are
too small to detect.

Plant the disease-free bedding
stock in a clean plant bed. Other-
wise, the sprouts from the disease-
free sweetpotatoes may become in-
fected with scurf from the bedding
soil.

Seed treatment with a corrosive
sublimate solution containing wet-
table sulfur partly reduces scurf
(p. 17), but treatment with plain
corrosive sublimate is not effective.

Root Rot

(Texas Root Rot)

Description

Root rot causes a firm brown rot,
resulting in complete destruction of
the sweetpotato (fig. 8). Above-
ground, the growth is within the

stem and may be detected by the
brown color produced. The causal
agent produces coarse brown or
gray strands of the fungus on the
surface of the roots that can be de-
tected easily with a hand lens.

10

Distribution, Prevalence, and Loss

Root rot occurs in Texas, New
Mexico, Oklahoma, Arizona, Cali-
fornia, Arkansas, and Nevada.
When the disease once gets into a
field a crop may be destroyed.
Large fields have been seen in which
not more than 10 percent of a crop
was produced. The growing crop
appeared normal when viewed from
a distance, but when harvested
nearly all of the sweetpotatoes were
found destroyed.

The causal organism lives from
one season to the next in the soil on
dead vegetable matter and probably
on growing winter crops and weeds.
It is killed by hard freezing, and
this alone probably restricts the dis-
ease to the Southern States. The
disease may be observed occasion-
ally as early as May or June, but it
does not become serious until Au-
gust when the vines are usually well
developed and the sweetpotatoes are
of considerable size. From August
on the disease increases in severity,
and by harvesttime in September
and October, a large percentage may
be destroyed. The disease may oc-
cur in spots of various sizes within
a field. Not all hills and not all
sweetpotatoes in a hill are neces-
sarily destroyed.

Cause

The root rot fungus lives from
one season to the next in the soil
and on seed sweetpotatoes. The or-
ganism gains access to the under-
ground parts of sweetpotato plants
and spreads in both directions, in-
vading the vines from 6 to 12 inches
above ground. It may enter the end
of the sweetpotato or may cause
spots of varying sizes on the surface.

Control

Root rot is more severe on black,
poorly drained soil and during wet
seasons. The disease is very difficult
to control or eradicate, because it

Figure 8. — The characteristic shriveling
produced by the root rot fungus.

11

occurs on a great variety of plants, tate crops, and use disease-free
It is particularly destructive on cot- sweetpotatoes for seed. Use grasses,
ton and alfalfa. To control root corn, and other cereals in the rota-
rot, cultivate deep and clean, aerate tion, as they are partially or com-
the soil, apply stable manure, ro- pletely immune to the disease.

Mottle Necrosis

Description

Mottle necrosis, a field disease of
sweetpotato, produces brownish,
somewhat sunken spots, which are
irregular in shape and size (fig. 9).
Usually the sweetpotato remains
more or less firm. Cutting the
root crosswise through one of the
brown, sunken surface spots reveals
the most striking symptom of the

Figure 9. — A sweetpotato with a large
part of the surface brown and some-
what sunken, a condition characteristic
of the advanced stage of mottle
necrosis.

Figure 10. — A cross section through a
sweetpotato, showing the characteristic
mottling of mottle necrosis.

disease : irregularly shaped patches
of chocolate-brown dead tissue
which appear to have no connection
with one another and give the cross
section a marbled appearance (fig.
10) . The entire sweetpotato may be
involved even though there is but
a small spot of diseased tissue on the
surface.

Distribution, Prevalence, and Loss

Mottle necrosis occurs in most
States where sweetpotatoes are
grown. It is not so prevalent, how-
ever, in the South as in the northern
part of the sweetpotato-growing
area. The loss varies from year to
year, depending upon soil and
weather conditions and on the va-
rieties grown. The entire loss
throughout the country is relatively
small. However, in certain isolated
districts where such varieties as the
Yellow Jersey are grown, losses as
high as 40 percent of the crop some-
times occur during seasons espe-
cially favorable to the disease.

12

Cause

Mottle necrosis may be caused by
either of two fungi. These fungi
probably enter through the small
fibrous roots and spread through all
parts of the sweetpotato.

Control

No method for the control of
mottle necrosis has been worked out.
The disease is most severe during

seasons of abundant rainfall and in
soils that are fairly light and sandy,
although some infection may occur
in fairly heavy soils. Very suscepti-
ble varieties are Triumph, Yellow
Jersey, Big-Stem Jersey, and
Georgia. Occasionally other va-
rieties may be slightly infected.
Do not plant susceptible varieties
in soils where mottle necrosis has
occurred in the preceding 3 or 4
years.

Soil Rot
(Pox, Ground Ret)

Description

Soil rot produces symptoms very
different from those of other sweet-
potato diseases. In a heavily in-
fested soil the plants are dwarfed
and often produce only one or two
short vines. The leaves are small,
thin, and pale green. The above-
ground symptoms are the result of
injury to the root caused by the dis-
ease. Any of the underground parts
of the plant may be attacked. Many
of the lateral feeding, or fibrous,
roots are destroyed, and those that
remain are often more or less mal-
formed. Nearly black flecks, or
spots, of varying sizes and appear-
ance, occur on the feeding roots and
underground part of the stem. The
decayed spots may occur on only one
side of the root or may girdle it,
thereby cutting off the food supply.
In the early stages of soil rot the
diseased spots seem to be covered by
the skin of the sweetpotato, which
later breaks, leaving conspicuous
holes or pits. On the swollen roots
these pits often attain a diameter of
one-half inch or more and have a
jagged margin (fig. 11). The en-
larged root is sometimes girdled;
the sweetpotato continues to enlarge
on each side of the point of infec-
tion and becomes dumbbell-shaped.

Figure 11. — A sweetpotato showing typi-
cal soil rot pits.

13

Distribution, Prevalence, and Loss

Soil rot occurs in California,
more or less generally in the South-
ern States, and in practically all
Northern States where sweetpota-
toes are grown. It has become a
limiting factor in sweetpotato pro-
duction in Louisiana. The disease
does not occur generally throughout
a State, but it is more or less local-
ized. It may be bad in one field or
locality and absent in another only
a few miles away.

The loss caused by soil rot may
range from practically nothing to
almost complete failure in different
fields and seasons. Losses are most
severe during dry seasons and on
poor soils. Soil rot is especially
severe on soils that are alkaline or
only slightly acid. It does not de-
velop much in fairly acid to acid
soils ; that is, with a soil reaction of
pll 5.2 or less.

Cause

The soil rot fungus lives in the
soil from one season to the next.
Most of the infections probably oc-
cur after the plants are set in the
field, although infection may result
from infested soil in the hotbed and
from infected seed sweetpotatoes.

Control

No adequate control measure is
known for soil rot. Some research
results indicate that application of
sulfur to reduce the soil reaction to
pH 5.2 or less will reduce the
amount of soil rot and increase the
yield. Apply sulfur with consider-
able care, as it may make the soil re-
action unfavorable for succeeding
crops. Apply the sulfur broadcast
and incorporate it thoroughly into
the soil 2 to 4 weeks before the
plants are set out. The amount of
sulfur you should use will depend
on the soil type and pH. Use stable
manure and green-manure crops to
improve the soil and rotate crops.

Avoid introduction of the soil rot
organism into new fields and new
localities. Since cattle or other live-
stock can carry the germs on their
feet, do not allow them to roam
from infested to disease-free fields.
Thoroughly clean plows and other
farm implements used to cultivate
infested fields before the equipment
is taken into clean fields. If you
purchase plants from outside
sources, be sure they are free of soil
rot. Obtain disease-free planting
stock for use on new fields by means
of vine or sprout cuttings (p. 18) .

Phyllosticta Leaf Blight

Phyllosticta leaf blight causes
roundish or angular brownish spots
one-eighth to one-half inch in diam-
eter on the upper side of the leaf
( fig. 12) . A number of black bodies
about the size of a pin point and just
visible to the naked eye are scat-
tered within the spots. The bodies
are slightly raised, round, and con-
tain numerous colorless spores.

The fungus does not live on any
other plant, nor does it occur on

other parts of the sweetpotato plant.
It probably lives through the winter
on the dead leaves. The disease oc-
curs every year in practically all the
Southern States, but it is less com-
mon in New Jersey, Delaware,
Maryland, Iowa, Kansas, and Illi-
nois.

Phyllosticta leaf blight has never
been serious enough to require reme-
dial measures.

14

Figure 13. — A sweetpotato leaf showing
white spots caused by the septoria leaf
spot fungus.

Figure 12. — Circular phyllosticta leaf
blight lesions on a section of a sweet-
potato leaf.

Septoria Leaf Spot

Septoria leaf spot is character-
ized by circular, white spots $bout
one-eighth inch in diameter on the
upper surface of the leaves (fig. 13) .
Within these white areas are one or
more black specks, just visible to
the naked eye. These specks* con-
tain numerous spores that, upon es-
caping, may be carried by insects or
other agencies to other leaves and
start new infections. Like the or-
ganism causing leaf blight, this fun-

gus does not live on other plants or
on other parts of the sweetpotato
than the leaves. It probably over-
winters on the dead leaves in the
field.

Septoria leaf spot is very widely
distributed, having been collected in
most States where sweetpotatoes are
grown. This disease is not serious
enough to require remedial meas-
ures.

White Rust
(Leaf Mold)

The first symptom of white rust
is the loss of the green color in
spots on the underside of the leaf
(fig. 14) . Later these spots become

brown and covered with a whitish,
viscid growth, which finally be-
comes more or less powdery. This
powdery white mass is made up of

15

Figure 14. — Sweetpotato leaf injury
caused by the white rust fungus.

numerous spores. These spores
fall on other leaves and under fa-

vorable conditions will cause new
infections. No great harm results
from the attack of this fungus,
though it may sometimes produce
swellings on the stems and petioles
(leaf stems) and cause malforma-
tions of the leaves and young shoots.
White rust is widely distributed and
occurs on a number of other plants,
among them- the wild morning-
glories.

White rust is more prevalent dur-
ing wet seasons. It is frequently
found on sweetpotato plants in most
of the Southern States. Under
favorable weather conditions it oc-
curs in New Jersey and other North-
ern States where sweetpotatoes are
grown. This disease has never been
serious enough to require remedial
measures.

Root Knot

Description

Root knot is a nematode disease
characterized by small galls or
swellings on the fine feeder roots,
stunting, and yellowish plants. The
vines are seldom killed. On the
sweetpotatoes this disease causes
surface blemishes, pitting, and
sometimes severe cracking. Inside
the sweetpotato and roots, the nema-
todes may cause brownish spots
about one-sixteenth inch or smaller
in diameter. Most of the spots oc-
cur within one-fourth inch of the
surface, but some spots may extend
as much as 1 inch deep.

Distribution, Prevalence, and Loss

Root knot occurs wherever sweet-
potatoes are grown, but it is usually
considered a minor disease of that
crop. In some fields it may cause
serious reduction in yield and qual-
ity.

The root-knot nematodes are able
to live from one season to another
in the soil and in sweetpotatoes in

storage. They are spread by means
of root-knot-infected seed sweet-
potatoes and plants, by infected
transplants of other kinds of plants,
and by farm animals, farm imple-
ments, drainage water, and wind.

Cause

Root knot is caused by root-knot
nematodes (sometimes called nemas
and eelworms) . The nematodes
feed on the roots of hundreds of
kinds of plants. While most com-
mon in the Southern States, root-
knot nematodes may occur in any
part of the country where sweetpo-
tatoes are grown. Soil in which in-
fected plants have been grown con-
tains numerous nematode larvae,
which are slender microscopic
worms about one- fortieth inch long.
These enter the roots and develop to
become males, which are slender
worms about one-twentieth inch
long, or females, which are pear-
shaped and about one-twentieth
inch long by one-thirtieth inch
wide. The males leave the roots,

8 Prepared by A. L. Taylor, nematologist, Agricultural Research Service.

16

but the females remain embedded
during their whole lives. Each
female may produce several hun-
dred eggs.

Control

For control of root knot in sweet-
potatoes use plants that are free of
nematodes and plant them in soil
free of nematodes. Plants grown
from infected seed sweetpotatoes or
in plant-bed soil infested with root-
knot nematodes will certainly be-
come infected and carry the infec-
tion to the field soil. Slice samples
from the lot of sweetpotatoes that
are to be used for seed and examine
for the brownish spots that indi-
cate the presence of root-knot
nematodes. If you find nematodes,
the lot should be discarded. If there
is any possibility that the soil to be
used for the plant bed has been in-
fested with nematodes, fumigate it
before use. For this purpose, fumi-
gants having as the active ingredi-
ent methyl bromide, ethylene di-
bromide, or a mixture of dichloro-
propene and dichloropropane are

satisfactory. These soil fumigants
are sold under various trade names
and should be used as recommended
by the manufacturer.

Control root-knot nematodes in
field soil by crop rotation or by soil
fumigation. Since suitable rotation
crops vary in different parts of the
country, consult your county agent
or the State agricultural experiment
station. For field-soil fumigation,
use either ethylene dibromide or
dichloropropene - dichloropropane
(D-D) fumigants as directed by the
manufacturer.

Several sweetpotato varieties
have been reported as being highly
resistant to root-knot nematodes at
various locations. However, it is
now known that sweetpotatoes may
be attacked by several different spe-
cies of root-knot nematodes, and
sweetpotato varieties that show re-
sistance at one location may not be
resistant at another location where a
different species of nematode is
present. Local experience is the
best guide in selecting resistant va-
rieties.

General Control Measures for Field Diseases
Seed Disinfection

Disinfect sweetpotato roots just
before they are bedded by dipping
them for 8 to 10 minutes in a solu-
tion made by dissolving 1 ounce of
corrosive sublimate (mercuric chlo-
ride) in 8 gallons of water. Use
only wooden vessels for the disin-
fecting solution.

To control scurf, add 5y 2 pounds
of wettable sulfur to 24 gallons of
the corrosive sublimate solution.
This treatment will not kill fungi
within the sweetpotato, but it will
destroy spores on the surface.
After about 10 bushels have been
treated in 24 gallons of solution, add
one-half ounce of corrosive subli-
mate dissolved in hot water and

make up the solution to the original
volume by adding water. Repeat
this process after the treatment of
each 10 bushels of seed until 30
bushels are treated. Then discard
the solution and prepare a fresh one.

If corrosive sublimate cannot be
obtained, disinfect the seed sweet-
potatoes by immersing them for 5
minutes in a 2- to 2i^>-percent solu-
tion of borax. Prepare this solu-
tion by dissolving 5 pounds of bo-
rax in 30 gallons of water. Borax
can usually be purchased at a gro-
cery store. The disinfecting quality
of the borax is not reduced by re-
peated use and it can be used in
metal vessels. However, if the vol-

324275°— 55 3

17

ume of the solution does not cover
the sweetpotatoes, add more solu-
tion.

Red the sweetpotatoes immedi-
ately after they are treated and
water them thoroughly. Otherwise
chemical injury and reduced sprout-
ing may result.

Excessive amounts of boron are
injurious to plants ; therefore, do
not pour the unused portion of

the borax solution on land to be
used for crop production. Corro-
sive sublimate is very poisonous.
Sweetpotatoes that have been
treated with either corrosive sub-
limate or borax should never be
fed to animals or used as food.
The chemicals and the solutions
remaining after treating should
be carefully disposed of or kept
out of reach of children and
animals.

Hotbed Sanitation

The repeated use of the same soil
year after year in the plant bed is
probably one of the chief means of
distributing many sweetpotato dis-
eases. Disease-producing organ-
isms multiply in the rotting roots
and manure. If the same soil is used
the next year, the sweetpotatoes are
at once exposed to infection. Fur-
thermore, when bedding sweetpota-
toes, farmers frequently throw the
diseased ones to one side. These
eventually become mixed with the
soil, and the disease germs may be
carried on the shoes and by chick-
ens and other agents to the plant
bed. As a result, plant beds that
might otherwise produce healthy
plants become badly infected.

Haul away all soil that has been
used once in the plant bed and all
the rubbish around the bed. Soak
thoroughly the framework of the
plant bed and the ground around it
with a solution of 1 pint of commer-
cial formalin in 30 gallons of water
or with a solution of 1 pound of

copper sulfate in 25 gallons of
water. Repeat this treatment after
about 24 hours. Obtain the soil, or
preferably sand, for the hotbed
from a place where sweetpotatoes
have never been grown. Rich soil
is not necessary for the hotbed; in
fact, some of the best results have
been obtained by using pure sand.
Use a grade of sand or soil that will
not bake or form a crust. Clean
the farm implements that were used
to handle and haul away old soil
and disinfect the implements with a
solution of formaldehyde before
new soil is handled.

The use of stable manure in the
hotbed is a questionable practice
unless you have taken care not to
feed discarded sweetpotatoes to
stock or throw them on the manure
pile. Do not throw diseased sweet-
potatoes in the yard where infected
parts may be carried around on the
feet of poultry, other farm animals,
and workers.

Vine or Sprout (Bed) Cuttings

You may produce disease-free
seed stock easily by planting vine
cuttings or sprout (bed) cuttings in
soil that has not been planted to
sweetpotatoes for 3 or 4 years.
Since such diseases as black rot and
scurf do not occur on the above-
ground parts, they are not carried
on the vine or bed cuttings as they

are on sprouts that are pulled in
the usual way.

Make vine cuttings by cutting the
sweetpotato vines into sections so
as to include at least two buds or
leaves. Then insert one end, usu-
ally the larger, into the ground. A
disadvantage with vine cuttings is
that the cuttings cannot be made

18

until the vines have grown to a con-
siderable length.

You can obtain sprout or bed cut-
tings almost as early as sprouts.
The sprouts are allowed to grow a
few inches longer than usual and
then cut about 11/2 inches above the
soil level instead of pulling them in
the usual manner.

To obtain good results from vine
or sprout cuttings, take the follow-
ing precautions :

1. Make cuttings from vines that
are not affected with stem rot. The

stem rot organism grows out into
the vines 3 to 5 feet from the hills
and its presence cannot always be
detected without pinching open the
vine.

2. Plant cuttings on new ground
or on ground on which sweet-
potatoes have not been grown for at
least 3 or 4 years.

3. Disinfect the seed stock and
bed it in a plant bed prepared ac-
cording to directions given on
page 18.

Storage Rots and Their Control

Soft Rot
(Ring Rot, Collar Rot)

Soft rot, caused by the bread
mold, is a very destructive disease
of sweetpotatoes in storage. It
may set in soon after the crop is
placed in storage and continue to
spread throughout the storage peri-
od, depending largely on the condi-
tion of the roots when stored and on
the management of the house. - The
decay begins usually at one end and
progresses rapidly, requiring only
a few days with favorable tempera-
tures and humidity to destroy the
entire sweetpotato. At first the
soft-rot-affected sweetpotatoes are
soft, watery, and stringy. After
decay and the escape of moisture,
they gradually become firm, hard,
shrunken, and brittle. Such dry
sweetpotatoes are frequently re-
ferred to by the farmer as being af-
fected with dry rot, which in reality
is a dried-up soft rot. If the skin
is broken while the sweetpotato is
still soft, a moldy growth, some-
times referred to as whiskers, forms
on the surface (fig. 15).

The soft rot disease often spreads
from one root to another by contact.
The spores of the black mold pro-
duced on the surface may be carried

Figure 15. — A sweetpotato showing the
moldy growth, or whiskers, of the fun-
gus causing soft rot.

19

Figure 16. — A sweetpotato infected with
ring rot.

by flies or wind currents to other
roots in the same house or may be
spread to them by handling. New
infections may take place if the
spores light on a wounded surface
and if the temperature and moisture
conditions are favorable.

Ring rot differs from soft rot in
that the decay begins at a point be-
tween the two ends of the sweetpo-
tato instead of at one end. From
the point of infection the decay
forms a ring, or collar, around the
sweetpotato, and then extends
slowly toward the ends. Under
conditions favorable to the mold the
sweetpotato may be wholly de-
stroyed. If conditions unfavorable
for its further development exist,
such as a relatively low humidity
and low temperatures, it may only
form a depressed ring, or collar (fig.
16) , varying in width from 1 inch
to 3 inches.

The losses sustained in storage
from soft and ring rots amount to
majiy hundreds of thousands of dol-
lars annually. The causal organ-
ism is found everywhere and will
grow on almost any decaying vege-
table matter. It is therefore im-
possible to exclude it from stor-
age houses. The fungus generally
gains entrance to the sweetpotato
through wounds and bruises caused
by rough handling or through
wounds made by rats and mice.

Black Rol

Black rot is very serious in stor-
age as well as in the field. The loss
caused by it in storage and in the
field probably equals that of all the
other sweetpotato diseases com-
bined.

When sweetpotatoes are dug,

black rot spots are comparatively
rare. Sometimes many potatoes
may be infected, but the point of in-
fection is so small that it is invisible
to the naked eye. In the storage
house, where the temperature and
humidity are relatively high, these

20

spots gradually enlarge. At the end
of a month or two they have formed
conspicuous, somewhat round, black
sunken spots on the root (fig. 3).
Near the center of these spots are
innumerable minute, flask-shaped
fruiting bodies from which myriads
of small spores escape. Although
black rot generally extends only a
short distance into the flesh, it may
penetrate as much as half an inch
after several weeks' storage (fig.
17). Cutting a sweetpotato cross-
wise through a black rot spot will
show that the flesh is black. A blu-
ish-black color will soon develop in
the tissues beneath the spot, some-
times almost to the center of the
sweetpotato.

The spores of the fungus readily
adhere to the bodies of insects and
may be carried to other sweetpota-
toes, where new infections may take
place at wounds if sufficient mois-
ture is present. The spores may
also be scattered by workmen pre-

Earing potatoes for the market and
y air currents inside the storage
house.

Sweetpotatoes that contain even
a small amount of black rot sflould

not be washed because of the dan-
ger of spreading the spores. Nearly
100 percent of the sweetpotatoes in
some commercial lots have become
infected following washing. Even
when the washed sweetpotatoes do
not show black rot when shipped,
they may be badly rotted before
they reach market or before the con-
sumer has time to use them.

Figure 17. — Cross section through a
sweetpotato with black rot lesion, show-
ing depth of penetration by the black
rot fungus.

Internal Cork

Internal cork is a virus disease
that causes dark-brown to blackish
corky spots in the flesh of the
affected sweetpotatoes (fig. 18).
The roots appear normal exter-
nally. Internally, the spots vary in ■
size and shape and may occur singly
or in groups at any point in the
fleshy tissues. The disease is most
- easily detected by cutting the po-
tatoes crosswise in slices about
inch thick. The hard discolored
spots make the roots undesirable
for food if the sweetpotatoes are
severely affected.
This disease was first recognized

in South Carolina in 1944. Since
then, it has been found in most areas
where sweetpotatoes are grown, but
it is most prevalent in South Caro-
lina, Georgia, and part of North
Carolina.

Some internal cork may be found
when the sweetpotatoes are dug, but
most of the damage occurs in stor-
age. Both the number and size of
the cork spots increase during stor-
age. The rate of increase is more
rapid at 70° F. than at the recom-
mended storage temperatures of 55°
and 60°.

21

Figure 18. — Crosswise and lengthwise sections of a sweetpotato, showing internal

cork spots in the flesh.

Surface Rot

In the early stages surface rot
is characterized by nearly circular
spots (fig. 19) on the surface of the
sweetpotato. These vary in number
and size. The rot is shallow, seldom
extending more than one-fourth to

one-half inch below the surface.
The sweetpotato shrinks later, es-
pecially at the margin of the spot.
Finally it becomes dry and mum-
mified.

Infection occurs at the base of the

22

small rootlets at about digging time,
especially if the ground is wet, or
early in the storage period. The in-
fected areas gradually enlarge in
storage and become conspicuous in
6 to 8 weeks. If the storage house
is kept rather warm and dry, mois-
ture escapes from the affected areas
and the sweetpotato gradually be-
comes dry and hard.

Surface rot has some character-
istics in common with black rot.
Surface rot spots may become an
inch in diameter and grayish brown
during storage. Black rot spots are
nearly black and may reach a diam-
eter of more than 2 inches. The sur-
face rot spots are more regular in
shape and size than those caused by
bruises.

The loss from surface rot is some-
times more than that of any other
storage disease. Occasionally the
sweetpotatoes are so badly shrunken
that they have no market value.
Some varieties shrink more than
others. Some strains of the Jersey
types, especially some of those with
dark-yellow skin, are only slightly
subject to surface rot. Light-
skinned Jersey types, on the other
hand, are more susceptible. There
are no immune varieties.

Figure 19. — A sweetpotato after several
weeks in' storage, showing a number
of circular lesions associated with sur-
face rot.

Dry Rot

Dry rot generally begins at the
end of the sweetpotato and produces
a firm brown decay. The sweet-
potato decays slowly and finally be-
comes dry, hard, and mummified
(fig. 20). Small domelike, or pim-
plelike, protuberances just visible to
the naked eye finally cover the en-
tire surface and contain large num-
bers of colorless spores of the fun-
gus. The tissue just beneath the

skin is coal black. Several weeks
are required under normal condi-
tions for the fungus to destroy a
sweetpotato completely.

The dry rot fungus grows on the
stems and vines aboveground under
field conditions, and probably some
sweetpotatoes become infected in
the field. Dry rot has also been
found on the stems of young plants
in hotbeds.

23

Dry rot, which is widely distrib- regarded as one of the more serious
uted throughout the country, is not storage disorders.

Figure 20. — The characteristic appear- Figure 21. — A sweetpotato showing the
ance of dry rot. dry, mummified condition produced by

the Java black rot fungus.

Java Black Rot

Java black rot, so called because
it was discovered on sweetpotatoes
grown from an importation from
Java, is a widely distributed storage
disease. It is more prevalent in the
South than elsewhere.

Java black rot is strictly a stor-
age disease. The affected sweetpo-

tatoes rot slowly and become dry,
hard, brittle, and coal black within,
and difficult to break ( fig. 21 ) . The
disease is spread by spores that de-
velop beneath the surface of numer-
ous pimplelike protuberances.
When the surface of the root is
broken, these spore bodies are set

24

free. Java black rot begins usually age conditions it takes the disease
at the end of the root and progresses f rom 4 to 8 weeks to destroy a sweet-
very slowly. Under normal stor- potato comj>letely.

Charcoal Rot

Charcoal rot is found in storage
houses throughout the country, but
is more prevalent in the South. The
characteristic black decay of the
roots differs from others of a similar
appearance by the production of
minute spherical resting bodies
throughout the interior of the sweet-
potato, but rarely on the surface.
These bodies are coal black and are

found buried in the tissue when the
skin is removed. They are visible
to the naked eye. Some shrinking
and drying of the sweetpotato fol-
low invasion by the fungus, and the
fleshy root may become a hard, dry,
charcoallike mummy. The loss
from this disease is comparatively
small.

Control Measures for Storage Rots

The first step toward preventing
storage rots is to control the field
diseases as completely as possible,
so that the sweetpotatoes will be
free of diseases when they are
stored. Some field diseases, such

as black rot, continue to develop
after digging and cause serious
losses in storage. Other field dis-
eases afford avenues of entry for
secondary decay-producing fungi.

Digging and Handling Sweetpotatoes

Dig and handle sweetpotatoes as
carefully as possible to avoid cut-
ting and bruising, since most decay-
producing organisms can enter only
at wounds.

Haul the roots to the storage
house and cure them at about 85° F.
and 90 percent relative humidity as
soon as possible after they are dug.
Curing should continue for 6 to 8
days. If you cure the roots at this
temperature and humidity shortly
after they are dug, the wounds will
heal before infection can take place.

After curing keep the storage
temperature at or as near to 55° F.
as possible and the humidity be-
tween 80 and 85 percent. Higher

storage temperatures are more fav-
orable to development of internal
cork and sprouting. However,
hold seed stock the first 6 weeks of
the storage period at 70°, so that lots
affected with internal cork can be
more easily detected and discarded.
Temperatures lower than 55° for
more than short periods cause chill-
ing injury and favor decay by fungi
that attack only weakened sweetpo-
tatoes.

Do not disturb sweetpotatoes
after they have been cured, because
they are easily injured and bruises
and cuts allow entrance for decay
organisms.

Management of the Storage House

Before the sweetpotatoes are put floor of the storage house or cellar

into storage, sweep out all the dirt and treat them with a fungicide to

and rubbish of the previous year, destroy the germs that are left on

Thoroughly clean the walls and them.

25

One treatment is to coat the walls,
bins, and floor thoroughly with
whitewash. Another is to spray the
inside of the house with a solution
made by dissolving 1 pound of cop-
per sulfate in 25 gallons of water.
After 1 or 2 days spray the walls
again with copper sulfate.

A third treatment is to fumigate
with gas generated by formalde-
hyde and potassium permanganate.
Three pints of commercial formal-
dehyde and 23 ounces of potassium
permanganate are required for each
1,000 cubic feet of storage space.
Place several containers (buckets,
crockery, or large cans), depending
on the size of the house, on the floor
and then divide the required
amount of potassium permanganate
among them. Set a can containing
the proportional amount of formal-
dehyde beside the container.

Beginning with the container
farthest from the door, pour form-
aldehyde on the potassium per-
manganate in each container. Keep
the house closed at least 24 hours

and then open and ventilate it
thoroughly.

The gas generated by the mix-
ture of the two chemicals is
very irritating to the eyes. Wear
gloves and goggles to protect
the hands and eyes in case of
accident.

Another effective method is to
fumigate by burning % to 1 pound
of ordinary flowers of sulfur to each
1,000 cubic feet of space. Set metal
containers on a base of brick to raise
them off the floor to avoid the dan-
ger of fire. Do not use containers
with soldered parts. Distribute the
required amount of sulfur in each
container and then set it afire,
("lose the house immediately and
continue the fumigation for 24
hours. If the inside of the house
is lightly sprinkled or sprayed with
water before fumigating with either
sulfur or formaldehyde treatments,
the treatment is more effective.
Disinfect the crates used for har-
vesting and storing by one of the
methods just described.

List of Causal Agents of Sweetpotato Diseases

Field diseases: Causal organism-
Stem rot Fusarium oxysporum f. batatas

Black rot Endoconidiophora fimbriata

Foot rot Plenodomus destruens

Scurf Monilochaetes infuscans

Root rot Phymatotrichum omnivorum

Mottle necrosis ' p V thium ultimum

mottle necrosis (p> 8cferoieic7lMm ,

Soil rot Steptomyces ipomoea

Phyllosticta leaf blight—. Phyllosticta batatas

Septoria leaf spot Septoria bataticola

White rust Albugo ipomoeae-panduratae

Root knot — Meloidogyne spp. (formerly Heterodera marioni)

Storage rots :

Soft rot RMzopus stolonifer

Black rot Endoconidiophora fimbriata

Internal cork Unidentified virus

Surface rot Fusarium oxysporum

Dry rot Diaporthe batatatis

Java black rot Diplodia thedbromae (D. tubericola)

Charcoal rot Macrophomina phaseoli

26

U. S. GOVERNMENT PRINTING OFFICE : 1955