I

m ü^!fíM^Market Diseases

Agricultural

Research

Service

Agriculture

Handbook

Number 155

of Beets, Chicory,

Endive, Escarole,

Globe Artichokes,

Lettuce, Rhubarb,

Spinach, and

Sweetpotatoes

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Abstract

Moline, H. E., and Upton, W.J. 1987. Market diseases of beets,

chicory, endive, escarole, globe artichokes, lettuce, rhubarb,

spinach, and sweetpotatoes. U.S. Department of Agriculture,

Agriculture Handbook No. 155, 86 p.

This handbook contains descriptions and color plates of

economically important postharvest diseases and disorders of

these nine commodities with recommendations for their control. It

is intended for use as a ready descriptive guide by inspectors, produce handlers, marketing specialists, and research workers. The

listed references, which are not intended to be all inclusive, supplement the subject for the user.

KEYWORDS: Beets, chicory, endive, escarole, globe artichokes,

lettuce, postharvest diseases and disorders, rhubarb, spinach,

sweetpotatoes.

Additional copies of this publi- Microfiche copies can be purcation can be purchased from chased from the National

the U.S. Government Printing Of- Technical Information Service,

fice, Washington, DC 20402. 5285 Port Royal Road,

When ordering by mail, ask for Springfield, VA 22161.

the publication by title and

series. Revised December 1987
United States

Department of

Agriculture

Agricultural

Research

Service

Agriculture

Handbook

Number 155

IVIarlcet Diseases of

Beets, Chicory, Endive,

Escarole, Globe

Artichokes, Lettuce,

Rhubarb, Spinach, and

Sweetpotatoes

H.E. Moline and W.J. Upton

USDA. National Agricultural Ubrary

WAL Bldg

10301 Baltimord Btvd

Beiltsvitie, MD 20705-23&1

veB^^

Preface

This is an extensive revision of Agriculture Handbook 155 by Glen

B. Ramsey, B.A. Friedman, and M.A. Smith, first published in 1959

and reprinted in 1967. This handbook is one of a series designed to

aid in the recognition and identification of market diseases, to provide information on factors causing the diseases, and to present

control measures for reducing the economic losses caused by

these diseases during marketing. This handbook is designed for

use by market inspectors, research workers, shippers, receivers,

carriers, and others concerned with maintaining the quality of fresh

produce during marketing.

Other publications in this series are—

28 Market Diseases of Tomatoes, Peppers, and Eggplants

(reprinted 1982).

184 Market Diseases of Cabbage, Cauliflower, Turnips, Cucumbers,

Melons, and Related Crops (out of print).

189 Market Diseases of Grapes and Other Small Fruits (out of print).

303 Market Diseases of Asparagus, Onions, Beans, Peas, Carrots,

Celery, and Related Vegetables (reprinted 1982).

376 Market Diseases of Apples, Pears, and Quinces (printed 1971).

398 Market Diseases of Citrus and Other Subtropical Fruits

(reprinted 1983).

414 Market Diseases of Stone Fruits: Cherries, Peaches, Nectarines,

Apricots, and Plums (printed 1972).

479 Market Diseases of Potatoes (printed 1978).

ç^^

111
Acknowledgments

For their advice and assistance we thank Clyde L. Burton, USDA,

East Lansing, Ml; Michael J. Ceponis, USDA, New Brunswick, NJ;

Adel A. Kader, University of California, Davis; and Edward J. Ryder,

USDA, Salinas, CA.

We are indebted to the following persons for photographs: Louis

Beraha, USDA (ret.), Chicago, IL; M.J. Ceponis, USDA, New

Brunswick, NJ; J.E. Gay, Georgia Agricultural Extension Service,

Tifton; A. Greathead, California Agricultural Extension Service,

Salinas; J.M. Harvey, USDA, Fresno, CA; D. Spalding, USDA, Miami,

FL; and J.K. Stewart, USDA (ret.), Fresno, CA.

IV
Contents

Beets 1

Bacterial soft rot 1

Black rot 2

Cercospora leaf spot 3

Scab 3

Internal black spot 3

Freezing injury 4

References cited 5

Chicory, endive, and

escarole 6

Bacterial soft rot 6

Black heart (brown heart) 6

Irradiation injury 7

References cited 7

Globe artichokes 8

Gray mold rot 8

Freezing injury 9

Irradiation injury 9

Mechanical injury 9

Wilting 10

References cited 11

Lettuce, whole 12

Bacterial soft rot 12

Downy mildew 14

Gray mold rot 15

Watery soft rot

(sclerotinia rot) 15

Viruses and related

diseases 16

Aster yellows 16

Big vein 17

Lettuce mosaic 17

Disorders and physical

injuries 19

Brown stain and heart

leaf injury 19

Butt discoloration 21

Freezing injury

(blistering) 22

Irradiation damage 23

Marginal senescence

(marginal browning) 23

Mechanical injury 24

Oxygen deficiency 25

Ozone damage 25

Pink rib 26

Rib discoloration

(brown rib) 27

Russet spotting 28

Solar injury 29

Tipburn 30

References cited 31

Lettuce, shredded 34

References cited 34

Rhubarb 35

Anthracnose 35

Bacterial crown rot and

bacterial soft rot 35

Gray mold rot 36

Phytophthora rots

(foot and crown rots) 37

Stem spot (leaf spot) 37

References cited 38

Spinach 39

Bacterial soft rot 39

Downy mildew 40

White rust 41

References cited 43

Sweetpotatoes 44

Bacterial soft rot 44

Black rot 45

Blue mold rot 46

Charcoal rot 46

Dry rot 47

End rots and

surface rots 48

Java black rot 49

Rhizopus soft rot 50

Scurf 51

Soil rot (pox) 52

Chilling injury 53

Freezing injury 54

Internal breakdown 54

Internal cork 55

References cited 56

Color plates 59
Market Diseases of Beets, Chicory, Endive, Escarole, Globe

Artichokes, Lettuce, Rhubarb, Spinach, and Sweetpotatoes

H.E. Moline and W.J. Lipton^

Beets

Garden beets (Beta vulgaris L.) are grown for their fleshy roots and

succulent tops, which may be used as greens. Edible roots are a

rich red, of moderate size, smooth and sweet, and with a fine

texture.

The early crop is usually marketed bunched in a fresh, crisp condition with tops attached and can be stored at 0°C2 for 10-14 days.

The leaves and crowns are susceptible to decay by bacterial soft

rot pathogens. Cercospora leaf spot can disfigure leaves, although

table beets are somewhat resistant. Foliar diseases, such as curly

top virus or rust (caused by Uromyces betae Tul. ex Kickx), may

severely damage leaves, and the reduced leaf area may cause

dwarfing and poor root development. Growing roots are susceptible

to black rot, crown gall (caused by Agrobacterium tumefaciens (E.F.

Smith and Townsend) Conn), fusarium rot (caused by Fusarium

spp.), and root rot (caused by Pellicularia filamentosa (Pat.) Rogers).

Topped beets free from diseases and without mechanical wounds

may be stored for 3 to 5 months at high relative humidity (95 percent) without significant losses. Occasionally stored roots are affected by black rot, blue mold rot, fusarium rot, internal black spot,

or scab.

Bacterial Soft Rot

(Causal Organism Erwinia carotovora subsp.

carotovora (Jones) Bergey et al. and Other

Related Bacteria)

Bacterial soft rot is most often found on damaged leaves and

crowns of young beets shipped and marketed as whole plants and

on beet tops marketed as greens. The rot first appears on damaged

leaf tissues as darkened, water-soaked areas, which may increase

rapidly in size at moderate temperatures. Decay tissues become

soft and slimy, giving off a disagreeable odor. (See also Spinach,

Bacterial Soft Rot, p. 39.)

1 Respectively, research plant pathologist, Research Service, U.S. Department of

Horticultural Crops Quality Laboratory, Agriculture, Fresno, CA.

Beltsville Agricultural Research Center, 2For °F, see conversion table on

Beltsville, MD 20705; retired, Agricultural p. 86.
Black Rot

(Causal Organism Phoma betae Frank)

Black rot may affect garden beets, but usually it causes only slight

loss except in topped beets that have been held in storage. The

disease is seldom found on young bunched beets on the market.

Decayed beets in storage usually show black rot at the tip of the

root, although decay may occasionally occur at the crown and at

wound sites. Because beet roots are dark red, small lesions can

easily be overlooked. Decay may not be observed until the affected

tissues become black and slightly sunken or until a flat, grayishwhite surface mycelial mat becomes evident. Internally the decayed

areas are dark brown to black, with a sharp line of demarcation between healthy and diseased tissues (pi. 1). Initially invaded tissues

appear brown and water soaked, but as decay progresses, the

tissues turn black and somewhat granular, eventually becoming dry

and spongy. With advanced decay, cavities lined with mycelium

may be observed within the spongy areas of larger lesions.

Although considerable grayish white mycelium develops on the surface of older lesions under humid conditions, pycnidia have not

been found on decayed beets.

The temperature range for growth of the black rot fungus in culture

is from 2° to 35°C, with an optimum of 24°. Beets artificially inoculated through small wounds develop decayed areas about 5 mm

in diameter at 7° and 10 mm at 13° within 6 weeks; however, decay

develops more rapidly on old beets than on young ones.

Because the disease also attacks seeds and seedlings, affected

plants that survive at these stages of development and produce

marketable roots can also have the fungus in the crown, where it

may remain dormant for some time. However, in storage it often

becomes active, penetrating the roots and causing a dry black rot.

Natural infection may also occur through wounds or follow infection caused by diseases.

Currently there are no approved chemical control methods, but

close topping of roots and storage at 0°C with high relative humidity

and adequate air circulation will retard black rot development.

Reference: 2.
Cercospora Leaf Spot

(Causal Organism Cercospora beticola Sacc.)

Cercospora leaf spot is an important field disease of beets. It is

also of importance at the market, because it disfigures infected

leaves; then bacteria may enter through these infected areas and

cause soft rot.

Lesions appear as small spots with gray to brown centers and

reddish-purple margins on leaves and petioles. Spots may drop out,

leaving ragged leaves that may collapse and die. The causal fungus

can survive from season to season on plant debris in the field.

Removal of beet trash and deep plowing are recommended control

measures. Garden beet varieties are somewhat resistant to the

disease.

Reference: 5.

Scab

(Causal Organism Streptomyces scabies

(Thaxter) Waksman and Henrici)

Scab is rare on beets, but occasionally it causes considerable

damage in heavily infested soil. The pathogen is also the cause of

common scab of potatoes. Scab lesions on infected beet roots are

superficial, much like those found on potatoes except that they are

more protruding and rounded. Affected roots are usually so conspicuously blemished that they are seldom offered for sale on the

market (pi. 2).

Beets should not be grown in soil that has produced scabby

potatoes. Acid soil tends to repress scab incidence. Varietal

resistance occurs.

References: 1, 3.

Internal Black Spot

Internal black spot is a serious disorder of garden and canning

beets in many areas. On the market, it can be found on topped and

stored beets but seldom on young bunch beets. The disorder appears

when plants are grown in soils deficient in boron. Although it can
occur in many soil types, it is most often serious in alkaline soils,

where boron is unavailable to the plants.

Internal black spot appears as irregular patches of dark tissues

near the center of the root. The necrotic areas are hard but do not

dry; they form cavities or cause unusual shrinkage. Occasionally affected tissues near the root surface may crack allowing secondary

decay fungi to invade. Beets may be severely affected without

showing external root symptoms; however, foliar symptoms such as

twisted, bent, uneven development of leaf blades may be observed

on young plants grown in boron-deficient soils. Beets showing

these symptoms should be marketed early as bunched beets. The

disease can be prevented by applying borax to the soil. Local horticultural authorities should be consulted for recommended rates.

Resistant beet varieties are also available.

Reference: 6.

Freezing Injury

Beets begin to freeze between - 1.1° and - 1.7°C. Frozen root

tissue may appear slightly water soaked with areas of white tissue.

Severe freezing causes external and internal water-soaking and may

produce blackening of the ring and rays of conducting tissue. These

beets become soft and susceptible to decay.

Beets should not be held at temperatures below 0°C; if frozen, they

should be thawed slowly and used as soon as possible. If frozen

beets are warmed quickly to temperatures above 5°, decay may

spread rapidly.

Reference: 4.
References Cited

1. Adams, M.J., Lapwood, D.H., and Rankin, B.

1976 The growth of red beet and its infection by Streptomyces

spp. Plant Path. 25:147-151.

2. Bugbee, W.M.

1973 Resistance in Beta vulgaris to phoma storage rot in the

North Central Region. Plant Dis. Rptr. 57:204-207.

3. Lapwood, D.H., Adams, M.J., and Crisp, A.F.

1976 The susceptibility of red beet cultivars to streptomyces

scab. Plant Path. 25:31-33.

4. Parsons, C.S., and Day, R.H.

1970 Freezing injury of root crops, beets, carrots, parsnips,

radishes, and turnips. U.S. Dept. Agr. Mktg. Res. Rpt.

866, 24 p.

5. Schneider, C.L, Sims, R.L., and Potter, H.S.

1980 Test of fungicides for leafspot disease control.

Fungicide and nematicide test. Amer. Phytopath. Soc.

35:61.

6. Walker, J.C.

1939 Internal black spot of garden beet. Phytopathology

29:120-128.
Chicory, Endive, and Escarole

Chicory {Cichorium intybus L) has small, torpedo-shaped, leafy

heads, which are blanched during growth in the darl<. The production of blanched chicory is minimal in the United States; however,

chicory is imported primarily from Belgium to supply the fancy food

trade. This type sometimes is called "witloof," which is Flemish for

white leafed. Endive and escarole both belong to the species

Cichorium endivia L. Endive has narrow, curled, finely divided

leaves, whereas escarole consists of a loose head composed of

broad leaves that tend to be thick and somewhat twisted and wavy.

All these salad vegetables are subject to the same diseases that affect lettuce, including lettuce mosaic, aster yellows, sclerotinia rot,

and gray mold rot. Consequently, several of them are discussed

under lettuce, including marginal browning.

Bacterial Soft Rot

(Causal Organism Pseudomonas marginalis

(Brown) Stevens)

Bacterial soft rot caused by P. marginalis is the principal disease of

chicory imported from Europe and also occurs on endive and

escarole. The disease may start at the leaf tips or margins and then

progress toward the center of the leaves. The infection also may

originate on the broad midribs if they have been injured by abrasion

or bruising or at the base of the small head. Eventually the entire

head may become dark brown and slimy. (Pis. 3 and 4.)

Bacterial soft rot spreads most rapidly between 24° and 27°C but

also between 2° and 9°. The incidence of the disease likely can be

minimized by (1) cleaning knives and any containers used during

harvest, (2) washing the heads with chlorinated water, followed by

removal of excess moisture, (3) careful handling of the product to

prevent physical injury, (4) prompt cooling to 0°, and (5) shipping

and holding it as close to this temperature as feasible throughout

marketing.

Reference: 2.

Black Heart (Brown Heart)

Black heart (brown heart) affects endive and escarole. The disorder

is characterized by marginal blackening or browning of the heart

leaves (pi. 23). In severe cases, the entire head may be affected. The

problem is analogous to tipburn in head lettuce and has a similar
etiology; that is, it is related to calcium nutrition and the water

status of the plant during growth. In endive and escarole, the

disorder can be prevented by spraying the plants during growth

with a solution containing calcium. Since the marginal tissue is

dead, it is subject to bacterial soft rot if temperatures are not ideal

and free moisture is present. Therefore, the postharvest consequences of black heart can be reduced by keeping the heads as

close to 0°C as possible throughout marketing. Even though drying

conditions might reduce the hazard of decay following black heart,

they also would lead to highly undesirable wilting of the heads.

References: 3-5.

Irradiation Injury

Gamma irradiation at 25 krad or more induces the development of

reddish-brown spotting along the midribs of the leaves, sunken

discolored areas on the leaves, and a browning of the heart leaves

(pi. 5). Consequently, gamma irradiation cannot be used to control

postharvest decay in chicory, endive, and escarole.

Reference: 1.

References Cited

1. Bramlage, W.J., and Lipton, W.J.

1965 Gamma radiation of vegetables to extend market life.

U.S. Dept. Agr. Mktg. Res. Rpt. 703, 15 p.

2. Friedman, B.A.

1951 Pseudomonas marginalis as the cause of soft rot of imported witloof chicory. Phytopathology 41:880-888.

3. Maynard, D.N., Gersten, B., and Vernell, H.F.

1962 The cause and control of brown-heart of escarole.

Amer. Soc. Hort. Sei. Proc. 81:371-375.

4. Ryder, E.J.

1979 Leafy salad vegetables. 266 p. AVI Pub. Co., Westport,

CT.

5. Westgate, P.J., and Forbes, R.B.

1959 Blackheart of chicory [endive]. Fla. Soil and Crop Sei.

Soc. Proc. 19:389-393.
Globe Artichokes

Globe artichokes are the flower buds of Cynara scolymus L. The

edible part consists of the fleshy, tender base of the bracts (scales)

and the fleshy receptacle (flower base).

Gray Mold Rot

(Causal Organism Botrytis cinérea Pers.: Fr.)

The light-brown to ainnost black lesions of gray mold rot may appear

anywhere on the bud, and the gray web of mycelium sometimes is

evident (pi. 6). The lesions most frequently occur on wounds, such

as abraded surfaces, the cut end of the stem, or the split tips of the

bracts. Frost-blistered surfaces also may be sites of infection. Gray

mold rot in the field tends to be particularly troublesome during

long spells of cool, wet weather. Temperatures near freezing,

whether before or after harvest, slow the spread of the disease.

Under humid conditions, the lesions of gray mold rot are moist to

wet, odorless, and reddish brown or brown, and the borders are

definite and slightly water soaked. Under dry conditions, the advancing edge is not water soaked, and the affected tissues are dark

brown, dry, and firm. Internally, decayed tissue is brown and firm in

the bracts and stem but somewhat spongy in the less fibrous, edible

base. The symptoms of gray mold rot and bruising of the bracts

resemble each other, but decay tends to penetrate deeper than

bruises.

Gray mold rot, by far the most common market disease of globe

artichokes, is caused by B. cinérea. This fungus is found wherever

vegetables are grown. Under humid conditions, spores are produced

abundantly and are carried about by air currents. Any type of

wound facilitates infection, but the fungus can readily penetrate

moist, unbroken tissue. Tissue that is dead or dying, weakened by

freezing or mechanical injury, is more readily infected than vigorous,

healthy tissue. The pathogen thrives under a wide range of temperatures. Its spores can germinate, grow, and sporulate between -2°

and 32°C, with an optimum of about 24° to 25°. The fungus grows

very little at -2° to 0° but still sufficiently to become established

in the plant tissue. Consequently, the artichokes must be kept at

desirably low temperatures throughout their marketing period.

Losses from gray mold rot can be minimized by rapidly cooling the

buds to below 5°C after harvest and by maintaining them between

8
0° and 3° throughout marketing. However, gentle handling is still

important, because physical injuries can accelerate decay development as much as a 10° rise in temperature. Optimum refrigeration

is particularly important for prepackaged artichokes, because the

humid conditions inside the packages are ideal for the rapid development of decay when temperatures are above about 5°.

References: 2-4.

Freezing Injury

Artichoke bracts freeze at -1.4° to -1.1°C. The freezing point of

the fleshy receptacle has not been reported. In slightly frozen buds,

the epidermis of the bracts becomes detached and forms whitish to

light-tan blisters. When the blisters are broken, the underlying

tissue turns brown. Light freezing detracts from the appearance of

artichokes, but it does not impair their edibility in any way. Severely

frozen buds turn black and become unsalable. Such severe freezing

discolors internal tissues near the juncture of the bud and stem

before other parts of the fleshy receptacle are affected (pi. 7).

The visible damage from freezing can be minimized by gentle handling

to prevent opening of the blisters and thus darkening of the exposed

tissue. To ascertain whether freezing has been severe, cut the buds

lengthwise to check whether or not the inner tissue is brown.

Reference: 5.

Irradiation Injury

Gamma irradiation of globe artichokes at doses of 100 krad or

above induces the development of dark sunken pits on the stem, a

brown scuffed-appearing discoloration on the outer bracts, reddishbrown spots on the inner bracts, and a blackening at the bases of

the florets (pi. 8). The internal discolorations develop at lower doses

than the externally visible injuries. Decay of globe artichokes cannot be controlled by gamma irradiation.

Reference: 1.

Mechanical Injury

During handling and packing, the surfaces of the artichoke bracts
are commonly abraded and the tips often are split. The abraded surface commonly turns an unsightly brown or black. Consequently,

rubbing the buds against each other or against the sides of the

container should be minimized. Splitting of the bract tips is serious

only when the buds are held under conditions favoring decay,

because decay frequently starts at the injured tips.

References: 3, 4.

Wilting

Wilted buds are soft and pliable. Severe wilting, induced by loss of

water, causes the bracts to fold inward and thus gives the entire

bud an unsightly appearance.

Normal washing or hydrocooling followed by prompt cooling to

below 5°C and by marketing at between 0° and 5° minimizes the

problem of wilting. Packaging in perforated plastic bags or box

liners also reduces the chance of wilting. However, the buds should

be cooled to the desirable temperature range before they are placed

in the bags or liners, and they must be maintained in that range or

else decay can become serious.

Reference: 4.

10
References Cited

1. Bramlage, W.J., and Lipton, W.J.

1965 Gamma radiation of vegetables to extend market life.

U.S. Dept. Agr. Mktg. Res. Rpt. 703, 15 p.

2. Link, G.K.K., Ramsey, G.B., and Bailey, A.A.

1924 Botrytis rot of the globe artichoke. J. Agr. Res.

21:85-92.

3. Lipton, W.J., and Harvey, J.M.

1960 Decay of artichoke bracts inoculated with spores of

Botrytis cinérea Fr. at various constant temperatures.

Plant Dis. Rptr. 44:837-839.

4. Lipton, W.J., and Stewart, J.K.

1963 Effects of precooling on the market quality of globe artichokes. U.S. Dept. Agr. Mktg. Res. Rpt. 633, 18 p.

5. Ryall, A.L, and Lipton, W.J.

1979 Handling, transportation and storage of fruits and vegetables. V. 1, Vegetables and melons. Ed. 2, 587 p.

AVI Pub. Co., Westport, CT.

11
Lettuce, Whole

All lettuce on the market, whether the head or leaf type, belongs to

the species Lactuca sativa L The major types are the crisphead,

butterhead, romaine or cos, and loose leaf or bunching. Most of the

information about the various diseases and disorders is based on

research with the crisphead type. However, all these types are subject to the bacterial and fungal diseases discussed here. In contrast, the physiological disorders do not affect all types of lettuce

similarly, and for most of the leafy types, relatively little information

is available.

Since all parts of a head, except the small stem, are edible, any

defect detracts from the market quality of the head. In crisphead

lettuce that is shipped with the outer, less desirable wrapper leaves

attached, the latter are often a source of decay that can spread to

the inner, desirable leaves. Removal of the outer wrapper leaves

tends to reduce the incidence of decay in the edible part during prolonged marketing periods. Thus, packaging of individual wrapped

heads limits spread of diseases. Particular care should be taken

with naked-packed lettuce to exclude heads that show any decay at

harvest. Careful handling and prompt cooling to and shipment between 0° and 2.5°C also limit the development of various diseases

that affect lettuce.

References: 8, 20.

Bacterial Soft Rot

(Causal Organisms Pseudomonas and

Erwinia spp.)

Bacterial soft rot is the most serious market disease of lettuce. Affected tissue is various shades of green and brown or tan. As the

decay develops, the tissue becomes soft and slimy when the

humidity is high or free moisture is present. Under dry conditions,

the affected areas may dry up and become papery in texture or fall

out and thus produce a shothole effect. In severe cases, the entire

head may become a slimy mass, but the disease does not cause a

putrid odor. (Pis. 9 and 10.)

The exúdate of bacterial soft rot from rotted tissue in contact with

underlying leaves sometimes produces spotting symptoms, which

resemble those of russet spotting. The ability to distinguish between these two defects may occasionally be important; this task is

aided by the following table.

12
Table 1

Characteristics and origin of spotting for bacterial

soft rot and russet spotting on lettuce

Origin of spotting

Characteristic Bacterial soft rot Russet spotting

Color Tan to brown; water soaked.

Shape Mostly elongated.

Surface Glistening; mostly sunken;

epidernnis always affected.

Other Frequently associated with

internally discolored veins;

obviously decayed tissue

often near or on adjacent

leaf.

Tan, brown, olive.

Irregular.

Dull, not always

sunken; epidermis

not always affected.

Veins not discolored

internally; not

associated with

decayed tissue.

Bacterial soft rot may be induced by one or more of the following

bacteria: Pseudomonas marginalis (Brown) Stevens, P. cichorii

(Swingle) Stapp, or Erwinia carotovora (Jones) Bergey et al.; other

bacteria occasionally may be involved.

Bacterial soft rot may occur on any type of lettuce, endive, and

related crops from any growing region. The bacteria attack tissue

that previously has been invaded by fungi, that has been injured in

some other manner such as mechanically, or that is dead. When lettuce is held above 3°C and moisture is available to the organism,

bacterial soft rot likely will affect lettuce that has been damaged

mechanically, that has been injured by exposure to freezing temperatures, or that exhibits some physiological disorders.

Bacterial soft rot develops at any temperature at which lettuce is

normally held. However, its development is much slower between 0°

and 3°C than at higher temperatures; it is very rapid above about 15°.

The incidence of this decay can be minimized by careful handling

of lettuce, by rapid precooling, and by maintaining it between 0°

and 2.5°C throughout marketing. Additionally, lettuce infected with

any type of disease should not be shipped, especially for long

distances.

References: 2, 3, 5, 6, 16, 18.

13
Downy Mildew

(Causal Organism Bremia lactucae Reg.)

In early stages of this infection in the field, light-green to yellowish

areas appear on the upper (adaxial) surface of wrapper leaves of

crisphead lettuce. Under conditions of high humidity and moderate

temperatures, a fuzzy, whitish-gray mold develops on the lower

(abaxial) surface of infected areas. The lesions eventually can turn

brown, may enlarge and coalesce, and may turn soft and slimy as

secondary decays enter. Generally, the lesions are well defined by

the veins. If transit temperatures are higher than desirable, downy

mildew may spread from the wrapper leaves to the outer head

leaves. (PI. 11.)

On butterhead lettuce, the lesions are less well defined by the veins

than on crisphead lettuce, but otherwise the symptoms are similar.

Downy mildew is induced by the fungus ß. lactucae, of which many

virulent forms have been identified. The fungus can be highly

destructive in lettuce grown under cool, moist conditions. In the

United States, it can be particularly serious in the lower Rio Grande

Valley of Texas in midwinter and in the Santa Maria Valley of

California in spring and fall. The disease has been found in all

lettuce-producing areas of the United States and Europe, but it is a

relatively minor problem on the market.

The most efficient means of controlling downy mildew is to select

resistant cultivars. Systemic fungicides also effectively control the

disease. However, any fungicide used on lettuce for this purpose

must be approved by the appropriate regulatory agencies. The postharvest development and intensification of the disease can best be

controlled by removing the wrapper leaves before the lettuce is

packed and shipped. This is particularly important for lettuce harvested in fields in which downy mildew is widespread. Additionally,

rapid precooling to the desired temperature and holding the lettuce

between 0° and 2.5°C throughout marketing will minimize the

adverse inpact of this disease.

References: 1, 5, 18, 21.

14
Gray Mold Rot

(Causal Organism Botrytis cinérea Pers.: Fr.)

Tissue affected by gray mold rot appears water soaked, grayish

green or brown, and, in severe cases, soft and slimy. At these later

stages of development, the characteristic smoky-gray mycelium and

spore masses frequently appear (pi. 12). Gray mold rot is incited by

the fungus B. cinérea. The fungus has a wide temperature tolerance

and grows between about -2° and 32°C, although it develops most

rapidly between about 20° and 25°. Gray mold rot is one of the two

most important diseases of head lettuce on the New York market;

however, it can develop wherever lettuce is grown, whether in fields

or in greenhouses. The disease also affects chicory, endive, and

similar salad vegetables.

There is no absolute control for gray mold rot because the fungus

grows under such a wide temperature range. However, its destructiveness can be minimized by rapidly precooling the lettuce and

holding it between 0° and 2.5°C during marketing. Development of

resistant cultivars holds some promise. Minimizing any type of

physical injury to the lettuce is an important control measure, since

spores of Botrytis do infect undamaged lettuce leaves only if a food

source, such as juice from wounded tissue, is available on the surface.

References: 7, 6, 18, 21.

Watery Soft Rot (Sclerotinia Rot)

(Causal Organisms Sclerotinia sclerotiorum

(Lib.) DBy. and S. /??/nor dagger)

Tissue affected by watery soft rot is water soaked and light or

pinkish brown. In severe cases, a white cottony mold may develop

on the surface of the leaves. Although watery soft rot may start

anywhere on a head, infection most commonly occurs near the soil

level. Severely affected heads are a wet leaking mass of tissue (pis.

13 and 14). On the market, it would be unlikely, but not impossible,

to see the black resting bodies of the fungus, called sclerotia.

Sclerotinia sclerotiorum is responsible for inducing much of the

15
watery soft rot seen on nnarkets, but S. minor is most common in

the Salinas Valley, CA.

Even though sclerotinia rot is found in all lettuce-growing areas, it

is of relatively minor importance on the market. Apparently either

affected plants are not harvestable, or if a mild field infection

exists, the lettuce would not be shipped long distances. Losses

from sclerotinia rot are most common under cool, moist conditions,

particularly when the soil surface is wet. Infection may occur between 0° and about 27°C, with an optimum of about 18° to 21°. The

presence of wounded tissue enhances the chance for infection, but

it is not an essential precondition.

Preventing infected lettuce from entering the marketing channels is

the best method to avoid major postharvest losses from sclerotinia

rot. Consequently, outer wrapper leaves should be carefully trimmed

before the lettuce is packed. Prompt and thorough precooling and

shipment between 0° and 2.5°C will slow development of the

disease. Maintaining a dry soil surface as far as possible, treating

the soil with fungicides, and developing resistant cultivars are ways

to combat sclerotinia rot before harvest.

References: 1, 5, 18, 21.

Viruses and Related Diseases

Aster Yellows

Lettuce heads seriously affected with aster yellows likely would not

reach the market, because the plants are stunted and thus would

not be harvested. If affected heads do reach the market, they would

be soft and have curled heart leaves. They may exude latex, which

then would dry and leave a pink to tan residue on the leaves.

Aster yellows is believed to be caused by a mycoplasma. The disease tends to be much more common in lettuce grown in the

Eastern and Midwestern United States than in the Far West. Since

the disease is spread by leafhoppers, eradication of these insects

and the weeds that harbor the infectious agent is the only means of

control.

Reference: 22.

16
Big Vein

Big vein is characterized by a translucent clearing of the veins,

which makes them appear unusually large and prominent. The

leaves of affected heads tend to be crinkly, stiff, and thicker than in

normal heads. Overall, the heads appear coarse; however, the disease does not seem to reduce the storage life of affected lettuce.

The agent that induces big vein is not known, but it appears to be a

viruslike entity introduced into the plant through the roots by a soil

fungus, Olpidium brassicae (Wor.) Dang. Symptoms of big vein are

most serious when the air temperature during growth is about 14°C.

Root temperature of 14° to 24° has no effect on the development of

the symptoms. When the air temperature is above 24°, almost no

symptoms develop, regardless of the root temperature. Consequently,

big vein is most commonly seen during cold, wet weather and in lettuce grown in heavy soils. It can occur in all lettuce-growing areas

of the United States and also has been reported from other

continents.

Planting of resistant cultivars provides the best control of lettuce

big vein. Fumigation of the soil and careful control of irrigation help

to reduce its incidence, but these procedures are not always commercially feasible.

References: 20, 21.

Lettuce Mosaic

This disease derives its name from the green and yellow mottling

that is noticeable in leaves of young plants. In mature heads that

normally would be on the market, the mottling usually is absent or

indistinct; if it is present, it would be most noticeable near the

margins. Even if the mottling is not evident, heads with lettuce

mosaic tend to be dull green to slightly yellow, and the marginal

serrations are apt to be particularly prominent in relatively young

leaves.

Lettuce mosaic is caused by a seed-transmitted virus that is spread

by the green peach aphid (Myzus persicae Sulzer). The means of

transmission allows for highly effective control measures if used.

They are (1) indexing of seed lots to contain 0 infested seeds in

30,000, (2) use of resistant cultivars, (3) prompt cultivation of aban17
doned fields, and (4) control of weeds on the periphery of fields,

since many weeds are hosts to the virus. There Is no clear evidence

that lettuce affected by nnosaic deteriorates more rapidly than

healthy lettuce, except in the cultivar 'Climax' and close relatives

(see Internal Rib Necrosis and Rusty-Brown Discoloration).

However, since the virus generally reduces the vigor of all lettuce, it

is reasonable to assume that virus-infested heads would have a

shorter market life than normal ones.

References: 1, 21.

Internal Rib Necrosis. Internal rib necrosis is characterized by a

diffuse, dark, gray-green or occasionally coal-black discoloration. It

usually starts at or near the junction of the leaf with the stem, but

it can extend several centimeters along the midrib. Since the

lesions are mostly subsurface, they have a diffuse appearance, but

they may reach the surface in severe cases. Most commonly the

cap leaves, outer head leaves, and some of the small inner head

leaves are affected; the wrapper leaves are affected only occasionally (pi. 15).

Internal rib necrosis is induced in the cultivar 'Climax' by lettuce

mosaic virus alone. However, in the presence of lettuce mosaic and

beet western yellow viruses, the cultivars 'Vanguard' and 'Vanmax,'

which are related to Climax, also may be affected. The symptoms

may be present at harvest, but the incidence and severity increase

thereafter at temperatures recommended for lettuce, but the lesions

tend to become darker as temperatures increase.

Internal rib necrosis may be controlled by replacing the susceptible

cultivars with resistant ones. Until this occurs, strict observance of

the seed indexing program for the control of lettuce mosaic will

provide the only sure control.

The discoloration associated with internal rib necrosis resembles

old lesions of rib discoloration, but the latter are mostly near the

middle of the midrib rather than near its base.

References: 16, 20, 21.

Rusty-Brown Discoloration. The name of this disease aptly

describes the symptoms. The discoloration may affect only the midribs, but entire leaves and heads may be affected. The discoloration

18
tends to follow the veins, but it is not confined to them. In sonne

cases, the veins may be normal and interveinal tissue discolored.

Generally only leaves in the outer half of the head are affected,

although the entire head may be involved. Only the epidermis and

one to four adjacent cell layers are normally affected, but distinct

sunken lesions may be present in severe cases. (PI. 16.)

Rusty-brown discoloration originates before harvest, but it can

become much worse thereafter, particularly at temperatures recommended for lettuce. The disease is confined to the cultivar 'Climax'

and is induced by infection of the heads with lettuce mosaic virus.

The symptoms are more intense in carbon dioxide-enriched atmosphere than in air.

Rusty-brown discoloration can be prevented by growing resistant

cultivars or by planting disease-free seeds.

References: 11, 16, 20, 21.

Disorders and Physical Injuries

Brown Stain and Heart Leaf Injury

Excess carbon dioxide (CO2) in the atmosphere surrounding lettuce

can result in several distinctly different symptoms of injury. The

principal ones are brown stain and heart leaf injury.

Typical lesions of brown stain measure about 0.5 by 1.0 cm and have

distinct margins that often are darker than the slightly sunken

centers. This appearance produces a halo effect. Brown stain occurs

on both leaf surfaces, often on or near the midrib and toward the

base of the leaf. Lesions most commonly develop on the head

leaves just under the cap leaf, but they also may develop on leaves

deeper in the head. The heart and wrapper leaves are not affected

by brown stain. Young lesions may show no discoloration, but as

they age they become tan, brown, or even black. When CO2 injury is

severe, these lesions may coalesce and thus be several centimeters

in length (pi. 17).

Brown stain lesions do not always have the characteristic halo appearance. They occasionally resemble russet spotting (see p. 28).

When uncertainty exists, examination of the heart leaves may

resolve the problem. Although the heart leaves show neither type of

19
spotting, they may have reddish-brown margins or be completely

discolored when excess CO2 causes the injury. Also, examination of

several heads may help resolve the issue, because some may show

the typical brown stain symptoms.

Brown stain is aggravated when lettuce is simultaneously exposed

to relatively high concentrations of CO2 and low concentrations of

oxygen (O2) (10 percent or below). Similarly, the addition of carbon

monoxide (CO) at about 1 to 1.5 percent also aggravates the development of CO2 injury. If high levels of CO2 and CO and low levels of

O2 coincide, the damage is most severe.

Injury from CO2 can be prevented readily by assuring that gas exchange in a transport vehicle or storage room is sufficient to prevent accumulation of CO2 produced during respiration. In situations

where gas exchange must be restricted, such as when controlled

atmospheres are deliberately used for the transit of lettuce, some

respiratory CO2 can be absorbed by placing hydrated lime in the

loading compartment.

Low O2 levels also can injure heart leaves, but in this case the discoloration is more brown than red and the injury commonly is accompanied by a flat, sweet flavor. Rusty-brown discoloration

resembles the broad discolored areas sometimes induced by high

levels of CO2 along the midribs; however, brown stain extends along

the veins into the leaf blades, whereas CO2 injury does not follow

the veins.

Lettuce differs substantially in its susceptibility to brown stain.

Cultivar and area of production seem to be the main factors involved.

Lettuce produced in the coastal valleys of California is more

susceptible than lettuce grown either in the desert areas of California or Arizona or in the San Joaquín Valley, CA. The basis for this

difference is unknown.

References: 13, 16, 20.

Heart leaf injury is sometimes the first and only symptom of CO2 injury. Heart leaves so injured have reddish-orange margins in mild

cases; in severe cases, the entire leaflet may be discolored. Heart

leaf injury is more frequent and more severe in soft than in firm or

hard heads. In soft heads, the margins of the inner, green leaves

20
also may be discolored, but they usually become grayish rather

than than rpdrlifih reddish orange. nrannfi.

Occasionally, excess CO2 induces a yellowish to reddish-brown discoloration of midribs and adjacent tissue. The dead areas lack a

definite margin and tend to extend along the main curvature of the

midribs so that they appear to be corrugated. However, this symptom by itself is not sufficient evidence that the lettuce has been injured by excess CO2. If lettuce is exposed to 20 percent CO2, a level

not normally reached during marketing, the leaves will become

water soaked and droplets of exúdate will appear on the surface.

(PI. 18.)

The various symptoms of CO2 injury can be induced by exposing

head lettuce to more than 1 percent CO2 while being held in the

temperature range most desirable for it. The longer the storage

period, the more likely the injury will occur. However, the injury

tends to be less severe at temperatures above the range that is

otherwise ideal for lettuce. The injury may be relatively mild when

the lettuce is removed from an atmosphere that contains undesirably high levels of CO2, but it may become very prominent after a

few days in air at about 10°C.

Butt Discoloration

The milky latex that leaks from the cut stem of a lettuce head

gradually turns pink and eventually reddish brown or brown. This

butt discoloration is due to the oxidation of the latex on exposure

to air and occurs rapidly at high and slowly at low temperatures.

However, other unknown factors affect the rate at which the discoloration occurs. In some heads the discoloration occurs within 1

week at temperatures recommended for lettuce, and in others the

same degree of discoloration may require 2 weeks. Consequently,

this symptom is not a reliable indication of lettuce freshness.

The postharvest darkening of the cut stem can be slowed by holding the lettuce in an atmosphere that contains 0.5 percent oxygen

(O2) or 1 to 3 percent carbon monoxide (CO). However, both of these

modifications are effective only while the lettuce is held under

these conditions. Once it is removed from the modified atmosphere,

discoloration proceeds fairly rapidly. Since 0.5 percent O2 approaches the lower limit of safe O2 concentration for lettuce and

21
since the use of CO is inherently dangerous to people, the use of

these procedures cannot be justified on an objective basis to control butt discoloration.

Reference: 20.

Freezing Injury (Blistering)

The separation of the epidermis from the underlying tissue, which

gives frozen tissue its characteristic blistered appearance, is the

most common sign of freezing in lettuce. When the skin is still intact, the Injury gives the lettuce a silvery appearance. Once the skin

is ruptured in any way, the dead cells below will turn tan; if the

cells are protected from drying, they tend to remain white. The

blistering may involve only a small area of a leaf or it may encompass the entire surface depending on the severity of the exposure

to freezing temperatures (pi. 19). Severely frozen lettuce is water

soaked and mushy once the tissue has been thawed (pi. 20). Lettuce freezes at -0.4° to -0.8°C. Freezing may occur in the field or

during vacuum cooling, transit, or storage.

The stage at which the freezing occurred can be determined with

reasonable confidence when a load of lettuce arrives with freezing

damage. Field freezing tends to be randomly distributed through

the load, and the blistering of the outer leaves is the main symptom. In-transit freezing tends to be confined to or is more severe

along the periphery of a box of lettuce or of the load, and the

leaves are typically water soaked and flaccid. If freezing occurred

during vacuum cooling, only a part of the load likely would show

the damage, because generally only part of a transport vehicle is

loaded from a given vacuum-cooling run. However, freezing during

vacuum cooling is very rare because of safeguards built into the

equipment.

Damage from freezing exposures in the field can be minimized if

harvest is delayed until after the lettuce has completely thawed.

Lettuce that froze in transit should be thawed at about 5°C before

being handled in any way, because frozen tissue is highly sensitive

to mechanical Injury. Such thawed lettuce will be marketable if

freezing was slight. Lettuce that has sustained freezing damage

must be held as close to 0° as feasible and utilized promptly,

because the damaged tissue tends to be susceptible to decay at

undesirably high temperatures.

22
Prevention of postharvest freezing depends on the operator of the

cooling, transit, and storage equipment, as the case may be.

Careful calibration, adjustment, and maintenance of thermostats

and other refrigeration-related equipment are essential.

References: 13, 16, 20.

Irradiation Damage

Gamma irradiation at a dose of 25 krad induces a reddish-brown

spotting, especially along the midribs. Similar spots often are

sunken when they occur on the leaves (pi. 21). The severity of the injury increases substantially as the dose is increased to 200 krad. Irradiation also tends to increase the severity of pink rib, especially

at the higher doses. Consequently, irradiation is not suitable for

controlling decay or insects in harvested lettuce.

Reference: 4.

Marginal Senescence (Marginal Browning)

Marginal senescence is characterized by a cream to brown discoloration of the margins of wrapper and outer head leaves (pis. 22 and

23). Initially the discolored parts are pliable, but unless humidity is

very high, they tend to dry out and become papery. The margins of

the discoloration are not always clearly defined. The etiology of the

development of marginal browning has not been thoroughly investigated. Stress induced in the leaves by excessive accumulation

of salts in the soil is one possible cause. Aging of the tissue also

may be a contributory factor because the disorder tends to be most

common in hard heads.

Since causes of marginal senescence have not been clearly identified, recommendations for control are lacking. The problem of

serious decay following marginal senescence can be minimized if

affected lettuce is precooled rapidly and then held at desirable

temperatures (0° to 2.5°C) throughout marketing. Lots of lettuce in

which marginal senescence is widespread should not be shipped.

Marginal senescence sometimes resembles symptoms of tipburn.

However, tipburn also occurs on inner head leaves, and its lesions

often show a dark-brown flecking, which is absent in marginal

senescence.

23
Reference: 7; J.K. Stewart, pers. commun.

Mechanical Injury

Mechanical injury of lettuce can take many forms. The heads or

leaves may be crushed, bruised, or torn; the surface may be abraded;

and occasionally the head leaves of crisphead lettuce may be

separated from the stem.

Mechanical injury can be caused by a multiplicity of procedures to

which the lettuce is exposed during marketing. During harvest, the

lettuce may be cut so high that some of the head leaves are detached

from the stem; therefore the cut surface of the leaves often

becomes discolored. The same type of damage may occur during

trimming of the lettuce immediately after harvest. Leaves may be

crushed during packaging or closing of the boxes, during their

loading into the transit vehicle, from overhead weight during transit,

or as a result of a sudden impact. Similar hazards exist during

handling of the boxes at wholesale and retail levels. Thus, lettuce

can be mechanically damaged anytime between harvest and

consumption.

Mechanical damage likely cannot be completely prevented. However, certain practices will minimize the problem: (1) Use containers

that will accommodate the lettuce without undue force being applied during packing and closing of the boxes; (2) use strong

enough boxes to protect the lettuce; they should not just surround

the lettuce; (3) institute handling methods that will minimize the

chance of rough treatment; and (4) provide sufficient and properly

trained supervisors. Injured tissue is particularly susceptible to invasion by decay organisms. Consequently, mechanical injury should

be prevented whenever feasible, but the virtual inevitability of some

mechanical injury occurring emphasizes the importance of maintaining the lettuce at desirable low temperatures (0° to 2.5°C)

throughout marketing.

The pink discoloration of bruised rib tissue sometimes resembles

pink rib. However, mechanical injury and pink rib can be readily

distinguished, because in the latter there is no evidence of external

physical damage.

References: 2, 9, 10, 20.

24
Oxygen Deficiency

Reddisii-brown discolored heart leaves sometimes are the first visible and only symptom of low oxygen (O2) injury. The wrapper and

cap leaves may have shiny to water-soaked, gray, dead patches

even when dead heart leaves are not obviously present. Young head

leaves may have shallow, reddish-brown spots on the midribs,

usually on the inner (adaxial) surface. A sweet odor and flat sweet

flavor also are characteristic of low O2 injury, but they disappear

almost completely within 3 days or less after the lettuce is removed

into the air. In contrast, the visible injuries become more intense

after removal of the lettuce to air. (PI. 24.)

The discoloration of the heart leaves can be induced when lettuce

is held at 10°C with less than 0.5 percent O2. Other injuries appear

only when the O2 level drops to 0.25 percent or less. These symptoms can develop within 1 week at temperatures recommended for

postharvest handling of lettuce.

Dangerously low O2 concentrations are unlikely to be reached during transport or storage of lettuce in normal atmospheres. However,

if lettuce is shipped or stored in controlled atmospheres, O2 deficiency must be prevented by careful monitoring of the O2 concentration throughout the period. If the atmosphere is adjusted only initially, leakage from outside must be adequate to maintain an O2

level of 1 percent or higher.

Low O2 injury of the heart leaves resembles that induced by high

CO2, but with high CO2 the injury is more red than brown and the

sweet and flat off-flavor is absent.

References: 16, 20.

Ozone Damage

Concentrations of ozone below 0.1 ppm maintained for 2 days or

longer at 2.5°C induce slight to moderate yellowing or browning of

areas between the veins. However, these mild symptoms are visible

only after about the fifth day following removal of the lettuce from

the ozone-enriched atmosphere. Exposure to 0.1 ppm ozone results

in brown flecking of midribs and yellowing of the interveinal tissue

after 3 days. Severe water-soaking follows exposure of lettuce to

25
ozone at 0.3 ppm or higher concentrations for 2 or more days. If the

exposure continues for 4 days, the tissue will appear transparent.

(PI. 25.)

Ozone gas, which is occasionally touted as a useful inhibitor of

decay in fresh produce, causes the injury; ozone, as contained in

snnog and applied to young lettuce plants, induces similar symptoms. The obvious solution is to prevent lettuce from coming into

contact with ozone at any concentration.

References: 19, 23.

Pink Rib

The name exactly describes the symptoms of the disorder. Three

types of pink rib have been described. The most common is a diffuse pink rib around air spaces commonly found in large midribs.

Although the pinkiness is mostly associated with the area around

air spaces, pink walls also may occur in groups of cells distant

from air spaces. This type of pink rib usually is confined to the

thick basal part of midribs in large head leaves and is more intense

on the inner (adaxial) than on the outer (abaxial) side of the rib. The

second type of pink rib involves the tubules that contain the milky,

white latex found in lettuce. In this type, the discoloration is due to

the oxidation of latex that has escaped from the laticifers (tubules

that contain the normally milky white latex). In the third type, xylar

pink rib, only the walls of some water-conducting elements are

pink. They may be scattered singly or in groups among normal

cells. Diffuse pink rib may occur by itself, but it always accompanies the other two types. (PI. 26.)

No specific cause of pink rib has been identified, but some preharvest and postharvest factors affect its incidence and severity.

Since diffuse pink rib frequently is associated with disrupted cells,

it is reasonable to assume that rapid growth affects eventual development. Also, senescence seems to be involved, because pink rib is

much more common on hard than firm heads and can occur on the

former at time of harvest. Also, higher than desirable postharvest

temperatures encourage the intensification of pink rib. Diffuse pink

rib is not aggravated by exposure to ethylene, and no microorganism has been implicated in its occurrence. Xylar pink rib may

possibly be associated with infection by a bacterium.

26
The incidence and severity of pink rib can be minimized by (1)

avoiding harvest of hard heads, (2) prompt precooling to between 0°

and 2.5°C, (3) maintaining the lettuce at that temperature range

throughout marketing, and (4) possibly instituting cultural practices

to mimmize excessively rapid growth. Holding lettuce in atmospheres with reduced O2 levels does not prevent pink rib occurrence; on the contrary, O2 at about 2 percent tends to aggravate its

development during 1 week at 10° or 1 month at 2.5° or 5°.

References: 16, 20.

Rib Discoloration (Brown Rib)

Circumstantial evidence strongly suggests that rib discoloration

and brown rib are the same disorder; consequently, they will be

treated as such.

The yellow to black lesions characteristic of rib discoloration

generally develop on the inner surfaces of the midrib and of secondary ribs of cap leaves and in one or more of the next four or five

younger leaves. Older or younger leaves sometimes also are affected. The discoloration commonly is found at the maximum curvature of the midrib and below the surface, although the epidermis

also can be involved. Sometimes a subsurface cavity is associated

with rib discoloration. The lesions tend to darken during storage; in

severe cases, they may be sunken and occasionally cracked. They

frequently become a focus for decay development during prolonged

storage. (PI. 27.)

Rib discoloration develops before harvest, and no specific cause

has been identified. The disorder appears more commonly when

day temperatures exceed about 27°C or when nights are between

13° and 18° than during cooler periods. This finding suggests that

conditions favoring rapid growth also tend to favor the development

of rib discoloration. Additionally, hard to overmature heads more

frequently are affected than less mature ones.

Lettuce from any growing region may develop rib discoloration, particularly during hot weather. Since rib discoloration appears before

harvest, postharvest control is not feasible. However, some measures

can be taken before or after harvest to reduce its incidence and

minimize its adverse effect on quality. Cooling the lettuce with

27
sprinklers during hot weather tends to reduce the Incidence of the

disorder. Avoiding harvest of hard or overmature heads serves the

same end. If rib discoloration is present, cooling the lettuce rapidly

and keeping it at 0° to 2.5°C will reduce the chance of decay

developing in the lesions. Resistant cultivars likely are the best

long-range solution to the problem.

Rib discoloration and pink rib superficially resemble each other, but

cross sections of the latter are decidedly pink, whereas those of

mild rib discoloration are yellowish to tan.

References: 12, 16, 20.

Russet Spotting

The name for the disorder is based on the small, tan, brown, or

olive spots that are randomly distributed over the affected leaf. In

severe cases, the spots may coalesce and form irregularly shaped

discolored areas. Spots on the midribs usually are longitudinal and

pitlike, but those on the blade appear shallow, roundish, and somewhat diffuse. They may be on the veins, between the veins, or both.

The spots tend to appear first on the outer (abaxial) side of midribs

of outer head leaves, where they also tend to be most severe.

Russet spotting affects leaf blades in advanced stages of the disorder. The spots generally are depressions of the surface, but internal spots may appear on the midribs and large veins. These subsurface lesions appear dark and diffuse as viewed through the outer

healthy cell layers. Even though russet spotting generally progresses Inward, absence of spots on outer head leaves does not

preclude their presence on inner leaves. Wrapper leaves seem to

escape this disorder. (Pis. 28 and 29.)

Russet spotting sometimes resembles the symptoms of carbon

dioxide excess and early infections of bacterial soft rot. The distinctions are clarifed in the relevant sections.

Exposure of lettuce to ethylene is the major cause of russet spotting.

Concentrations as low as 0.1 ppm can induce the disorder in some

lots of lettuce. The rate and severity at which the disorder develops

increase as the concentration increases up to about 10 ppm.

Russet spotting can develop even when ethylene is not introduced

Into the atmosphere that surrounds the lettuce. The following condi28
tions favor development of the disorder: (1) Shipping heads that are

hard and old at harvest; (2) holding lettuce with decayed produce;

(3) storage temperatures between about 3° and 5°C; (4) storage that

exceeds 10 days; (5) field temperatures that exceed 30° for 2 or

more consecutive days at about 9 to 14 days before normal harvest;

and (6) possibly, infection of the head with lettuce mosaic during

develGipJTient.

Russet spotting occurs sporadically in lettuce from any source,

although cultivars differ in their susceptibility to the disorder.

Russet spotting can be prevented, or at least minimized, by observing the following precautions: (1) Keep lettuce in surroundings that

are essentially free of ethylene. This means that lettuce should not

be stored or shipped with any products, such as apples, pears or

plums, or melons, that emit substantial amounts of ethylene at

temperatures recommended for lettuce. Additionally, electricpowered vehicles should be used in confined spaces, since engines

powered by fossil fuels emit ethylene during operation. (2) Avoid

shipping hard lettuce. (3) Avoid holding decayed or sound lettuce

with other produce that shows substantial decay. (4) Maintain

temperatures between 0° and 2.5°C. (5) Ship or store lettuce with a

high potential for developing russet spotting In an atmosphere that

contains 2 to 6 percent oxygen Instead of the normal 21 percent.

References: 15, 16, 20.

Solar Injury

Head lettuce grown In areas with high levels of solar radiation may

show thin papery or bronze areas on the surface most directly exposed to the direct rays of the sun. Most commonly, it is the cap

leaf that shows the damage. The next underlying leaf may have only

a small yellow-tan spot on the outer surface immediately below the

more severely Injured leaf.

Solar injury likely will develop only during clear weather when

mature or nearly mature heads are exposed to air temperatures of

about 24°C or higher. Under those circumstances, parts of the head

facing south or southwest may exceed 38°. Since the defect ¡s induced in the field, nothing can be done to alleviate It after harvest.

Refer^ce: 20.

29
Tipburn

Tipburn is characterized by light-tan to dark-brown margins of

leaves. The dead margins may be very narrow or extend inward 3

cm or more in severe cases. The lesions may be confined or they

may encompass a major part of the margin. Tipburn usually is

found on head leaves of various ages, but the outer leaves that

have not yet folded over in crisphead types of lettuce occasionally

may be affected. The youngest leaves in the center of a head rarely

show tipburn in crisphead lettuce, although they may be encompassed

in other types of lettuce. The large veins near the leaf margins may

sometimes discolor before the typical symptoms appear. (Pis. 30

and 31.)

The development of tipburn is very complex; however, distribution

of calcium in the plant is the chief factor involved. Insufficient

calcium in cell walls weakens them and thus causes their collapse.

The discoloration itself is due to the oxidation of latex that apparently spreads through the tissue from ruptured laticifers due to

high pressure within them. The disorder develops most commonly

during periods of rapid growth.

Tipburn occurs sporadically but can cause severe losses in almost

all lettuce-growing areas, whether the crop grows outdoors or in

greenhouses. Objectionably severe tipburn of head lettuce increases only slightly after harvest at normal temperatures during

marketing. However, individual lots may diverge appreciably from

this general trend. The best guarantee against objectionably high

levels of tipburn on the market involves shipment of lettuce that is

free or nearly free of tipburn at the time of harvest. When there is a

substantial difference in incidence of tipburn at the time of harvest

and the time of arrival at the market, two factors may be involved.

First, the discoloration may have been so slight at harvest that it

was not noticed, but darkening occurred during transit. Second,

since the incidence of tipburn varies greatly from location to location, even within a given field, sampling done by regulatory agencies may not always be truly representative of the entire field. Since

examination for tipburn destroys the head, the size of sample

naturally must be limited.

Lettuce with tipburn is not necessarily more prone to decay than

lettuce without tipburn. The hazard of decay development on tipburned tissue is substantial primarily when temperatures are

30
undesirably high, when free moisture is present, or both. Aggravation of tipburn and development of decay In affected lettuce can be

minimized during transit by holding the lettuce between 0° and

2.5*'C throughout marketing.

There is no postharvest control for tipburn. The best preventative is

to plant cultivars that are resistant to the injury. Harvest before full

maturity tends to lessen the incidence of tipburn, because it has

had less chance to develop. Even though tipburn is induced by a

deficiency of calcium in some cells, spraying calcium on crisphead

lettuce plants is not useful, because the mineral will not reach the

inner leaves, although it may be effective on leaf lettuce.

References: 7, 14, 17, 20, 22.

References Cited

1. Bardin, R.E.

1971 Diseases of lettuce in the Salinas-Watsonville area of

California. 62 p. Monterey County Dept. Agr.,

Salinas, CA.

2. Beraha, L, and Kwoiek, W.F.

1975 Prevalence and extent of eight market disorders of

western-grown head lettuce during 1973 and 1974 in

the greater Chicago, Illinois area. Plant Dis. Rptr.

59:1001-1004.

3. Berger, R.D.

1967 Marginal leaf blight of lettuce. Fla. State Hort. Soc.

Proc. 80:134-138.

4. Bramlage, W.J., and Upton, W.J.

1965 Gamma radiation of vegetables to extend market life.

U.S. Dept. Agr. Mktg. Res. Rpt. 703, 15 p.

5. Ceponis, M.J.

1970 Diseases of California head lettuce on the New York

market during the spring and summer months. Plant

Dis. Rptr. 54:964-966.

31
6. Ceponis, M.J., Kaufman, J., and Butterfield, J.E.

1970 Relative innportance of gray mold rot and bacterial soft

rot of western lettuce on the New York market. Plant

Dis. Rptr. 54:263-265.

7. Collier, G.F., and Tibbitts, T.W.

1982 Tipburn of lettuce. Hort. Rev. 4:49-65.

8. Harris, CM.

1980 Effect of interior carton coatings on the retention of

lettuce quality. U.S. Dept. Agr. AAT-W 13, 7 p.

9. Hinsch, R.T., and Rij, R.E.

1980 Mechanical damage and losses to crisphead lettuce

during marketing. U.S. Dept. Agr. AAT-W-9, 16 p.

10. Hinsch, R.T., and Rij, R.E.

1980 Physical injury to crisphead lettuce during box closing.

HortScience 15:657-658.

11. Ilker, Y., Kader, A.A., Ilker, R., and Morris, L.L

1977 Anatomy of lettuce tissue affected by three physiological disorders. J. Amer. Soc. Hort. Sei. 102:426-428.

12. Jenkins, J.M., Jr.

1962 Brown rib resistance in lettuce. Amer. Soc. Hort. Sei.

Proc. 81:376-378.

13. Kader, A.A., Brecht, P.E., Woodruff, R., and Morris, L.L

1973 Influence of carbon monoxide, carbon dioxide, and oxygen levels on brown stain, respiration rate, and visual

quality of lettuce. J. Amer. Soc. Hort. Sei. 98:485-488.

14. Upton, W.J.

1963 Postharvest changes in amount of tip burn of head lettuce and the effect of tip burn on incidence of decay.

Plant Dis. Rptr. 47:875-879.

32
15. Lipton, W.J., and Harvey, J.M.

1977 Compatibility of fruits and vegetables during transit in

mixed loads. U.S. Dept. Agr. Mktg. Res. Rpt. 1070, 7 p.

16. Lipton, W.J., Stewart, J.K., and Whitaker, T.W.

1972 An Illustrated guide to the identification of some

market disorders of head lettuce. U.S. Dept. Agr.

Mktg. Res. Rpt. 950, 7 p., 19 pis.

17. Misaghi, I.J., and Grogan, R.G.

1978 Effect of temperature on tipburn development In head

lettuce. Phytopathology 68:1738-1743.

18. Ramsey, G.B., Friedman, B.A., and Smith, M.A.

1959 Market diseases of beets, chicory, endive, escarole,

globe artichokes, lettuce, rhubarb, spinach, and sweetpotatoes. U.S. Dept. Agr. Handb. 155, 42 p., 19 pis.

19. Relnert, R.A., Tingey, D.T., and Carter, H.B.

1972 Ozone Induced foliar injury in lettuce and radish

cultlvars. J. Amer. Soc. Hort. Sel. 97:711-714.

20. Ryan, A.L, and Lipton, W.J.

1979 Handling, transportation and storage of fruits and

vegetables. V. 1, Vegetables and melons. Ed. 2, 587

p. AVI Pub. Co., Westport, CT.

21. Ryder, E.J.

1979 Leafy salad vegetables. 266 p. AVI Pub. Co.,

Westport, CT.

22. Ryder, E.J., and Whitaker, T.W.

1980 The lettuce industry in California: A quarter century of

change. Hort. Rev. 2:164-207.

23. Spalding, D.H.

1966 Appearance and decay of strawberries, peaches, and

lettuce treated with ozone. U.S. Dept. Agr. Mktg. Res.

Rpt. 756, 11 p.

33
Lettuce, Shredded

Browning of the cut surface is the primary synnptom of deterioration in shredded lettuce. It is caused by the oxidation of the cell

contents that are exposed to air. This disorder can be minimized by

a combination of the following procedures: (1) Cut the lettuce when

it is between 0° and 2°C; (2) use a very sharp blade (this reduces

the exudation of cell sap); (3) have the surface of the lettuce dry

when cut; (4) place the lettuce in a sealed, permeable plastic film,

such as 0.005-mm (2-mil) thick polypropylene. (This type of packaging results in a desirable low oxygen concentration (2 to 3 percent

O2); the concurrent accumulation of carbon dioxide, up to at least

10 percent, does not appear to injure shredded lettuce, even though

it can cause severe injury in entire heads.) (5) Hold the lettuce between 0° and 2° until it is used; increasing the temperature above

2° results in rapid loss of quality; and (6) just before the lettuce is

used, rinse it with ice-cold water for 5 minutes to restore freshness.

Use of chemical rinses either does not result in better quality or is

detrimental. Reduced pressure storage also is harmful, because it

induces severe browning and loss of desirable texture.

References: 1-3.

References Cited

1. Bolin, H.R., Stafford, A.E., King, A.D., Jr., and Huxsoll, C.C.

1977 Factors affecting the storage stability of shredded lettuce. J. Food Sei. 42:1319-1321.

2. Krahn, T.R.

1977 Improving the keeping quality of cut head lettuce. Acta

Hort. 62:79-92.

3. Priepke, P.E., Wei, LS., and Nelson, A.I.

1976 Refrigerated storage of prepackaged salad vegetables.

J. Food Sei. 41:379-382.

34
Rhubarb

Rhubarb (Rheum rhaponticum L) is grown for its thick, fleshy leaf

stalks (petioles). Their marketability depends primarily on their

tenderness and crispness, which are related to age, and on freedom

from blemishes and decay. Anthracnose, foot and crown rots, and

stem spot affect the leaf petioles and may cause losses on the

market. Gray mold rot is the most important market disease of

rhubarb.

Anthracnose

(Causal Organism Colletotrichum erumpens

(Sacc.) Sacc.)

Anthracnose has been found on rhubarb in the field in several areas

and on the market, where it is damaging because the lesions

detract from the appearance of the product and may also allow invasion by secondary decay-producing organisms.

Lesions appear as soft, watery, translucent spots on leaf petioles.

When the oval lesions attain a diameter of about 10 mm, numerous

small black specks (spore-producing acervuli) appear in their

centers. In advanced stages of infection, the whole petiole may be

soft and rotted and covered with acervuli. This stage of decay is

most often found on old wilted petioles in the field. Only the small

spots that escape inspection by the packer are found on the market.

The pathogen produces abundant spore masses in the acervuli.

Spores are spread by raindrops and insects from one petiole to

another in the field and may be spread by contact even in the

absence of wounds. Humid, warm weather favors infection and

decay development.

Postharvest spread of the disease can be minimized by prompt,

thorough cooling of petioles to near 0°C and by keeping them cold

during marketing.

Bacterial Crown Rot and Bacterial Soft Rot

(Causal Organisms Erwinia rhapontica

(Millard) Burkholder, E. carotovora subsp.

carotovora (Jones) Bergey et al., and Other

Associated Bacteria)

35
Rhubarb grown continuously in one place for many years may be

subject to a serious bacterial rot of the crown buds. After death of

the crown bud, laterals grow out, some of which may become infected and cause rot at the base or in the lamina. Crown rot is a

field disease and diseased petioles are unmarketable. Bacterial soft

rot (BSR) can cause serious losses in all areas where rhubarb is

grown; however, it is usually present as a secondary decay, causing

decomposition of stalks invaded by other pathogens, such as

Phytophthora spp. and Botrytis sp. (see Phytophthora Rots, p. 37). It

can quickly become the predominant cause of decay by the time

such infected stalks reach the market.

Transit and marketing losses resulting from BSR can be minimized

by rapid cooling of stalks and shipment at temperatures below 5°C.

Rhubarb showing any evidence of phytophthora infection should

not be shipped as BSR may follow and render stalks unmarketable.

References: 1, 3, 4.

Gray Mold Rot

(Causal Organism Botrytis sp.)

Gray mold rot is the most serious transit and market disease of

rhubarb. Although the causal fungus is present wherever the crop is

grown, it seldom affects vigorously growing plants in the field.

However, injured plants and old leaves become infected readily

under humid conditions. The causal fungus produces great

numbers of spores, which contaminate leafstalks during harvesting

and packing.

On the market, gray mold rot lesions in their early stages appear as

small red spots on the sides of leafstalks and as water-soaked

areas at the base of the petiole or in injured tissue elsewhere. At

this stage, mycelium and spores are not visible. Decay lesions

enlarge rapidly, soon involving large parts of the petiole (pi. 32).

Grayish mycelium and grayish-brown, granular masses of spores on

the large lesions are characteristic signs of this disease. Most infections occur at the base of cut petioles that have been wounded during harvest, but they may also occur in apparently healthy tissues.

Maintenance of transit and storage temperatures near 0°C will

retard but not stop development of gray mold rot during marketing.

36
References: 3, 5.

Phytophthora Rots (Foot and Crown Rots)

(Causal Organisms Phytophthora spp.)

Foot and crown rots are widespread in all areas where rhubarb is

grown connmercially and are caused by one or more species of

Phytophthora. Although different causal agents are involved, disease symptoms are very similar and may be considered as one

disease.

The phytophthora foot and crown rots are primarily field diseases,

but occasionally infected petioles are found on the market.

Bacterial soft rot (BSR) follows these rots closely, and by the time

diseased stalks reach the market, BSR may be the major cause of

decay loss. The two diseases have often been confused on the

market. Phytophthora rots are watery, greenish-brown, sunken lesions

that appear first at the base of the petiole and progress rapidly

through the petiole causing a brown decay. Secondary bacterial

decay usually causes rapid decomposition of affected stalks.

The pathogens responsible for most of the foot and crown rots of

rhubarb are Phytophthora parasítica Dast. and P. cactorum (Leb. &

Cohn) Schroet. However, a closely related fungus, Pythium ultimum

Trow, and other species of Pythium have also been associated with

stalk rots in some areas.

Sorting to remove infected rhubarb at harvest will minimize the

possibility of decay occurring during transit or storage. Transit and

storage temperatures below 5°C will retard decay development.

References: 3, 5.

Stem Spot (Leaf Spot)

(Causal Organism Phyliosticta straminella

Bres.)

Stem and leaf spot disease is occasionally of importance on

rhubarb from California, Illinois, and other North Central States. On

the upper surfaces of infected leaves, small, greenish-yellow spots

appear, which enlarge and become tan with wine-red margins as

37
the lesions mature. Eventually parts of the dead tissues drop out,

leaving ragged holes. When the stems are infected, the most

serious loss generally occurs at the first cutting. Spots on infected

stems are small, oval to oblong, and reddish brown. With age some

spots may become elongate and extend 10 mm or more lengthwise

along the stem. Small, black fruiting bodies (pycnidia) eventually

appear in the dead tissue of stem and leaf spots. Other fungi may

also invade stem spot lesions.

Infection may occur through healthy, uninjured tissues wherever the

fungus comes in contact with moist stems. However, lesions

develop so slowly that no new spots are likely to appear during

transit.

Losses in transit and marketing of rhubarb may be reduced by not

shipping spotted stems and by keeping the temperature near 0°C.

References: 2, 3.

References Cited

1. Bradbury, J.F.

1977 Erwinia rhapontici Millard. Commonwealth

Mycological Institute, Surrey, CMI Descriptions of

Pathogenic Fungi and Bacteria, Set 56, No. 555, 2 p.

2. Chupp, C.

1950 Early development of rhubarb leaf spot in New York.

Plant Dis. Rptr. 34:240.

3. Horst, R.K.

1979 Rhubarb. In Wescott's Plant Disease Handbook. 803

p. Von Nostrand Reinhold Co., NY.

4. Letal, J.R.

1976 Crown rot of rhubarb in Alberta. Canad. Plant Dis.

Survey 56:67-68.

5. Wells, A., Potter, H.S., and Bird, G.W.

1978 Control of insects, diseases and nematodes on commercial vegetables. Mich. State Univ. Coop. Ext. Bui.

1978, 312 p.

38
Spinach

Spinach (Spinacia olerácea L.) is liarvested and shipped to nnari^et

as leaves and crown or as leaves and may be shipped loose or

packaged in perforated plastic bags. Quality spinach should be

dark, green, fresh, crisp, and free from blemishes and decay.

The more important field diseases of spinach include anthracnose

(caused by Colletotrichum spinaciae Ell. & Halst.), bacterial soft rot

(BSR), damping off (caused by Pythium spp., Phytophthora spp., and

Pellicularia filamentosa (Pat.) Rogers), downy mildew, heterosporium

leaf spot and other leaf spots, orange rust, white rust, wilt (caused

by Fusarium spp.), and several virus diseases. In areas where air

pollution is a problem, spinach may be severely injured by airborne

chemicals. Nutritional diseases often characterized by leaf

chlorosis are caused by the unavailability or deficiencies of

magnesium, boron, and manganese. On the market, BSR, downy

mildew, and white rust are of major importance.

Damage to the young leaves in the center of the plant caused by

feeding of the seedcorn maggot larvae (Delia platura (Meigen)), the

so-called budworm injury, and holes made on older leaves by feeding of the spotted cucumber beetle (Diabrotica undecimpunctata

howardi Barber) constitutes important insect injuries observed on

the market. Aphids on fresh spinach are occasionally a problem in

the canning and frozen industries and in the prepackaging of fresh

spinach.

Bacterial Soft Rot

(Causal Organism Erwinia carotovora subsp.

carotovora (Jones) Bergey et al. and Other

Species of Bacteria)

Bacterial soft rot (BSR) is the most important disease of fresh

spinach on the market. Affected tissues are water soaked, muddy

green, or greasy. Rapid softening and disintegration follow so that

the decayed tissues soon appear wet and mushy and eventually

have a putrid odor (pi. 33). Where the air is dry and moisture is lost

rapidly from infected tissue, the decay may be checked, and the affected tissues become dry and brittle.

The soft-rotting bacteria are common in the soil and on plant

debris. Spinach becomes contaminated with the bacteria while in

39
the field or during harvesting and packaging. Infection ainnost

always occurs through mechanical injuries, insect wounds, disease

lesions, or other skin breaks or abrasions.

A combination of high humidity and high temperature favors development and spread of decay. Decay progresses rapidly above 7°C

and at high humidity.

Harvested spinach should be handled as carefully as possible to

minimize chances of mechanical injury. Rapid precooling to remove

field heat and maintaining temperatures near 0°C during transit and

marketing are essential to minimize losses from E. carotovora, the

most serious postharvest pathogen of spinach. Removal of free

water from the surface of washed leaves by centrifugation and

subsequent vacuum cooling also reduce incidence of BSR.

References: 3, 5, 6.

Downy Mildew

(Causal Organism Peronospora effusa (Grev.)

Ces.)

Downy mildew Is found in all commercial spinach-growing areas.

Significant field losses occur regularly, especially in the coastal

areas. During cool, rainy weather, the disease may be so serious

that fields are left unharvested. Downy mildew is an important disease of fresh market spinach, because the leaf spotting detracts

from the appearance and quality of harvested leaves, and lesions

may allow entry of secondary soft rot bacteria.

The appearance of pale-yellow, irregularly shaped areas without

distinct margins is the first sign of the disease (pi. 34). The spots

are apparent on both upper and lower leaf surfaces (pi. 35). At high

humidity, whitish-gray mycelium develops over the infected areas

on either surface, although it appears first on the lower leaf surface

and may precede discoloration of the leaf. In older lesions, purplishgray fruiting bodies cover the mycelium. At very low humidity,

yellowing may occur without signs of the mycelium. Severely infected leaves may dry up; under wet conditions they become water

soaked and brownish and subsequently decay. Infection primarily

occurs in the field, but the disease can become more prominent

40
after harvest. Temperatures between 5° and 25°C and relative

humidity above 85 percent favor infection.

The pathogen, P. effusa, is an obligate parasite and can grow and

reproduce only while associated with living tissue. It attacks only

spinach. The fungus reproduces primarily by means of spores (conidia), which form in great abundance under humid conditions and

are spread by wind and rain. It is the presence of conidia and the

stalks (conidiophores) on which they are borne that gives mildew its

purplish-gray color.

Moisture on the leaf surface is necessary for germination of conidia

and subsequent infection. Mildew is likely to be important following

periods of fog or heavy dews and temperatures ranging from 5° to

25°C. At 15°-18°, infection of spinach leaves may occur within 3

hours after inoculation. Fruiting of the fungus on these new lesions

may occur within 6 days of inoculation.

Oospores, or resting spores, which are produced in diseased leaf

tissue, may be the means by which the fungus survives in the soil

between crops and possibly on contaminated seed. The fungus may

also survive in mild climates on volunteer spinach plants.

Spinach affected by mildew should not be shipped, and all spinach

should be kept near 0°C during transit and marketing. Resistant

cultivars provide the best long-range means of preventing losses

from downy mildew.

References: 1, 4.

White Rust

(Causal Organism Albugo occidentalis

G.W. Wils.)

White rust causes serious field losses in some seasons in Texas,

Oklahoma, and Louisiana. The disease has also been reported in

Arkansas and Virginia. On the market, white rust is at times an important disease of Texas-grown spinach.

The fungus develops within the leaf tissues and later produces

numerous, tiny, blisterlike pustules (sori) on the lower leaf surface,

41
which are filled with whitish masses of spores (pi. 36). Pustules

may have a concentrically zonate pattern. A slight yellowing of the

adjacent tissues that is apparent on both leaf surfaces accompanies the white pustules. The yellowed areas are indefinite in

outline, and when seen from the upper leaf surface are similar to

those caused by downy mildew (pi. 37). White rust usually appears

first near the margins of the outermost leaves of the plant. Later

the lower surfaces of all leaves may bear a few to many sori. Occasionally sori may appear on the upper leaf surface and petioles.

Oospores, or resting spores, may develop and give a blackish appearance to affected tissues. Sometimes there is a yellowing and

mosaiclike discoloration of affected leaves without the formation of

sori. Leaves that are severely infected may develop brown, necrotic

spots, and the entire leaf may turn brown.

The disease is favored by periods of clear, relatively warm, dry days

with cool nights. The fungus apparently overwinters in the soil.

Some weeds are susceptible to the white rust fungus, but spinach

is the only commercial crop affected.

Removal of crop residues immediately after harvest to prevent

oospore development in unharvested plants can minimize damage

to subsequent spinach crops. Some resistant spinach varieties are

available also.

Reference: 2.

42
References Cited

1. Byford, WJ.

1981 Downy mildews of beet and spinach. In Spencer, D.IVI.,

ed., The Downy Mildews, p. 531-543. Academic Press, NY.

2. Dainello, F.J., Heineman, R.R., and Jones, R.K.

1981 Relative white rust (Albugo occidentalis) resistance and

adaptability of spinach varieties in southwest Texas.

Tex. Agr. Expt. Sta. June 1981, 7 p.

3. Friedman, B.A.

1951 Control of decay in prepackaged spinach. Phytopathology 41:709-713.

4. Jones, R.K., and Dainello, F.J.

1982 Occurrence of race 3 of Peronospora effuse on spinach

in Texas and identification of sources of resistance.

Plant Dis. 66:1078-1079.

5. Smith, M.A.

1944 Bacterial soft rot of spinach. Phytopathology

34:747-752.

6. Smith, W.L, Jr.

1955 Streptomycin sulfate for the reduction of bacterial soft

rot of packaged spinach. Phytopathology 45:88-90.

43
Sweetpotatoes

The edible part of the sweetpotato (Ipomoea batatas (L) Lam.) consists of the thickened roots of the plant. There are several varieties

used for food in the United States. Roots should be firm, smooth,

well shaped, and free from blemishes and decay.

Many diseases that attack plants in the field not only lead to reduced

yield but also may affect the quality of the crops by causing poorly

shaped, rough, or blemished potatoes. Feeding by the root-knot

nematode (Meloidogyne spp.) may injure growing roots, but it is of

minor importance on the market as damaged roots are culled at

harvest. Some of the diseases that affect the growing plants also

cause blemishes and decay of the roots during storage and marketing. Other diseases affect sweetpotatoes only after they have been

harvested. Many of the storage diseases are less serious if sweetpotatoes are cured for 4 to 5 days at 29°C and a relative humidity

of 85 to 90 percent and subsequently stored at 13° to 15° and a

relative humidity of 85 percent or higher.

The most important parasitic diseases of sweetpotato in storage

and on the market are black rot, charcoal rot, end and surface rots,

Java black rot, rhizopus soft rot, scurf, and soil rot. The major

causes of nonparasitic losses are bruising, chilling, and freezing injuries, growth cracks, and internal breakdown.

Bacterial Soft Rot

(Causal Organism Erwinia chrysanthemi

Burkholder et al.)

Bacterial soft rot (BSR) is normally of minor importance as a sweetpotato storage disease; however, isolated outbreaks have caused

extensive losses of harvested and stored roots (pi. 38). The

pathogen is spread to the field by Infected, symptomless vine cuttings. High field temperature and moisture are predisposing factors

to BSR. Sweetpotatoes that have become infected in the field decay

rapidly during the curing process. The bacteria cause rapid decay

of inoculated roots at 29°-32°C but cannot decay roots stored at

15°.

The initial symptoms of decay are dark water-soaked lesions that

increase rapidly in size. The affected tissues soon become soft and

watery; infected roots may be reduced to a watery mass within 4-5

days at 30°C. Decay can spread from infected to apparently healthy

44
roots by contact during curing, because the bacteria produce extracellular enzymes capable of digesting host cell walls. Decayed

tissues may appear hard and dried if temperature and humidity are

reduced soon after infection begins. In such cases, lesions are surrounded by dark stains where watery exúdate has oozed from the

infected area or dripped from other infected roots (pi. 39).

Sweetpotatoes showing decay at harvest should be kept at 15°C

and marketed promptly without curing, because decay will increase

at the high temperature and humidity required for curing.

References: 14, 16.

Black Rot

(Causal Organism Ceratocystis fimbriata Ell.

et Halst.)

Black rot is one of the most serious and most widely distributed

diseases of sweetpotatoes. Although it affects the plants in the

seedbed and in the field, most damage occurs in storage and during marketing.

The early symptoms of black rot appear as circular, brown, slightly

sunken, superficial spots about 5 mm in diameter. As they enlarge,

they become black to greenish black (pi. 40). Small, black fruiting

bodies (perlthecia) are often present at this stage. To the unaided

eye, they appear as black bristles. The rot is shallow and firm, rarely

penetrating the center of the root. Affected internal tissues are dark

(pi. 41) and have a bitter taste that affects the entire root when

cooked.

Infections by black rot occur in the field through wounds, dead root

hairs, or apparently healthy tissues. The fungus persists from

season to season in sweetpotato fields in roots, plant debris, and

soil. Infection is favored by wet soil and warm temperatures.

Although obviously infected potatoes are discarded at harvest,

many black rot lesions are so small that removal of all infected

roots is impossible. In storage, these small lesions may enlarge to

about 10 mm in diameter within 4 to 6 weeks and account for the

substantial decay losses in lots thought to be free of decay. Decay

can also spread to healthy sweetpotatoes if healthy roots are

washed in water with sweetpotatoes affected with black rot.

45
Control of black rot in storage can be increased by careful handling

and discarding all visibly infected sweetpotatoes before storage.

Roots harvested from soils known to be infested should be kept

separate from those growing in disease-free fields and should be

marketed early.

Special high-temperature curing (35°C for 4 days) reduces black rot

development in storage. Heat treatment at 40° to 43° for 24 hours

will prevent the development of black rot but may adversely affect

root quality. Chemical treatments have not been consistently effective for black rot control.

References: 12, 17.

Blue Mold Rot

(Causal Organism Pénicillium spp.)

Blue mold rot is often found in chilled or frozen sweetpotatoes. The

fungus may also be present as a secondary pathogen on roots

decayed by other fungi such as Rhizopus. Fungal spores are in the

air and on the surface of most fruits and vegetables, but they are

unable to cause infection except through wound-damaged tissues.

Blue mold rot is a soft decay generally involving large areas surrounding mechanical injuries or lesions of other diseases. In chilled

or frozen sweetpotatoes, the whole root Is soon decayed, and the

characteristic white and blue-green tufts of mycelium break through

the epidermis. When many potatoes are infected, the lot has a

musty odor.

The disease may be avoided if care is used to prevent chilling and

mechanical injuries and if proper temperature and humidity are

maintained.

Reference: 11.

Charcoal Rot

(Causal Organism Macrophomina phaseolina

(Tassi) G. Gold.)

Charcoal rot is most important as a storage disease of sweet46
potatoes grown in the South, but it causes some loss in storage

wherever sweetpotatoes are grown. The fungus nnay injure growing

plants in the field when the stem becomes infected at the soil line.

Decay most often starts at the upper end of the sweetpotato and

progresses throughout its length. In early stages, infection is

characterized by a light-brown discoloration of the surface as well

as the internal tissues. The discolored areas may be any size or

shape, but there is a sharp line of demarcation between the diseased and healthy tissues. As the decay progresses, the affected

areas remain firm and become dark brown. The skin later begins to

shrivel as water is lost by evaporation. In the final stages of this

disease the root is converted into a hard, dry, black mummy (pi. 42).

When a decaying potato is cut, usually three rather distinct color

zones are in the affected tissues. The margin or advancing edge of

the decay is light or cinnamon brown and slightly spongy in texture,

the intermediate zone is reddish brown and firm, and the oldest part

of the lesion (root end of sweetpotato or center of lesion) is grayish

black to black, dry, and firm. On close examination of the grayishblack area, very small, black sclerotia are visible within the tissues.

No sclerotia or fungal hyphae are visible on the surface of diseased

sweetpotatoes.

Charcoal rot develops rapidly on uncured sweetpotatoes marketed

soon after harvest. Slight infections at broken ends will develop in

transit and result in substantial loss at the market. Optimal growth

temperature of the pathogen is 31.5°C, but it will grow at 7.8° to

42°. Field infections will progress slowly at recommended storage

temperatures, but the potential for rapid decay remains when temperatures are raised during transit and marketing.

Losses from charcoal rot may be reduced by not storing or shipping

sweetpotatoes with even the slightest evidence of decay at broken

or injured ends. Roots free from decay should be cured promptly by

rapid wound barrier formation to protect them from decay.

References: 11, 20.

Dry Rot

(Causal Organism Diaporthe batatatis Harter & Field)

Most losses from dry rot occur during storage and marketing. The

47
disease ¡s widely distributed but seldom causes extensive losses of

sweetpotatoes in the field. The fungus attacks potatoes as a result

of infections on slips in the seedbeds and diseased vines in the

field.

Dry rot, even in its early stages, is characteristically a dark-brown,

firm decay. In practically all cases, decay starts at the stem end of

the potato. As infection progresses, the decayed tissues lose water

quickly and the affected part becomes withered (pi. 43). Soon the

diseased area becomes black and hard and the surface of decayed

roots is covered with black, pimplelike pycnidia. These numerous

black fruiting bodies in and beneath the skin of a sweetpotato with

a hard, dry, black decay are diagnostic of this disease.

Dry rot does not develop as rapidly or cause as much loss as black

rot, but inoculated sweetpotatoes may be completely decayed

within 6 weeks at recommended storage temperatures.

Promptly curing under optimal temperature and humidity conditions

favors rapid wound-cork formation and can prevent infections after

harvest.

Reference: 10.

End Rots and Surface Rots

(Causal Organisms Fusarium spp.)

Although end rots and surface rots may be caused by several different organisms that attack sweetpotatoes, most of them found on

the market are due to various species of Fusarium. The pathogens

live in the soil and invade roots through wounds, growth cracks,

and small rootlets at harvesttime and during storage. Infection

takes place most readily when potatoes are harvested during wet

weather; consequently, end and surface rots are most serious in

storage after wet seasons. Decay develops so slowly that conspicuous lesions are not usually evident until about 6 weeks after

storage. Sweetpotatoes with many lesions shrivel badly in storage

and are not marketable.

The early stages of infections are characterized by small, circular,

light-brown, superficial spots. As the disease progresses, lesions

enlarge and become slightly sunken, with sharp margins caused by

48
the drying and shrinkage of affected tissues (pi. 44). Sonne rots remain superficial, not penetrating nnore than a few millimeters into

roots, whereas others may spread to involve a major part of the infected root; however, all decayed tissues appear dry and sunken

unless invaded by other organisms (pi. 45).

Although new infections may occur in storage if sweetpotatoes are

not properly cured, it appears doubtful that new lesions develop in

storage. Incipient infections do enlarge, however.

Sweetpotatoes should not be harvested during wet weather if it can

be avoided. Prompt curing and proper storage conditions will

reduce losses by assuring rapid healing of wounds made during

harvesting.

References: 7, 18, 19.

Java Black Rot

(Causal Organism Lasiodiplodia theobromae

(Pat.) Griff, et Maubl.) (Synonyms Diplodia

tubericola (Ell. & Ev.) Taub, and

Botryodiplodia theobromae Pat.)

Java black rot affects sweetpotatoes during storage and marketing

and may cause heavy losses. It can be found in most sweetpotato

storages, but losses are usually more severe in the Southern United

States and tropical areas than in more northern growing areas.

Within 10 days of infection, symptoms begin to appear, and within

1 month, inoculated sweetpotatoes can be completely decayed. Initially the decay appears brown and moderately firm. As it progresses, all the tissues become involved. The central part is light

brown, and the skin and tissues just beneath change from dark

brown to black as pycnidia form and push up through the skin (pi.

46). Within 3 to 4 weeks, these fruiting bodies form in such sufficient numbers that they create domelike elevations on the root surface. Eventually the root becomes a hard, black mummy.

Lasiodiplodia ttieobromae occurs in soils and in plant debris. No

root infection appears to take place in the field except through

wounds from digging and during handling for storage and market49
ing. The fungus grows at temperatures from 12° to 37°C, with an

optimum of 29°-31°.

Careful handling of sweetpotatoes to avoid all unnecessary wounds

at harvest and storage time and prompt curing under optimal conditions can minimize damage caused by Java black rot.

References: 7, 2.

Rhizopus Soft Rot

(Causal Organisms Rhizopus stolonifer

(Ehr.: Fr.) Vuill. and R. tritici SaWo)

Rhizopus soft rot is the principal cause of sweetpotato loss during

storage and marketing. In most markets, it produces more wastage

than all other sweetpotato diseases combined. All varieties are

susceptible to the disease, and it occurs in roots from all production areas. Handling and storage methods probably have a greater

effect on the subsequent development of decay than any natural

susceptibility because of varietal characteristics. Since infections

by Rhizopus spp. are dependent on wounds or injuries produced by

other diseases, rhizopus rot is not an important field disease, but it

may affect potatoes in the seedling bed.

Affected tissues first appear soft and watery, but change little in

color. Later infected areas become light to dark brown. Freshly

decayed areas will yield a yellowish-brown liquid when broken, but

as water is lost from the decayed tissues, the area becomes

withered and firm. Sweetpotatoes can be completely decayed within

4 to 6 days. Surface mold growth under moist conditions appears

as coarse, white mycelium with numerous black sporangia. This

growth is conspicuous at the ends and through breaks in the skin

of infected sweetpotatoes held in a humid atmosphere (pi. 47).

When roots decay under dry conditions, surface mold may not

develop.

Infection by Rhizopus may occur at broken ends of the roots or at

any unhealed wound. A high incidence of infection usually indicates that roots have been improperly handled or exposed to external stress after harvest. There is a positive correlation between

weight loss during curing at 26°C and decay development after 5

months of storage at 15°. In research studies, weight loss was

50
reduced by increasing the relative humidity fronn 80-90 to 90-97 percent for curing. When weight loss was less than 4 percent during

curing, only 1 percent decay developed in storage. When curing

resulted in weight loss of about 10 percent from injuries and low

relative humidity, decay was as high as 25 percent after 5 months

of storage.

Careful handling of roots during harvest and transport, together

with prompt curing at optimal temperature and humidity, is of the

greatest importance in preventing rhizopus soft rot during storage.

When roots are washed, sorted, and packed after storage, additional wounding may occur. Several fungicide treatments have been

developed that will reduce infections at new wounds when they are

applied to roots during their preparation for market. Re-curing roots

for 2 days at 29.5°C after being packed for market has been

reported to reduce decay; however, this method has not been practical. Prompt shipment and marketing of washed, treated roots are

the best means of minimizing losses.

References: 4, 21.

Scurf

(Causal Organism Monilochaetes infuscans

Ell. & Halst.)

Scurf is one of the most common diseases of sweetpotatoes. It occurs on all varieties and appears to some extent on stocks from all

shipping regions. Although the causal fungus may produce a

brownish discoloration of any or all underground parts of the growing plant, the chief damage results from a reduced market value of

the sweetpotatoes because of their blemished appearance. Otherwise, field, storage, and marketing losses are of little consequence.

On marketable roots, scurf appears as small, grayish-brown spots

and blotches that are only skin deep. They may be found anywhere

on the sweetpotato, but usually most are near the stem end. When

numerous infections occur, the discolored spots often coalesce,

making a continuous brown area (pi. 48). In extreme cases, the skin

may crack when such extensive areas of the skin are killed; the

sweetpotato loses moisture rapidly and becomes unmarketable. A

few discolored spots cause no appreciable damage and are generally overlooked at the market.

51
Most of the infections occur in the field, but sonne new infections

may be noted under humid storage conditions. Spots already present may also enlarge slightly during storage and transit. Usually

shrinkage of the roots is slow in storage unless the temperature is

high and the humidity low. Severely infected sweetpotatoes in hot,

dry storage often crack and wither and become worthless within 4

to 6 weeks.

The causal fungus first infects seed roots, then slips, and on these

it is spread to the field. Heavy soils containing abundant organic

matter favor the development of scurf.

Losses from scurf can be avoided by such field control measures

as careful selection of disease-free seed stock, use of vine or

sprout cuttings instead of pulled sprouts, and planting in soil that

has not been infested by M. infuscans. Heavy, black, wet soils with

much organic matter should be avoided if possible.

References: 13, 17.

Soil Rot (Pox)

(Causal Organism Streptomyces ipomoeae

(Person and W.J. Martin) Waksman and Henrici)

Soil rot or pox is found in all important sweetpotato-growing

regions of the United States. Although losses vary considerably

from season to season, in many areas soil rot is considered one of

the most important field diseases of sweetpotatoes. It seriously

curtails the growth and yield of infected plants and blemishes roots

so that their value may be greatly reduced.

Sweetpotatoes with evidence of soil rot on the market are characterized by having dry, brown pits or pox marks of irregular size

and shape (pi. 49). The infected areas may vary in size from less

than 4 to 12 mm in diameter. In the early stages, the superficial,

brown, circular spots are smooth or slightly sunken, but as they

enlarge, the epidermis cracks exposing a cavity with rough lining

and irregular margins. These exposed tissues are firm and dry when

found on mature roots at the market. Ordinarily they are not followed

by soft rot or other secondary decays. Occasionally the root may be

almost girdled by these blemishes, and in extreme cases the

52
growth of tissues around the deep pits may result in misshapen

potatoes.

The optimal growth temperature of S. ipomoeae is 31.7*'C, with

growth at 20° to 40°. The high soil temperatures in the Southern

States appear to favor development of soil rot.

The causal organism inhabits the soil and invades potatoes through

the small secondary rootlets. Most of the dissemination of the disease appears to be by transportation of infested soil on seed

potatoes and by farm implements, animals, drainage water, or windblown dust. Experimental evidence indicates that the disease is not

transmitted by using diseased roots for seed purposes if they have

no infested soil on them.

Soil rot is most serious during dry seasons. When sufficient

moisture is available, plants are able to continue to grow and produce some marketable potatoes even though they are diseased.

If disease-free soil is not available for growing sweetpotatoes, soil

rot in infested soil can be controlled by adding sufficient sulfur to

make it acid (pH 5.0). Sweetpotato varieties resistant to soil rot are

also available.

References: 15, 16.

Chilling Injury

Sweetpotatoes are subject to chilling injury when held at temperatures below 13°C for 10 days or more. These low temperatures

cause tissue breakdown, which results from physiological changes

in the sweetpotato. Internal discoloration from brown to slightly

black is the principal symptom of chilling injury. Discolored areas

may be somewhat scattered but are usually associated with the

vascular elements in the central part of the sweetpotato (pi. 50).

Flavor of cooked potatoes is also affected. In addition to tissue

discoloration, chilling injury leads to the development of "hardcore"

areas in cooked sweetpotatoes. Exposure of uncured roots to 1° for

as little as 3 days causes significant hardcore. Properly cured

sweetpotatoes are more resistant to chilling injury than noncured

ones.

53
The danger from chilling and from associated decays can be

prevented by avoiding exposure to temperatures below 13°C at any

time during storage, transport, and marketing. Varieties also vary in

their susceptibility to chilling injury.

References: 5, 6, 8.

Freezing Injury

The average freezing points for 10 sweetpotato cultivars ranged

from -1.9° to - 1.2°C. Soluble solid content of roots is not consistently correlated with the freezing point.

Roots that have been only slightly frozen have a characteristic

yellowish-brown discoloration of the vascular ring and internal

vascular elements. The roots also appear yellowish green and water

soaked (pi. 51). When freezing exposure has been so prolonged that

ice crystals have formed, the tissues collapse immediately when

thawed, and the sweetpotatoes become soft and flabby as water

evaporates. Badly frozen roots are often invaded by rot-producing

fungi before they dry up.

Freezing injury seldom occurs in storage because temperatures

must be maintained well above freezing to prevent chilling injury.

However, freeze damage does occur occasionally during winter

transport to northern markets and during delivery to retail markets

in severe weather. Heated vehicles for transport and market delivery

during cold weather are essential to protect sweetpotatoes from

both chilling and freezing injury.

Reference: 9.

Internal Breakdown

Sweetpotatoes in storage sometimes develop a breakdown of internal tissues. In advanced cases, affected roots can be detected by

their light weight and spongy texture. Cavities may form in roots as

the tissues separate, and the spongy tissues may appear white or

light yellow (pi. 52).

Pithiness, with subsequent internal breakdown, has resulted from

excessive intercellular space in certain roots. This characteristic

varies with the cultivar and growing conditions. Intercellular space

54
may increase during storage and is most prevalent wiien roots are

stored at temperatures above those recommended or at low relative

humidity.

Curing sweetpotatoes longer than 1 week at 30°C increases the

development of intercellular space, as does storage above 13°-15°.

Cultivars vary in their susceptibility to internal breakdown. The

disorder Is least prevalent in cultivars with high dry-matter content,

low intercellular space, low water-loss rate, and low respiration rate

at harvest.

Measurement of intercellular space at harvest provides a useful index of susceptibility to internal breakdown. Because pithiness and

breakdown occur during storage, those lots with a high proportion

of intercellular space at harvest should be marketed early. Accurate

control of storage temperature, humidity, and airflow can reduce

the severity of internal breakdown.

Reference: 11.

Internal Cork

Internal cork is a virus disease of sweetpotato characterized by

dark-brown to black, hard, corky spots of irregular size and shape in

the flesh of infected roots (pi. 53). The disease was first recognized

in South Carolina and has since been reported in all States that

grow sweetpotatoes commercially.

Occasionally a surface depression indicates a corky area beneath

the skin, but usually the disease cannot be detected without slicing

the root. The hard corky spots are sharply outlined and the surrounding tissue shows no sign of deterioration or decay. The spots

remain hard and gritty even after baking, but the color, flavor, and

texture of surrounding tissues are not affected. Slight symptoms

may be visible at harvest, but they increase substantially during

storage, especially at temperatures above optimum for sweetpotato

storage. Cultivars vary in their susceptibility and symptoms. Sweetpotatoes known to be infected with internal cork should be marketed soon after harvest.

Reference: 3.

55
References Cited

1. Arinze, A.E., and Smith, I.M.

1979 Production of a polygalacturonase complex by

Botryodiplodia theobromae and its involvement in the

rot of sweetpotato. Physio!. Plant Path. 14:141-152.

2. Arinze, A.E., and Smith, I.M.

1982 Effect of storage conditions on the resistance of

sweetpotato tissues to rotting by Botryodiplodia

theobromae Pat. and other fungi. J. Stored Prod. Res.

18:37-41.

3. Aycock, R., Hughes, M.B., Cooley, J.S., and Smith, W.L., Jr.

1953 The relation of temperature to internal cork symptom

expression in stored sweetpotato roots. Phytopathology 43:50-51.

4. Boukamp, J.C., Scott, LE., and Kantzes, J.G.

1970 Control of soft rot in cut root pieces of sweetpotatoes

with 2,6-dichloro-4-nitroanaline. Plant Dis. Rptr.

55:1097-1099.

5. Broadas, E.W., Collins, W.W., and Pharr, D.M.

1980 Incidence and severity of hardcore in sweetpotatoes

as affected by genetic line, curing, and lengths of 1°C

and 2rC storage. Sei. Hort. 13:105-113.

6. Buescher, R.W., and Baimoori, M.R.

1982 Mechanism of hardcore formation in chill-injured

sweetpotato (Ipomoea batatas) roots. J. Food

Biochem. 6:1-11.

7. Clark, C.A.

1980 End rot, surface rot, and stem lesions caused on

sweetpotato by Fusarium solani. Phytopathology

70:109-112.

8. Daines, R.H., Ceponis, M.J., and Hammond, D.F.

1974 Relationship of chilling to development of hardcore in

sweetpotatoes. Phytopathology 64:1459-1462.

56
9. Ezell, B.D., and Wilcox, M.S.

1967 Frost injury systemic in sweetpotatoes. J. Agr. and

Food Chem. 15:729-735.

10. Harter, L.L., and Field, E.G.

1913 A dry rot of sweetpotatoes caused by Diaporthe

batatatis. U.S. Bur. Plant Indus. Bui. 281, 38 p.

11. Harter, L.L., and Weimer, J.L.

1929 A nnonographic study of sweetpotato diseases and

their control. U.S. Dept. Agr. Tech. Bui. 99, 117 p.

12. Kojinna, M., and Uritani, I.

1978 Studies on factors in sweetpotato root which induce

spore germination of Ceratocystis fimbriata. Plant

Cell Physiol. 19:91-97.

13. Lawrence, G.W., Moyer, J.W., and Van Dyke, CO.

1981 Histopathology of sweetpotato roots infected with

Monilochaetes infuscans. Phytopathology 71:312-315.

14. Martin, W.J., and Dukes, P.D.

1977 Bacterial stem and root rot of sweetpotato. Plant Dis.

Rptr. 61:158-161.

15. Martin, W.J., Hernandez, T.P., and Hernandez, T.P.

1975 Development and disease reaction of Jasper, a new

soil rot resistant sweetpotato variety from Louisiana.

Plant Dis. Rptr. 59:388-391.

16. Moyer, J.W.

1982 Postharvest disease management for sweetpotatoes.

In Villareal, R.L., and Griggs, T.D., eds.. Sweet Potato

Proceedings of the First International Symposium, p.

176-184. Asian Vegetable Research and Development

Center, Tainan, Taiwan, China.

17. Nielsen, LW.

1977 Thermotherapy to control sweetpotato sprout-borne

root knot, black rot, and scurf. Plant Dis. Rptr.

61:882-887.

57
18. Nielsen, L.W., and Johnson, J.T.

1974 Postharvest tennperature effects on wound healing and

surface rot in sweetpotato. Phytopathology 64:967-970.

19. Nielsen, L.W., and Moyer, J.W.

1979 A fusarium root rot of sweetpotatoes. Plant Dis. Rptr.

63:400-404.

20. Steinbauer, CE., and Kushnnan, L.J.

1971 Sweetpotato culture and diseases. U.S. Dept. Agr.

Agr. Handb. 388, 74 p.

21. Thonnpson, D.P.

1979 Phenols, carotene, and ascorbic acid of sweetpotato

roots infected with Rhizopus stolonifer. Ganad. J.

Plant Sei. 59:1177-1179.

58
Color Plates

1 Black rot of beet, internal discoloration.

2 Beet scab.

59
3 Bacterial soft rot of witloof chicory; normal head

in center.

4 Bacterial soft rot of endive.

60
5 Gamma irradiation injury of endive; normal head

on left.

6 Gray mold rot on artichoke buds; gray mycelium

on some decayed bracts.

61
7 Freezing injury on artichoke buds; blistering on surface

of bracts and darl<ening of young bracts at center

of bud.

8 Irradiation injury causing discoloration at bases

of florets induced by 100-400 krad doses

of gamma irradiation.

62
9 Bacterial soft rot of fiead lettuce, early stage.

10 Bacterial soft rot of fiead lettuce, advanced stage.

63
11 Downy mildew lesion on head lettuce.

12 Gray mold rot on head lettuce.

64
13 Severe watery soft rot of head lettuce.

14 Watery soft rot of witloof chicory; normal head above.

65
15 Internal rib necrosis on 'Climax' head lettuce.

16 Rusty-brown discoloration of 'Climax' head lettuce.

66
17 Brown stain, caused by excess carbon dioxide,

on inner leaf of head lettuce.

18 Heart leaf injury on head lettuce caused by excess

carbon dioxide.

67
19 Field freezing causing blistering of epidermis

on fiead lettuce.

20 Field freezing causing water-soal<ing of midribs

and lamina.

68
21 Gamma irradiation injury visible as spotting

on inner leaves.

22 Marginal senescence on head lettuce.

69
23 Marginal browning of endive leaves.

24 Low oxygen injury on wrapper leaves of head lettuce.

70
25 Ozone injury on outer leaves of head lettuce.

26 Pink rib.

71
27 Rib discoloration on inner leaf surface.

28 Russet spotting on head lettuce.

72
29 Russet spotting on leaf of head lettuce.

30 Tipburn on outer leaf of head lettuce.

73
31 Tipburn on inner leaf of head lettuce.

32 Gray mold rot on rhubarb.

74
34 Downy mildew lesions on spinach leaf.

75
35 Downy mildew on upper and lower spinach leaf

surface.

36 White rust pustules on spinach leaf.

76
37 White rust on spinach leaves.

38 Bacterial soft rot of sweetpotato roots.

77
39 Bacterial soft rot lesion on sweetpotato.

40 Sweetpotato black rot, external symptoms.

78
41 Sweetpotato black rot, internal symptoms.

42 Sweetpotato charcoal rot.

79
43 Sweetpotato dry rot.

44 Sweetpotato fusarium surface rot.

80
45 Sweetpotato fusarium end and surface rot.

46 Sweetpotato Java black rot.

81
47 Sweetpotato rhizopus soft rot.

48 Sweetpotato scurf.
49 Sweetpotato soil rot (pox).

50 Sweetpotato chilling injury followed by fusarium rot.

83
51 Sweetpotato field freezing.

52 Sweetpotato pithy breal<down.

84
53 Sweetpotato internal cork.

85
Conversion Table

-1.1 30

0 32

1.7 35

4.4 40

7.2 45

10.0 50

12.8 55

15.6 60

18.3 65

21.1 70

23.9 75

Conversion:

°F = °Cx9/5 + 32

°C = °F-32x5/9

26.7 80

29.4 85

32.2 90

35.0 95

37.8 100

40.5 105

43.3 110

46.1 115

48.9 120

51.8 125

86