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Farmers' Bulletin No. 1059 




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 


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 


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 

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) 


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. 


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. 


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


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

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 

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. 


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 

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 

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. 


Black Rot 

(Black Shank, Black Root) 


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. 


ing from them will likely have the 

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. 


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 


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. 


Foot Rot 

(Die Off) 


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 

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. 


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. 


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. 


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. 


(Soil Stain, Rust, Jersey Mark) 


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 


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


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. 


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 

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) 


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. 


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. 


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. 


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. 


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 


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 

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 

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. 



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. 


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 

Soil Rot 
(Pox, Ground Ret) 


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. 


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. 


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. 


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- 

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


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 

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- 

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 


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- 

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 

Root Knot 


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- 

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. 


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. 


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


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 

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- 

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 


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

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 

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 


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 

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. 


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- 

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 


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 

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 

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 

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°. 


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- 

Infection occurs at the base of the 


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 

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. 


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 


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 

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- 

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

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. 


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 

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 

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