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SWEETPOTATO CULTURE

AND

DISEASES

Agriculture Handbook No. 388

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í73

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Agricultural Research Service

UNITED STATES DEPARTMENT OF AGRICULTURE
Some of the information given in this handbook is similar to that formerly contained in U.S. Department of Agriculture Farmers' Bulletin 2020,

"Commercial Growing and Harvesting of Sweetpotatoes," and in Farmers'

Bulletin 1059, "Sweetpotato Diseases." Both of these reports are now out

of print.

Mention of a trademark name or a proprietary product does not constitute a guarantee or warranty of the product by the U.S. Department of

Agriculture and does not imply its approval to the exclusion of other

products that may also be suitable.

PRECAUTION

Pesticides used improperly can be injurious

to man, animals, and plants. Follow the directions and heed all precautions on the labels.

Store pesticides in original containers under

lock and key—out of the reach of children and

animals—and away from food and feed.

Apply pesticides so that they do not endanger

humans, livestock, crops, beneficial insects, fish,

and wildlife. Do not apply pesticides when there

is danger of drift, when honey bees or other

pollinating insects are visiting plants, or in

ways that may contaminate water or leave illegal residues.

Avoid prolonged inhalation of pesticide

sprays or dusts; wear protective clothing and

equipment if specified on the container.

If your hands become contaminated with a

pesticide, do not eat or drink until you have

washed. In case a pesticide is swallowed or gets

in the eyes, follow the first aid treatment given

on the label, and get prompt medical attention.

If a pesticide is spilled on your skin or clothing,

remove clothing immediately and wash skin

thoroughly.

Do not clean spray equipment or dump excess

spray material near ponds, streams, or wells.

Because it is difficult to remove all traces of

herbicides from equipment, do not use the same

equipment for insecticides or fungicides that

you use for herbicides.

Dispose of empty pesticide containers

promptly. Have them buried at a sanitary landfill dump, or crush and bury them in a level,

isolated place.

NOTE: Some States have restrictions on the

use of certain pesticides. Check your State and

local regulations. Also, because registrations of

pesticides are under constant review by the

U.S. Department of Agriculture, consult your

county agricultural agent or State extension

specialist to be sure the intended use is still

registered.

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POU.OW THC LABBI. '

U.S. »trAITMINT or ACIICttlTUM
SWEETPOTATO CULTURE

AND

DISEASES

Agriculture Handbook No. 388

Agricultural Research Service

UNITED STATES DEPARTMENT OF AGRICULTURE

Washington, D.C. Issued June 1971

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402 — Price $1.25
CONTENTS

Part I: CULTURE

Page

Introduction 1

Importance 2

Uses 3

Botany 5

Varieties 7

Varietal types 7

Varietal improvement 8

Properties and characteristics of important varieties 9

Growth requirements of sweetpotatoes _

Climatic adaptation

Soils for sweetpotato culture

Crop rotation

Land preparation

Ridge height

Fertilizers

Composition and rates of application

Nutrient deficiency symptoms

Fertilizer placement

Types and sources of planting materials

Use of clean seed stock

Foundation, registered, and certified

seed

Producing plants

Location of plant beds

Types of plant beds

Preparation of seed stock for bedding

Bedding procedures and bed care

13

13

13

14

15

15

16

16

17

17

18

19

20

21

21

21

24

25

Page

Amounts of seed stock required 26

Field planting 28

Methods of transplanting 28

Preplanting fungicidal treatment of

plants 29

Size of plants 30

Time of planting 30

Plant spacings 30

Weed control and cultivation 32

Weed control by cultural methods 32

Chemical weed control 33

Water requirements and irrigation 34

Harvesting the crop 35

Methods and equipment 35

When to harvest 38

Handling and grading in the field 41

Selecting and storing seed stock 42

Curing and storing 42

Purposes and requirements 42

Storage house operation 44

Preparing sweetpotatoes for the fresh

market 46

Washing 47

Sorting and grading 47

Treating to prevent decay 47

Waxing 47

Packaging 47

Shipping 48

Part II: DISEASES

Agents causing sweetpotato diseases 49

Fungi 49

Nematodes 49

Viruses 49

General procedures for controlling sweetpotato diseases 49

Field diseases 49

Storage diseases 49

Field diseases and their control 50

Stem rot 50

Black rot 53

Foot rot 55

Scurf 56

Root rot 57

Mottle necrosis 58

Soil rot 59

Phyllosticta leaf blight 60

Septoria leaf spot 60

White rust 61

Nematodes and their control 61

Storage rots and their control 64

Soft rot 64

Black rot 65

Surface rot 66

Java black rot 66

Dry rot 68

Virus diseases and their control 68

Internal cork 68

Chlorotic leafspot 69

Yellow dwarf 69

Feathery mottle 70

Russet crack 70

Literature cited 71

Appendix 72
SWEETPOTATO CULTURE AND DISEASES

By CLARENCE E. STEINBAUER, research horticulturist, retired,

and

LEATON J. KUSHMAN, research plant physiologist,

Plant Science Research Division Agricultural Research Service

Part I: CULTURE

INTRODUCTION

The sweetpotato (Ipomoea batatas (L.)

Lam.) has been used as human food for manycenturies. However, early peoples of Africa,

Asia, Australia, and Europe apparently did not

use, or know of, the sweetpotato, and there is

no available evidence that this plant was known

to the ancient Arabs, Chinese, Egyptians,

Greeks, Hebrews, Indians, or Romans. Primitive peoples in tropical areas of Central and

South America, as well as in numerous tropical islands in the Pacific, were using this plant

long before first written historical records came

into existence. In spite of the more or less restricted areas of the world in which the sweetpotato was known in early times, the subsequent

spread of the plant throughout other areas of

the world has been very extensive. This crop

plant has been referred to under numerous

names. Some of these synonyms are listed in

the Appendix of this handbook.

It is not possible to accurately determine in

just what part of the world the sweetpotato

originated. Some authorities believe it evolved

in tropical America; others believe it evolved

somewhere in the tropical South Pacific Islands.

The evidence appears to favor American origin,

however, because a considerable number of

other closely related wild plant species belonging to the same botanical genus (Ipomoea) as

the sweetpotato are native to the American

tropics and subtropics. Among these other species are believed to be the progenitors of the

sweetpotato.

Wherever it originated, the spread of this

crop plant to other climatically suitable areas

of the world has been very extensive. Today

sweetpotatoes are grown in tropical, subtropical, and warmer temperate areas throughout

the world. The Food and Agriculture Organization of the United Nations, in Production Yearbooks in 1963 and 1965 estimated the world production and yields of sweetpotatoes and yams

from 1960 through 1965 (i,5).^ Sweetpotatoes

are of prime importance in the food supplies

and economies of many countries. (See Appendix.) Production is especially large in the African, Far Eastern, and Latin American regions ;

is moderate in the North American region ; and

is notably smaller in the European, Near Eastern, and Oceanic regions (table 1).

The sweetpotato was used in tropical regions

of Central and South America long before the

Spanish explorations. Columbus found the natives in the West Indies using it, and he is

reported to have presented a sweetpotato to

Queen Isabella upon his return to Spain. Other

Spanish explorers later found the plant in Mexico and in South America. It probably was not

cultivated in what are now the 48 contiguous

States of the United States before the time of

Columbus. It may have been introduced into

this country later through trading among Central and North American Indian tribes. Early

American settlers probably obtained sweetpotatoes during trading with the West Indians. The

crop was introduced into Virginia sometime before 1560.

Early records give only sketchy, incomplete

descriptions of the sweetpotato. Very likely the

forms referred to in those early records were

notably different from the sweetpotatoes grown

commercially in the United States today. Selection and propagation of naturally occurring

sports or mutations over the years have resulted

in numerous productive forms (varieties) possessing desirable horticultural characteristics.

Some early varieties probably arose through

selection of naturally occurring hybrids among

flowering types. Since 1937, with controlled

breeding and selection programs, rapid progress

^ Italic numbers in parentheses refer to Literature

Cited p. 71.
AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

TABLE 1.—Estimated average annual production and yields of sweetpotatoes and yams

in various regions, from 1960 through

1965^

Region

Production

area

Yield

per acre Production

Africa

Far East

Latin America

North America

Oceania

Europe

Near East

1,000 acres

2 6,323

4,150

1,131

200

3 49

35

11

Cwt.

2 61

89

65

81

4 67

108

175

1,000 cwt

2 392,040

367,048

74,096

16,214

4 3,300

3,740

1,892

^ Calculated to acres and hundredweights from metric

units shown in Production Yearbook 1963 (4, table 26)

and Production Yearbook 1965 (5, table 27).

2 Data for period from 1948 through 1953.

3 Data for period from 1958 through 1962.

4 Data for period from 1960 through 1962.

has been made in the United States in developing improved, disease-resistant varieties.

Importance

For many generations the sweetpotato has

been an important crop in the United States.

Until about the time of World War II, it ranked

second only to the Irish potato in importance

among vegetable crops. Maximum acreage and

production was in 1932 when 47.6 million hundredweight was produced on 1,059,000 acres.

Since that time, acreage planted, total production, per capita consumption, and relative importance of sweetpotatoes among vegetable

crops have decreased. These changes have occurred in spite of rapid increases in population

and substantial increases in yields of sweetpotatoes produced per acre (table 2).

A number of factors have contributed to

changes in status of the crop. Increased urbanization of our population and mechanization of

our industry and agriculture have reduced the

overall average physical labor requirements and

the needs for high-calorie foods. As a result of

this and of changed dietary concepts, the per

capita consumption of all starchy foods, including sweetpotatoes, has declined. The per capita

consumption of sweetpotatoes in the United

States was approximately 31 pounds in 1920; it

had dropped to about 21 pounds by 1940; and

it was only 5 to 6 pounds by 1968.

Greatly increased efficiency through progress

in mechanization of large farm units has sharply reduced not only the numbers of small farm

units but also the size of rural populations,

which formerly consumed sizable proportions

of the sweetpotatoes produced. In 1940 consumption of sweetpotatoes in homes on farms where

the sweetpotatoes were grown was approximately 12 million hundredweight; in 1962 it

had declined to less than 2 million hundredweight. Much of this decline has been associTABLE 2.—Sweetpotato acreage, production, yield, value, and price in the United States

in selected years, from 1910 through 1969

Year Planted Production

Yield per

harvested acre Value of crop

Average price

per cwt.

1910

1915

1920

1925

1930

1932

1935

1940

1945

1950

1955

1960

1961

1962

1963

1964

1965

1966

1,000 acres

641

731

992

778

670

1,059

944

652

652

499

352

207

201

229

204

187

172

161

149

151

154

1,000 cwt.

32,966

41,601

57,159

34,372

30,017

47,627

44,687

28,434

33,692

27,269

21,608

14,858

14,415

17,120

14,356

12,969

15,524

13,697

13,658

13,763

13,958

Cwt.

51

57

58

44

45

45

47

44

52

56

63

79

79

85

84

86

92

87

93

92

95

1,000 dollars

40,216

46,980

117,834

85,554

57,751

40,650

54,545

44,160

124,306

85,589

72,392

61,358

61,924

61,488

57,835

65,980

65,628

62,399

61,204

66,526

62,342

Dollars

1.22

1.13

2.04

2.49

2.02

.86

1.25

1.59

3.64

2.99

3.27

4.09

4.30

3.54

4.03

5.09

4.21

4.98

1967

1968

1969

4.49

4.90

4 44
SWEETPOTATO CULTURE AND DISEASES

ated with the decrease in number of farm

family units. According to the U.S. Census, in

1939 about 1,200,000 farms were reported as

producing sweetpotatoes. By 1959 the number

of such units had decreased to 310,000, and the

downward trend still continues.

Relatively less mechanization has occurred

with sweetpotatoes than with many other farm

crops. The high requirement for hand labor

during production and harvesting has put

sweetpotatoes in a poor position for the reduced,

but increasingly more expensive, available labor

supply. Costs of production and harvesting,

therefore, remain relatively high and contribute

to the comparatively high prices of sweetpotatoes at the retail level. In the highly competitive

food market, high prices tend to reduce sales.

Fresh sweetpotatoes have a relatively high

perishability rate. They are easily bruised and

often injured by storage at low temperatures.

Both conditions lead to excessive losses from

rots and shrinkage. Distribution personnel

sometimes lack the proper appreciation of the

necessity of careful handling and temperature

control for maintenance of quality. This lack

has often resulted in unappealing displays on

retail counters and in poor edible qualities of

the product. As a consequence, customer purchases of sweetpotatoes have declined. Recent

trends toward the preparation and sale of

sweetpotatoes in processed forms (canned,

frozen, flaked) better adapted to the needs of

the modern housewife and toward the attractive

prepackaging in consumer-size containers and

the retailing of a better graded and sized fresh

product have helped to curtail the rate of decline in consumer purchases of sweetpotatoes.

Along with the changes in overall production,

consumption, and yields of sweetpotatoes, some

significant shifts in production between regions

and among States within the several regions

have occurred. On the whole, acreage and actual

production have increased in the Western

States (mostly in California) but have decreased considerably in the three main production regions in the Central States and in South

Atlantic and Central Atlantic Coast States.

Table 3 shows that during the period from 1934

through 1966 the proportions of the total production for the country increased in the Central

Atlantic Coast and Western States but decreased in the Central States and South Atlantic Coast States.

Uses

Sweetpotatoes are used primarily for human

food, but they may also be used for animal feed,

for starch production, and for various other

industrial purposes. Of the sweetpotatoes proTABLE 3.—Shifts in proportions of national

sweetpotato output originating in leading

production areas, from 19SU through 1968

Production region

Average proportions of

national production

1934-37 1965-68 Shift

Central Statesi

Percent

54

32

12

2

Percent

52

23

19

6

Percent

-2

South Atlantic Coast States2

Central Atlantic Coast

States.3

Western States^

-9

+4

1 Alabama, Arkansas, Illinois, Indiana, Iowa, Kansas,

Kentucky, Louisiana, Mississippi, Missouri, Oklahoma,

Tennessee, Texas.

2 Florida, Georgia, North Carolina, South Carolina.

3 Delaware, Maryland, New Jersey, Virginia.

4 Arizona, California, New Mexico.

duced each year in the United States, approximately 60 to 70 percent are used as human food.

Approximately 5 percent of the crop must be

retained for "seed" (plant-producing) purposes.

The other 25 to 35 percent of the potatoes are

lost through disease or shrinkage either at the

farm or during preparation and marketing of

the fresh or processed products or are used in

small amounts for animal feeding or for industrial purposes.

For Human Food

A large part of the crop is prepared by the

housewife from fresh roots, but increasingly

larger proportions of the sweetpotatoes consumed in the United States are prepared commercially in various canned, frozen, or dehydrated forms (table 4). In 1965 over 11 million

cases (each case equivalent to about 1 bushel

of sweetpotatoes) were canned, over 9 million

pounds were frozen, and additional large quantities were preserved as various dehydrated

products, including the recently developed

sweetpotato flakes. Some of the factors in the

progressively increasing popularity of processed products with both housewives and institutional users are assurance of products with

acceptable levels of quality, convenience in storage, ease of preparation of the processed products, and avoidance of the high rates of spoilage frequently encountered with fresh market

sweetpotatoes.

The nutritive value of sweetpotatoes is high.

They are good sources of energy-supplying

sugars and other carbohydrates; of calcium,

iron, and other minerals ; and of vitamins, particularly of Vitamin A in the popular highcarotene, orange-flesh varieties and of Vitamin

C (tables 5 and 6).
AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

TABLE 4.—Forms in which sweetpotatoes are

processed for food

Manner of

preservation and Characterization

type of product

Canning :

Sirup packs Small whole roots, chunks, or

mixed types packed and processed

in sugar sirups of varying concentrations.

Vacuum packs Roots, as above, processed under

vacuum without addition of sirups.

Solid packs Mashed stock packed solidly in

cans and heat processed.

Purees Comminuted, strained, precooked

stocks, usually unflavored, heat

processed.

Baby foods Specially prepared and blended,

heat-processed purees for infant

and child feeding. Usually without

additives.

TABLE 5.—Proximate composition of edible

portion of raw sweetpotatoes ^

Freezing :

Strips, slices,

dices, chunks.

Mashed, souffle

Purees

Dehydration :

Strips, dices

Flakes

Flours

'*Alayam" and

similar specialty products.

Cooking in oil :

Chips

Variously prepared from peeled or

unpeeled potatoes, frozen with or

without sirups, and with or without cooking or baking.

-Stock prepared as for solid-pack

canning, but preserved by freezing.

_Pureed product preserved by freezing.

„Prepared pieces blanched, dehydrated with heat and vacuum with

or without antidarkening treatments.

-Peeled, preheated stock, sliced,

cooked, pureed, double-drum dried,

flaked, packaged in low-oxygen atmospheres.

-Stock washed and dehydrated fresh

or after cooking, ground and sifted.

Prepared prebaked roots pulped,

pureed, other additives incorporated, extruded onto trays, baked in

ovens. Used directly as cookies,

snacks, or confections; or ground

to give "Alamalt" flour for use in

ice creams or other products.

_Prepared raw slices of suitable

thickness and shapes immersed in

hot cooking oils, drained, salted as

desired, packaged in low-oxygen

atmospheres.

For Animal Feed

Limited quantities of sweetpotatoes are used

as high-carbohydrate feeds for cattle, hogs,

poultry, and other domestic animals. Most of

these sweetpotatoes are fed raw on farms where

they are grown and most are cull or offgrade

and offsize roots not suitable for fresh market

Component Percentage

Solids 31.5

Total carbohydrate 27.9

Protein 1.8

Mineral matter (ash) 1.1

Fat 07

^ Calories per pound of peeled tuber—565.

use. Considerable portions of the cull roots formerly used for animal feed because of their unmarketability as first-grade stock are now being

used in preparation of various processed sweetpotato food products. Vines and foliage, as well

as storage roots, are usable for animal feed but

are not widely used for this purpose. A satisfactory, nutritious silage can be prepared from

vines, leaves, and roots. Present methods of

silage preparation, however, are not economical.

The extent to which sweetpotatoes can be fed

raw is restricted by the handling and storage

problems resulting from the high water content, bulkiness, and perishability of the sweetpotato. These limitations on use for animal feed

can be largely overcome by partly drying or dehydrating the materials before storage. Satisfactory methods have been worked out for

sun-drying or artifically dehydrating chopped,

shredded, chipped, or macerated sweetpotatoes

for feed. However, because of the relatively

high costs of growing, harvesting, and handling

sweetpotatoes, as compared with costs for corn

and other high-carbohydrate feed crops, the

production and dehydration primarily for

animal feed probably will remain economically

unfeasible in most of the United States. Dehydration for animal feed will be feasible only in

locations in the South favored with long growing seasons and abundant sources of cheap fuel,

and for growers who are able to produce raw

stock at lowest possible costs and who are located reasonably close to available dehydrating

facilities. Maximum mechanization of all production and handling operations will be necessary to keep costs of raw stock for feed dehydration at economically workable levels. One

limited source of dehydrated feed is the byproduct obtained by dehydrating culls and trimmings

from canning, freezing, or other processing

operations.

For Starch Production or

Other Industrial Purposes

Starch can be manufactured from white- or

very light-ñesh sweetpotato varieties but at

present (1970) there is no commercial production of sweetpotato starch in the United States.
SWEETPOTATO CULTURE AND DISEASES

TABLE 6.—Nutritive value of sweetpotatoes prepared in various ways

Nature and size of prepared sweetpotato

Nutrient

Baked, peeled after baking

1 medium about 6 ounces

(5 by 2 inches)

110 grams

Baked, peeled after boiling

1 medium about 6 ounces

(5 by 2 inches) '

147 grams

s

lg a

;^

CO

Canned, vacuum or solid pack

1 cup

218 grams

Food energy cal

Protein g

Fat g

Carbohydrates g

Calcium mg

Iron mg

Vitamins :

A LU

Thiamin mg

Riboflavin mg

Niacin mg

C mg

155.0

2

1

36

44

1

8,970

.10

.07

.7

24

170.0

2

1

39

47

1

11,610

.13

.09

.9

25

295.0

2

6

60

65

1.6

11,030

.10

.08

.8

17

235.0

4

Trace

54

54

1.7

17,110

.12

.09

1.1

30

SOURCE: Nutritive Value of Foods (19),

In the late 1930's and early 1940's interest

was aroused in the South in the possibilities of

developing a commercial sweetpotato starch industry. Federal and State agencies successfully

solved the technical aspects of starch manufacture from this crop, and plant breeders developed productive varieties with high starch contents. In small-scale commercial trials sweetpotato starch proved quite suitable for use in

certain adhesives, in bakery products, in textile

sizings, in cosmetic manufacture, and in laundry products. However, the costs of producing

raw stock remained so high that farmers could

not grow and handle the crop at prices the extraction plants could afford to pay, and starch

production on a commercial basis failed to

materialize. Other crops, such as waxy corn and

waxy sorghum, contain starches similar to

those of the sweetpotato, and they can be grown

for starch production and handled by mechanized methods far more cheaply than sweetpotatoes. Also, seeds of grain crops are more

easily handled and stored for prolonged periods

than are sweetpotatoes.

In some countries, Japan, for example, sweetpotatoes are the basic stock used in the fermentation industry to produce industrial alcohol and

associated byproducts. Such use has not been

promoted in the United States where such products can be obtained more economically from

other sources.

Botany

The sweetpotato (Ipomoea batatas (L.)

Lam.) is placed by taxonomists in the botanical

family Convolvulaceae, which includes various

morning-glories, bindweeds, dodders, and several hundred other species found widely distributed throughout the world. It is a perennial

in its growth habit but in crop practice it is

treated as an annual.

The sweetpotato is an herbaceous plant with

stems that vary greatly in thickness, length,

spacing of nodes, and general growth habit.

Forms that have more or less twining and trailing, long stems of slender to moderate thickness

and moderately to widely spaced leaves are most

prevalent. It is not uncommon, however, to find

sweetpotato types with relatively short, thick

stems and short internodes, semierect to erect

growth habits, and little or no tendency to

twine. Stem fasciation is sometimes observed

and this condition is characteristically more

prevalent in certain varieties and selections

than in others.

Leaves in the sweetpotato are alternately and

spirally spaced along the stems in a pattern

known as 2/5 phyllotaxy ; that is, for two leaves

to be located in the same vertical (longitudinal)

plane on the stem, five leaves will be so spaced

that the stem will be spirally encircled twice in

order for the fifth leaf to lie directly above or
AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

below the first. The spiralling direction may be

either clockwise or counterclockwise. There are

no stipules at the junction of leaf petioles with

stems. Leaf shapes and sizes in /. batatas varywidely. In outline, leaves may be shouldered,

toothed, entire, deeply cleft (parted), variously

lobed or fingered, or sagittate (fig. 1). Petioles

also vary in length, thickness, and degrees of

erectness.

Colors and pigmentation patterns in leaves,

stems, and petioles of sweetpotatoes range

widely and these are useful characteristics for

identifying particular varieties and selections.

Flowers of this plant have a form and appearance very similar to that of the ornamental

morning-glories (fig. 2). They are axillary and

borne either singly or in cymes on short peduncles. The corolla is funnel shaped, about V/2 to

2 inches long and 1 to li/o inches wide at the

mouth, made up of five fused petals, and colored

various shades of lavender to light purple. The

stamens, typically five in number, are attached

to the base of the corolla. The calyx is persistent, and made up of five greenish imbricated

sepals about one-half inch long. The ovary is

superior, bicarpellate, two to four celled and

four ovuled. One to four slightly flattened or

somewhat angular brown to black seeds are

borne in the capsúlate fruit. Seedcoats become

very hard when seed is fully matured and dried,

«».'i'»^«

SHOULDERED TOOTHED ENTIRE

PARTED LOBED

FIGURE 1.—Some leaf shapes found in sweetpotatoes.

BN-36818

FIGURE 2.—Flowering sweetpotato plant growing

in a greenhouse.

necessitating some form of scarification to insure prompt seed germination. Both colors and

sizes of flowers and flower parts and seeds vary

somewhat among sweetpotato varieties and selections.

The feeding root system of the sweetpotato

is more or less fibrous and is relatively extensive both in depth and in lateral spread. The

edible sweetpotato is variously referred to as

a "root," a "root-tuber," or a "tuber." Although

a few earlier investigators considered this

organ to be a true tuber, that is, a thickened,

modified underground stem, more recent careful anatomical studies of the developing young

sweetpotato have proved conclusively that this

is a fleshy thickened root. The fleshy thickening

occurs principally through the activity of a primary cambium located in a more or less continuous peripheral vascular ring, and through

that of secondary and tertiary cambiums located in numerous vascular strands embedded

within the extensive central core of thin-walled

storage parenchyma tissue.
SWEETPOTATO CULTURE AND DISEASES

In the very young sweetpotato storage root

the outermost protective layer is a single-celldeep epidermis. This is soon replaced by a periderm layer composed of relatively large, thinv^alled cells. The outer cells in this layer become

partly lignified and are progressively sloughed

off as the root grows and expands. A phellogen

(cork cambium) forms nev^s cells to replace

those lost in the sloughing process. This outer

periderm layer with its regenerative cambium

is very important to the handling and storage

physiology of sweetpotatoes. Postharvest regulation of factors that favor proper development

of this layer and adjacent tissues developed at

sites of surface wounds is essential to the successful handling, storage, and marketing of

sweetpotatoes.

In the more mature sweetpotato storage root,

the protective periderm layer is underlain by a

cortical layer composed of large starch-filled

parenchyma cells among which are dispersed

phloem conductive elements—sieve tubes and

their so-called companion cells. Adjacent to

some of the sieve tubes are latex vessels that

contain the white, viscid latex fluid. Below this

cortical layer and external to the more or less

continuous peripheral secondary xylem ring,

lies the main cambium from which both the

adjacent cortex and xylem ring arise. Beneath

the xylem ring lies the large central, parenchymatous-celled storage area, with its embedded

vascular strands, which constitutes the main

bulk of the fleshy sweetpotato.

Both skin and flesh colors vary widely in the

fleshy sweetpotato root. Skin colors may range

from almost pure white through cream colored,

yellow, orange, pink, red, to very dark purple.

Flesh colors may be white or various shades of

cream, yellow, salmon, or orange, depending

largely on the amounts and proportions of various carotenoid pigments present. The flesh may

also contain red or purple anthocyanin or other

pigments, which can impart an undesirable

greyish appearance to the cooked sweetpotato.

In the United States, types having roots with

any appreciable amounts of purple pigmentation in the flesh are looked on with disfavor.

In some countries, however, types with such

purple pigmented flesh are actually preferred.

VARIETIES

Many varieties of sweetpotatoes have been

grown in the United States but only a few have

been important commercially. Productiveness,

appearance, and acceptable edible qualities were

usually the main bases for selection of most of

the older varieties. With the advances in mechanization, specialization in production, processing, handling, and marketing operations and

with the increased competition between this

crop and others, numerous other characteristics

have become essential for acceptability in good

commercial varieties. These additional characteristics include such attributes as uniformity

of root size, shape, and flesh color, nutritional

properties, processing characteristics, storage

and reproductive capacities, and resistance to

serious diseases.

Of some 40 distinct varieties of sweetpotatoes

grown commercially in the United States in

about 1920 only two or three, or selected strains

of those two or three, furnish an appreciable

part of the current sweetpotato supply. A number of improved varieties developed in recent

years in breeding and improvement programs

now make up the major portion of the commercial crop (table 7).

Varietal Types

Sweetpotato varieties can be classified into

two types, according to their principal uses—

(1) food types and (2) feed or industrial types.

In practice many varieties are used for both

fresh market and processing. The forms in

which they are used are often determined

larerely by the economics of supply and demand

and by the comparative prices being offered for

the fresh stock.

Food type varieties, particularly those for

fresh market use, are often designated as **dryflesh^' or "moist-flesh^* according to the feel

sensation experienced in the mouth during mastication of the cooked or baked sweetpotatoes.

The terms dry-flesh and moist-flesh do NOT

refer to the percentages or proportions of water

in the roots of the two types. Studies have

shown that the moisture content of freshly harvested roots of a dry-flesh variety such as Big

Stem Jersey is often higher than that of roots

of a moist-flesh variety such as Porto Rico

produced under the same conditions. It is the

difference in the character of the solids in the

cooked roots of the two types rather than the

difference in actual moisture contents that

causes the dissimilarity in the feel sensation in

the mouth. Varieties that tend to convert most

of their starch to sugars and dextrines during

cooking become quite sweet and soft and are

referred to in the trade as moist-flesh or "yam"

types ; those that tend to convert the starch less

completely are usually not as sweet as these

moist-flesh types, are firmer, and are referred

to as dry-flesh. Many varieties are intermediate

with respect to these classes.
AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

TABLE 7.—Acreage and production of principal food-type sweetpotato

varieties grown commercially in the United States, 1965

Acreage Production

Variety

Acres

Percentage of

U.S. total 1,000 cwt.

Percentage of

U.S. total

Allgold

Carogold

Centennial

Coastal Sweet

Earlysweet

Gem

Georgia Red

2,125

300

96,615

300

2,000

460

10,600

26,090

8,470

800

3,050

920

100

16,950

3,280

1,340

10,475

800

1,180

1,000

4,710

300

1.1

0.1-

48.9

0.1-

1.0

0.2

5.4

13.2

4.3

0.4

1.5

0.5

0.1-

8.6

1.7

0.7

5.3

0.4

0.6

0.1-

2.4

0.1-

187

22

9,148

30

200

75

1,463

1,813

643

95

250

86

10

2,103

401

129

665

54

127

87

447

30

1.0

0.1-

49.4

0.1

1.1

0.4

7.9

Goldrush

"Jerseys"!

Julian

Kandee

Lakan

Nancy Hall

9.8

3.5

0.5

1.4

0.5

0 1

Nemagold

Nugget

Oklamar

Porto Ricos2

11.4

2.2

0.7

3 6

Redgold

Sunnyside

Tanhoma

Velvet

Virginian

0.3

0.7

0.5

2.4

0 1

Total—varieties

listed.

U.S. total—all

varieties.

191,865

197,700

96.7

100

18,065

18,502

97.7

100

1 "Jerseys" includes Jersey Orange, Orange Little Stem, Orlis, Maryland Golden, Big

Stem Jersey, Little Stem (Yellow) Jersey, and Red Jersey.

2 Porto Ricos includes both vining and bunch types.

The term "yam" is commonly used by the

trade for sweetpotatoes of the moist-flesh type

but strictly speaking this terminology is incorrect since a true edible yam is an entirely different plant from the sweetpotato. The true yam is

grown in the Continental United States on only

a limited scale in Florida.

In the United States attractiveness of skin

and flesh colors, uniformity of root shapes,

pleasing flavor and texture of flesh, high vitamin and nutritional values, and productiveness,

all are important attributes in food type sweetpotatoes. High yielding capacities, high percent solids and carbohydrates, including high

starch content and recovery, and superior propagative and storage properties are important

attributes in varieties intended primarily for

feed or industrial uses.

Certain very light-colored, relatively starchy

varieties such as Triumph and Hayman (Southern Queen) are sometimes used as early market

food types. In some countries, particularly

where maximum production of solids and carbohydrates is essential in the food economy, the

light-flesh varieties are actually preferred for

food over the orange- or deep-yellow-flesh varieties, high in vitamin content and generally

preferred in the United States. On the other

hand, the deep-yellow-flesh varieties can be used

for production of high-quality feeds or for industrial use where economic factors permit. As

an example, highly nutritious, high vitamin

content, dehydrated sweetpotato meal for feed

is sometimes produced as a byproduct from

culls and trimmings in the canning and processing operations.

Varietal Improvement

New varieties of sweetpotatoes may originate

either by chance genetic mutation or by breeding. Vegetative propagation of desirable genetic

mutants has been practiced for many years. In

recent years seedlings originating by pollinating flowering types in breeding programs have

provided greater varietal improvement than is

possible through selecting mutant forms.

Mutations of various kinds occur frequently

in the sweetpotato. Changes may be desirable

or undesirable and may involve various charac-
SWEETPOTATO CULTURE AND DISEASES

teristics such as skin or flesh colors, general

growth habits, shape of roots, or even productivity factors. Just v^hy these sudden and sometimes relatively large deviations or changes in

type occur in certain individuals of a plant

population is not adequately understood. Whatever the cause (s), the changes involve heritable

alterations in the nuclear makeup of the plant

cells.

Either through chance observation or by conscious search over the years, many naturally

occurring, desirable, mutant forms have been

collected, propagated, tested, and grown as new

varieties. The opportunities for subsequent improvement of forms originating through mutations are limited. Propagation of such forms by

vegetative means involves thousands of somatic

or vegetative divisions of plant cells, each normally identical in its genetic constitution with

the cell from which it arose and therefore ultimately identical with the first mutated or

changed cell that started the new mutant form.

Thus, each vegetatively propagated plant of a

clone or variety is made up of many thousands

of cells all having identical heritable genes. The

only opportunity for further improvement depends on the rare, fortuitous occurrence and

detection of possible subsequent favorable alterations in the genie makeup of propagatable

plant cells.

Greater opportunities for varietal improvement than are provided through mutations are

offered through the sexual method in which new

combinations of desirable characteristics are

obtained by hybridization or crossing of parent

stocks that possess known heritable characteristics and properties. Flowering of parent

stocks, either naturally or by induced conditions, is essential to use of this method. Sweetpotatoes may flower rather readily under field

conditions in tropical and subtropical habitats

but flowering is usually sparse and virtually

absent in many varieties and selections in the

temperate areas of the United States. Several

techniques have been found effective for inducing flowering. These include grafting on rootstocks of other related species, girdling of

stems, treating with certain chemicals, adjusting the photoperiod, and regulating temperatures and nutritional conditions. Through the

use of one or more of these techniques, nearly

all sweetpotato clones used in breeding programs have flowered and set seed.

An active sweetpotato breeding program was

initiated at the Louisiana Agricultural Experiment Station in 1937. Subsequently, additional

breeding programs were undertaken by State

experiment stations in California, Georgia,

Maryland, Mississippi, North Carolina, Oklahoma, South Carolina, and Virginia, and by the

United States Department of Agriculture at

Beltsville, Md., and at Meridian, Miss. Many

thousands of seedlings have been produced and

evaluated through the cooperative efforts of

these and other experiment stations. After

thorough testing and evaluation, the best of the

selections are named and released to the trade

as new varieties. Among the many characters

observed are plant type and growth characteristics such as yields, earliness, root set, placement, shape, uniformity, skin and flesh colors,

and sprout-producing properties; edibility and

nutritive values (including vitamin contents) ;

solids and carbohydrate composition; reaction

to important diseases, nematodes, and insect

pests; shipping, handling, and storing properties; and suitability for various processing or

other uses. In the period from 1935 through

1966, more than 40 new varietal releases were

made to the trade (table 8). Some of the commercially important properties and characteristics of a number of these varieties currently in

production are indicated in table 9. More detailed descriptions of the most important of

these varieties have been published by Steinbauer (16), Roots of some varieties are illustrated in plates I and II.

Properties and Characteristics of

Important Varieties

Varieties Used Primarily for Food

BIG STEM JERSEY.—Mainly fresh market but

sometimes processed. Dry-flesh type. Roots

mostly fusiform and fairly regular in shape;

set close to hill; surface generally smooth but

sometimes veined; small to large. Skin russetyellov^, flesh amber- to straw-yellow. Quality

very good, sweet and mealy when baked. Vines

long, trailing, moderately thick, green. Leaves

medium size, entire to shouldered, green. Moderate foliage density. Yields moderate to heavy.

Propagates well, producing an abundance of

draws or sprouts in the plant bed. Roots have

relatively short storage life. Excessive shrinkage and development of pithiness by midwinter

necessitates early marketing. Susceptible to

stem rot (fusarium wilt), scurf, black rot, soil

rot, and russet crack diseases ; somewhat resistant to internal cork and root-knot nematodes.

Maryland Golden, an orange-flesh mutant selection from Big Stem Jersey, was in extensive

commercial production for several years but

because of disease susceptibility, inadequate

sprouting properties, and excessive tendency

for root cracking, this variety is now (1970)

unimportant.
10 AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

TABLE 8.—Origin and principal uses of new sweetpotato varieties developed in the United States,

from 19S5 through 1966

VarietyAcadian

Allgold .

Apache .

Australian Canner

Canbake

Carogold

Centennial

Georgia Bunch Porto

Rico (Cliett Bunch).

Coastal Sweet

Earlyport

Earlysweet

Gem

Georgia Red

Goldrush __.

Heartogold

Hopi

Jersey Orange

Julian

Kandee ^

Lakan

Murif Bush Porto Rico

N.C. Porto Rico 198 ___

Nancy Gold

Nemagold

Nugget

Oklamar

Orange Little Stem

Orlis

Pelican Processor

Unit I Porto Rico

Queen Mary

Ranger

Redcliff __

Redgold

Red Nancy

Rols

Shoreland

Sunnyside

Tango

Tanhoma

Virginian

Whitestar

Principal usesi

Fresh market

Fresh market,

processing.

Fresh market

Canning

Fresh market,

processing.

Fresh market

Fresh market,

processing.

do _

Fresh market

do

Early fresh market

Fresh market,

processing.

Fresh market

Fresh market,

processing.

Fresh market

do

Fresh market,

processing.

do _

Fresh market

do _

do _

Fresh market,

processing.

Fresh market

Fresh market,

processing.

do

Fresh market __

Fresh market,

processing.

Fresh market __

Feed, industrial

Fresh market,

processing.

Fresh market __

Fresh market,

processing.

do

Fresh market __

do

do

do _

Fresh market,

processing.

Fresh market

do

do

Feed, industrial

Origin

Breeding-selection

do

-_ do

Selection, openpollinated seedling.

Breeding-selection __

do

do

Hill selection from

Vining Porto Rico.

Breeding-selection __

do

do

do

do

do

do

do

Mutation from Little

Stem Jersey.

Breeding-selection ___

do

do

Mutation from Vining

Porto Rico.

Flesh color mutation

from Porto Rico.

Mutation from

Nancy Hall.

Breeding-selection ___

do

do

Mutation from Little

Stem Jersey.

-—_ do

Breeding-selection __

Hill selection

Breeding-selection

do

do

do

Mutation from

Nancy Hall.

Mutation from Orange

Little Stem.

Breeding-selection

do

do

do

do

do

Originating agency2

Louisiana

Oklahoma

U.S. Dept. of

Agriculture.

U.S. Dept. of Agriculture, Mississippi.

Georgia

South Carolina

Louisiana

Private grower, Georgia

Georgia Coastal Plain _

Louisiana

Georgia Coastal Plain _

North Carolina

Georgia Coastal Plain _

Louisiana

do

U.S. Dept. of Agriculture, New Mexico.

Kansas

Louisiana

Kansas, Louisiana

Louisiana

Private grower, Texas

North Carolina

Kansas

Oklahoma

North Carolina

Oklahoma

Kansas

do

Louisiana

do

do

do

South Carolina

Oklahoma

Kansas

do

Maryland

U.S. Dept. of

Agriculture.

U.S. Dept. of

Agriculture, Missouri.

Oklahoma

Virginia Truck

Experimentstation.

U.S. Dept. of

Agriculture.

Year

released

as

variety

1957

1952

1959

1948

1958

1958

1960

1949

1965

1953

1956

1964

1956

1951

1947

1965

1947

1964

1958

1958

1965

1950

1958

1961

1957

1950

1950

1944

1935

1944

1944

1966

1953

1950

1950

(3)

1953

1957

1960

1949

1948

^ Processing = canning, freezing, or flaking.

2 Named but not officially released.

2 Unless otherwise indicated, these varieties were developed at

Agricultural Experiment Stations.
SWEETPOTATO CULTURE AND DISEASES 11

TABLE 9.—Some commercially important properties and characteristics of sweetpotato

varieties in production, 1967 ^

Variety

Most important

commercially :

Centennial

Georgia Red _

Goldrush _.

"Jerseys"^

Nemagold

Nugget __-

Porto Rico

(strains).''

Other varieties:

Allgold

Carogold

Coastal Sweet

Earlyport

Earlysweet

Gem

Heartogold

Julian

Kandee

Lakan

Oklamar

Redgold -

Sunnyside

Tanhoma _

Virginian _

Relative

yield

****

**

Skin

color2

Orange

Red ___

Copperorange.

Tan to

orange-tan,

red.

0range-tan_-

Light copper to tan.

Copper

Orange

do

Copper-red

Copper

Cream

Light

copper.

Tan

Salmon

Orange

Reddishtan.

Purple

Reddishpurple.

Tan

Copper

Copper-red

Flesh

color2

Orange _

Light

orange.

Orange _

Yellow,

orange.

Orange _

do _

Salmonorange.

Salmonorange.

Orange _

do _

do _

do _

do _

do _

Deep

orange.

Salmonorange.

Deep

orange.

Salmon _

Salmonorange.

do _

Salmon _

Orange _

Flesh

type

Moist

do

do

Dry to

semi-dry.

Semi-moist

Semi-dry _

Moist

Moist

do

do

do

do

___ do

do

do

Moist _

do

do

do

do

do

do

Canning

quality

Fair

Poor

Good

Fairly

good.

Good

do

do

Fairly

good.

do

do

do

Good

do

do

Good

Fair

Good

Good

Fair

Poor

Storage

life3

Good

do

do

Short

do

Goods _

do

Good

do

do

do

Fair __

Good __

do

do

Good

do

Fairly

good.

do

Good

do

do

Plant

production^

Fair.

Good.

Do.

Do.

Do.

Fair.

Excellent.

Fairly good.

Poor.

Fairly good.

Good.

Do.

Do.

Do.

Poor.

Good.

Do.

Do.

Fair.

Fairly good.

Good.

Fair.

^ For reaction to diseases, see table 21, page 50 and descriptions of individual varieties beginning on page 9.

2 Colors vary somewhat, depending on soil type, culture, and handling practices used.

3 Rated with respect to development of "spongy" or "pithy" condition.

4 Relative abundance and vigor of plants.

5 Includes the older, yellow-flesh varieties Big Stem Jersey and Little Stem Jersey and the orange-flesh varieties

Orange Little Stem, Orlis, Jersey Orange, and Red Jersey.

6 Roots remain sound but often develop "blister" spots in storage.

7 Include both vining and short vining types. Root characteristics similar in most strains but Velvet (Red Velvet)

has reddish-purple skin and sometimes some purple in flesh.

LITTLE STEM JERSEY (YELLOW JERSEY).—

Similar in most respects to Big Stem Jersey but

has somewhat more slender vines, smaller

leaves, and roots that are sometimes shorter

and not quite so large.

ORANGE LITTLE STEM, ORLIS, JERSEY ORANGE.

—Color-mutant selections from Little Stem Jersey. Flesh less *'firm'' or ''dry" than Little Stem

Jersey. Roots have orange-tan skin and orange

flesh. Other root and plant characteristics very

similar to those of Yellov^ Jersey. All susceptible to stem rot, black rot, scurf, russet crack,

and soil rot diseases.

CENTENNIAL.—Fresh market and processing.

Moist-flesh type. Roots variably tapered to

cylindrical, medium to large, orange skin and

deep-orange flesh. Good quality. Vines vigorous,

thick, long, trailing, reddish-purple except

green at terminal ends. Leaves large, entire to

lightly toothed, light-green. Very proliñc, heavy

yielder. Stores well. Only fair sprout producer.

Moderately susceptible to stem rot and internal

cork; susceptible to black rot, scurf, soil rot,

and root-knot nematodes.

GEORGIA RED.—Fresh market. Moist-flesh

type. Roots slightly variable tapered-spindle,

purplish-red to copper-red skin and light-orange

flesh. Excellent quality v^hen baked. Vines long.
12 AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

moderately vigorous, trailing. Heavy foliage

density. Leaves large, toothed to shouldered

with heart-shaped base, green v^ith light-bronze

tips. Moderately heavy yielder. Stores and propagates well. Susceptible to stem rot, black rot,

scurf, internal cork, and root-knot nematodes.

Susceptible to (but somewhat tolerant of) soil

rot disease.

GOLDRUSH.—Excellent quality for fresh market and processing. Moist-flesh type. Roots

tapered-spindle, uniform, attractive copperorange skin and orange flesh. (Strains with

more copper-colored skin have largely replaced

the original lighter orange-skinned form.) Skin

smooth. Vines short to medium length, moderately vigorous, smooth, semibunch growth

habit, vines and petioles deep reddish-purple.

Moderately heavy foliage density. Leaf blades

sagittate with heart-shaped base. Yields about

same as Porto Rico. Stores and propagates well.

Very resistant to stem rot under usual field

conditions. Susceptible to black rot, scurf, soil

rot, internal cork, russet crack, and root-knot

nematodes.

JULIAN.—Fresh market and processing.

Moist-flesh type. Roots smooth, tapered to

spindle-shaped, with salmon skin, deep-orange

flesh, and high carotene content. Good appearance. Makes very heavy set of roots with a

large proportion in the small to medium canning sizes. Good canning and culinary quality.

Vines vigorous, coarse, moderate length, trailing, green. Leaves entire or lightly toothed in

outline. Moderately heavy foliage density.

Stores satisfactorily. Overall yields moderate

to heavy. Poor plant producer. Resistant to

stem rot in the field ; appears to be resistant to

internal cork. Susceptible to black rot and rootknot nematodes.

NEMAGOLD.—Fresh market and processing.

Semimoist flesh type. Roots medium-size, short

to long spindle, with orange-tan skin and orange

flesh. Skin usually smooth, but sometimes

veined. Good quality. Vine type similar to that

of Little Stem Jersey. Leaves distinctly shouldered, small but slightly larger than those of

Little Stem Jersey. Possesses certain other

characteristics of Jersey-type varieties such as

heavy root set, relatively uniform root size

and shape, and relatively short storage life.

Propagates well. Susceptible to stem rot and

scurf. Resistant to root-knot nematodes. Somewhat tolerant to soil rot and internal cork.

Best adapted in Jersey-type variety production

areas, especially the Eastern Shore of Delaware-Maryland-Virginia.

NUGGET.—Fresh market and processing.

Semidry-flesh type. Roots medium size, smooth,

slightly blocky tapered to cylindrical. Skin lightcopper to tan and flesh orange. Excellent quality. Vines long, medium thickness, trailing,

green. Somewhat light foliage density. Leaves

rather small, entire, green. Yields well. Stores

well. Produces sprouts late and in only fair

numbers. Moderately resistant to stem rot,

black rot, and soil rot. Symptomless carrier of

internal cork virus.

PORTO RICO.—Fresh market and processing.

Moist-flesh type. Roots variable spindle to

globular and irregular. Skin smooth, light-copper to copper; flesh orange-yellow to salmon,

often lighter colored at root end ; excellent edible quality. Vines medium to long, trailing,

coarse, vigorous, reddish-purple. Moderately

heavy foliage density. Leaves medium to large,

shouldered to sagittate, green except purple at

base of blade and on veins. Moderate to heavy

yielding. Very susceptible to stem rot, black rot,

scurf, soil rot, internal cork diseases, and rootknot nematodes. Extensively grown, but less

well adapted in northern areas than in southern.

A number of strains of Porto Rico, selected

for better and more uniform skin and flesh

color, root shape, and yielding capacity, have

been develoDed and released as separate varieties. One of these, designated UNIT I PORTO

RICO, developed through hill selection, has been

one of the most extensively grown of the Porto

Rico varieties.

The GEORGIA BUNCH PORTO RICO (CLIETT

BUNCH) variety has shorter vines than vining

Porto Rico and is usually earlier and has more

uniform roots than most other strains. Good

plant producer. Otherwise like vining Porto

Rico.

The VELVET and RED VELVET varieties are

selections of Porto Rico with reddish-purple to

dark reddish-purple skin and sometimes with

considerable purple pigment beneath the skin.

In some strains some purple streaking in flesh.

Otherwise like vining Porto Rico. Production

mainly in California and in the Southwest.

The N. C. PORTO RICO 198 variety is a fleshcolor mutation selection from a high-yielding

Porto Rico clonal line. Flesh color deeper and

more uniform than that of regular Porto Rico.

Other characteristics similar to those of vining

Porto Rico.

Varieties Used Primarily for Feed, Starch, or

Other Industrial Purposes

PELICAN PROCESSOR.—Feed and industrial

type with high yield and high starch content.

Roots medium to large, fusiform-cylindrical,

with cream skin and light-cream flesh. Good

keeper but only fair plant producer. Vines vigorous, intermediate length, trailing, green.

Heavy foliage density. Leaves entire to sagit-
SWEETPOTATO CULTURE AND DISEASES 13

täte with heart-shaped base, dark-green. Resistant to internal cork. Moderately tolerant to

stem rot. Susceptible to black rot, scurf, soil

rot, and root-knot nematodes. Best adapted in

Gulf Coast States area.

WHITESTAR.—Feed and industrial type with

roots spindle-cylindric to chunky, medium to

large, with russet-white skin and cream-colored

flesh. Heavy yielding, moderate solids and

starch contents. Good keeper and satisfactory

propagator. Vines green, medium length and

vigor, trailing. Leaves dark-green, lobed to

doubly sagittate with heart-shaped base. Resistant to internal cork. Slightly susceptible to stem

rot. Susceptible to black rot, scurf, and soil rot.

GROWTH REQUIREMENTS OF SWEETPOTATOES

Climatic Adaptation

The sweetpotato is native to the Tropics, and

accordingly, as a crop plant, it thrives best

under tropical or subtropical climatic conditions. The warmer parts of the United States

are best suited to its culture but commercial

crops are produced over wide areas. Most extensive production occurs from New Jersey

south in States bordering the Atlantic Coast

and the Gulf of Mexico and westward to California (table 10). There is sizable, but less

extensive, production of sweetpotatoes in "Middle States" extending north of the Gulf Coast

area and west as far as Kansas and Oklahoma.

Limited production occurs even farther north

and west in certain favorable locations in Idaho,

Illinois, Indiana, Iowa, Michigan, and Washington.

Because the sweetpotato is a tropical plant,

both warm days and warm nights are essential

for its best development. For good commercial

production mean summer temperatures should

TABLE 10.—Average yields, acreages harvested,

and production of sweetpotatoes in princi^

pal producing States, from 1965 through

1969

State

Maryland

Virginia

North Carolina

California

Tennessee

New Jersey

New Mexico —

Kansas

Mississippi —

Georgia

Louisiana

Alabama

Arkansas

South Carolina

Texas

Oklahoma

Average

yield

per acre

CwU

137

120

112

98

98

97

93

89

85

84

82

81

78

75

71

66

Acreage

harvested

IßOO acres

3.7

13.6

20.8

8.3

3.5

6.2

0.6

1.1

12.5

8.5

51.8

5.7

2.0

3.2

12.0

0.8

Production

1,000 cwt.

429

1,617

2,448

814

341

599

49

95

1,037

714

4,253

464

153

237

856

54

average above 72° F., and there should be ample

sunshine, warm nights, a minimum of cool,

cloudy weather, at least 1 inch of either rainfall or moisture supplied through irrigation per

week, and a minimum frost-free growing season of 4 to 5 months.

Average yields of sweetpotatoes produced

per acre are usually considerably higher for

States in the Atlantic Coast area extending

from New Jersey south through North Carolina than for States in other areas (table 10).

However, sweetpotatoes of the same varieties

grown on mineral soils in more southern parts

of the production areas usually have higher

total solids and starch contents than do those

of the same varieties grown in the northern

parts, even when the length of the growing

period is the same in each part.

Varieties differ considerably in their adaptation to climatic, soil, and other conditions

existing in different producing areas within the

United States. Jersey-type varieties and others

such as Nemagold, with more or less similar

characteristics, grow and produce best in the

more northern producing States (Iowa, Kansas, Maryland, New Jersey, Oklahoma, and

parts of Virginia) but produce poorly in most

areas in the Deep South. On the other hand,

some varieties such as Goldrush and Porto

Rico, which are well adapted to production in

most southern areas, often fail to produce well

in northern areas. Very few, if any, varieties

are sufficiently wide in their adaptation to produce equally well in all producing areas.

Soils for Sweetpotato Culture

Types

Sweetpotatoes may be grown on a wide variety of soils but production of high-quality roots

with desirable sizes, shapes, appearance, and

yields is best on certain fertile, well-drained,

moderately deep, friable, fine sandy loams,

sandy loams, and loamy fine sands underlain

by firm, friable, heavier subsoils such as clay

loams or sandy clay loams. The principal sweetpotato soils are the sandy loams of the Norfolk,
14 AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

Ruston, Marlboro, and Sassafras series and

other series with similar characteristics.

Surface layers of best sweetpotato soils

should range from about 8 inches to 24 inches

deep. Where the surface layers are too light

and deep, roots tend to become too long and to

penetrate too deeply. Then harvesting is difficult and injuries to roots are often excessive.

Where subsoils are too heavy and tight, they

interfere with proper soil drainage. Sweetpotatoes produced on heavy soils with silty or

clay surface layers tend to be rough and poorly

colored, and are usually difficult to free of adhering soil. All fine-texture surface soils such

as clay loams, silt loams, silty clays, clay soils,

and deep sands are poorly adapted for sweetpotato culture.

Reaction

Sweetpotatoes are fairly tolerant of variations in soil reaction (within the ranges usually

encountered in crop production). The sweetpotato grows reasonably well at soil reactions

ranging from about pH 4.5 to pH 7.5 and yields

and plant growth are usually best where the

soil reaction ranges from about pH 5.6 to 6.5.

Under some conditions where the more acid

soils are deficient in available calcium or magnesium, or both, liming may make those elements more soluble and available to improve

the growth and quality of sweetpotatoes. Land

should be limed the year before the sweetpotato crop is planted. Avoid overliming and do

not raise the soil reaction too near to neutrality

(pH 7.0). Infections by certain sweetpotato

disease organisms, such as those causing the

soil rot (pox) and scurf diseases, are more

severe as the soil reaction approaches neutrality. Such disease infections are controlled to a

practical degree when the soil reaction ranges

from about pH 4.5 to 5.5. Where land is used

primarily for production of sweetpotatoes,

applications of sulfur to increase soil acidity to

within this range may become necessary to

attain practical levels of disease control.

Crop Rotation

Sweetpotatoes are grown over a wide range

of territory on farms of many different kinds

and with different cropping practices. Even

though soils on these farms may be well adapted to sweetpotato culture, market prospects,

availability of labor, and other economic considerations determine which crops are to be

grown on the land in any particular year.

Studies in Louisiana and Maryland have

demonstrated that where serious disease and

pest infestations are absent, continuous production of sweetpotatoes on the same land is possible without undue soil impoverishment, provided sufficiently large quantities of fertilizers

are applied and suitable green manures and

coyer crops are properly incorporated into the

soil. Even so, continuous cropping with sweetpotatoes is to be considered as an exceptional

practice to be applied only under special conditions and only where disease and pest infestations are absent or of minor consequence.

Sweetpotatoes cannot be grown continuously,

year after year, even on land of suitable type,

without serious danger of building up in the

soil various disease-causing organisms or pests

that sooner or later reduce both yield and quality of the sweetpotato crop. Some pathogenic

organisms, for example, Fusarium oxysporum

f. batatas that causes the stem rot or wilt

disease and Ceratocystis fimbriata that causes

blackrot, persist in soils for at least 2 years.

Therefore, sweetpotatoes should be grown in

rotation with other suitable crop plants or soilbuilding cover crops. Except under special conditions, sweetpotatoes should not be planted on

the same land more often than once in 3 or 4

years. Sweetpotatoes should not be included in

rotations that include other crop plants that

are susceptible to, or are carriers of, the same

disease or diseases as sweetpotatoes. Growing

stem rot (wilt) susceptible varieties of sweetpotatoes in rotations that include any wilt susceptible type of tobacco, for example, should

be avoided because both crops are susceptible

to some of the same pathogenic races of the

Fusarium fungus that causes the sweetpotato

wilt disease.

Sweetpotatoes can successfully follow a wide

variety of farm and truck crops, especially on

land where soil pests or diseases have not become serious problems and where the preceding

crop does not leave the land high in nitrogen

content. Excessive nitrogen encourages excessive vine growth in sweetpotatoes, which often

results in poor storage root growth and poor

root shape. For these reasons, legume crops, as

a rule, should follow but not immediately precede sweetpotatoes in the cropping sequence.

Suitable cover crops and green manures must

be included in the cropping program both as a

conservation measure by preventing erosion of

the generally relatively light-texture soils on

which this crop is usually grown and as a means

of maintaining soil organic matter at moderate

levels. Various legumes such as soybeans, cowpeas, crimson clover, and vetch, as well as winter cereal grains such as rye, wheat, or oats,

are used for such soil improvement. To avoid

excessive nematode buildup in soils and serious

nematode damage to sweetpotatoes, plant only
SWEETPOTATO CULTURE AND DISEASES 15

more or less root-knot resistant varieties of

such soil-improving crops, especially in the

South. Rye is much better than other cereals

for preventing nematode buildup. The time of

digging the sweetpotato crop will determine to

some extent what type cover crop is planted.

Crimson clover or winter vetch, for instance,

can be planted after an early harvest but winter rye or wheat will be better suited to planting conditions after late harvest.

Land Preparation

Plow land at least 6 to 7 inches deep when

the soil is sufficiently dry to pulverize well.

Plow early enough to allow for decomposition

of cover crop or other plant residues before

planting time. Follow this plowing with prompt

and thorough disking and harrowing. Since

sweetpotato planting and cultural operations

are now usually mechanized, to avoid any interference of bulky residue materials in subsequent fitting, planting, and cultural operations,

any plant residues from preceding crops must

be chopped up and adequately incorporated into

the soil before or during plowing and disking.

Land preparation may involve incorporation

of chemicals for control of nematodes, insects,

and weeds. Use of herbicides is discussed on

page 33. The nematicides and the insecticide

registered for use on sweetpoptatoes are listed

in table 11. Apply the chemicals after the land

has been plowed and prepared, before ridges

are constructed, and sufficiently in advance of

planting time to insure that the chemicals do

TABLE 11.—Nematicides and insecticide used

for pest control in sweetpotatoes ^

Nematicide Insecticide

About 50% 1,3-dichloro- Ö,ö-Diethyl 0-(2-isopropylpropene and about 6-methyl-4-pyrimidinyl )

50% 1,2-dichloropro- phosphorodithioate

pane mixture (DD). (diazinon).

100% 1,3-Dichloropropene [Apply 4.0 lb. actual/acre

and related Cs hydro- as a preplanting

carbons (1,3-D). broadcast soil

83% Ethylene dibromide application. Tolerance

(EDB). 0.1 p.p.m.]

79% 1,3-Dichloropropenes

and 19% ethylene

dibromide mixture

(1,3-D-EDB).

Sodium methyldithiocarbamate (SMDC).

80% DD and 20%

methyl isothiocyanate

(DD-MENCS).

1 Toxic residue tolerances have been established by the

Food and Drug Administration for most of these pesticides, subject to revisions whenever such are found necessary. Application and use of the materials should conform to the tolerance'limitations and label directions.

not remain to injure the sweetpotato plants

after they are set in the field. Because the usefulness of these pesticides and the amounts of

undesirable residues left in the soil and on the

crop are influenced by numerous factors such

as soil characteristics, moisture, rainfall, and

concentration of chemicals used, recommendations of State or local specialists based on label

requirements and local experience should be

followed.

Make the first application of fertilizer 10

days to 2 weeks before planting time. For most

producing areas, the fertilizer is applied before the planting ridges are constructed. Methods of applying and placing fertilizers and of

constructing the ridges vary somewhat in different producing areas and with the type and

power of equipment used. To a large extent,

single- or multiple-row, tractor-mounted equipment of various designs applies the fertilizer

at desired rates and placements, mixes it with

the soil, and constructs ridges uniformly and

to the desired heights, often in a single operation. Various designs of disk-hillers, listers, or

middlebuster-type equipment are used in constructing the ridges. Sometimes the fertilizer

is applied broadcast as a separate operation

and ridges constructed afterward with the

horse- or tractor-powered equipment. The

ridges are allowed to settle or firm-up until

shortly before planting ; then the ridge tops are

flattened off slightly with a pole or plank or

with a roller drawn by a horse or tractor, either

as a separate operation or as a part of the mechanical planter operation. Some mechanical

planters have a built-in "plow'' or levelling

blade to flatten the ridge tops to the desired

height ahead of the plant-setting equipment.

(See fig. 9, p. 28.)

Ridge Height

In most areas sweetpotatoes are grown on

ridges. Soil texture and the amount and intensity

of rainfall are important factors determining

the best ridge height to use. On heavy sweetpotato soils in areas where rainfall may be

heavy and drainage slow, ridges 12 to 15 inches

high are often needed. On light sandy loams,

somewhat lower ridges, averaging about 10

inches high, may be adequate. On light sandy

soils, ridge heights are usually 8 inches or less.

In a few areas, such as the Eastern Shore area of

Maryland and Virginia where the soils are very

light and sometimes in other areas where rainfall is erratic, sweetpotatoes are planted ''on

the flat," without ridges. Even though some soil

is pulled toward the rows during cultivation

in these areas, thus forming essentially low

ridges by digging time, for most other grow-
16 AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

ing areas this method of planting on the flat is

usually unsatisfactory. Roots in low ridges will

sometimes set so deeply that they are cut and

damaged severely during digging.

Row spacings must be wider where ridges

are high than where they are low. Spacings of

42 to 48 inches between rows are often used

with high ridges; spacing of only 30 inches

may be adequate on light soils and with low

ridges.

FERTILIZERS

Composition and Rates of Application

High yields of high-q.uality sweetpotatoes

can be obtained only on suitable soils with a

moderately high supply of available nutrients.

For the most part soils best suited to production of this crop are more or less sandy and

have only low to moderate levels of natural

fertility. Addition of fertilizers is, therefore,

essential for best production of high-quality

roots. Good soil management practices, proper

handling of manures and cover crops, and

attention to soil nutrients supplied to other

crops grown in rotation with sweetpotatoes can

aid materially in providing part of the needs

of the crop. The heavy feeding requirements of

this crop, however, usually make additional

liberal applications of commercial fertilizers

necessary for optimum profitable production.

At the Maryland Agricultral Experiment Station studies showed that about 100 pounds per

acre of nitrogen, 40 pounds of phosphorus (as

P20r,), and 210 pounds of potassium (as K2O)

were taken up by a crop of the Maryland Golden

variety during the growing season (15) (table

12). Approximately half each of the nitrogen

and potassium was used for vine growth; the

other half was removed in the root crop harvested. Two-thirds of the phosphorous was removed in the root crop. In addition to the

fertilizer elements actually used by the crop,

additional amounts must be provided to compensate for losses due to leaching and to ''fixing" in the soil in forms not available to the

plants.

The composition and amounts of commercial

TABLE 12.—Mineral uptake by a 400-bushel-peracre crop of Maryland Golden sweetpotatoes

Minerals

Nitrogen

Phosphorus (P2O5)

Potassium (K2O) _ Calcium

Magnesium

Uptake

Vines

Pounds

per acre

49.7

13.7

107.6

24.6

5.2

Roots

Pounds

per acre

53.4

26.8

102.2

5.9

4.7

Total

Pounds

per acre

103.1

40.5

209.8

30.5

9.9

SOURCE: The Mineral Uptake by the Sweet Potato (15).

fertilizers, the timing of applications, and the

methods of placement recommended for best

production of sweetpotatoes v^ill vary from

place to place because of the variations in climatic conditions, in amounts and frequency of

rainfall or irrigation, in physical and chemical

characteristics of the surface soils and subsoils

used, in cropping practices, and even in the

varieties grown. Because of these many variables, no really satisfactory, generally applicable,

recommendations are possible. More reliable

and specific advice for fertilization should be

obtained from the local county agent or State

agricultural experiment station or college representatives who are familiar with particular

combinations of soil and climatic conditions

and cropping practices in a given locality or

area. Many different fertilizer formulas and

methods and rates of application have been published by the various experiment stations (table

13). Generally they have included only moderate amounts of nitrogen and relatively high

proportions of phosphorus and potassium. Most

profitable yields generally have been obtained

from the use of 1,000 to 1,500 pounds per acre

of mixtures containing 3 to 5 percent nitrogen,

6 to 8 percent phosphoric acid, and 8 to 12 percent potassium.

Sweetpotatoes require moderate amounts of

nitrogen for good crop production. On soils

where previous cropping and soil management

practices have provided good residual quantities of nutrients, sometimes only 20 to 30

pounds of nitrogen per acre need be added in

the fertilizer mixture used with the sweetpotato crop. Under conditions of high rainfall

or with irrigation, particularly on soils favoring high leaching rates, higher rates of nitrogen application may be required for best crop

production.

Potassium is used in relatively large amounts

by the sweetpotato and is often easily leached

from soils suitable for this crop, consequently

the amounts supplied in fertilizers also must be

relatively large. Studies at the Maryland Agricultural Experiment Station showed that sweetpotato yields can be increased significantly by

heavy applications of potassium but that the

higher yields are accompanied by a lowering in

the solids content of the roots so produced.
SWEETPOTATO CULTURE AND DISEASES 17

TABLE 13.—Sweetpotato fertilizers and rates of

application recommended in publications of

agricultural experiment stations in 8 States

State and formula ^

California :

8-10-12

Georgia :

3-9-9 _

4-8-8 _.

Louisiana:

4-12-8 _

5-10-10

6-12-6 _

Maryland :

3-9-12 _

Mississippi :

6-6-6 —

6-8-8 —

8-8-8 __

New Jersey:

2-8-10 -

3-9-9 _-

3-12-15

Texas :

4-8-8 __

4-12-8 -

6-10-7 -

Virginia :

2-8-10 _

3-3-15 -

3-9-9 _-

3-9-15 -

Rate

Pounds per acre

750

800-1,000

800-1,000

500-800

500-800

500-800

1,000-2,000

800

500-1,000

600

1,200

1,0001-1,800

800

800-1,000

400-1,000

400H1,000

1,500

1,000

1,000

1,200

1 Percentages of nitrogen, phosphorus, and potassium,

in order shown.

Phosphorus is used by sweetpotatoes in much

smaller quantities than are nitrogen and potassium. On many of the sandy soils used for this

crop, most of the phosphorus remains readily

available to the plants and the rates of application could probably be lower than are often

recommended and still be adequate. However,

on some heavier types of sweetpotato soils,

significant proportions of the phosphorus become tied up in the soil in forms unavailable or

only slowly available to plants. On such soils

it is necessary to supply phosphorus at moderately heavy rates.

In addition to nitrogen, phosphorous, and potassium, smaller amounts of a number of other

nutrient elements, such as calcium, magnesium,

and boron, are required for normal growth and

development of sweetpotatoes. An inadequate

supply of these elements from the soil will result in abnormal growth and development of

characteristic deficiency symptoms in the plant.

Some calcium is supplied as calcium phosphate and limestone ''fillers'' in low-analysis

fertilizers. When high-analysis fertilizers are

used or the soils become calcium deficient for

other reasons, additional calcium may be supplied in the form of gypsum or limestone.

Sweetpotatoes require small amounts of

boron for satisfactory growth and root development. However, even a small excess of this element in the soil can injure this crop. Caution,

therefore, must be used in supplying this element. As little as 5 pounds of borax per acre,

applied well mixed in the regular fertilizer,

may adequately satisfy the boron requirements

of sweetpotatoes in soils where insufficient

amounts of this element are available.

Nutrient Deficiency Symptoms

The following are some of the symptoms by

which serious deficiencies in the supply of the

main nutrient elements needed by sweetpotato

plants may be recognized :

Boron—Restriction and stunting of terminal

growth and shortening of internodes ; leaf petioles become curled; storage roots become

slender and elongate and show necrotic brown

internal spotting.

Calcium — Reduction in plant growth; new

leaves are light green, reddish, turning to

brown in some basal leaves ; small yellow leaves

develop on thin lateral branches.

Magnesium—Interveinal areas of leaves gradually turning yellow from margins inward;

veins very dark green ; new stems become light

bluish green.

Nitrogen—Reduced vine growth ; basal leaves

develop red margins, turn yellow or yellowish

green; necrotic areas appear; leaves abscise;

tip leaves turn light green.

Phosphorus—Defoliation; young leaves are

dark green; old leaves develop large yellow

areas, turn purple, then brown.

Potassium — Reduced growth ; defoliation ;

few stems develop ; marginal areas of old leaves

turn brown; leaf blades become yellow and

puckered.

Fertilizer Placement

Many tests have been conducted on the time

and methods of application and the placement

of fertilizers for sweetpotatoes. Although there

were exceptions, results were usually best when

about one-half of the fertilizer was applied

before planting, either directly under the ridge

or a few inches to the sides of the center line

of the ridge and on a level with or a little below

the depth at which the plants were set. The

rest of the fertilizer was applied as side or top

dressings 2 or 3 weeks after planting.

Side dressings are often applied with special

fertilizer attachments during the second or

third cultivation. In some places where sweet-
18 AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

potatoes were grown "on the flat" instead of on

ridges, side dressing all or a part of the fertilizer 4 to 5 inches from the row and 3 to 5 inches

deep resulted in better yields than in places

where the fertilizers were broadcast before

plowing. When herbicides are used for weed

control and cultivation is greatly reduced, fertilizer may be applied as surface side dressings

along one or both sides of the row. This procedure is most effective when the fertilizer

application is followed by irrigation or rainfall.

The principal advantage of side placement of

fertilizers is the reduced amounts of plant injury. Whatever the placement pattern used, it

is not wise to apply fertilizers close to the root

area of the plants. At the heavier rates of

application the fertilizer must be placed farther

from the plant root zone to avoid injury. Nitrogen and potassium salts, especially, cause injuries when the fertilizer is placed too close.

Phosphorus salts are less soluble and are less

likely to cause injury under the same conditions.

In studies in Texas, plant stands were significantly better where fertilizers were applied

entirely as side dressings 3 weeks after plants

were set than where fertilizers were applied

entirely under the ridge 5 weeks before plants

were set.

TYPES AND SOURCES OF PLANTING MATERIALS

Two types of planting materials are commonly used in the commercial propagation of

sweetpotatoes : transplants and vine cuttings.

The largest part of the crop is from plants

grown from roots that have been placed in

warm soil or sand. Such plants are often called

draws, slips, or transplants. When they have 6

to 10 well-developed leaves (fig. 3), the plants

are pulled from the propagating beds and set

in the field.

Use of transplants permits early field plantings but makes necessary the storage and bedding of considerable quantities of *'seed'' roots.

A minimum of 10 to 12 bushels of medium-size

roots of a good sprouting variety such as Porto

Rico must be bedded to insure sufficient strong

plants at a single pulling to set an acre with

average row and plant spacings. More roots

must be bedded for varieties that produce

sprouts less abundantly. Both the earliness and

numbers of plants produced can be improved

by presprouting roots before bedding.

The quantities of seed roots bedded can be

reduced if well-developed sprouts from second

and third pullings are also set in the field. However, although equally strong sprouts from late

pullings have productive capacities equal to

those from the first pulling, the delay needed

to develop these plants in the beds means a

corresponding delay in making the field planting. A delay in planting usually results in a

reduction in yield; the shorter the growing

season, the more serious the yield reductions

occasioned by the delay.

Another disadvantage of propagation by

transplants is the danger of transmitting certain diseases that may be on or in the seed

roots. Black rot, soil rot (pox), scurf, certain

virus diseases, and even nematodes may be

spread from diseased roots to the new plants

as they grow in the bed. The infestations are

then carried into the field with the plants.

To reduce the amounts of seed stock needed

and the costs of building and operating large

plant beds and to avoid the disease hazard of

plants from ''mother" roots, many growers in

the South plant a part of their acreage with

vine cuttings (sometimes also referred to as

''slips'') made from the early vine growth of

early field plantings set with transplants. The

cuttings are set in the field the same as bedproduced plants. There is less danger of carrying disease organisms into the field on or in

cuttings taken from healthy vines of earlytransplanted plants than on plants pulled from

the average plant bed containing average seed

stock. This is a very important advantage of

vine cuttings over transplants. Use of vine

cuttings causes a considerable delay in planting,

however, because the mother plants must be

grown first.

"Bed cuttings," a special type of plants made

by cutting off 10- to 12-inch-long shoots just

above the soil surface in regular plant beds,

are also sometimes used in an effort to avoid

the time delay needed in obtaining regular vine

cuttings. The number of plants obtained from

bedded roots is reduced when bed cuttings are

made because additional sprouts are not produced as quickly as where plants are pulled

from the roots.

Growing a seed stock plot or a field from

vine cuttings to help insure against the spread

of diseases is highly recommended. Except in

the southernmost areas, however, any other

advantage of cuttings is very doubtful. Whereever the use of cuttings delays planting enough

reduce the value of yields much more than the

saving in cost of seed stock, cuttings should be

used only for the production of a clean seed

stock plot.
SWEETPOTATO CULTURE AND DISEASES 19

Obtaining the vine cuttings from the mother

patch requires considerable labor, and cuttings

are harder to handle in most planting machines

than are transplants. Furthermore, early removal of one-half to three-fourths of the vine

growth for cuttings reduces the yield of the

mother patch about 15 to 20 percent. Later and

heavier prunings of the mother patch plants

will reduce yields from the mother patch even

more.

Planting small pieces of seed roots directly

in the field, in a manner comparable to the

planting of Irish potato tubers, has been suggested and investigated. Propagation in this

way would eliminate the high costs involved in

producing plants in beds and should permit

better timing of planting. Studies have shown,

however, that plant stands were invariably

poorer, weed control was more difficult, and total

and marketable root yields were lower from

direct seed piece plantings than from later

plantings made with transplants obtained from

comparable roots bedded in unheated field beds

at the same time the seed pieces were planted

in the field. From 10 to 20 percent of the total

yields from the seed piece plantings were

greatly enlarged, unmarketable, rough, mother

roots resulting from continued growth of the

original seed pieces. This method of propagation cannot be recommended.

Use of Clean Seed Stock

For bedding, the grower should use only seed

stock that is known to be free of any signs of

disease and that has been carefully selected in

BN-36819

FIGURE 3.—Sweetpotato plants (draws, slips, transplants) for transplanting. All four of these plants are good

growing quality, but the one at the left is too short for easy handling. The second and fourth plants are

satisfactory, but the third plant is the best size. Scale is in inches.
20 AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

the field at harvest for seed purposes. Only at

that time can the proper precautions be taken

to insure that the roots have come from productive hills that are true to the variety and free

from disease. Furthermore, seed stock should

have better care than is usually given the bulk

of the crop. Sortinp out seed roots from fieldrun or commercial lots at the packing house or

storage house is a bad practice. It is desirable,

wherever feasible, to grow a separate patch or

field especially for seed purposes.

The procedures described above for selecting

clean seed stock are basic requirements that

entail much skill, care, and effort. Some growers

concerned mainly with producing stock for the

market or for processing may not feel justified

in spending the time and effort demanded for

an effective seed stock maintenance program.

If the grower has saved no clean seed stock of

his own, he should try to get it from another

grower who is known to produce seed of dependable quality, reasonably free from disease.

Lacking such a source, he should inquire

through his county agricultural agent or State

agricultural extension service representative

about sources of certified seed or plants. For the

small grower who needs only a few thousand

plants, purchase of plants from a dependable

source may be less expensive than producing

his own.

Foundation, Registered, and Certified Seed

For careful growers, particularly those engaged in extensive commercial production on a

permanent, self-sufficient basis, or for private

or public agencies distributing high-quality

selected seed stocks of established or new

varieties, in addition to producing seed for

general cropping uses, it is essential to follow

a continuing program for developing and maintaining superior stocks by best possible procedures. Repeated and continuing reproduction

by vine cuttings; reselection on individual hill

bases for trueness to varietal types, colors, and

other characteristics and for freedom from

diseases; and observance of adequate sanitary

precautions during growing, storage, and handling, all are essentials in effective seed programs.

New varieties are usually developed by State

or Federal agricultural experiment stations.

The original or basic stocks of these varieties

are maintained and controlled as foundation

seed by the originating agency directly, or

through designated public or private foundation

seed-producing agencies. Registered seed stocks

are produced directly from the foundation

stocks by selected growers. Certified seed stocks

must be produced either from the foundation

stocks or directly from registered stocks. In

other words certified stocks must be no further

than two successive steps from the basic

originator's seed.

High-quality seed stocks may be distributed

as registered or certified only after they have

met exacting requirements and been subject to

careful inspections by qualified State inspectors

for trueness to type, form, and color and freedom from diseases and pests in the plant bed, in

the field, and in storage. Several States maintain certification services and have well-defined

regulations and procedures to insure that seed

stocks and plants sold shall be virtually disease

free and have true varietal characteristics

(table 14).

TABLE 14.—Production of foundation, registered, and certified sweetpotato seed stocks, 1965

Varieties

reported

Acreage grown Production

State Foundation Registered Certified Foundation Registered Certified

Georgia

Louisiana

Mississippi

North Carolina

South Carolina

Virginia

Number

5

11

5

9

4

5

Number

38.0

14.3

4.4

13.1

3.3

4.1

Number

0

0

3.8

118.0

2.0

0

Number

0

558.3

.8

134.6

12.3

6.1

Cwt.

4,625

1,220

876

838

260

456

Cwt.

0

0

750

16,125

0

0

Cwt.

0

23,10,5

150

14,450

1,436

890

Totals 77.2 123.8 712.1 8,275 16,875 40,031

^ Prepared from data assembled by the International Crop Improvement Association.
SWEETPOTATO CULTURE AND DISEASES 21

PRODUCING PLANTS

Location of Plant Beds

Plant beds, whether temporary or permanent,

shoud be conveniently located on well-drained

sites where there is no danger of contamination

by water draining off higher lying, diseaseinfested land. Sites with southern exposure

and some wind protection are desirable. The

sand or sandy soil used in plant beds should be

in or obtained from a field or other source

where sweetpotatoes have never been grown

or have not been grown for at least 4 or 5

years, and from a field or some other source

that is free of possible contamination by

diseases.

A good source of water is essential to the

location of a good plant bed. About 15 square

feet of plant bed space per bushel of mediumsize roots bedded are required for all types of

plant beds.

Types of Plant Beds

Hotbeds

In the cooler production areas plants are

usually produced in some type of hotbed. Wherever it is necessary to provide any structure for

protection of the plant bed and to raise its temperature, heat is supplied most commonly by

specially designed electric heating elements.

Electric heating can be readily adapted to any

size hotbed operation (fig. 4). Other, less frequently used means of supplying heat are by

fermenting manure or other suitable organic

material and by heated flues or pipes. These

are discussed in Farmers' Bulletin 2020 (2). In

this handbook only general features of electricFlGURE 4.—Large electrically heated hotbeds. Alternate

sashes are opened for ventilation.

ally heated hotbeds are discussed. Details of

hotbed construction can be found in Farmers'

Bulletin 1743 (17) and Leaflet 124 (Í5). Details

concerning electrical wiring, heating equipment, and controls and estimated costs of electric heating should be obtained directly from

qualified representatives of local power companies or cooperatives.

Plant beds to be heated electrically should be

well built and thoroughly insulated to hold the

power cost down to a reasonable figure. Even

at relatively low rates for electricity, a poorly

built hotbed will be too costly because of the

power that will be wasted. Where electric heat

is used, permanent structures have many advantages over temporary ones. In the cooler

areas where much heating is necessary, fairly

elaborate provisions against heat loss are justified. Suitable conditions can usually be supplied

at a lower cost for a period of years by a

well-planned, well-built structure that does not

have to be rebuilt every year.

Permanent locations for electrically heated

beds are also desirable to avoid the considerable

costs of relocating power lines that serve the

beds whenever locations are changed. For permanent beds, good weatherproofed electrical

installation and fittings will save much time and

trouble over a period of years. Crude, temporary installations may be satisfactory for any

one year, but they require more labor for repeated installations and also are shorter lived

than permanent ones.

Because of the increasing costs and diminishing supplies of farm labor and the accompanying increasing mechanization of farm operations, permanent-type hotbeds (and coldframes)

are now sometimes constructed with removable

ends and semirigid "floors" that permit direct

filling and removal of bedding media by frontloading tractor scoops or other mechanical

means.

Hotbed frames are covered with cloth in

warm areas and with glass sash or glass substitutes in cool areas. Glass sash protects the

bed more efi'ectively from cold and allows much

more sunlight to reach the bed than does a

cloth cover. Glass sash sheds hard rain, much

of which may otherwise go through cloth and

thus chill the beds and make them too wet. The

initial cost of glass sash is much higher than

that of cloth covers, but sash can be used for

many years if handled with care. Cloth covers

can rarely be used more than two or three

seasons.
22 AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

In areas too far north for cloth covers to be

suitable, glass sash is better than the commonly

available plastics or other "glass substitutes"

because it transmits light better than the other

materials. The sturdiness of glass sash makes

it much easier to put on and take off straw or

mats needed for additional protection during

cold periods. In areas where much heating is

required, much of the higher cost of glass sash

will be offset through savings in heating costs.

Use of a standard, lead-sheathed or plasticcoated hotbed heating cable is recommended.

This cable is made so that single elements or

"loops" must be 60 feet long for operation on

110 to 115 volts, or 120 feet long for operation

on 220 to 230 volts. Under most conditions in a

well-built bed, a 60-foot element is recommended for each two-standard sash, or 6 by 6

feet of bed. Similarily, a 120-foot element will

heat the equivalent of four-standard sash, or 6

by 12 feet of bed. Figure 5 shows one practical

method of arranging the cable in the bed to

give uniform distribution of heat. The loops of

the cable should be no more than 7 to 8 inches

apart in the bed.

Automatic temperature control is furnished

by effective thermostats that are available at

reasonable cost. In locating thermostats and

laying the heating elements in large beds, it is

generally better to arrange the thermostats by

groups of sash than to have part of the area

under any group of sash controlled by one

thermostat and part by another.

In preparing the hotbed in cool areas, excavate the bottom of the bed about 1 foot below

the soil level. Put about 4 inches of coarse

gravel or coarse cinders in the bottom to keep

the heated part of the bed well away from the

cold, wet soil below. Cover this with a layer of

clean straw or other coarse litter that will

settle down to â thickness of about 2 inches

when soil and sand are put on top if it. The

coarse litter gives further insulation from the

cold bottom of the bed and thus keeps much

heat from being lost downward. Cover the

litter with a thin layer of medium-texture soil,

just enough to keep the bedding sand or light

soil from sifting down through the litter.

Finally, carefully level an inch of sand over the

entire bed. Arrange the heating cable on the

FIGURE 5.-

AGR. ENG. NEC. 30108

-One practical method of arranging hotbed-heating elements in a bed 6 feet wide.
SWEETPOTATO CULTURE AND DISEASES 23

surface of the sand and complete the electrical

connections (fig-. 5). Add 4 inches of sand or

sandy soil over the cable and put the thermostat bulbs and thermometers in place. After the

thermostat is adjusted so that the electric current will be turned on and off as near 80° F. as

possible and the glass or other bed cover is put

on, the bed is ready for operation.

Some variations in the directions iust given

may be made, depending upon local weather

conditions and materials available for preparing the hotbed. However, good drainage, tight

construction, tight covers, and thorough insulation must always be provided.

Coldframes

Coldframes are used in certain areas that are

somewhat warmer than those in which hotbeds

must be used. In the Eastern Shore area of

Maryland and Virginia, for example, sweetpotatoes are commonly bedded in coldframes

placed in more or less protected locations but

without any sources of bottom heat. Muslin or

other cloth or plastic covers are used to protect

against low air temperatures. In general practice in most other areas, however, coldframes

are not widely used for sweetpotato plant

production.

Field Beds in the Open

In the warmer areas of the South, where little

freezing occurs, seed sweetpotatoes are usually

bedded in the open on ridges or in field beds,

with no frame protection or heat applied (figs.

6 and 7).

For best results with field beds, choose only

well-drained, light soils that warm up early and

that have had no sweetpotatoes on them for

several years. When bedding is to be done on

land where root-knot or other nematodes are a

problem, after the land is worked up but before

bedding, treat the soil with a registered nematicide or soil fumigant, following the manufacturer's directions. Proceed with bedding

when the nematicide has disappeared sufficiently

to cause no injury to the roots. Additional information on application of nematicides has

been published by Good and Taylor (6).

In preparing ridge-type field beds, plow the

soil deep in late January or February, disk

thoroughly, then construct ridges about 12

inches high and 3 feet apart. Work about 500

to 600 pounds per acre of suitable fertilizer,

such as 4-12-4, 5-10-5, or 6-12-6 mixtures, into

the top 5 or 6 inches of soil in the rows. Let the

ridges settle and the soil warm up for 2 or 3

weeks, open shallow furrows atop the ridges,

FIGURE 6.—Plastic-covered open field beds with 15-foot-wide plastic sheets covering three field beds.

(Photo courtesy of North Carolina State University.)
24 AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

FIGURE V.—Sweetpotato plants in open field beds. The crew in the background is pulling- plants.

(Photo courtesy of North Carolina State University.)

and place the seedroots a few inches apart,

lengthwise of the ridges, and cover them with

about 2 inches of soil.

In preparing broader field beds, plow and

thoroughly prepare the bedding area, then rake,

drag, or otherwise smooth and level the land

into beds 5 to 6 feet wide and as long as desired.

Place seed potatoes by hand, leaving 6 to 12

inches on each side of the bed, cover them

uniformly with about 2 inches of soil, and then

apply and work fertilizer into the bed surface.

These beds are usually covered with plastic

kept in place by soil plowed over the edges. The

beds may be covered individually, or a single

wide sheet of plastic may be used to cover two

or more beds (fig. 6). When sprouts begin to

emerge, remove the covers. Alleys about 3 feet

wide are needed between adjacent beds to provide soil for covering the roots and to permit

access for bed care and plant-pulling operations.

Open field beds should be relocated each year

to prevent buildup of disease-causing organisms

in the bedding area.

Plants are produced slower in field beds than

in hotbeds, but at less cost. Earlier sprouts can

be obtained from open beds if the seed roots are

presprouted before bedding. (See p. 26.)

Preparation of Seed Stock for Bedding

Upon removal from storage, sort seed stock

to eliminate all roots that show any evidences

of disease or that do not have a bright, smooth,

firm, external appearance. "Nick" or notch each

root with a knife by cutting a small wedge

through the cortical layer at the stem end (end

usually showing some sprouts) and discard any

roots that have inferior flesh colors. This practice is especially important in producing foundation, registered, or certified seed.

Before roots are bedded, they should be

treated with a registered, effective, chemical disinfectant to eliminate any surface contamination by disease organisms. Such treatment aids

in preventing seed-piece decay in the plant beds.

In cases where presprouting heat treatments or

cutting of seed roots may be used to increase

the earliness or numbers of plants produced

(pp. 26 and 27), suitable disinfectants for use

are applied after the preparatory treatments

and immediately before bedding. A number of

chemicals such as sodium o-phenylphenate tetrahydrate (SOPP) are effective for seed-root

treatment when used in proper concentrations.

Manufacturers' directions should always be

followed.
SWEETPOTATO CULTURE AND DISEASES 25

Bedding Procedures and Bed Care

Previously used plant hotbeds and coldframes

should be cleaned out and disinfected with a

registered, effective fungicide and all old bedding materials removed from the bedding area

before starting the bedding operation.

Where bedding is in hotbeds, the bed temperatures should be observed carefully and

adjusted to 80° F. before the roots are placed

in the beds and covered. Thereafter the temperature should be maintained between 75° and

80°. A good supply of plants should grow in

about 6 weeks in this temperature range. Bury

the elongated thermostat bulb horizontally in

the bed at the level of the lower part of the

bedded roots. Place it near the middle of the

area controlled by the thermostat, and set a

good soil thermometer next to it with the

thermometer bulb at the same depth. Read the

thermometer from time to time to insure that

the thermostat is properly adjusted. One or 2

inches of pine shatters, wheat straw, or other

clean litter applied over the bed surface will

help conserve moisture. It should be removed

when sprouts appear.

Whatever the type of bed used, as far as

practical, bedding should be done on sunny,

warm days and covers should be placed over

beds as promptly as possible to avoid unnecessary chilling of roots and beds.

Bed roots of similar size together so that they

can be covered to equal depths. Mixed sizes result in unequal covering. Place roots on the sand

or soil of the bed with about a one-half-inch

space left between roots (fig. 8). Cover with

about 2 inches of clean sand or sandy soil.

Water the beds to settle the soil about the roots.

On bright, warm days the temperature will rise

rather high under glass sash or plastic covers.

When this occurs, lift some of the sash or open

the ends of plastic covers to keep the temperature down (fig. 4). It should not exceed about

95° F. As large numbers of good plants develop,

bed covers should be left off whenever the

weather is warm. For about 10 days before

transplanting starts and thereafter, covers

should be left off all the time unless there is

danger of an untimely frost or near frost.

The beds will need watering from time to

time, the frequency depending on the amount of

FIGURE 8.—Bedded sweetpotato seed roots ready to be covered with sand or soil.

AGR. ENG. NEC. 30167
26 AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

ventilation given to help control the temperature. The bedding material around the roots

should be examined through its depth every

few days to insure that it does not become too

dry for plant growth. Excess watering tends to

chill the beds, waste heat, and check plant

development. Whenever possible, watering

should be done during warm, bright periods to

avoid chilling the beds or plants. Before the

sprouts appear, very little water is needed. As

large numbers of plants develop, the need for

water increases rather rapidly.

After each pulling of plants, the beds are

usually fertilized lightly, then watered to resettle the soil. Most growers use some commercial fertilizer in their plant beds. This is usually

applied and worked into the surface soil after

roots have been covered in the bed. As a means

of saving labor, however, where open field beds

are used, sometimes the fertilizer is applied and

worked into the soil during land preparation

before the beds are laid out. Fertilizer composition and application are varied according to

local experience and practice. In experiments

conducted in South Carolina, about 17 percent

more plants were produced when 4-8-4 fertilizer

was thoroughly mixed with the bedding soil at

a rate of 300 pounds per acre than when no

fertilizer was added.

The North Carolina Experiment Station

recommends use of % pound of 8-8-8 fertilizer

per square yard, raked lightly into the soil after

bedding in open field beds, supplemented by 1/5

pound of nitrate of soda per square yard applied

before watering after each pulling of plants.

Amounts of Seed Stock Required

The number of plants required per acre varies

with the spacings used within and between

rows (table 15). For average recommended

spacings of 12 to 15 inches in the row and about

3^2 feet between rows, roughly 10,000 to 12,-

000 plants are needed for setting one acre. As a

general guide, a good sprouting variety may be

expected to produce, in the first one to two pullings, up to 1,000 plants per bushel of goodquality, medium-size seed roots bedded. For

later plantings about 500 additional plants per

bushel can be expected from each of one or two

later pullings spaced at 7- to 10-day intervals.

Factors to consider in determining more definitely the quantities of seed stock to be bedded

in particular cases include:

(1) LOCATION OF PRODUCTION AREA.—Abundant supplies of early sprouts are especially

important for plantings made in intensive culture areas having relatively short growing

seasons. In such locations planting at earliest

safe dates is imperative for high yields. This

means bedding sufficient roots to supply the

desired plants at one early pulling. A delay of

only 10 days to 2 weeks in planting, occasioned

by waiting for sprouts to develop for second

or third pullings, can mean differences in marketable yields of 20 to 30 or more bushels per

acre for each planting delay. In the warmer

areas of the South where the growing season

is longer, plants from successive pullings can

still give reasonably high yields and therefore

less bedding stock will be needed for supplying

plants.

(2) VARIETAL SPROUTING CHARACTERISTICS.

—Ideally, a good sweetpotato variety should

produce an abundance of strong plants within

a reasonably short time after bedding of good

stock. The best of the older, commercially important varieties such as the Porto Ricos and

Jerseys possess this characteristic. Unfortunately, however, some of the recently developed,

high-yielding varieties sprout slowly or do not

develop sprouts in suflScient abundance (table

16). To properly estimate amounts of stock to

bed, the grower must consider the sprouting

characteristics of the particular variety to be

grown.

Special presprouting and seed-piece cutting

techniques can be applied to increase plant production and thus to reduce the amounts of stock

to be bedded. These are especially useful with

poorer sprouting varieties. When stored roots

are held under conditions of high humidity and

temperatures of about 80° F. until sprouts are

about 1 to 2 inches long before bedding, the

numbers of sprouts available at the first pullings are considerably increased. In most cases

TABLE 15.—Approximate numbers of plants required to set 1 acre at different plant and

row spacings

Spacing

within

rows

(inches)

6

8

10

12

14

15

16

18

21

24

Plants required to set

1 acre at row spacings of—

36

inches

Number

29,040

21,780

17,420

14,520

12,440

11,610

10,890

9,680

8,300

7,260

40

inches

Number

26,130

19,620

15,680

13,070

11,200

10,430

9,800

8,710

7,470

6,530

42

inches

Number

24,890

18,670

14,960

12,440

10,670

9,960

9,330

8,300

7,110

6,220

44

inches

Number

23,760

17,820

14,490

11,870

10,180

9,500

8,910

7,920

6,780

5,940

48

inches

Number

21,780

16,310

13,080

10,890

9,330

8,710

8,170

7,260

6,220

5,440
SWEETPOTATO CULTURE AND DISEASES 27

TABLE 16.—Relative abundance and earliness of

sprout production of sweetpotato varieties

compared in 10 States, 1965 and 1966 ^

Mean ratings for—

VarietyPlant

production 2 in— Earliness 3 in—

1965 1966 1965 1966

Centennial

Gem ^

3.2

3.5

4.0

2.0

2Td

5.0

3.7

3.0

4.4

2.1

5.0

3.0

5.0

2.0

1.7

1.5

2.5

2TÖ

1.0

1.6

2.1

Goldrush 1.4

Julian 2.6

Nemagold

Nuereret

2.0

3.0

Porto Rico 1.0

1 Unpublished data : National Sweetpotato Cooperator

Group.

2 Plant production rated 1 (poor) to 5 (good).

3 Earliness of plant production rated 1 (early) to 3

(late).

the total numbers of sprouts developed for all

pullings are also increased by the treatment.

The average total number of plants produced

per bushel of presprouted and nonpresprouted

roots of three varieties in eight tests conducted

in North Carolina v^as 1,673 in 1960 and 862

in 1961. Table 17 shoves results obtained in one

test v^ith the Goldrush variety in 1960. Presprouting is particularly advantageous where

coldframes and plastic-covered open field beds

are used. Presprouting permits delaying of bedding until soils have warmed up sufficiently to

promote rather rapid plant growth. Plant production almost as good as that from heated

beds is attained without the much more expensive construction and operation costs involved

with the heated facilities.

Cutting seed roots crosswise into about equal

TABLE 17.—Plant production of presprouted

and nonpresprouted Goldrush sweetpotatoes bedded in a plastic-covered field

bed at Wake Forest, N.C., April 11, 1960

Plants per bushel of seed roots

Date of pulling Presprouted Nonpresprouted

May 16

Number

1,300

175

300

Number

400

May 26 425

June 8 300

Total 1,775 1,125

parts before bedding increases the number of

sprouts produced by a given quantity of stock.

Although some reduction in average plant size

usually accompanies the increased numbers of

plants produced with this procedure, except

with very small stock, plant size is usually adequate for field planting. When seed roots are

crosscut, the cut roots must be treated with a

registered, effective fungicide before bedding

to prevent excess rotting of the seed pieces. Results of crosscutting seed roots of the Canbake

variety in one test at the Georgia Experiment

Station in 1960 are shown in table 18.

(3) SIZE OF SEED ROOTS BEDDED.—Small to

medium-size bedding roots produce more plants

per bushel of bedded stock than do larger roots

although the larger roots produce more and

somewhat larger plants per bedded root. Ordinarily it will be more economical to use the

small to medium-size selected roots for production of plants. Where larger size roots are used,

larger quantities of bedding stock must be provided and more bed space will be required to

furnish enough plants for planting a given

area.

TABLE 18.—Number and size of plants produced from whole roots and

plants produced by crosscutting Canbake sweetpotatoes before bedding

at Experiment, Ga., 1960 ^

Cutting

treatment

Roots

used Plants pulled

Average

plants

produced

per

root

Average

weight

per

plant

May 5 May 17 June 10 Total

Whole roots

Number

272

272

Number

319

649

Number

1,430

2,113

Number

741

826

Number

2,490

3,558

Number

9.1

13.2

Grams

11.6

Crosscut roots 8.9

^ All roots of comparable size, bedded April 6 in electrically heated beds, after being

dipped in a fungicide.

SOURCE: Cross-Cut Bedding Sweet Potatoes to Increase Sprouts (3),
28 AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

FIELD PLANTING

Methods of Transplanting

Sweetpotatoes are set in the field either by

hand or with machine transplanters. Handsetting is generally a relatively slow and inefficient method of planting this crop and requires

much greater expenditures of labor for setting

a given area than machine transplanters require. Handsetting is used mostly in planting

small areas or on small farms where machine

transplanters are not available. Most large

sweetpotato operations today use single- or

multiple-row machine transplanters (figs. 9

and 10). A number of difl'erent makes and types

suitable for sweetpotatoes are available. Small

growers who cannot buy a machine transplanter

or cannot make it profitable in their small

operations should consider the joint purchase

and operation of a machine with a few of their

neighbors.

In handsetting, the plants are dropped by one

person at the desired intervals on the prepared

ridges. One or more workers follow and do the

actual planting with the aid of trowels, dibbles,

or special "sticks." Handsetting is best done

when the soil contains adequate moisture.

Machine transplanters not only save man

labor but also transplant the plants more uniformly and generally more satisfactorily than

handsetting. These machines are equipped with

watering devices that apply a small quantity

of water around each plant just before the soil

is drawn around it. Under some conditions

rapid recovery of plants following transplanting is facilitated when dilute "starter" nutrient

solutions are used instead of plain water. Highanalysis, soluble starter fertilizers are available

commercially for use, at manufacturers' recommended rates, in the transplanting water supply, or 2 to 3 pounds of a moderately high

nitrogen content, regular fertilizer may be dissolved for use in 50 gallons of water. Where the

available nutrient levels in sweetpotato soils

are adequate, it is not always economical or

necessary to use starter fertilizer but, in general, water should be used when transplanting

by machine because the soil is usually dry

enough at planting time for the plants to benefit from the water applied. The low cost of

application of water by machine makes the

grower less dependent on the weather in determining when he can plant. In view of the losses

in yield that follow late planting (or delayed

starting of growth after planting), this advantage of the machine is often an important one.

Three men with a one-row planter can set

3,500 to 4,000 plants per hour—one man driving

and two men handling plants. With a two-row

planter, five men can set 7,000 to 8,000 plants

per hour. If the soil is moist enough for setting

FIGURE 9.—Single-row machine transplanter. Note ridge-leveling plow and water supply tank.

BN-36821
SWEETPOTATO CULTURE AND DISEASES 29

FIGURE 10.—Planting with 4-row machine transplanter. (Photo courtesy oí North Carolina State University.)

the plants without water, considerably larger

numbers can be set per hour because stops for

filling the water tanks are unnecessary.

For efficient results with machine transplanters, the work crews must have some experience and skill, the field must be well

prepared, the water tanks must be refilled

quickly, and the plants must be of medium size

and neatly arranged for rapid handling. Waiting for plants or water is costly. Water should

be brought to the field in large tanks or barrels. Plants that are too large or too small or

that are tangled or disorderly in the bundles

can easily waste 25 percent or more of the time

of the planting crew.

Plants that are delayed in starting active

growth fail to catch up with neighboring plants

that start growth promptly. Imperfections in

the plants or the transplanting operation and

cloddy or trashy spots in the ridges where

plants are set may retard plants enough to make

them unproductive laggards throughout the

entire season. Plants next to skips often produce oversize roots too large to be marketable.

Planting skips or replacing dead or defective

plants 2 or 3 weeks after the field has been

transplanted is unprofitable. Late replacements become so crowded by their early starting neighbors that they rarely yield enough to

pay for the trouble of transplanting them. It

is most important to get a full stand of good

plants all at a single transplanting.

Preplaiitiiifi Fungicidal Treatment of Plants

Normally, sweetpotato plantings are made

with clean plant materials on soils believed to be

essentially free of serious sweetpotato diseaseproducing organisms. No preplanting fungicidal treatments of plants are ordinarily used.

Sometimes plantings must be made in fields

where certain disease-producing organisms,

such as those causing stem rot (wilt) or scurf,

are suspected of being present or are known

to be present in small amounts. Under such

conditions dipping the basal parts of clean

plants in a solution of a registered, effective

fungicide immediately before planting often

gives measurable protection against early field

infections by some pathogenic organisms.

Sweetpotatoes should never be grown on land

known to be heavily infested with disease-
30 AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

producing organisms or from plants produced

in disease-contaminated plant beds. Preplanting

fungicida! applications can be of little practical

value under such adverse conditions.

Size of Plants

Good, sturdy plants are essential in obtaining full stands in the field. Small, weak plants

with few leaves have a much poorer survival

rate and give poorer yields and for these reasons should be discarded when they are pulled

at the plant beds. About 20 percent of the plants

are ordinarily this small, weak type, not suitable for field setting.

Best plants for field setting of most varieties

should average about 8 inches long (including

the rooted parts of the plants). (See fig. 3.)

Sturdy, disease-free plants ranging from about

6 inches to 10 inches long and each having 6

to 8 leaves handle best in transplanting, establish well, and give best stands and overall production. Plants less than 6 inches and more

than 10 inches long are usually somewhat more

diflicut to handle, particularly where they are

machine transplanted. Plants that are excessively long before pulling may have to be

trimmed off at the basal ends to facilitate handling. As far as practicable, plants that are

excessively long should be avoided by timely

and judicious regulation of plant bed temperatures, ventilation, and watering practices.

Plants of those varieties having *'bunch'' or

''bush'' growth habits usually do not grow long

enough for most effective handling during field

planting operations. Therefore, plantings of

these varieties are made with stocky, vigorous,

but short plants.

Vine cuttings for field planting should be

made only from vigorously growing, healthy

plants in field beds or in early-transplanted

fields. Cuttings suitable for planting should be

about 8 to 10 inches long and made from terminal or middle portions of vigorously growing

vines. Cuttings made from the older, more or

less hardened, basal portions of vines do not

root and establish well in the field, and as a

rule resulting plants are not sufiSciently productive.

Bed cuttings are made by cutting off the 6-

to 8-inch tops of vigorous sprout plants at or

near the surface of the media in plant beds.

Time of Planting

Time of planting is very important in sweetpotato production. Total and marketable yields

are best from plantings made as early in the

spring as plants are available and weather is

warm enough after the frost-free date for the

particular area. Delay in planting reduces both

the total crop yields and the yields of prime

market-size potatoes. Early planting is especially important in areas where the normal effective growing season is comparatively short.

Yields are satisfactory but not maximum with

some delays in planting times in warmer parts

of the South where the growing season is sufficiently long.

In an experiment conducted with two varieties at four locations over a 3-year period, a

2-week delay in planting after the earliest

planting dates reduced total yields nearly 40

bushels per acre and yields of No. 1 grade 30

bushels per acre. A 4-week delay reduced total

yields 86 bushels per acre and yields of No. 1

grade 43 bushels per acre. Further delays reduced yields even more.

Setting the plants closer together in the rows

will not make up for the yields lost as a result

of late planting (table 19). The importance of

transplanting as soon as the weather is warm

enough can hardly be overemphasized. Planting

on time will greatly improve average yields,

with only a moderate increase in cost of producing the crop, mainly in the higher cost of

producing plenty of early plants.

Plant Spacings

On most well-drained, light soils adapted to

intensive sweetpotato culture, rows for sweetpotatoes are usually spaced from 3 to 31/2 feet

apart. Under these conditions the crop is grown

on ridges of low to moderate heights—up to

about 1 foot. Where suitable soils are somewhat

heavier and where somewhat higher ridges may

be required for adequate drainage, rows are

spaced as far as 4 feet apart.

A number of factors must be considered in

determining the plant spacings to be used within sweetpotato rows: (1) Growth habit and

root-setting characteristics of the variety

grown; (2) sizes of roots needed for the principal end use(s) for which the crop is produced; (3) type and fertility level of soil and

intensity of fertilization program followed;

(4) length of growing period; and (5) economic considerations : relationships of expected

net returns to labor, plant material, and other

production costs involved for cropping at varying plant spacings.

Some varieties, usually certain vigorously

growing types, have inherent tendencies to set

their storage roots early and to produce significant proportions of their crop as large roots,

particularly from early plantings and when

harvest is delayed beyond the optimum digging
SWEETPOTATO CULTURE AND DISEASES 31

TABLE 19.—Effect of hill spacing on total yield and yields of No. 1, No. 2, and Jumbo grades of

Porto Rico sweetpotatoes from 5 successive plantings i

Hill spacing and

grade of

sweetpotato

Yield per acre for— Average yield

per acre

for spacing

First

planting

Second

planting

Third

planting

Fourth

planting

Fifth

planting

8-inch:

Total

Bushels

329

161

86

23

319

147

81

36

319

139

79

50

Bushels

304

138

73

23

287

126

72

31

271

118

69

40

Bushels

242

100

56

14

240

104

53

23

223

97

51

26

Bushels

199

99

50

6

206

99

46

8

184

86

43

13

Bushels

134

56

29

1

128

50

32

1

126

49

30

3

Bushels

242

No 1 111

No. 2 59

Jumbo 13

12-inch:

Total 236

No. 1 105

No. 2 57

Jumbo 20

16-inch:

Total 225

No. 1 98

No. 2 j

Jumbo

54

26

Average yield per

acre for spacing :

Total 322

149

82

36

287

127

71

31

235

100

53

21

196

95

46

9

129

52

27

2

No. 1

No. 2

Jumbo

1 Coordinated tests conducted from 1940 through 1942 at Blackville, S.C., Experiment, Ga., Gilmer, Tex., and

Laurel, Miss. Row spacings 3i^ feet. First plantings soon after average dates of last killing frost in spring, with

successive plantings at intervals of approximately one-twelfth of normal growing season for each location. Yields

in 55-pound bushels.

SOURCE: Regional Studies of Time of Planting and Hill Spacing of Sweetpotatoes (1),

period for the variety. Closer spacing of plants

in the row, slightly delayed planting tirnes, and

earlier harvests usually reduce the yields of

oversize roots of such varieties v^ithout significantly reducing total yields or yields of marketable and canning-size roots.

High root yields are alv^ays important. Root

size requirements, hov^ever, vary v^ith the principal intended use(s) for the crop. They are

different for fresh market use, for vv^hole-root

canning, freezing, or other food-processing

uses, or for use in feed production or industrial

processes. For the fresh market, high yields of

uniform-size and uniform-shape roots of the

No. 1 and No. 2 market-size grades are very

important. For whole-root canning and freezing, large parts of the crop need to be roots

with small diameters (1 to 2 inches) and short

to medium lengths (3 to 6 inches). For feed

production, industrial processes, and some foodprocessing uses such as baby food puree canning, dehydrated flake manufacture, and frozen

dice stock preparation, root size characteristics are less critical, but large roots are often

preferred because of greater ease of handling

and relatively less waste losses during peeling

and other preparation operations.

Plant spacings of 12 to 15 inches in the row

are generally preferable for economical production of roots of best grades for the fresh market. Results of extensive plant-spacing experiments have shown that for a designated

planting period, the average total yield per acre

and yields of No. 1 grade and small roots are

increased and yields of Jumbo and oversize

roots are decreased by decreasing plant spacings within the row (table 19).

Because of the costs of additional plants and

problems associated with transplanting and

handling plants at close spacings, it may not

always be economical to use spacings much less

than 12 inches as a means of attaining desired

yield increases of market-size roots. (See table

15 for numbers of plants required per acre at

different plant and row spacings.) Where increased yields of small roots are needed for

canning, assuming that adequate returns can

be expected for these roots, increasing plant

densities makes attainment of this objective

possible without materially altering production

of market-size roots. Yield effects obtained by

setting two plants per hill at twice the distance

between hills can be the same as yields obtained

by uniform close spacing of single-plant hills.
32 AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

Practical considerations make machine planting difficult at uniform, very close plant spacings.

When production of large roots is desired

for special food-processing, dehydrating, industrial, or feed purposes, widening the plant

spacings makes it possible to obtain greater

proportions of the total production as large

roots. Total production of roots of all sizes.

however, may be smaller than at closer plant

spacings.

Plant spacings can be somewhat closer on

good, fertile soils than on less desirable, less

fertile soils. On soils not adequately supplied

with plant nutrients, apply enough fertilizers

to support efficient plant densities rather than

depend on wider spacings but lower yields to

insure adequate root development of this crop.

WEED CONTROL AND CULTIVATION^

Control of weeds and maintenance of ridges

at proper height and shape are the two main

purposes in cultivation of sweetpotatoes. Many

annual and perennial weeds, including pigweed,

ragweed, jimsonweed, crabgrass, goosegrass,

and nutsedge, infest sweetpotato fields. The

most serious weeds are annual and perennial

grasses and nutsedge. Effective control of weeds

is particularly important during the early part

of the growing season when the sweetpotato

plants are getting established and before vines

have grown extensively. One or two hand hoeings are often needed to destroy weeds in the

rows when the sweetpotato plants are small.

If weeds are allowed to get ahead of the sweetpotatoes in this early period, much labor and

expense are needed later to remove these

weeds. Because of the spreading growth habit

of sweetpotatoes and their luxurious foliage

that shades the weed seedlings during the

middle and late parts of the season, sweetpotatoes compete comparatively well with

weeds at these times. Sweetpotatoes also develop extensive root systems and therefore compete strongly with the weeds for moisture and

nutrients. The average cost of weed control by

usual cultural methods is approximately $20 to

$30 per acre. Use of improved control methods,

particularly the use of registered herbicidal

chemicals, should reduce this cost significantly.

Weed Control by Cultural Methods

As far as possible, sweetpotato land should

be free of weed seedlings at the time of planting. To keep down new grass and other weed

growth as much as possible and to prevent any

serious crusting of soil on the crowns of rows,

the first cultivation should be within a week or

10 days after planting. Two to four subsequent

cultivations are usually required to control

2 The section "Weed Control and Cultivation" was prepared in cooperation with L. L. Danielson, plant physiologist, Crops Protection Research Branch, Crops Research Division, Agricultural Research Service.

weeds adequately and to maintain the ridges.

If for any reason the ridges become grassy,

especially during the early part of the season,

the sides should be barred off with shovels,

disks, or small plows and the soil pulled back

to the ridges with sweeps or other suitable

tools. Make the last or "lay-by'' cultivation

when vine growth becomes so extensive that

further cultivation is difl^cult. Throw the soil

well up toward the rows to smother out small

grass and other weeds and to maintain the

ridges.

The types and arrangements of tools to be

used for cultivation will depend on the kind of

power and equipment available and on the particular soil and field conditions. Various

arrangements of sweeps, disks, and shovels are

used successfully on both horse-drawn and

tractor-powered cultivators. Sweeps are particularly useful tools in weed removal and ridge

maintenance. Sweeps of the type designed for

plow stocks are often mounted to follow disk

hillers. The disks are set to cut away part of

the ridge and throw the weeds into the middle.

The sweeps immediately follow, restoring the

soil to the ridge and weeding the upper part of

the ridge at the same time. Today cultivation

is usually done with tractor-powered equipment. Each design of tractor and of tools used

with it presents a separate problem that must

be worked out according to the particular field

conditions and the job to be done. By trial and

adjustment and by careful control of tractor

speed, work can be done very accurately. Effective cultivation and ridge maintenance with

tractor tools demand that ridge construction

and transplanting be done in a regular, precise

manner. Precision is especially necessary for

two-row equipment or larger. Tractor tools are

so mounted that when the tractor is in motion,

the principal control lies only in the driving of

the tractor. Wavering of the row on the ridge

and variations in distances between rows and

in ridge height seriously interfere with doing

a job satisfactorily. Tractor tools are not flexible enough to deal with such irregularities.
SWEETPOTATO CULTURE AND DISEASES 33

Chemical Weed Control

Herbicides are generally used on sweetpotatoes to control weeds early in the season to

reduce the amount of cultivation needed and to

maintain weed control after the lay-by cultivation with a minimum expenditure for hand

labor. Herbicides are usually used in combination with mechanical cultivation operations for

weeding, ridging, and fertilizing the crop.

There are situations in which herbicides used

under favorable conditions immediately after

transplanting will effectively control weeds

throughout the growing season without supplemental cultivation.

A large number of registered, effective herbicides are available for control of weeds in

sweetpotatoes. Several of these are used successfully in field practice. These include:

Sodium salt of AT-l-naphthylphthalamic acid

TABLE 20.—Weeds controlled by preemergence applications of herbicides

Weeds

Herbicides

<

<

O

<

PL,

Q

Co

Ö

PA

o

H ñ

>

Broadleaf weeds:

Carpetweed __

Chickweed

Cocklebur

Coifeeweed ___

Deadnettle

Dock

Florida pusley

Goosefoot, nettleleaf

Jimsonweed

Knotweed

Lambsquarters

Morningglory, ivyleaf

Mustard

Nightshade, hairy

Pigweed, prostrate

Pigweed, redroot

Purslane

Ragweed

Shepherdspurse

Smartweed

Spurge, spotted

Velvetleaf

Weed Grasses:

Annual bluegrass

Barnyardgrass

Cheat

Crabgrass

Foxtail, giant

Foxtail, green

Foxtail, yellow

Goosegrass

Johnsongrass (seedling)

Lovegrass

Quackgrass

Sandbur

Stinkgrass

Witchgrass

Sedge :

Nutsedge, yellow

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

1 X = Satisfactory control.
34 AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

(naptalam) ; A^,A^-dialIyl-2-chloroacetamide

(CDAA) ; S-ethyl dipropylthiocarbamate

(EPTC) ; dimethyl tetrachloroterephthalate

(DCPA) ; 3-amino-2,5-dichlorobenzoic acid

(amiben) ; A^,iV-dimethyl-2,2-diphenylacetamide

(diphenamid) ; and >S-propyl dipropylthiocarbamate (vernolate). These herbicides kill germinating weed seeds, and vernolate and EPTC

also control sprouting weed rootstocks.

Climate, soil composition, and cultural practices are critical factors in the successful use

of herbicides. Each herbicide kills certain weeds

and requires certain specific conditions and

methods of application for best results. Choice

of a herbicide will therefore depend on the particular weed problem. Crops and herbicides

should be rotated to reduce the number and

kinds of weed seed in the soil. Using difl'erent

herbicides on sweetpotato plantings and other

crops in the rotation in successive years will

control many diñ'erent kinds of weeds and

minimize the possibility of accumulating herbicide residues in the soil.

Information in table 20 shows which preemergence herbicides effectively control the

most commonly encountered broadleaf weeds,

weed grasses, and nutsedge. This information

can be used as a guide in choosing the herbicide

for controlling weeds in particular fields. In

general, preemergence herbicide treatments

should be made when soil moisture and temperature are optimum for rapid weed seed

germination.

The method of application depends on the

herbicide. The herbicides listed in this handbook are usually available for making spray or

granular application. Some herbicides are

applied to the soil and deeply incorporated by

disking before planting. Others are applied

immediately after planting or after clean cultivation soon after planting. When late-season

weeds that emerge after the lay-by cultivation

are a severe problem, the herbicide treatments

may be repeated immediately after the lay-by.

^ Carefully planned use of registered herbicides can improve sweetpotato production efficiency. Herbicides are being constantly improved and methods of application are changing

as new research information is developed. To

insure correct and most effective use and to keep

residues ivitJiin tolerance limits, you must carefully follow the directions 07i the label of the

container. State experiment station weed specialists can help the grower to select herbicides

and methods of use that have proved most

successful for local conditions. All precautions

for storage and use of herbicides should be

carefully observed.

WATER REQUIREMENTS AND IRRIGATION

The sweetpotato is not a dry-weather crop

although it may be considered as a moderately

drought-tolerant one. Once growth is well established, the sweetpotato can survive rather long

dry spells during the summer, and with subsequent rainfall is able to resume growth and

produce storage roots. Drought tolerance is

made possible, at least in part, by the rather

extensive and moderately deep root system that

enables the plants to obtain some water even

when the surface soil layers become seriously

moisture deficient. However, quality and yields

of roots will be poorer when inadequate moisture levels develop during the growing season.

The most severe yield reductions result from

prolonged water shortages 50 to 60 days after

planting, when vine elongation is slowing down

and storage root initiation has begun. Yield

reductions are less severe when droughty

periods occur either a little earlier or a little

later than this. Drought reduces the number of

storage roots more than it reduces their size.

The total solids, total carbohydrate, and starch

contents are lower and the moisture content is

higher in roots of a variety produced in very

dry years (without irrigation) than in roots of

the same variety produced in years of adequate

or abundant rainfall. Apparently, where the

moisture supply is deficient, plants are unable

to synthesize and to accumulate the normal

amounts of carbohydrates and other solids in

the storage roots.

Highest yields of best quality sweetpotatoes

can be expected only where the crop is planted

in sufficiently moist soil and where adequate

supplies of soil moisture are maintained

throughout the growing season up to about 2

or 3 weeks before harvest. Approximately

18 to 24 inches of water, well distributed

throughout the growing season, supplied by

rainfall or by irrigation or by rainfall supplemented by timely irrigation, are needed for

best sweetpotato production. Irrigation water

may be supplied either by overhead sprinkler

systems or, where conditions permit, by furrow

irrigation methods.

In Louisiana the average daily water requirement for a sweetpotato crop is approximately

one-tenth acre-inch per day during the early

part of the growing season. This increases to

about one-fourth acre-inch per day in midsummer.

When to apply irrigation water to keep the

moisture supply adequate has been the subject
SWEETPOTATO CULTURE AND DISEASES 35

of much research. A sweetpotato grower who

is thoroughly familiar with the growth of the

crop on his soil and under his growing conditions will learn to recognize the symptoms of

moisture shortage in his plants and can gage

irrigation accordingly. Various types of soil

moisture measuring equipment such as moisture meters and tensiometers are available to

aid the grower in ascertaining when irrigation

is needed. In studies in Alabama and Arkansas

where irrigation was provided only when the

available soil moisture level in the main rooting

zone fell to about 20 percent of the total available moisture capacity for those particular soils,

yields of marketable roots were as high, or almost as high, as when irrigation was applied

more frequently (when the available soil moisture level in the main rooting zone fell to more

than 20 percent of the total available moisture

capacity).

The major part of the sweetpotato crop in

the United States is grown in more or less

humid areas where the annual rainfall is at

least 40 inches and where a good part of this

amount is received fairly regularly during the

growing season. The advantages of irrigation

to supplement the moisture supplied by rainfall

are now widely recognized and practiced by

good sweetpotato growers.

Portable overhead sprinkler-type irrigation

is widely used in humid areas. Lightweight,

seamless, aluminum pipe with quick coupling

joints is often used for movable feed lines and

laterals. A smaller but increasingly larger part

of the crop in the United States is produced

under irrigation in otherwise suitable, but lowrainfall areas of the West and the Southwest.

Furrow irrigation methods are widely used in

those areas.

Frequently, facilities for irrigation are already in use on farms where other crops are

produced. Where new installations are proposed, competent engineering advice should be

obtained from private or local public sources.

Nature and topography of land and technical

engineering details, as well as costs of installation and operation, must be considered for any

proposed installation. A dependable water supply is essential to any successful irrigation system.

HARVESTING THE CROP

Methods and Equipment

Methods used in digging and handling sweetpotatoes during harvest greatly influence subsequent storage and market quality of the crop.

Care should be exercised in all operations to

keep injuries to a minimum. The sweetpotato

root has a thin, delicate skin that is easily

broken and flesh that is readily bruised, cut, or

otherwise wounded. Injuries provide opportunity for entrance of decay-producing organisms, and may constitute grade defects or lower

the quality of the roots from an appearance

standpoint.

The usual steps in harvesting sweetpotatoes

are: (1) Vine cutting or removal; (2) either

(a)plowing out or digging, followed by scratching out of roots and picking up into field containers (usually with some selection for grade)

or (b) digging with a combine-type harvester

unit that permits placing roots directly into

field containers; (3) loading field containers

and hauling them to the packing shed or storage house; and (4) either (a) grading and

packing the roots preparatory to direct shipment to the fresh market or (b) placing them

in the curing room preparatory to storage.

Practically all varieties of sweetpotatoes

grown extensively in this country produce a

vigorous vine growth. Vine cutting or removal

from the îrown of the rows is necessary, especially for eflTicient use of mechanical harvesters.

Even with simpler types of digging equipment

such as middlebusters or plows, unless vines

are cut or removed, they become tangled around

the equipment and digging operations must be

stopped frequently. Vines are cut or removed

either as a separate operation before digging

or as a part of the digging operation itself.

Numerous devices are used for cutting vines

but are of three general types: (1) Vertical

cutting tools such as fixed knives, colters, or

disks ; (2) rotary horizontal cutting blades ; and

(3) flail- or beater-type equipment.

Frequently fixed knives, colters, or disks are

tractor mounted or are attached to the plow or

some other type digger. These cut the vines

shallow and about 6 inches from row centers

on one or both sides of the rows, just ahead of

the digging tool. Where they can be used,

shielded types of colters about 8 inches in diameter are preferred because the cutting depth

can be kept shallow. Unshielded, larger colters

or disks must be properly adjusted or they may

run too deeply and thus cause extensive cutting

of roots.

In large-scale operations vines are often removed by one- or two-row, tractor-operated,

rotary-type mowers (fig. 11) in a separate

operation ahead of the actual digging. Successful use of this type equipment depends on
I'LATE I AíiKICT'LTlRE HANDHOOK 388, U.S. DEPARTMENT OF AGRICULTURE

Color and shape of typical roots of some commercially

important sweetpotato varieties : A, Centennial

(Louisiana) ; B, Centennial (Maryland) ; C, Goldrush

(Copperskin) ; D, Nemagold ; and E, Georgia Red.
ACiKICrLTfKE HANDBOOK 388. I'.S. DEl'ARTMKXT OF AGRICULTURE PLATE II

Color and shape of typical roots of some commercially

Important sweetpotato varieties : A, Unit I Porto

Rico; B, Julian; C, Nugget; D, Jersey Orange; and

E, Yellow Jersey.
38 AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

FIGURE 11.—Modified chain-type harvesters with sorting

conveyers, digging sweetpotatoes. Two-row, rotaryblade vine cutter in operation to left of center

harvester. (Photo courtesy of The Progressive

Farmer.)

having uniform-height ridges so that vines are

cut off uniformly at or very near to the row

surface without significantly cutting the sweetpotatoes that may be growing at shallow depths

or rnay even be slightly exposed. This type cutter is not satisfactory for use on land where

many stones are present.

Occasionally tractor-powered, flail- or beatertype equipment has been used in vine removal.

On the whole, this type has not been too satisfactory for use on sweetpotatoes, particularly

where metal flails are used. Costs of equipment

and operation are high and damage to sweetpotatoes may be excessive where ridge heights

are uneven.

Removal of vines by herbicidal chemicals has

been tried experimentally but results have not

been encouraging. In the tests, foliage was

killed by the chemicals but the vines usually

became tough and even more diflicult to cut and

remove by vine-cutting devices.

Horse-drawn equipment such as smaller turnplows and middlebusters is used to a limited

extent in harvesting sweetpotatoes, particularly

on small farms, but most of the crop is harvested with tractor-powered equipment. Much

hand labor has always been required in sweetpotato harvesting, and decreasing the amount

of hand labor required has been a prime objective in the continuing search for better digging

equipment.

The most widely used, reasonably satisfactory, tractor-powered equipment for digging

sweetpotatoes is the modifications of large (14-

to 18-inch) moldboard turnplows and large

middlebusters. These are usually power-lift

operated and drawn deeply enough to avoid

excessive cutting of marketable-size roots.

Moldboards or middlebuster wings are often

modified to better expose the sweetpotato roots.

Moldboards are usually shortened somewhat to

leave the plowed-out row only partly turned

over and roots better exposed. In other modifications, parts of the moldboard or middlebuster wings are replaced by various designs

and arrangements of rods that allow more of

the soil to drop away during digging, thus

leaving more of the sweetpotatoes on the soil

surface (fig. 12).

Chain-type Irish potato diggers have been

modified for digging sweetpotatoes. Both towed

and tractor-mounted types have been developed

(fig. 13). Various machines have been developed for digging the crop and permitting sorting of the roots on" a conveyor without letting

them drop back on the ground (fig. 11). Although these chain-type diggers usually remove

more of the potatoes from the soil, generally

they cause more skinning and bruising than do

tractor-mounted turnplows or middlebusters

modified for sweetpotato digging. For the fresh

market, root appearance is a prime concern and

skinned and bruised areas impair appearance

and keeping quality. Where the crop is to be

canned or otherwise processed shortly after

harvest, bruising is a relatively minor concern

unless damage has been severe; consequently,

combine-type machines can be used for harvesting roots for these uses (fig. 14).

No completely satisfactory mechanical harvester is yet available, not even for digging

stocks for canning, processing, or industrial

uses. The greatest need, however, is for better

machinery to harvest fresh market stocks that

will avoid most of the skinning and bruising

and significantly reduce the amounts of hand

labor needed to recover and handle the roots.

For additional information concerning harvesting machinery, consult publications by Park,

Powers, and Garrison (13) and Poole (li).

When to Harvest

The bulk of the sweetpotato crop in the

United States is normally harvested in October

and early November. Earlier harvests are

made, however, and often begin in July in the

southernmost producing areas. Roots harvested

in July, August, and September usually are

shipped directly to market. Although the bulk

of the crop harvested in October and November is placed in storage, nevertheless, many

sweetpotatoes dug in this period are also

shipped directly to market.

Early harvests made in July and August are

possible from early plantings of varieties that
SWEETPOTATO CULTURE AND DISEASES 39

BN-36822

FIGURE 12.—Modified plow with moldboard shortened and revolving spiral rod attachment added to better expose

sweetpotatoes during digging.

produce marketable-size roots quickly. The

times of making- such early harvests depends

largely on sufficiently high market prices and

on yields of marketable-size roots sufficient to

insure a reasonable net profit. Sometimes a

grower profits more by harvesting at least a

part of his crop early when yields are only fair

but when market prices are high than by harvesting all of it later when yields are higher but

selling prices are substantially lower. Under

very favorable growing conditions, fair yields

may be obtained about 100 days after planting

but usually profitable yields are not obtained

until about 120 days after planting.

The time for harvesting sweetpotatoes for

storage usually is when the highest yields of

No. 1 grade roots can be expected for the particular field and growing conditions and when

the grower can complete all harvesting by the

time the frost kills the vines, or soon thereafter. Usually 130 to 150 days from planting

are needed to give best yields. The storage roots

will continue to grow until frost kills the vines

but yields will not increase after that. The

danger of losing all or part of the crop still in

the field increases after frost has killed the

vines. Such losses may result from a hard

freeze that damages roots near the surface of

the soil or from chilling injury to all roots as

the soil temperature drops to 50° F. or lower.

The greatest danger from delayed digging,

however, is in the effect that wet soil has on the

roots. Excessive moisture in the soil, either

before or after frost kills the vines, causes

physiological changes in the roots. Under such

conditions, the roots cannot heal injuries properly, and decay-producing organisms gain entrance into the roots. Before frost, warm

weather and evaporation of moisture from the

soil and vines help to overcome the excessively

wet condition of the soil. After frost, the soil

does not dry out easily and this further delays

digging and increases the possibilities of root

damage.

Where sweetpotatoes are known to have been

exposed to chilling soil temperatures or to un-
40 AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

BN-36823 and !

FIGURE 13.—Chain-type sweetpotato diggers: A, Tractor-mounted form; B, towed form.
SWEETPOTATO CULTURE AND DISEASES 41

BN- 3CS2r>

FIGURE 14.—Combine-type harvester digging sweetpotatoes for canning and processing or industrial

uses.

favorably wet soil conditions, immediate disposal for fresh market use is recommended

rather than storage of the crop for later sale.

In general, therefore, harvesting should be

done Vichen the soil is in good vv^orking condition

and the weather is fair, due consideration

being given to the condition of the crop and

to its anticipated use.

Handling and Grading in the Field

Whatever the digging equipment used (except that of the combine type), after plowing

out, those roots remaining in the soil must be

scratched out by hand and the sweetpotatoes

picked up and placed in the containers in which

they will be transported or stored. Care should

be exercised during the scratching out, picking

up, and handling operations to avoid unnecessary skinning and bruising of roots, which will

result in poor appearance, rotting, and inferior

quality. In handling the crop, avoid practices

such as throwing the sweetpotatoes into "heap

rows" in the field, throwing them directly into

trucks or wagons for transport, or carrying

them from the field in bags—all of which lead

to unnecessary skinning and bruising.

Someone has aptly said that for the best

quality sweetpotatoes the roots should be

handled "like eggs." The amount of handling

can be lessened if the grower will at least partly

grade the crop in the field by putting the roots

of separate grades directly into the baskets or

crates in which they will be stored or marketed.

The sound, uninjured, best quality sweetpotatoes should be picked up and stored separately

from smaller ones and from those that have

been cut or are otherwise inferior.

Any sweetpotatoes harvested from field areas

where black rot infected roots are found

should be sorted, graded, and shipped to market

as soon after harvest as possible. It is impossible to tell how many sweetpotatoes from such

field areas were infected during harvesting and

handling. Curing of infected roots will not prevent development of black rot if the roots are

stored.

To facilitate surface drying and removal of

adhering soil, some exposure of roots after

digging will usually be required before the crop

is graded and packed into field containers. However, care must be exercised not to allow the

roots to be injured by too long exposure. As

little as 30 minutes' exposure in bright sunshine during hot weather may sunscald freshly

dug sweetpotatoes. This detracts from their

appearance and often causes them to decay.

Vines placed over filled containers will protect

the sweetpotatoes from exposure until they can

be hauled from the field. Because sweetpotatoes

are injured by exposure to low temperatures,

the harvested roots should not be left in the

field overnight when air temperatures are likely

to drop to near freezing.

Several kinds of containers are used for picking up, hauling, and storing sweetpotatoes. The

most commonly used containers are tub-bottom

bushel baskets, various types of wirebound

straight-side wooden crates, and either nailed

or nailed and metal-reinforced field boxes.

Roots are usually bruised more when packed

and handled in bushel baskets than in crates

or field boxes. This bruising occurs because

baskets, which are usually stacked in a pyramidal manner both in hauling and in storage,

are packed with a bulge in the lid and have

somewhat less rigid sides than crates or boxes.

Consequently, the sweetpotato roots must support more of the load weight in the baskets.

Some of the various types of wooden handling containers such as the James and Hybrid

crates are used not only for transporting roots

from the field and for storing but also for shipping to market. The Durabox, designed essentially to use in palletized handling from the

field and in storage, is also used for shipping. In

some of the larger sweetpotato operations, filled

field crates or boxes are loaded in the fields onto

pallets placed on truck beds, in a systematic

pattern that gives stability to the stacks of
42 AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

loaded containers and permits unitized handling

with forklift equipment during subsequent curing, storing, and handling operations (fig. 15).

Some or the bruising of roots that occurs during rehandling of individual containers is thus

avoided.

Sv^eetpotatoes for processing may be transported in baskets or crates but more often are

transported to the processing plant in bulk in

open-bodied trucks or vans. Sometimes the roots

for processing are dumped into one-half-ton

capacity field pallet boxes and hauled from the

field on trucks. The boxes are subsequently

handled v^ith forklift equipment at the processing plant.

Selecting and Storing Seed Stock

As far as possible, fields planted for production of sv^eetpotato seed stock (bedding roots)

should be grown from vine cuttings to minimize

the amount and possibility of spread of diseases. In addition, any plants showing any

symptoms of disease should be removed from

the field during the growing period. Seed stock

must be dug and selected before the plant stems

have suffered frost damage.

Hills that are much less productive than those

nearby should be looked upon with suspicion.

Disease is often, but not always, the cause of

such unproductiveness. Choosing seed stock

from only the productive hills, however, helps

to avoid disease that is not otherwise evident.

The smaller sweetpotato roots (not ''strings")

from a clean^ productive hill are just as valuable for seed as the medium-size ones and are

preferred to th^ large ones, which will produce

fewer plants per bushel of stock bedded than

medium and small roots. (See p. 27.)

Recommendations often have indicated that

the incidence of stem rot (fusarium wilt) in

seed roots may be significantly reduced by

splitting the stems at harvest and eliminating

roots from hills showing discoloration in the

stems. While some reduction in wilt incidence

may be obtained with this procedure, the improvement is usually not as good as desired.

Too many infested hills escape detection and,

in addition, roots are infected from the wilt

organism present in the soil. It is more practical to discard or sell roots from fields showing significant amounts of stem rot than to try

to clean them up for use for seed by splitting

stems. Obtain a new source of stock known to

be essentially free of stem rot. Use of seed

produced in certified seed programs is especially recommended.

Wherever possible, seed stock should be

dug and selected during warm, sunny, dry

weather. It should be handled carefully to

avoid unnecessary bruising or skinning, placed

in clean containers, then transported promptly

to a clean storage house, and properly cured

and stored. Roots should not be injured by unnecessary handling, once they are placed in

storage.

To allow a safe margin for shrinkage and

other possible losses in storage and for roots

that might have to be discarded because of

defects at bedding time, at least 15 bushels of

seed stock should be saved for each acre to be

planted with transplants the next year. (See

discussion on amounts of seed stock required,

pp. 26 to 27.)

CURING AND STORING

Purposes and Requirements

The primary purpose of curing and storing

sweetpotatoes is to keep them in good condition

for marketing during the winter and spring

and to preserve seed roots to be used in producing plants for the next crop.

Regardless of how carefully sweetpotatoes

are handled during harvesting, some wounding

of the roots is inevitable. To prevent infection

of wounds by disease-producing organisms, the

roots are brought into storage as soon after

digging as practical and are cured for 4 to 7

days at from 80° to 85° F. and from 85- to

90-percent relative humidity. Under these conditions injuries are healed rapidly through the

formation of new wound-cork layers beneath

the wounded areas. The rate of healing is

slower at temperatures below 85° or above 95°

F. and at relative humidities below 85 percent.

Slow healing allows more decay-producing

organisms to gain footholds in the roots than

does rapid healing. Rapid healing of the

broken ends, skinned areas, and other wounds

immediately after harvest usually prevents such

injuries from becoming unsightly blemishes

that cause roots to be placed in lower grades

or to be passed up by the housewife on the

retail display counter. Providing good curing

conditions should not, however, be used as a

substitute for observing good harvesting and

handling practices that keep root injuries at a

minimum.

The conditions to which the roots are subjected for the first 3 or 4 days after harvest

determine the degree of success to be attained
SWEETPOTATO CULTURE AND DISEASES 43

in controlling decay. Extending the curing

time beyond about 7 days cannot compensate

for the use of less effective temperatures or

humidities. Even with optimum conditions, the

time of curing should not be extended unnecessarily because this leads to excessive sprouting

and shrinkage and to shortened storage life of

the roots. Roots that have been properly cured

usually have produced a fev^ very short sprouts

and have a somewhat velvety feel.

Where roots are to be shipped soon after harvest, the curing period may be extended and

the humidity reduced during the last few days

as a means of ^'setting'' the skin so that the

roots may be graded, packed, and shipped without excessive loss of skin.

In addition to healing wounds, curing also

speeds up those physiological changes that make

the sweetpotatoes more palatable after cooking than they are if they are cooked immediately after harvest. The major change involves

the conversion of starch to sugars and dextrins. Freshly dug roots convert less of their

starch to sugars and dextrins during cooking

than do comparable roots after they have been

cured. Although the sugar content of the raw

roots usually increases somewhat during curing and increases still more during storage at

recommended temperatures, the greatest

change occurs during cooking.

Varieties of sweetpotatoes differ in the extent

of conversion of starch to sugars and dextrins during cooking and these differences are

recognized in the trade when classifying varieties as moist-flesh (or *Vam'') or dry-flesh type.

(See p. 7.)

After curing, the storage room temperature

is lowered to 55° to 60° F. but the relative

humidity is kept at 85 percent to 90 percent

for the duration of storage. The temperature

is lowered to reduce the physiological activities

of the roots and thus to keep the sweetpotatoes

in good condition as long as possible. The

lowered temperature also reduces the activity

of most decay-producing organisms.

Although lowering the storage temperatures

below 55° F. will reduce the physiological activities of the roots and retard decay-producing

organisms still further, sweetpotatoes cannot

be stored for long periods at temperatures

much below 55°. Sweetpotatoes are damaged

by temperatures below this point and the degree

of injury is about proportional to the time and

temperature of exposure. Damage resulting

from a few days' exposure at 50° often may

not be noticeable, but that from 1 or 2 days'

exposure at 32° usually is. Chilling temperatures can result in internal discoloration and

breakdown, increased susceptibility to decay.

poor cooking quality, and reduction of the

sprouting ability of the roots. Roots will freeze

and become worthless at temperatures below

30°.

Although rapid healing of wounds by curing

at the recommended ranges in temperature and

humidity will help prevent decay-producing

organisms from gaining entry into sweetpotato

roots, there is one noteworthy exception. Where

black rot (see p. 53) is in the field at harvest,

it may cause many roots to develop the black

spots typical of this disease in storage, even

though the roots are properly cured. Therefore,

when black rot is present, the crop should be

marketed as soon as possible after harvest.

Fortunately, this disease, which was once very

troublesome, is not often found today because

effective control measures are usually used during production of the crop. Black rot can be

controlled reasonably well during marketing by

good sanitation practices in the packinghouse

and by treatment of roots with SOPP or other

registered, effective fungicides. (See "Preparing Sweetpotatoes for the Fresh Market," p.

46.)

When infected with the internal cork virus

disease, storage roots of most sweetpotato

varieties, such as Centennial, Georgia Red,

Goldrush, and Porto Rico, develop characteristic corky flesh lesions if they are stored at

temperatures above 60° F. (See p. 68.) Although some lesions may be present at the time

of harvest, especially when harvest is preceded

by a period of warm weather, most lesions develop and enlarge during storage when temperatures are kept higher than recommended.

Some sweetpotato varieties, however, are symptomless carriers of the internal cork virus and

do not develop corky lesions in infected roots

even if they are stored at temperatures above

60°.

During curing, sweetpotatoes may lose up to

5 percent of their weight. The weight loss continues during storage but at a much slower

rate. It is not uncommon for sound roots that

have been properly cured and stored to lose

approximately 15 percent of their weight during 5 months' storage. Decay adds to the weight

loss.

The development of pithiness is accelerated

by the same conditions that cause weight loss.

High temperatures during storage increase

respiration and sprouting of roots, thus increasing pithiness. Low relative humidities

during storage also increase weight losses and

accelerate development of the pithy condition.

Varieties differ in the rates at which they lose

weight or volume and consequently differ considerably in the rates at which they become
44 AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

pithy. (See table 9, column 7 and footnote 3.)

Roots of the varieties Goldrush, Nugget, and

Porto Rico, for example, become pithy slowly

—even after 6 or more months in storage;

whereas roots of the varieties Gem, Jersey

Orange, and Nemagold become pithy quickly—

often after only 2 or 3 months in storage. Since

pithiness is evidence of reduced cooking quality,

varieties that become pithy quickly should be

marketed soon after harvest or after short

periods of storage.

Storage House Operation

In managing a storage house the objective

should be to provide the most effective conditions of temperature, humidity, and ventilation

for proper curing and storing of the roots. This

is more difficult than might be supposed.

The construction of the storage house will

affect to a large extent the manner of house

operation. Weather conditions, type of storage

containers used, and nature of market outlets

will also affect the operations. Storage facilities and containers should be clean. They are

usually fumigated with suitable materials and

procedures, especially when black rot or other

decay-producing organisms have caused noticeable losses during the preceding storage season.

From the standpoint of good management, it

is usually recommended that the storage house

be divided into several compartments or rooms.

One room at a time should be filled with sweetpotatoes within a period of a week or less.

Proper conditions for curing should be provided

during the filling period and for a few days

afterward, then the temperature should be reduced to the level required for the duration of

storage. This sequence is repeated with each

storage room, and avoids holding some of the

sweetpotatoes under curing conditions throughout an extended harvest period, a situation that

cannot be avoided where all of the roots from

an extensive operation are cured in one large

room. The stored roots should not be disturbed

more than necessary after curing is completed

because unhealed injuries open the way for infection and decay.

A recently devised system offers considerable

opportunity for improved curing and storage

house management. Under this system, the

roots are handled from the field in field boxes

or pallet boxes and placed on pallets, then the

palletized units are placed in a curing room

operated at proper temperature and humidity.

At the end of the curing period, the palletized

units are picked up by fork-lift equipment and

moved to areas where proper storage conditions are maintained (fig. 15). Handling in

FIGURE 1,5.—Palletized handling of sweetpotatoes with

fork-lift equipment. (Photo courtesy of North

Carolina State University.)

this way is so gentle that the cured roots are

injured very little and wounding is usually not

detectable. This system permits close control

of curing time and of curing and storage conditions, requires less expensive construction than

compartmentalized storage, and ofli'ers opportunities for segregating the palletized units

according to grower, field of production, variety, and grade.

In the late summer and early fall, especially

in the warmer producing areas, the weather

at harvest and shortly thereafter is often so

warm that no heat need be supplied to the storage to obtain temperatures approximating

those needed for curing. Later in the season

and in cooler growing areas, some heat is

usually supplied. The routine opening and closing of doors usually provides sufficient ventilation to take care of the oxygen requirements

of the respiring roots and the dissipation of

carbon dioxide produced by them. Except in

particularly dry seasons or in dry climates, the

stored roots, adhering soil, and storage containers usually provide sufficient moisture to

keen the humidity up to the required level.

When heat is added to the storage, it should
SWEETPOTATO CULTURE AND DISEASES 45

be supplied and distributed in a manner that

will provide essentially uniform warming- of

all the stored roots as the heat rises in the

storage room. To accomplish this, heat is released under a slatted false floor, from registers located either in the cement floor or low

on the walls or, in a more recently devised procedure, from trenches in the floor. Trenches

permit better distribution of heat than can be

obtained with a few scattered registers, but

they do not distribute the heat as well as when

the heat is released under a slatted false floor.

Trenches, usually measuring about 12 inches

wide and 12 to 16 inches deep, permit the use

of fork-lift trucks whose weight would not be

supported by the usual slatted false floor. The

trenches are covered with short pieces of 2- by

6-, 2- by 8-, or 2- by 10-inch lumber recessed

into the cement floor to make the floor level.

These pieces are spaced to allow the heated air

to rise in the amounts estimated to be needed

in the various parts of the storage room. Some

mechanical circulation of air is usually desirable if the filled containers are stacked more

than about 10 feet high.

To provide ventilation for cooling the storage or for removing condensation, air intake

ventilators are usually placed near the floor or

below the level of a slatted false floor of the

storage room, and exhaust vents are placed in

the ceiling or roof. The warm air at the top of

the storage rooms is often exhausted with fans.

The introduction of cold air into the bottom of

storage rooms often causes chilling injury to

roots stacked near the intake vents while temperatures at the top of the rooms may become

too high. The warm temperatures near the top

of the rooms are usually accompanied by high

relative humidities, conditions that often induce

excessive sprouting of roots in this part of the

storage.

Another ventilating system has been installed in some storage facilities (flg. 16). In

this system fans blow air into the top of the

storage rooms at a rate of 1 to 2 cubic feet per

minute per bushel of storage capacity. Exhaust

vents are on outside walls at ceiling level. A

baffle is usually used to distribute the cold air

over the stacked sweetpotatoes. The fans are

controlled by time clocks and thermostats that

permit them to operate when the room is

warmer than desired (above 60° F.), but only

at night or whenever the outside air temperature is cooler than the room temperature near

the ceiling. This system uses the well-known

principles that warm air rises and cold air

settles. Heat, when needed, is introduced as uniformly as possible into the bottom of the storage. Cold air is introduced as uniformly as

possible into the top to help maintain more

nearly uniform temperatures throughout the

storage. The introduction of cold air into the

FIGURE 16.—Modern curing and storage facility, designed for palletized handling of sweetpotatoes. Curing area at

left; storage rooms on right provided with top air intake and exhaust ventilation. (Photo courtesy of North

Carolina State University.)
46 AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

top puts the cold air where it is most needed

to control temperature without danger of causing- chilling injury. Since cold air can carry

relatively less moisture, the introduction of cold

air reduces the relative humidity in the top of

the storage, but usually not below the required

level. The reduction of temperature and humidity in the top of the storage helps reduce

excessive sprout growth of roots stored in the

upper parts of the room.

A small part of the sweetpotato crop is

usually stored until the succeeding new crop is

available. Mechanical refrigeration is usually

used when the crop is stored through the summer months because summer air temperatures

are too high to maintain satisfactory storage

temperature control by ventilation alone. The

refrigerated air is distributed over the top of

the stacked sweetpotatoes in a manner similar

to that provided in the overhead ventilation system described above. Since the refrigerated air

is continually recirculated, some air is introduced from the outside to maintain fairly normal oxygen and carbon dioxide levels in the

storage atmosphere. Construction details,

weather conditions, and product loads influence

the amount of mechanical refrigeration needed.

With reasonably good insulation many refrigerated storage rooms operate with a ton of refrigeration capacity per 1,000 bushels of sweetpotatoes. Qualiñed refrigeration engineers

should be consulted for speciñc recommendations for a particular installation.

Storage temperatures are easily measured

and fairly easily controlled by adding either

heated or cold air. However, relative humidities are not easily measured and usually less is

done to control them. Immediately after harvest, the roots give off moisture through injuries and moisture is added to the storage by

soil adhering to the roots and sometimes by the

storage containers that have picked up moisture while sitting on damp soil in the field. As

a storage room is filled with sweetpotatoes, the

volume of air left in the room is correspondingly reduced. Since 100 pounds of roots (about

2 bushels) will give off approximately 3 pounds

of water during curing, most of this during the

first few days, and since 1 pound of water is

enough to completely saturate 500 cubic feet of

dry air, one can easily understand why the relative humidity in a storage room is usually sufficiently high, and often so high that some ventilation is needed to prevent condensation. When

wounds have healed, moisture is given off less

readily by the roots. However, less moisture is

required to maintain the relative humidity near

85 to 90 percent at the lowered storage temperature.

In arid climates or where considerable heat

must be added to the storage, the relative humidity may drop below desired levels. Often

sprinkling water on the floors of the storage

rooms, especially on dirt floors, increases the

humidity suflSciently. Sometimes mechanical

atomizing units controlled by humidistats are

used. In storages using trenches, water can be

added in the trenches or in the furnace duct

system.

Proper insulation of the walls and ceiling of

the storage structure is very important in the

maintenance of high relative humidity. Moisture will condense on inner wall or ceiling surfaces of the storage room if the temperature

at these surfaces drops to 78° F. at any time

while the curing temperature of 85° is being

maintained in the room and while the relative

humidity of the air adjacent to the wall surfaces is above 80 percent. As far as practical,

therefore, storage buildings should be sufficiently insulated to prevent the inside surface

temperatures from becoming cold enough to

allow such moisture condensation. Some ventilation of the room may be required where

moisture condenses appreciably on storage

walls and ceilings.

PREPARING SWEETPOTATOES FOR THE FRESH MARKET

Sweetpotatoes should be carefully prepared

for market. Operations involved in such preparation vary considerably, depending principally upon market demand and custom. Some

sweetpotatoes are transported directly to the

fresh market, following digging and grading in

the field. (See p. 41.) More often they are

taken to a packing shed or storage house on

the farm or to a commercial packing house for

additional preparatory treatment either before

transport directly to market or after a period

of storage.

Whether performed on the farm or in a commercial packinghouse, the usual preparatory

operations include washing, sorting and grading, sometimes treating with registered fungicides to prevent decay, packaging, and finally

loading for shipment. Sometimes wax is applied

to enhance the appearance of the roots for

certain market outlets. Equipment used in these

operations should be constructed and used in

a manner that will keep injuries to the sweetpotatoes at a minimum. Skin is easily removed

from freshly harvested roots and such skinning
SWEETPOTATO CULTURE AND DISEASES 47

detracts from their appearance. Sweetpotatoes

removed from storage are also rather easily

injured, allowing decay to develop.

Washing

The sweetpotatoes are first washed. They

may be dumped directly from the field containers onto a roller conveyor or into a tank

of water with an inclined roller conveyor for

removing them. In either case the sweetpotatoes

then pass under several nozzles on the washing

equipment, which spray water on the sweetpotatoes while they are scrubbed from below

with pintle rubber-, nylon-, or tampico fibercovered rollers. The number of nozzles and

rollers used depends to some extent on the type

of soil adhering to the roots.

Washing aids in subsequent detection and

removal of defective sweetpotatoes, especially

those showing insect injuries or skin discolorations caused by the scurf disease.

Sorting and Grading

The sweetpotatoes next pass from the washer

onto a conveyor where defective and diseased

or other cull roots as well as roots too large or

too small for the fresh market are first sorted

out and removed.

Where sweetpotatoes are treated with registered fungicides to reduce losses from decay

during marketing (discussed below), the treating unit follows this first conveyor. The remaining roots then pass to a second conveyor

from which they are separated, according to

required standards of size and other characteristics, into desired market grades. Federal

grade standards are usually followed (20).

However, since the quality requirements of

specific market outlets vary a great deal, sorting and grading often are done according to

the demands of the specific market, following

local or State regulations applicable to the

sorting and grading operations. At this stage

the roots are packed in suitable shipping containers or are set aside for later final grading

and packing.

Sorting and grading are very important, and

if done improperly, can result in buyer dissatisfaction and direct monetary loss, depending

upon whether sorting is too strict or not strict

enough.

Treating to Prevent Decay

Some injury to sweetpotatoes is bound to

occur during handling, washing, sorting, and

grading. The bruised or crushed tissues offer

favorable locations for decay to develop. Soft

rot causes most decay but other diseases such

as black rot, surface rot, and fusarium rot may

also cause losses during marketing. To prevent

or greatly reduce such losses, the sweetpotatoes

are often treated with registered fungicides

after washing and before the roots are graded

and packed for market. One effective treatment

is to thoroughly wet the roots with a solution

of SOPP. This treatment is followed by a brief

rinse with water or wax emulsion (see ''Waxing'' below). SOPP is effective against the soft

rot and black rot organisms. Another material,

2,6-dichloro-4-nitroanaline (Botran), is effective against the soft rot organism but does

not control black rot. Botran is simple to apply,

relatively inexpensive, and less likely than

SOPP to cause root discoloration.

Instructions supplied with each fungicide

should be followed carefully to avoid injury to

the sweetpotatoes or to the people working in

the packinghouse, and to comply with pesticide

residue requirements.

Waxing

Waxes registered by the Food and Drug

Administration may be added to the roots to

meet the demands of some market outlets. Wax

may be sprayed on as a water-wax emulsion or

brushed on the sweetpotatoes with a hot wax

applicator.

The spray application of wax may serve as

the rinse recommended after treatment of the

roots with SOPP.

Packaging

Sweetpotatoes may be packaged in bushel

baskets, wirebound crates (James, Hybrid, 3-

way Durabox), or corrugated cardboard boxes

for shipment to market from either farm or

the commercial packinghouse. Whatever container is used, the roots in the top layer are

usually arranged in an orderly manner

(''faced'') to present an attractive pack (fig.

17). The wirebound James and Hybrid crates

are stacked on their sides to prevent damage

to roots in the bulge that develops in the top

of the container w^hen it is filled. Because corrugated boxes do not injure the roots as much

after they have been treated to prevent decay

during sorting and grading, decay occurs less

frequently during shipment of roots in these

boxes than in some other types of containers.

Cost, convenience, and market preference

also inñuence the choice of shipping container.

Sweetpotatoes vary considerably in specific

gravity, and a package labeled to contain **50

pounds net'' must contain 200 to 300 cubic

inches more volume for roots that have been
48 AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

BN-36826

FIGURE 17.—Orderly arranging of sweetpotato roots

("facing") on top layer of marketing container to

present an attractive pack.

stored 4 to 5 months than a package containing 50 pounds of freshly harvested roots. Most

of the shipping containers can provide this

much flexibility. Because of weight loss in

transit and handling, a package labeled "50

pounds net" is usually packed with 51 or more

pounds of sweetpotatoes at the shipping point.

Shipping

A large part of the sweetpotato crop is

shipped to the fresh market from the farm or

the commercial packinghouse by truck. Special

provision for air circulation in or around the

packages in a load is usually unnecessary with

bushel baskets or wirebound crates. Loads

shipped in corrugated boxes, however, should

be stacked to maintain air channels along the

side walls and through the bunker so that air

can circulate, especially in winter months when

heat is added to the load to prevent chilling or

freezing. Because of the sensitivity of sweetpotatoes to chilling injury, temperatures should

not be allowed to drop below 55° F.

Additional detailed information concerning

these various operations, the equipment involved, and precautions to be observed in treating, grading, handling, and shipping may be

obtained by consulting Marketing Bulletin 38,

"Preparing Sweetpotatoes for Market" (10),

and other publications of the U.S. Department

of Agriculture (.9, 11, 12, 20).

Part II: DISEASES'

Sweetpotatoes are damaged by a number of

important fungus and virus diseases and by

nematodes. Such damage results in serious

losses in yield and quality. Some of the diseases

affect sweetpotatoes primarily in the field,

others primarily during storage, handling, and

marketing. Some are serious both in the field

and during storage.

Damage to the crop may be through a stunting or lowering of the vitality of the plants and,

as a consequence, in reduced yield ; or damage

may be through reductions in the quality and,

thus, marketability of the sweetpotatoes by

causing less desirable root shapes, appearance,

colors, textures, and flavors. Losses may be

heavy during storage as a result of soft rots

and other diseases, which may either make

damaged roots unsalable or at least materially

shorten the storage life of the roots and the

3 The section "Diseases" was prepared by revising and

supplementing information contained in U.S. Department of Agriculture Farmers' Bulletin 1059, "Sweetpotato Diseases" (8), now out of print. E. M. Hildebrand, plant pathologist, assisted in preparing the

discussion on virus diseases.

period of time over which they may be marketed.

Since about 1945 several virus diseases have

become increasingly serious in sweetpotato

production. Some progress has been made in

learning the nature of some of these virus

diseases, the modes of dissemination, and procedures that aid in their control. More information is needed, however, regarding these

aspects of the virus diseases, particularly those

regarding efi'ective, practical measures that can

be applied by growers to control or eliminate

the diseases. We must find sources and means

of providing growers with adequate supplies

of virus-free propagating stocks. Virus diseases

are usually diflicult to diagnose in the field since

sweetpotatoes ordinarily fail to exhibit foliage

symptoms characteristics of the diseases until

midseason or even later, if at all.

Organisms causing the diseases considered

in this publication are identified in the tabulation that follows. Still other diseases can injure

sweetpotatoes in the field and during storage

and handling. However, economic losses from

these other diseases are usually less important

than from the diseases discussed here.
SWEETPOTATO CULTURE AND DISEASES 49

AGENTS CAUSING SWEETPOTATO DISEASES

Fungi

Disease Causal organism

Field diseases:

Stem rot Fusarium oxysporum /.

batatas

Black rot Ceratocystis fimbriata

(formerly Endoconidiophora fimbriata)

Foot rot Plenodomus destruens

Scurf Monilochaetes infuscans

Root rot Phymatotrichum omnivorum

_, ,,, . {PythiuTYi ultiynum

Mottle necrosis [p scleroteichum

Soil rot Streptomyces ipoTnoea

Phyllosticta leaf blight __Phyllosticta batatas

Septoria leaf spot Septoria bataticola

White rust Albugo ipomoeaepanduratae

Storage rots:

Soft rot Rhizopus stolonifer

Black rot Ceratocystis fimbriata

(formerly Endoconidiophora fimbriata)

Surface rot Fusarium oxysporum

Dry rot Diaporthe batatatis

Java black rot Diplodia theobromae

{Botryodiplodia

theobromae)

Charcoal rot Macrophomina phaseoli

Nematodes

ComTYion name Scientific name

Root-knot nematodes Meloidogyne spp. (formerly

Heterodera marioni)

Lesion nematodes^ Pratylenchus spp.

Reniform nematodes^ Rotylenchulus reniformis

c. . , . j 1 ( Helicotylenchus spp. and Spiral nematodes! [ Scutellonema spp.

Dagger nematodes^ Xiphinema americanum

Lance nematodes^ Hoplolaimus galeatus

Sting nematodes^ Belonolaimus longicaudatus

Ring nematodes^ Criconemoides spp.

! Cause root decay.

2 Cause root pruning.

Viruses

Internal cork

Chlorotic leafspot

Yellow dwarf

(Feathery mottle)

Russet crack

GENERAL PROCEDURES FOR CONTROLLING SWEETPOTATO DISEASES

Several general procedures for controlling

sweetpotato diseases have been considered in

various preceding discussions in this handbook.

The following outline summarizes these practices :

Field Diseases

(1) Plant on essentially ''clean" land.

Ordinarily, sweetpotatoes should be

included only every 3rd or 4th year

in rotations with other crops that are

not hosts for diseases affecting sweetpotatoes.

(2) As far as practical, plant roots that

are: (a) Produced on plants grown

from vine cuttings or bed cuttings;

(b) carefully selected in the field for

type and freedom from diseases; and

(c) stored separately from other

sweetpotato stocks.

(3) Clean and treat the beds and bedding

media with registered fungicides to

free them of sweetpotato diseaseproducing pathogens.

(4) At bedding time treat roots with registered fungicides to control disease

organisms present on root surfaces.

(5) Grow disease-resistant varieties when

available. In the absence of such resistance, propagate only from clean stocks

and control insects or other vectors

that may spread disease infections in

the field.

Storage Diseases

(1) Store only sweetpotatoes that: (a)

Are as free as possible of diseases;

(b) have been carefully dug and

handled ; and (c) have been placed in

clean field and storage containers.

(2) Store in properly constructed and

cleaned storage houses.

(3) Cure freshly harvested roots at effective temperatures (80° to 85° F.) and

relative humidities (85 percent to 90

percent) to insure healing of wounds

and minimal development of soft rots

or other rots. Maintain storage thereafter at 55 to 60° F. and 85 to 90-

percent relative humidity.

(4) Move stocks only when necessary during the storage period to keep at a

minimum injuries that may result in

losses by decay.

Specific field and storage diseases and methods for their control are considered in more

detail in the sections that follow. The reactions of 23 commercially grown sweetpotato

varieties to six important diseases are indicated in table 21.
50 AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

TABLE 21.—Reaction of 23 commercially important American sweetpotato

varieties to 6 diseases ^

Variety

Allgold

Carogold

Centennial

Georgia Bunch Porto

Rico.

Earlyport

Earlysweet

Georgia Red

Goldrush

Jersey Orange

Kandee

Nancy Hall

Nemagold

Nugget

Oklamar

Orange Little Stem

Unit I Porto Rico

Redgold

Sunnyside

Triumph

Virginian

Yellow Jersey

Pelican Processor 4

Whitestar ^

Disease reaction of ^-

Stem rot Black rot Scurf Soil rot Internal cork ^ Root knot

S

I

I

S

S

I

s

R

S

s

S

S

I, s

I

s

s

I

s

R

S

s

R, I

R, I

s

S

s

S

s

s

s

s

s

s

I, s

I

s

s

I

I

s

I

s

s

s

s

s

s

s

s

s

s

s

s

I

s

s

s

s

R

s

s

s

s

s

s

s

s

s

s

I, s

I

s

s

s

s

s

s

R

s

I

s

R, I

I, s

s

s

R

R

R

I

R

s

I

R

I

R

R

R

R

S

R

S

I

S

s

I

R, I

~R~

s

s

I

s

s

R

S

s

s

s

1 Ratings supplied by researchers in National Sweetpotato Cooperator Group.

2 Reaction symbols used: S = Susceptible (severe symptoms under a wide range of

conditions) ; I = Intermediate (mild disease symptoms ordinarily, but ranging up to

severe under some conditions) ; R = Resistant (very mild or no disease symptoms under

a wide range of conditions).

3 Tentative ratings pending further confirmatory tests.

4 Industrial-type varieties, low in carotene.

FIELD DISEASES AND THEIR CONTROL

Stem Rot

(Fusarium 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. 18). The stems of

diseased plants darken inside and sometimes

split open near the ground level. This discoloration of the stems sometimes extends 3 to

5 feet from the hill, a sure sign of stem rot.

The fungus causing stem rot may also invade

the fleshy roots and cause a blackened ring

about one-fourth inch below the surface (fig.

19). Sprouts produced 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 generally be detected by the purplish tint

that shows through the white basal part of the

stem and by the yellow color of the leaves.

Prevalence and Loss

Stem rot is an important field disease. It

occurs in every State where sweetpotatoes are

grown. From 10 to 50 percent of the crop of

susceptible varieties may be destroyed in infested plantings, and stem rot has been known

to kill 95 percent of the plants in some fields.

Means of Distribution

The stem rot fungus can overwinter in the

soil on the remains of dead sweetpotato vines

and in the roots in storage. The disease may be

carried from one field to another in the same

locality by insects, farm animals, farm implements, drainage water, wind, and discarded

diseased roots dumped on the fields, either before or after being fed to stock.
SWEETPOTATO CULTURE AND DISEASES 51

FIGURE 18.—Stem rot (fusarium wilt) symptoms of a sweetpotato plant.

BPI-1348

The disease is spread from one locality to

another primarily by the exchange or sale of

infected seed sweetpotatoes and plants.

Cause

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 sweetpotato.

Infection takes place both in the plant bed

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. Healthly 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

conductive vessels of the stem. After the vines

die, the fungus lives on the decaying vegetation. Numerous very smah fruiting bodies, or

spores, develop on the dead vines. The spores

are readily carried by the wind and insects or

on tools to infest other fields.

Control

Other than growing sweetpotato varieties

possessing more or less inherent resistance to

stem rot, the most important factors in control

of this disease are using clean, healthy seed

stocks for plant production and growing the

crop in rotation with other crop plants not

attacked by the stem rot fungus on land as free

as possible of stem rot contamination.

No varieties are known that are completely

immune to attacks by the stem rot fungus. Although some varieties, grown formerly for

market use, were sufficiently tolerant of stem

rot to permit their culture on infested soils,

some of the most important and most widely

grown varieties and improved selections developed from them have been highly susceptible to infection and damage by this disease. In

recent years plant breeders have developed
52 AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

FIGURE 19.—Section through a sweetpotato showing the blackened ring caused by the stem rot fungus.

several improved sweetpotato varieties that

possess practical levels of resistance to stem

rot under usual field conditions (table 21).

Genetic sources for resistance to this disease

are available and are being used in breeding

programs to incorporate high levels of resistance in new, horticulturally superior, productive varieties. Wherever possible, resistant

varieties should be grown in areas where stem

rot is a serious problem.

The importance of proper selection, care, and

use of clean, healthy seed stock for the control

of stem rot and other diseases cannot be overemphasized where susceptible varieties must

be grown. Since the stem rot fungus overwinters in stored infected sweetpotatoes, plants

grown from infected seed roots are very likely

to be diseased also, hence the necessity of using

seed roots free of infection. Surface disinfection with registered fungicides at the time of
SWEETPOTATO CULTURE AND DISEASES 53

bedding will not control stem rot within infected roots. Similarly, stem rot within infected

plants cannot be controlled by fungicidal dip

treatments applied at the time plants are set

in the field.

Applications of lime and gypsum to the soil

are of no value in controlling stem rot.

The stem rot fungus will live in the soil for

several years, even in the absence of sweetpotatoes. For that reason, as a general rule,

sweetpotatoes should not be planted on the

same ground oftener than once in 3 or 4 years.

This rotation will not eradicate the fungus but

will reduce the losses. No other crop except

tobacco is known to be attacked by this fungus ;

therefore, any crop except wilt-susceptible

types of tobacco commonly grown in the region may precede sweetpotatoes in the rotation. In some areas stem rot-resistant varieties

of tobacco are planted after sweetpotatoes in

the cropping system.

Black Rot

(Black Shank, Black Root)

Description

Black rot may occur on any of the underground parts of the plant. Infection on the

young plants begins as small black spots on the

underground part of the stem. The spots enlarge until the whole stem is rotted off. Frequently the infection extends up the underground part of the stem to the surface of the

soil (fig. 20). The name ''black shank" is commonly applied to this phase of the disease.

Black rot produces dark to nearly black,

somewhat sunken, circular spots on the surface

of sweetpotatoes (fig. 21). At first these spots

are small and nearly round, but under favorable

conditions they enlarge and often cover nearly

the whole sweetpotato. Fruiting bodies, 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 black. If sweetpotatoes affected with

black rot are used for seed, the plants produced

from them are likely to have the disease.

Prevalence and Loss

Black rot occurs in most States where sweetpotatoes are grown. The disease can occur on

the plants or sprouts in the hotbeds, in the

fields, and on the roots in storage houses in the

winter. When disease-infected roots are put in

storage, under favorable conditions the black

rot develops rapidly, rendering the sweetpotatoes unfit for sale and causing heavy losses.

Sweetpotatoes affected by black rot have a very

disagreeable, bitter taste when cooked.

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

infected roots that appear sound when stored

may become badly affected within a few weeks.

Cause

The black rot fungus overwinters on the dead

vines and other decayed vegetable matter in the

soil and on the sweetpotatoes in storage. Other

than sweetpotatoes, no host plants, are known.

If roots affected by black rot are used as planting stock, the fungus usually grows into the

plants in the plant bed. Infection also takes

place through the roots after the plants are set

in the field. Plants that become infected early

soon die, and those that continue to grow rarely

produce sweetpotatoes.

Control

Black rot is controlled by using disease-free

seed stock, root disinfection, and clean plant

beds and bedding media to produce clean transplants. The crop should be planted on clean land

in rotation with other plants. The most practical way to obtain disease-free bedding stock is

to produce the seed sweetpotatoes on plants

grown from vine cuttings or sprout (bed) cuttings. (See p. 18.) Since black rot affects only

the underground parts of the plant, these cuttings will be free of the disease. If the cuttings

are planted in fields that are free of black rot,

clean bedding roots should be produced. Sorting

out black rot-infected sweetpotatoes, either at

harvesttime or before bedding in the spring,

is not a satisfactory way to obtain clean bedding stock. Many small infections escape detection and the disease continues to develop on

such roots in storage or in the plant bed.

Seed roots should be disinfected with SOPP

or other registered, effective fungicides to destroy disease spores on the root surfaces, then

they should be bedded promptly after treatment

in a properly prepared clean plant bed. (See

p. 25.) Disinfection does not kill the fungus

inside of black rot-infected roots.

Although none of the present (1970) commercial varieties of sweetpotatoes are highly

resistant to black rot, a few of the recently

developed varieties are moderately resistant to

infection under usual field conditions (table

21).

Plantings should be made on new ground or
54 AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

/

BPI-1339-P

FIGURE 20.—Small sweetpotato plants showing the characteristic blackening of the underground parts

of the stems caused by the black rot fungus.
SWEETPOTATO CULTURE AND DISEASES 55

in fields that have not been used for sweetpotatoes for at least 2 or 3 years. Rotation of

sweetpotatoes with other crops helps control

black rot because the fungus does not affect

BPI-6772-A

FIGURE 21.—Black circular spot on sweetpotato caused

by the black rot fungus.

other crops and is only able to live about 2 years

in the soil.

Although animal manures are no longer

generally available for use on sweetpotatoes, in

those cases where they are used, care must be

taken that they have not come from sources

where sweetpotatoes infected with black rot

have been fed to livestock. The black rot organism is able to survive the digestive process in

animals and can subsequently reinfect sweetpotatoes if contaminated manure is spread on

the land.

Fool Rot

(Die Off)

Description

In the field, foot rot infections usually appear first as small brown to black spots on the

stem of the plant near the soil line. The grow'th

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, black fruiting bodies

of the fungus, just visible to the naked eye,

appear in the diseased areas. This disease progresses rather slowly from field infections and

the plants do not die off until about midsummer

or later.

Often the affected plants bear no sweetpotatoes even though they may have produced long

vines. In those hills in which sweetpotatoes

develop, the infection may spread from the

stem to the roots and cause a brown, rather

firm rot. Later, fruiting bodies develop close

together on the root surface in the form of

pimplelike protuberances (fig. 22). Many

wounds and bruises on sweetpotatoes in storage become infected by the foot rot fungus.

Prevalence and Loss

Foot rot is distributed in the same way as

stem rot and black rot. It occurs in several

States in various parts of the United States.

Because it is not as widely distributed as stem

rot or black rot, the total loss from foot rot is

much less than from stem rot or black rot.

In a few places, however, it has caused heavier

losses than either.

Cause

Most of the initial infection by the foot rot

fungus takes place in the hotbed, spreading

from diseased seed roots to the plants. When

set in the field, such infected plants usually die

early in the season. Although infection occurs

primarily through the roots and underground

parts of the plant, vines may become infected
56 AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

at the nodes several feet from the plant stem

during wet periods when the growth in the

field is very luxuriant.

Spores, borne in great numbers, escape from

the pimplelike protuberances of the diseased

tissue and are carried by insects or other agencies to other plants, where new infections 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 rot and black rot, therefore,

diseased seed stock produces 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.

BN-23861

FIGURE 22.—Sweetpotato rotted by the foot rot fungus.

Scurf

(Soil Stain, Rust, Jersey Mark)

Description

Scurf produces a brown to black surface

discoloration of the root (fig. 23). The discolored areas may take the form of spots of

diñ"erent sizes and shapes with no definite outline, or there may be a uniform rusting of the

surface of the sweetpotato. The skin of the

sweetpotato is not broken by scurf and the

brown or black discoloration is only skin deep

and can be scraped ofi" easily with the fingernail.

Prevalence and Loss

Scurf occurs in most places where sweetpotatoes are grown and on all varieties.

The losses in production caused by scurf are

relatively small in comparison with those caused

by some other diseases. On the other hand, the

economic losses resulting from this disease are

large. The market prices offered for scurfy

sweetpotatoes are significantly lower than for

clean ones, even though the food value of roots

is not impaired by the presence of scurf.

Scurf may damage the sweetpotato skin.

Then when the storage house becomes rather

dry, the root loses moisture and becomes shriveled and dried.

Cause

The scurf fungus overwinters in storage and

on the decayed vines and other decayed vegetable matter in the field. If infected sweetpotatoes are used for seed, the fungus grows up on

the stem of the plants and is carried on the

plants to the field. Later, the organism in the

field grows or is washed by rains down onto

the roots. Scurf damage is most severe on

heavy soils and on those rich in organic matter. It is likewise more severe during a wet

season and on low, wet ground.
SWEETPOTATO CULTURE AND DISEASES 57

BN-23965

FIGURE 23.—Sweetpotato showing discoloration caused

by the scurf fungus.

Control

Use clean planting stock, produced wherever

possible from cuttings grown in scurf-free soil.

Since scurf affects only the underground parts

of plants, the cuttings will be free of the fungus. Bed the scurf-free sweetpotatoes produced

from the cutting plants in a clean plant bed and

pull the sprouts in the usual manner.

Selection of seed roots by sorting is impractical and not very effective if any scurf is present in the stock because many of the scurf spots

are too small to be easily seen.

Make field plantings in 2- to 3-year rotations

with other crops on lighter sandy and sandy

loam soils, or in 3- to 4-year rotations on heavier

soils and those rich in organic matter. The

scurf organism survives longer in these latter

types of soils.

Root Rot

(Texas Root Rot)

Description

Root rot causes a firm, brown rot and results

in complete destruction of the sweetpotato

(fig. 24). Above ground, the growth is within

the stem and may be detected by the brown

color produced. Coarse brown or gray strands

of the fungus on the surface of the roots can

be detected easily with a hand lens.

Prevalence and Loss

Root rot has been found in Arizona, Arkansas, California, Nevada, New Mexico, Oklahoma, and Texas. When the disease once gets

into a field, a crop may be destroyed. The growing crop may appear normal when viewed from

a distance, but by harvesttime nearly all the

sweetpotatoes may be 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 disease to the

Southern States. The disease may be observed

occasionally as early as May or June, but it

does not become serious until August 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 of the roots may be destroyed. The

disease may occur in spots of various sizes

within a field. Not all hills and not all sweetpotatoes in a hill are necessarily destroyed.

Cause

The root rot fungus lives from one season to

the next in the soil and on seed sweetpotatoes.

The organism gains access to the underground

parts of sweetpotato plants and spreads in

both stems and roots, invading the vines from

6 to 12 inches aboveground. It may enter the

end of the sweetpotato or may cause spots of

varying sizes on the surface.

Control

Root rot is most severe on black, poorly

drained soil and during wet seasons. The disease is very difficult to control or eradicate

because it affects a great variety of plants. It

is particularly destructive on cotton and al-
58 AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

faifa. To control root rot, cultivate deep and

clean, aerate the soil, apply stable manure,

rotate crops, and use disease-free sweetpotatoes

for seed. Use grasses, corn, and other cereals in

the rotation because they are partly or completely immune to the disease.

Mottle Necrosis

Description

Mottle necrosis, a field disease of the sweetpotato, produces brownish, somewhat sunken

spots, which are irregular in shape and size,

on the surface of the roots. 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 disease: irregularly shaped

patches of chocolate-brown dead tissue, which

appear to have no connection with one another

and give the cross section a mottled, or marbled,

appearance (fig. 25). The entire root may be

diseased even though there is only one small

spot of diseased tissue on the surface.

Prevalence and Loss

Mottle necrosis occurs in most States where

sweetpotatoes are grown. It is not as prevalent,

however, in the southern as in the northern

part of the sweetpotato-growing area. The loss

varies from year to year, depending upon soil

and weather conditions and upon the varieties

grown. The entire loss throughout the country

is relatively small. However, in certain isolated

districts where varieties such as the Yellow

Jersey are grown, as much as 40 percent of the

crop may be lost during seasons especially

favorable to the disease.

Cause

Mottle necrosis may be caused by either of

two fungi. These fungi probably enter through

BPI-70143

FIGURE 24.—Characteristic shriveling of sweetpotato

caused by the root rot fungus.

BN-36827

FIGURE 25.—Cross section through a sweetpotato root

severely infected with mottle necrosis.
SWEETPOTATO CULTURE AND DISEASES 59

the small fibrous roots and spread through all

parts of the sweetpotato.

Control

No method for the control of mottle necrosis

is known. 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. Triumph, Yellow Jersey, and Big Stem Jersey are

very susceptible varieties. Occasionally other

varieties may be slightly infected. Susceptible

varieties should not be planted in soils where

mottle necrosis has occurred in the preceding

3 or 4 years.

Soil Rot

(Pox, Ground Rot)

Description

Soil rot produces symptoms very different

from those of other sweetpotato diseases. In a

heavily infested soil the plants are dwarfed

and often produce only one or two short vines.

The leaves are small, thin, and pale green. The

aboveground symptoms are the result of injury

to the roots, caused by the disease. 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 malformed. Nearly black

flecks, or spots, of varying sizes and shapes

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 are covered

by the skin of the sweetpotato, which later

breaks and leaves conspicuous holes or pits. On

the swollen roots these pits are often one-half

inch, or more, in diameter and have jagged

margins (fig. 26). The enlarged root is sometimes girdled; the sweetpotato root continues

to enlarge on each side of the point of infection,

often growing more or less in the shape of a

dumbbell.

Prevalence and Loss

Soil rot occurs in practically all States where

sweetpotatoes are grown. It has become a limiting factor in sweetpotato production in Louisiana. The disease does not occur generally

throughout a State, but is more or less localized. It may be severe in one field or locality

and absent in another only a few miles away.

The loss caused by soil rot may range from

virtually no loss to almost complete failure in

different fields and different seasons. Losses are

most severe during dry seasons and on poor

soils. Soil rot is especially severe on soils that

*»

BPl-68797

FIGURE 26.—Sweetpotato showing typical soil rot pits.
60 AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

are alkaline or only slightly acid. It does not

develop much in fairly acid to acid soils; that

is, with a soil reaction of pH 5.2 or less.

Cause

The soil rot fungus lives in the soil from one

season to the next. Most of the infections probably occur after the plants are set in the field,

although infection may result from infested

soil in the plant bed and from infected seed

sweetpotatoes.

Control

No fully adequate control measure for soil

rot is known. Some research results indicate

that application of sulfur to reduce the soil

reaction to pH 5.2 or less will increase acidity,

reduce the amount of soil rot, and increase the

yield. Sulfur should be applied with considerable care; otherwise, it may make the soil too

acid for other succeeding crops. The amount

of sulfur to be used will depend on the soil

type and pH. Apply the sulfur broadcast and

incorporate it thoroughly into the soil 2 to 4

weeks before the plants are set out. Other

control measures are the use of stable manure

and green-manure crops to improve the soil

and rotation with other crops.

Avoid introduction of the soil rot organism

into new fields and new localities. Since cattle

or other livestock can carry the germs on their

feet, do not allow them to roam from infested

to disease-free fields. Plows and other farm

implements used to cultivate infested fields

should be thoroughly cleaned before they are

taken into clean fields. Use disease-free bedding

stock for producing clean transplants to be set

in new fields. (See p. 18.) If plants are obtained

from outside sources, be sure they are free of

soil rot.

Pliyllosticta Leaf Blight

Phyllosticta leaf blight causes circular or

angular, brownish spots one-eighth to one-half

inch in diameter on the upper sides of the

leaves (fig. 27). A number of black bodies about

the size of a pinpoint and just visible to the

naked eye are scattered within the spots. The

bodies are slightly raised and round and contain numerous colorless spores.

The fungus does not live on any other plant,

nor on any other part of the sweetpotato plant.

It probably overwinters on the dead leaves in

the field.

The disease occurs every year in practically

all the Southern States, but less commonly in

bN-23968

FIGURE 27.—Circular phyllosticta leaf blig-ht lesions on

a section of sweetpotato leaf.

some of the more Northern sweetpotato-producing States.

Septoria Leaf Spot

Septoria leaf spot is characterized by circular, white spots about one-eighth inch in diameter on the upper surface of the leaves (fig. 28).

Within these white areas are one or more black

specks, just visible to the naked eye. These

specks contain numerous spores that, upon escaping, may be carried by insects or other

agents to other leaves and start new infections.

Like the organism causing phyllostica leaf

blight, this fungus does not live on any other

plant nor on any other part of the sweetpotato

plant. It probably overwinters on the dead

leaves in the field.

Septoria leaf spot is very widely distributed
SWEETPOTATO CULTURE AND DISEASES 61

BN-2396B

FIGURE 28.—Sweetpotato leaf showing white spots

caused by the septoria leaf spot fungus.

and has been observed in most States where

sweetpotatoes are grown.

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. 29). Later these spots become brown

and covered with a whitish, viscid growth, which

finally becomes more or less powdery. This

BN-23967

FIGURE 29.—Sweetpotato leaf injury caused

by the white rust fungus.

powdery white mass is made up of numerous

spores. If these spores fall on other leaves,

under favorable conditions they will germinate

and cause new infections. No great harm results from the attack of this fungus, although

it may sometimes produce swellings on the

stems and petioles (leaf stems) and cause malformations 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 during wet

seasons than dry. It occurs on sweetpotato

plants in most of the Southern States. Under

favorable weather conditions it also occurs in

Northern States where sweetpotatoes are grown.

NEMATODES AND OTHER CONTROL^

Description

Plant-parasitic nematodes reduce yield and

quality of sweetpotatoes, destroy root systems,

and cause inefficient use of nutrients and water.

The vines are seldom killed, but usually have

poor growth and are yellowish. On hot days

the vines often will wilt.

* The section "Nematodes and Their Control" was

prepared by J. M. Good, nematologist, Crops Research

Division, Aji-ricultural Research Service.

Several species of root-knot nematodes cause

galling (or knots) on the feeder roots, roughening and frequent cracking of tubers, and

generalized decay of the entire fiberous root

system (fig. 30). Root-knot nematodes develop

in colonies in the tuber, which appear as small

discolored areas about one-sixteenth inch, or

smaller, in diameter. However, these areas

sometimes occur deep in the fleshy tissue.

Lesion nematodes cause irregular darkbrown, sunken areas of decay on the surface
62 AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

BN-36828

FIGURE 30.—Tuber and feeder sweetpotato roots showing heavy root-knot infection. Note the small galls on the roots

caused by root-knot nematode, and the decay, cracking, and roughness of the developing tuber.
SWEETPOTATO CULTURE AND DISEASES 63

of the tuber. These spots contain the minute

nematodes.

Prevalence and Loss

One or more species of plant-parasitic nematodes occur in all sweetpotato-producing areas.

Crop losses often exceed 50 bushels per acre on

heavily infested land.

Five species of root-knot nematodes (Meloidogyne spp.) attack sweetpotatoes. The southern

and cotton root-knot nematodes (M. incognita

and M. incognita acrita) cause greatest damage

in the Southern States. The northern root-knot

nematode (M. hapla) occurs in both southern

and northern climates. The peanut root-knot

nematode (M. arenaria) and the oriental or

peach root-knot nematode (M. javanica) occur

most frequently on farms v^here peanuts and

peaches are grown or in the extreme Southern

and Western United States.

Reniform nematodes (Rotylenchulus reniformis) cause severe losses in Louisiana and

in parts of Alabama and Mississippi. Other

nematodes such as lesion (Pratylenchus spp.),

spiral {Helicotylenchus spp. and Scutellonema

spp.), dagger (Xiphinema americanum), lance

(Hoplolaimus galeatus), sting (Belonolaimus

longicaudatus), and ring {Criconemoides spp.)

cause an unknov^n amount of damage to sv^eetpotatoes, especially those grov^ing in the Atlantic and Gulf Coastal Plain soils.

Causes

Plant-parasitic nematodes are microscopic

round worms that live part of their lives in

soil. Some types feed externally on the plant

roots; others enter the root and tuber tissues

and feed there. All disrupt root development

and interfere with translocation and transport

of plant nutrients and water. They often incite

decay of fiberous roots and tubers by providing

entry, into otherwise healthy tissue, for other

micro-organisms that cause decay. Plant-parasitic nematodes are spread on farms in drainage and irrigation water, by the wind, and by

movement of farm animals and implements.

Root-knot and lesion nematodes are frequently

introduced into noninfested farmland in nematode-infected seed sweetpotatoes and plants, or

in other infected transplanted crops.

Eggs, larvae, and adults of all nematodes

except the root-knot nematode are found in

soil. After hatching larvae and adults feed on

plant roots. The lesion nematodes penetrate

fiberous roots or tubers and migrate during

feeding within the root tissues. This migration

causes extensive necrosis or decay. Most other

plant-parasitic nematodes feed externally on

the roots. During feeding they puncture root

cells, which usually results in death of the injured cells. Continued feeding and death of

hundreds of cells prevents normal root development and function.

The larvae of root-knot nematodes enter the

roots as slender worms about one-twentieth

inch long ; the female becomes embedded in the

root tissues and feeds in one site. Each female

produces several hundred eggs. The feeding of

the root-knot nematode at one site causes the

plant to produce the characteristic root-knot

gall around the bodies of one or more females.

Control

To prevent nematode damage the grower

must control these pests in both seedbeds and

field plantings. Failure to control nematodes

in the seedbed will assure damage to field plantings because the nematodes can be carried from

the seedbed to the fields in seed sweetpotatoes

or plants. Seedbeds should be treated with a

registered nematicide. If root-knot-resistant

varieties are not used, field soil should also be

treated with a registered nematicide to prevent

losses when nematode populations are high.

Nematicides that can be used in seedbeds and

in the field include DD; 1,3-D; EDB; 1,3-DEDB; SMDC; and DD-MENCS (table 11).

Of several control methods available, nematicides and soil fumigants provide the best

means of controlling all types of nematodes

that attack sweetpotatoes. However, because of

the relatively high cost, nematicides cannot be

used routinely as a production practice. These

chemicals are practical and increase yields and

profits only when nematodes are present in

sufficient numbers to seriously limit production.

The best means of determining whether nematodes are reducing yields and profits is to apply

nematicides on a trial basis to each field where

sweetpotatoes are grown. Do not apply nematicides to the entire field; leave untreated rows

to determine if nematode control is profitable in each field. If high yield increases are

obtained, similar increases can be expected

from use of nematicides whenever sweetpotatoes are grown. This is because once nematodes

become established in soil they seldom disappear, but remain a problem year after year.

Nematicides do not permanently rid the soil

of nematodes, but reduce their numbers so that

a crop can be grown profitably without significant damage. Re-treatment of soil will usually

be necessary each time sweetpotatoes are

grown.

Details concerning methods of applying nematicides and soil fumigants have been published by Good and Taylor (6).
64 AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

Strict adherence to the manufacturers' directions for applying nematicides is a necessity.

Application of insufficient amounts of chemicals will not control nematodes. Application of

excessive amounts of chemicals may injure

plants and reduce yields, or may contribute to

unlawful chemical residues in the sweetpotatoes.

In the field, all types of nematodes cannot

be controlled by crop rotation. Since suitable

rotation crops vary in different parts of the

country, you should consult your county agent

or State agricultural experiment station before

starting a nematode-controlling rotation. Rotations can be designed to control one or more

species of nematodes. Root-knot nematodes are

more easily controlled by crop rotations than

are other nematodes.

Nematode infestations can be reduced in field

soil by removing all potatoes from the field at

harvest. Unmarketable potatoes that are left

on the ground contain large numbers of nematodes that will reinfest the soil and cause severe

damage to subsequent crops. Unmarketable

potatoes should be burned or buried some distance from cultivatable land.

A number of sweetpotato varieties have resistance to the southern and cotton root-knot

nematodes, but none have complete resistance

to all species of root-knot nematodes or even to

all populations of the southern and cotton rootknot nematodes that occur in all parts of the

country. A few of the root-knot-resistant

varieties are listed in table 21. The varieties

Heartogold and Nemagold have the best resistance to all species of root-knot nematodes. The

root-knot-susceptible variety Goldrush has moderate resistance to the reniform nematode;

otherwise, resistance to the many other types

of nematodes is not available. When root-knot

nematodes are the principal cause of damage,

use of resistant varieties can reduce yield losses

and will prevent increase of soil populations

of nematodes that damage subsequent crops

grown on the land.

STORAGE ROTS AND THEIR CONTROL

Soft Rot

(Ring Rot, Collar Rot)

Soft rot, caused by the so-called bread mold,

is a very destructive disease of sweetpotatoes

in storage. It may start soon after the crop is

placed in storage and continue to spread

throughout the storage period. The decay begins in unhealed wounded areas, often at the

ends of the root. It progresses rapidly. Only a

few days with favorable temperatures and humidity are needed to completely destroy the

sweetpotato root. At first the soft rot-affected

sweetpotatoes are soft, watery, and stringy. A

copious growth of spore-bearing fungus strands,

sometimes referred to as '^whiskers," usually

develops quickly on the ruptured root surface

(fig. 31). After decay and the escape of moisture, the shriveled roots gradually become firm,

hard, shrunken, and brittle.

Soft rot sometimes spreads from one root

to another by contact. Spores of the mold produced on the root surface may be carried by

flies, rodents, or air currents to other roots in

the storage house. The spores also may be

spread in handling the potatoes. New infections

may occur if the spores fall on fresh, unhealed,

wounded surfaces, especially those of crushed

tissues, when the temperature and moisture

conditions are favorable for development of

the fungus.

In the ring rot or collar rot forms of soft

rot, both caused by the same organism, the

decay usually begins at one or more points between the ends of the root. From the points of

infection, the decay forms rings or collars

around the sweetpotato and then extends slowly

toward the root ends. Under conditions favorable to the mold the root may be entirely destroyed. If conditions unfavorable for the mold's

further development exist, such as relatively

low humidity and low temperatures, only depressed rings or collars varying in width from

about 1 inch to 3 inches may be formed (fig.

32).

The soft rot causal organism is found everywhere and is therefore impossible to exclude

from storage houses. The fungus generally

gains entrance to the sweetpotato through

fresh wounds and bruises caused by digging,

rough handling, or bites of rats and mice. Soft

rot develops in roots injured by cold or wet

soils and is often most serious following such

damage. Timely digging of sweetpotatoes during fair weather, allowing surface drying of

roots before picking up in the field, handling

roots carefully to keep bruising and injuries at

a minimum, promptly transferring the crop

from the field to storage, and maintaining

proper temperature and humidity during curing and storage periods, all are important factors in keeping soft rot losses at a minimum.
SWEETPOTATO CULTURE AND DISEASES 65

BPI-6G70-A

FIGURE 31.—Sweetpotato showing the moldy growth, or

"whiskers," of the fungus causing soft rot.

Black Rot

Where adequate precautions are taken to

eliminate black rot infection or keep it at minimal levels during propagation and culture of

sweetpotatoes in the field, this disease should

ordinarily cause only minor losses during storage and subsequent marketing of the crop.

However, where appreciable infection and

spread of the disease occur in the field for any

reason, then the dangers of serious losses during and after storage may be great. For this FIGURE 32.-BPI-68794-

-Sweetpotato with ring rot.
66 AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

reason, roots from fields where black rot is

present should be marketed as soon after harvest as possible.

At harvesttime, roots from contaminated

field plantings may have relatively few readily

visible black rot lesions. Large numbers of

roots, however, may have infections with lesions too small to be seen with the naked eye.

If these roots are placed in storage where temperature and humidity are favorable for development of the black rot fungus, the lesions will

gradually enlarge. They may grow to be an

inch or two in diameter and penetrate as much

as one-half inch in depth after a month or two

in storage (fig. 33). The number of infections

and the size of the lesions increase rapidly at

temperatures between about 60° and 85° F.

(Note that temperatures during the curing

period are within this range.) Development of

black rot infections is slow when temperatures

are near or below about 55°.

From the fungus lesions, innumerable fruiting bodies are developed, from which immense

numbers of one-celled spores are exuded. Infections are spread during storage by these

spores being carried from root to root by rats

and mice, insects, and possibly air currents;

by settling of roots in containers; or by handling of stock during preparation for market.

Washing sweetpotatoes in preparation for

marketing can be a potent means of spreading

black rot infections if the disease is present on

any roots. Adequate precautions must be observed to sterilize washing equipment and to

prevent build-up of the black rot spore load

in the wash water. Treatment of roots with

a registered, effective fungicide such as SOPP

and sanitizing grading equipment with 27.6

percent sodium dimethyl dithiocarbamate and

2.4 percent sodium 2-mercaptobenzothiazole

(Vanicide 51) between lots in connection with

the washing operation can greatly reduce losses

from black rot during marketing.

Surface Rot

In the early stages, surface rot is characterized by nearly circular, somewhat sunken spots

(fig. 34) on the surface of the sweetpotato.

These vary in number and their size increases

with time after infection. The rot is shallow,

seldom extending more than one-fourth to onehalf inch below the surface. The sweetpotato

shrinks later, especially at the margins of the

spots.

Infection occurs at about digging time, especially if the ground is wet, or early in the storage period when small decayed spots can often

be found at the base of the small rootlets. The

infected areas gradually enlarge in storage and

become conspicuous in 6 to 8 weeks. If the

storage house is kept rather warm and dry,

moisture escapes from the affected areas and

the sweetpotato gradually becomes hard and

mummified. Injury from exposure of roots to

sun and wind after digging increases the loss

from this rot.

Surface rot has some characteristics in common with black rot. Surface rot spots may become grayish brown and 1 inch in diameter

during storage. Black rot spots are nearly black

and may become 2 inches in diameter. The

surface rot spots are more regular in shape

and size than spots caused by bruises.

The loss from surface rot is sometimes more

than that of any other storage disease. Occasionally the sweetpotatoes are so badly shrunken

that they have no market value. Some varieties

are much more susceptible to surface rot damage than are others. No varieties are completely

immune to this disease.

Proper handling, curing, and storage practices will reduce losses from this disease.

BPI-6769-A

FIGURE 33.—Cross section through sweetpotato with

black rot lesion, showing depth of penetration by

the black rot fungus.

Java Black Rot

Java black rot, so called because it was discovered on sweetpotatoes grown from an im-
SWEETPOTATO CULTURE AND DISEASES 67

portation from Java, is a widely distributed

storage disease. It is more prevalent in the

South than elsevi^here.

Java black rot is strictly a storage disease.

The aiïected svs^eetpotatoes rot slowly and become dry, hard, brittle, and coal black within

and are difficult to break (fig. 35). The disease

is spread by spores that develop beneath the

BPI-e8795-P

FIGURE 34.—Sweetpotato after several weeks in storage,

showing a number of circular lesions associated

with surface rot.

FIGURE 35.-—Sweetpotato showing the dry, mummified

condition caused by the Java black rot fungus.
68 AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

surface of numerous pimplelike protuberances.

When the surface of the root is broken, these

spore bodies are set free. Java black rot begins

usually at the end of the root and progresses

very slowly. Under normal storage conditions,

the disease can completely destroy a sw^eetpotato in from 4 to 8 weeks.

Careful handling and prompt curing are valuable methods for reducing losses from this

disease.

Dry Rot

Dry rot generally begins at the ends of the

sweetpotato and produces a firm, brown decay.

The sweetpotato decays slowly and finally becomes dry, hard, and mummified (fig. 36).

Small domelike or pimplelike protuberances

just visible to the naked eye finally cover the

entire surface and contain large numbers of

colorless spores of the fungus. The tissue just

beneath the skin is coal black. Under normal

conditions, the fungus can completely destroy

a sweetpotato in several weeks.

The dry rot fungus grows on the stems and

vines aboveground in fields, 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, although widely distributed throughout the United States, is not regarded as one

of the more serious storage disorders.

„J^.

¿^ ,, . <• ''*'*' ^<

mE¿r^^-'^¿»

fl

^ ^ •^^^H 1

l^^^^^^^i,,.^^ -¡^^^^SÊSêR

1^^B^^^K^^^^^ÊÊi^^'' ' *

^^^^^BHHKM'^ ' 1

^BBBBHBiP^»-^:^'*,*^^f '

FIGURE 36.—Characteristic appearance of sweetpotato

decay caused by dry rot.

VIRUS DISEASES AND THEIR CONTROL

Internal Cork

Internal cork was first reported in 1944, in

South Carolina. This disease occurs in every

State where sweetpotatoes are grown commercially in the United States, and its occurrence

has also been reported in many other countries.

Internal cork symptoms have been observed in

most of the numerous varieties and selections

examined in the United States. Some varieties

are very susceptible to infection by this disease,

whereas others are much less susceptible (table

21).

A few varieties are carriers of the internal

cork virus without themselves developing the

corky lesions found in roots of most varieties

infected with this disease.

Plants infected with the cork virus have

yellow spots on the leaves when the latter are

about 1 month old. These spots become masked

and less discernible as the leaves grow older.

Yellow spots that develop on the new growth

later in the growing season may be surrounded

by purple-pigmented borders (fig. 37).

Internal cork is primarily a root lesion disease. The virus causes formation of dark-brown

to black, hard, corky spots or lesions in the

flesh of susceptible varieties (fig. 38). Small,

corky lesions sometimes may be found in some

roots at digging time. Usually, however, extensive development of the corky lesions occurs

only after storage at relatively high temperatures (60° F. or higher) for periods of from

several weeks to about 6 months. The corky

spot may occur singly or in groups and are

most readily detected by cutting the roots into

thin slices. Severely diseased roots may develop

so many corky lesions that they become practically useless for food.

Two viruses, internal cork and chlorotic

leafspot (see below), always occur in both the

leaves and roots of plants showing internal

cork symptoms. These two viruses persist from

one season to the next in fieshy sweetpotato

roots. Both are spread in the field by aphids,

especially during the cool weather early in the

growing season.
SWEETPOTATO CULTURE AND DISEASES 69

BN-36829

FIGURE 37.—Purple-bordered chlorotic leaf spots of internal cork disease on sweetpotato leaves.

Chlorotic Leafspot

Chlorotic leafspot is one of the most widespread virus diseases of sweetpotatoes. It occurs in all States where sweetpotatoes are

grown commercially and in all varieties that

have been examined. In some cases it has infected every plant in a field during the growing season.

When only the chlorotic leafspot virus is

present in sweetpotato plants, it causes a mild

chlorosis on older leaves with occasional spots

along veins having a "feathered" outline.

Chlorotic leafspot and internal cork symptoms in foliage are so much alike that ordinarily they cannot be clearly differentiated by

observation alone.

Use of virus-free stocks is the recommended

method of control for both chlorotic leafspot

and internal cork viruses. In experimental

trials new vine growth free of both viruses has

been obtained by growing plants continuously

at 100° F. for 6 months.

Yellow Dwarf

Yellow dwarf virus causes a stunting of both

plants and roots (fig. 39). In the field it is

BN-23963

FIGURE 38.—Cross and long sections through sweetpotato roots infected with internal cork, showing

appearance of lesions.
70 AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

FIGURE 39.—Left, Healthy sweetpotato plant; right, sweetpotato plant infected with yellow dwarf virus.

Note the lighter color and smaller size of the infected plant.

BN-36830

always found in mixture with the internal cork

and chlorotic leafspot viruses. Dwarfed plants

produce roots that are few, dwarfed, misshapen,

and practically worthless for food or for propagation. The virus lives from one season to the

next in roots of diseased plants. It apparently

is not seedborne (in true seed). Under controlled conditions in the greenhouse, plants infected with yellow dwarf virus show systemic

vein clearing and nonchlorotic leaf-dwarfing

symptoms.

Yellow dwarf is spread in the field by the

banded-wing whitefly (Trialeurodes abutilonea), an insect that feeds and reproduces on

weeds commonly found in sweetpotato-growing

areas, and by the sweetpotato whitefly (Bemisia tabaci). Yellow dwarf-free planting stock

should be used to avoid introduction of this

disease into an area.

Feathery Mottle

When the internal cork, chlorotic leafspot,

and yellow dwarf viruses are acting together,

the disease complex is called feathery mottle.

Leaves of infected young plants are commonly

mottled with green and yellow. There may be

a feathery yellowing of older leaves. In some

sweetpotato varieties the leaves may become

distorted and sometimes fan shaped. In later

stages of the disease, growth is dwarfed (from

yellow dwarf virus) and leaves are yellowed

and spotted (from the internal cork and chlorotic leafspot viruses). Roots produced by

plants diseased by this complex are few, small,

and worthless.

Russet Crack

The first observations of russet crack were

reported from Maryland and New Jersey

about 1960. Since then, it has been found in

other sweetpotato-producing States. The virus

that is the primary cause of this disease is

spread rather rapidly in the field by the green

peach aphid (Myzus persicae), which appears

to be the principal insect vector. Losses from

russet crack can be heavy because the disfiguring lesions on the surface of the roots render

the sweetpotatoes unfit for sale. All sweet potato varieties tested to date appear to be susceptible to the root-lesion phase of the disease.

The most conspicuous symptoms of russet

crack occur on the enlarged fleshy roots. The

characteristic symptoms, as they occur on roots

in the field, appear to result from secondary

infections by rhizoctonia and other fungi. These

infections discolor the tissues around less conspicuous primary lesions caused by the russet

crack virus. Less readily observable symptoms

can be found on leaves, stems, and feeder roots

of infected plants. Where the fleshy roots are
SWEETPOTATO CULTURE AND DISEASES 71

BN-36831

FIGURE 40.—Hill of Yellow Jersey sweetpotatoes

severely infected with russet crack disease.

only mildly affected, symptoms appear as small

to moderate-size, dark-brown to grayish spots,

or transverse bands with fine longitudinal

cracks in the skin and cortical layers of the

roots. In moderately infected roots, lesions are

darker, with deeper, more extensive cracking.

These lesions usually extend in transverse

bands of varying width, sometimes incompletely

but often entirely circling the roots. Sometimes

the encircling lesions may cause constrictions

in the roots. In very severely infected roots, the

damaging lesions cover the entire surface of the

roots (fig. 40). Cracking and discoloration also

occur in the smaller feeder roots of infected

plants or of sprout plants grown from infected

bedding stock. Grayish-black spots or streaks

sometimes occur on vines and leaf petioles of

infected plants. The disease causes a characteristic necrotic spotting of older leaves in some

varieties, such as those of the Jersey types.

Until completely effective control of insect

vectors is devised, it will probably be impossible

to prevent spread of russet crack (as well as

other virus diseases discussed above) in field

plantings if infection sources are present. Clean

plants for propagation can be obtained only if

bedding stock is entirely free of the disease

because any sprouts produced from infected

bedded roots will also be infected.

LITERATURE CITED

(1) ANDERSON, W. S., COCHEAN, H. L., EDMOND, J. B.,

and others.

1945. REGIONAL STUDIES OF TIME OF PLANTING

AND HILL SPACING OF SWEETPOTATOES.

U.S. Dept. Agr. Cir. 725, 20 pp.

(2) BOSWELL, V. R.

1950. COMMERCIAL GROWING AND HARVESTING

OF SWEETPOTATOES. U.S. Dept. Agr.

Farmers' Bui. 2020, 38 pp.

(3) DEMPSEY, A. H.

1961. CROSS-CUT BEDDING SWEET POTATOES TO

INCREASE SPROUTS. Ga. Agr. Res. 2(3) :

10-11.

(4) FOOD AND AGRICULTURE ORGANIZATION OF THE

UNITED NATIONS.

1963. PRODUCTION YEARBOOK, V. 17. Rome.

(5)

1965. PRODUCTION YEARBOOK, V. 19. Rome.

(6) GOOD, J. M., and TAYLOR, A. L.

1965. CHEMICAL CONTROL OF PLANT-PARASITIC

NEMATODES. U.S. Dept. Agr., Agr.

Handb. 286, 28 pp.

(7) GROTH, B. H. A.

1911. THE SWEET POTATO. Univ. of Pa. Bot.

Lab. Contrib. v. 4, No. 1, pp. 21-32.

(8) HILDEBRAND, E. M., and COOK, H. T.

1959. SWEETPOTATO DISEASES. U.S. Dept. Agr.

Farmers' Bui. 1059, 28 pp.

(9) KUSHMAN, L. J.

1961. FUNGICIDAL CONTROL OF DECAY IN PORTO

RICO SWEETPOTATOES. A PROGRESS REPORT.

U.S. Dept. Agr., Agr. Market. Serv.

AMS-452, 17 pp.

(10)

(11)

1967. PREPARING SWEETPOTATOES FOR MARKET.

U.S. Dept. Agr., Market. Bui. 38, 14 pp.

HARDENBURG, R. E., and WORTHINGTON,

J. T.

1964. CONSUMER PACKAGING AND DECAY CONTROL OF SWEETPOTATOES. U.S. Dept.

Agr., Agr. Market. Res. Rpt. 650, 15 pp.

(12) - -WRIGHT, W. R., KAUFMAN, J., and HARDENBURG, R. E.

1965. FUNGICIDAL TREATMENTS AND SHIPPING

PRACTICES FOR CONTROLLING DECAY OF

SWEETPOTATOES DURING MARKETING. U.S.

Dept. Agr. Market. Res. Rpt. 698, 12 pp.

(13) PARK, J. K., POWERS, M. R., and GARRISON, O. B.

1953. MACHINERY FOR GROWING AND HARVESTING

SWEETPOTATOES. S.C. Agr. Expt. Sta.

Bui. 404, 39 pp.

(14) POOLE, W. D.

1963. HARVESTING SWEET POTATOES IN LOUISIANA. La. Agr. Expt. Sta. Bui. 568,

24 pp.
72 AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

(15) SCOTT, L. E., and OGLE, W. L.

1952. THE MINERAL UPTAKE BY THE SWEETPOTATO. Better Crops With Plant Food

36(8):12-16, 50.

(16) STEINBAUER, C. E.

1962. PRINCIPAL SWEETPOTATO VARIETIES DEVELOPED IN THE UNITED STATES, 1940 TO

1960. Hort. News (N.J.) 43(1): 123-

124, 129.

(17) U.S. DEPARTMENT OP AGRICULTURE.

1952. HOT BEDS AND COLD FRAMES. U.S. Dept.

Agr. Farmers' Bui. 1743, 28 pp.

(18)

(19)

(20)

1958.

1960.

1963.

SASH GREENHOUSES.

Leaflet 124, 8 pp.

U.S. Dept. Agr.

NUTRITIVE VALUE OF FOODS. U.S. Dept.

Agr. Home and Gard. Bul. 72, 36 pp.

UNITED STATES STANDARDS FOR GRADES OF

SWEETPOTATOES. [Effective July 1, 1963.]

U.S. Dept. Agr., Agr. Market. Serv. (28

F.R. 5252), 4 pp.

APPENDIX

TABLE 22.—Some synonyms for the sweetpotato, used in writings

concerning this plant ^

Ahe (Choctaw Indian)

Amotes

Apichu (Peru)

Artichaut des Indes (France)

Bátala

Batatas (Malaya)

Bataten-Winde (German)

Bâtâtes douces

Battades

Battatas

Boga

Cacamotic

Camote

Canange

Chillagada

Convolvulus (several species names)

Cumala

Cumar

Dankali

Doukali

Fiasi (Wanika-land)

Gajar lahori

Getica (Brazil)

Grasugada

Gumalla

Gumara

Hantsoa

Hetich

Hoanxy (Cochinchina)

Igname

Ino (Japan)

Ipomoea batatas

Ipomoea tuberosa

Jetica (Brazil)

Kan-chu

Kara-imo (Japan)

Kiasi

Kimhella

Kindolo

Kitaiti

Kumana (New Zealand)

Kumara (New Zealand)

Lardak-lahori (Persia)

Lohita

Maby (Antilles)

Mankutu

Mawandres

Mita-alu (Hindustan)

Myonk-ni (Burma)

Obi-djawa (Java)

Pappas

Pararo

Patales

Patata

Patatas

Patate Jaune (France)

Patates

Pendaloo (Hindustan)

Potades

Potates

Potato, Virginian (or Carolina)

Potato, sweet

Potatoes (Spain)

Potatos

Potatus

Ranga

Ratalu

Rukta-kunda

Ruktaloo

Ruktupindaloo

Sakaria

Schumbalino

Shakarkand (Punjab)

Shukar-kandu (India)

Sisarum Peruvianorum

Skirrets (Peru)

Sukkarag-vullie

Truffe douce (France)

Ubi (Java)

Ubitora (Malaya)

Umara (South Sea Islands)

Vallikilangu

Veeazee (Central Africa)

Ycam (Peru)

Yeti

Zardak-lahori (Persia)

1 Partial list, adapted from The Sweet Potato (7).
SWEETPOTATO CULTURE AND DISEASES 73

TABLE 23.—Estimated average annual world production and yields of

sweetpotatoes and yams from 1960 through 1965 ^

Continent and country

EUROPE:

Italy

Portugal (Azores)

Spain

NORTH and CENTRAL AMERICA :

Antigua

Barbados

Cuba

Dominican Republic

Guadeloupe

Haiti

Hawaii

Honduras _

Jamaica

Martinique

Mexico

Panama

Puerto Rico

St. Lucia

St. Vincent _

Production

area

Trinidad and Tobago

United States

SOUTH AMERICA:

Argentina

Bolivia

Brazil

British Guiana

Columbia

Ecuador

French Guiana

Paraguay

Peru

Uruguay

Venezuela

ASIA:

Brunei

Cambodia

Ceylon

China (Taiwan)

Hong Kong

India

Indonesia

Japan

Korea, Republic of

Laos

Lebanon

Malaysia :

Malaya

Philippines

Ryukyu Islands

Sabah

1,000 acres

5

20

M5)

5

2151

*(10il)

6

2(490

2

85

9

48

35

18

22

^6

200

89

HIO)

361

M7)

13

^2

21

439

337

30

6

32

582

7

385

51,080

785

208

45

21

364

17

2

Yield

per acre

Cwt.

146

2(121)

111

M50)

89

233

M18)

110

M45)

^30

24

49

499

60

3(161)

32

535

^21

Production

81

H 69)

96

38

no7

86

^9

350

67

55

77

37

112

85

59

560

176

79

^56

138

2(93)

45

146

(74)

1,000 Cwt.

669

2(880)

2,191

3(242)

475

24,972

n,723

704

M2,178)

522

66

4,167

n,ioo

2,292

3(814)

581

*51

5110

5(418)

16,192

7,960

244

34,632

mo

M2,904)

524

n6i

1,769

^3,080

21,826

1,923

22

502

1,162

65,072

638

22,735

551,955

142,402

17,103

^205

22

2(4,356)

16,249

2,561

(264)

See footnotes at end of table.
74 AGRICULTURE HANDBOOK 388, U.S. DEPARTMENT OF AGRICULTURE

TABLE 23.—Estimated average annual world production and yields of

sweetpotatoes and yams from 1960 through 1965 i—Continued

Continent and country

ASIA—Continued

Malaysia—Continued

Singapore

Thailand

Viet-Nam, North

Viet-Nam, Republic of _

AFRICA:

Algeria

Cameroon :

East Cameroon

West Cameroon

Central African Republic.

Production

area

Chad

Congo (Brazzaville)

Congo (Democratic Republic)

Dahomey

Ethiopia

Gabon

Ghana

Guinea

Ivory Coast _

Madagascar

Malawi

Mali

Mauritius __

Niger

Nigeria

Reunion

Ruanda-Urundi (former)

Rwanda

Senegal

Sierra Leone

South Africa

Southern Rhodesia ^

Tanzania :

Tanganyika

Zanzibar

Togo

Uganda

United Arab Republic

Upper Volta

Zambia

OCEANIA:

Australia "'' British Solomon Islands.

Fiji Islands

New Caledonia

New Guinea (Australia Administration).

Pacific Islands (U.S. Administration)^

Papua

Tonga

WORLD TOTAL

lyOOO acres

2

2457

111

M2)

^32

M7)

40

223

425

(141) 5177

32

5(158)

310

^612

n48

249

212

5

3(3,223)

2353

218347

7

M25)

5(193)

55

3210

641

11

60

2(5)

^12)

5

25

55

52

22

211

39,322

Yield

per acre

CwU

76

569

240

53

2(89)

529

3(73)

33

246

448

(104)

572

^27

318

^(67)

3124

567

*38

218

2124

579

111

^(68)

107

264

271

^62

23

3(36)

^38

5(27)

437

385

M92)

171

23

2(44)

575

M93)

313

561

44

552

2119

Production

1,000 Cwt,

101

57,711

218,381

5,861

2(242)

53,432

3(572)

1,320

21,078

n,ioo

3(6,952)

512,683

1,122

344

no,604

51,716

540,766

55,753

2902

21,540

522

537

2335,500

66

222,660

211,858

4693

198

41,419

322

25,225

4169

317,776

4(46,882)

1,870

1,373

2(220)

533

4(1,056)

5(330)

344

5242

88

544

21,353

2,604,800

^Calculated from data in Production Yearbook 1963 (4) and Production Yearbook

1965 (5). Figures shown in parentheses are for period from 1948 through 1953.

2 Average 2 years. 3 One year only. 4 Average 3 years. ^ Average 4 years.

^ For farms and estates only. "^ Excluding New South Wales.

Ù U.S. GOVERNMENT PRINTING OFFICE: 1971 O 389-924