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Agrici

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No. 221

Agricultural Research Service

U.S. DEPARTMENT OF AGRICULTURE
CONTENTS

Page

Botany and genetics 2

Lettuce breeding 5

Classification and adaptation of

lettuce varieties 8

Crisphead varieties 8

Butterhead varieties 8

Cos, or romaine, varieties 9

Looseleaf, or bunching, varieties. 9

Stem varieties 9

Description of the important varieties of lettuce 10

Crisphead varieties 10

Varieties of Imperial type 10

Varieties of Great Lakes type_ 11

Other crisphead varieties 13

Butterhead varieties 14

Cos, or romaine, varieties 15

Looseleaf, or bunching, varieties. 16

Seedbed preparation, planting, and

thinning 17

Soil management and fertilization. 20

Green manures 21

Animal manures 21

Commercial fertilizers 21

Rotations 22

Irrigation 23

Cultivation and weed control 24

Harvesting and marketing 25

Seed production 29

Page

Diseases of lettuce and their control 32

Damping-off 32

Mosaic 32

Spotted wilt 35

Aster yellows 35

June yellows 36

Big vein 38

Downy mildew 39

Powdery mildew 40

Tipburn 40

Sclerotinia 41

Anthracno^e 42

Bottom rot 42

Botrvtis 42

Rib blight 43

Insect enemies of lettuce and their

control 43

Insects affecting the marketlettuce crop 43

Cutworms and armyworms.. 43

Cabbage looper 44

Corn earworm 45

Aphids 46

Leafhoppers 46

Other insects 46

Insects and mites affecting the

lettuce-seed crop 47

Aphids 47

Spider noites 47

Hyaline grass bug 47

Literature cited 48

This handbook replaces the material formerly contained in U.S.

Department of Agriculture Farmers' Bulletin 1953, Lettuce Varieties

and Culture.

Washington, D.C. Issued March 1962

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

Washington, 25, D.C. — Price 25 cents
Lettuce and Its Production.

By THOMAS W. WHITAKEE and E. J. glYDEK, getieticists. Crop« Research Division,

•ice\

and ORíN A.QIILLS, entomologist,rÉntomology Research Division, Agricultural

Research Service \ ^

Lettuce {Lactuca sativa L.) is an

important vegetable crop in this

country ; it exceeds in farm value all

fresii vegetable crops except potato

and tomato. During the 1950's the

acreage for harvest fluctuated between 204,000 and 233,000 acres.

The farm value of the crop fluctuated between $96 million in 1950

and about $142 million in 1957

(SS).' These figures do not include

the collectively huge amount of lettuce grown in home gardens, for

roadside stands, and for local

markets.

One can find at least tliree, and

frequently four, types of lettuce in

most of the large markets and many

of the smaller ones in nearly every

city of the country. There is always

a variety of the crisphead type, commonly a variety of butterhead, often

a variety of cos, and sometimes a

looseleaf variety. Moreover, lettuce

is offered to the consumer in all

seasons.

The ancient Greeks and Romans

had lettuce, but most of the evidence, as De Candolle shows, indicates that lettuce is a comparative

1 The authors are much indebted to

K. Bardin, G. W. Bohn, V. E. Boswell, W.

H. Gabelman, B. G. Grogan, J. F. Harrington, J. M. Jenliins, .Jr., D. G. A. Kelbert, .J. McCabe, L. L. Morris, W. D. Pew,

G. J. Raleigh, J. E. Welch, and F. W.

Zink, Jr., ior their help in the preparation

of this handbook.

'Italic numbers in parentheses refer

to Literature Cited, p. 48.

newcomer to our assemblage of cultivated plants. For instance, there

is no Sanskrit or Hebrew name for

the plant. Likewise, Bretschneider,

an authority on the cultivated

plants of China, states that lettuce

was introduced from the West into

China, but not until 600 to 900 A.D.

Herodotus records that lettuce was

cultivated by the Persians as early

as the sixth century B.C. Several

writers report that it was grown by

the Greeks in the fifth and fourth

centuries B.C. Reasoning from this

scattered information it seems

highly probable that lettuce was

domesticated within historic time,

and certainly did not become a

widespread cultivated plant until

the beginning of the Christian era.

The food value of lettuce is confined mostly to minerals and vitamins. Compared with other vegetables it is a good source of calcium,

iron, and vitamin A; it is low in

energy, protein, ascorbic acid (vitamin C), thiamine (vitamin Bi),

riboflavin (vitamin B^), and niacin.

In comparing the food value of 32

vegetables on a pound, acre, and

man-hour basis, MacGilHvray and

others (SO) placed these crops in 4

groups, in descending order, based

upon their efficiency in producing 9

nutrients. Lettuce falls in the third

group with such crops as celery,

asparagus, and snap beans.

Lettuce has taste appeal for most

people, and it is easily prepared for

the table. Since it is consumed
AGRICULTURE HANDBOOK 221, U.S. DEPT. OF AGRICULTURE

raw, none of the nutrients are lost

in cooking or in preparing it for

table use. Vitamin C, however, is

lost if the leaves are not kept cool

and moist. Because the calorie

content is low, it has become a

common ingredient of most lowcalorie diets.

In the United States most of the

commercial crop of lettuce is produced in several areas in each of

three regions (fig. 1) :

1. The western region, which includes Washington, Oregon, California. Arizona, Xew- Mexico, Colorado, and Texas.

2. The northern region, which includes Maine, Massachusetts, Connecticut, Xew York, New Jersey,

Pennsylvania, Ohio, Michigan, and

Wisconsin.

3. The southeastern region, which

includes Virginia, Xorth Carolina,

South Carolina, Georgia, and

Florida.

Since 1949 approximately 90 percent of the acreage harvested for

market has been grown in the western region. About 87 percent of

the lettuce produced in the western

region conies from Arizona and

California.

BOTANY AND GENETICS

All domesticated varieties of lettuce belong to the species Lactuca

sativa L. The genus Lactuca is a

member of the very large sunflower,

or thistle, family, the Compositae,

which also includes: Helianthus,

simflower; Soru^hits, sowthistle;

Taraxacum., dandelion; C'ichoriwm,

endive ; Tragopogón, salsify ; and

Cynara, artichoke.

Species of Lactuca occur all over

the world. The basic chromosome

numbers are n=8 and 9, but there

are several species, mostly in the

New World, that have a basic number of 17. These probably arose as

natural amphidiploids from crosses

between 8 and 9 chromosome species. Cytogenetic studies support

this hypothesis.

Cultivated lettuce is an annual

plant with panicled heads of yellow

flowers. The flower is composed of

many florets surrounded by several

series of bracts, called, as a group,

an involucre. Each floret has a

strap-shaped corolla. The anthers

are fused and form a tube that surrounds the style. The style is cleft

into two branches at its apex. The

single-celled ovary contains a single

ovule that matures into a dry fruit

called an achene. The achene is

flattened and beaked and crowned

with a pappus. Although commonly referred to as a seed, as in

this handbook, the achene is botanically a fruit.

Cultivated lettuce was probably

deri\-ed from the so-called wild, or

prickly, lettuce, L. sernola L. Wild

lettuce and cultivated varieties hybridize readily without any decrease

in the fertility of the progeny in the

Fi or ensuing generations; this indicates a close genetic affinity. Furthermore, both L. serviola and L. ,

sativa have nine pairs of chromosomes. There is much evidence for

a continuous flow of genes between

them. Although the Fi hybrids

between the two forms are so conspicuous in cultivated seed fields

that they are immediately seen when ,

the fields are rogued, an inspection

of individual plants of wild lettuce

adjacent to abandoned fields of

market lettuce or seed fields often

indicates that introgression has occurred. Broad basal leaves, few ¡i

spines along the midrib of the leaf, ,'

and the general conformation of the -

plants suggest that such individuals

are recent derivatives of hybridization betîveen cultivated varieties ^

and wild lettuce.
LETTUCE AND ROMAINE HARVESTED FOR SALE

ACREAGE. 1954

UNITED STATES TOTAL

197.979

US. DEPARTMENT OF COMMERCE

I DOT=IOO ACRES

(COUNTY UNIT BASIS)

MÛP NO A54-239 BUREAU OF THE CENSUS

FiGUBE 1.^—^Map of the United States, showing the Lmi>ortant lettuce-production areas

and the approximate acreage planted to the crop in each area.
AGRICULTURE HANDBOOK 221, U.S. DEPT. OF AGRICULTURE

According to Vavilov (36), L.

serrlola is indigenous to the liuge

area encompassed by inner Asia

Minor, Transcaucasia, Iran, and

Turkestan. We can tlierefore assume tluU I'ultivated lettuce was

domesticated in this general area.

Genetic information about lettuce

is relatively sparse. Data on a fewgenes have been reported in the

literature, however, and they are

listed with their authorities in

table 1.

In addition, several crosses between i. sémola and varieties of L.

sativa have been studied. The gene

B. i for shattering vs. nonshattering seed heads was described by

Whitaker and McCollum in 1954

(il ) and by Lindqvist in 1956 (17).

Durst ( 7, pp. 329-341) studied spininess and described the gene S, s

for spines vs. no spines, in 1930.

For both characters, the wild parent carried the dominant alíele.

Other genetic studies have been

extremely limited. Thompson (25)

found the C and R loci linked with

a recombination value of 36 percent.

No other linkages have yet been

fully demonstrated. In quantitative inheritance investigations.

Durst (7) analyzed several size

characters and concluded only that

many genes were responsible for

each trait.

Research on chromosome structure, fertility in interspecific

crosses, and polyploidy has produced some significant information.

These studies indicate the close genetic I'elationship of L. sativa, L.

serriola. L. saligna L., L. virosa L.,

and L. altaica Fisch, & Mey. These

species form a single compatibility

group. There are at least two other

compatibility groups in the genus,

but they are not cross fertile with

the L. sativa group.

TABLE 1.—Genes of lettuce (L. sátira)

Description of trait

Tl', w

Y,y

G,g

C, c

T, (

R, r', r

(Unnamed)

Si, si

U, u

A', k

(Unnamed)

Black vs. white seed color ^

Yellow vs. white seed color 2

Dark-green vs. yellow-green leaf color

Anthocyanin present vs. absent in leaves > —

Anthocyanin present vs. absent in leaves ^

Intense-red vs. spotted-red vs. tinged-red

leaves.^

Normal-green vs. chlorophyll-deficient leaves

Netted-vein vs. striate-vein leaves

Lobed vs. unlobed leaves *

Nonheader vs. header

Downy mildew resistance (race 5) vs. susceptibility.

Durst (7, pp. S!

HI).

Thompson (2S)

Thompson {29)

Thomspon (îi)

Thompson {2S)

Thompson (ÎS)

Thompson (2S)

Whitaker (3S).

Whitaker and Bolin

CO)-- Whitaker (39).

Lindqvist {18).

Pearson {iS).

Jagger and Whitaker

(13).

1 A capital letter denotes dominant alíele.

2 F, ¡Í is epistatic to 11', ¡r, givine 9 black:, 3 yellow, 4 white in F^.

3 C, c and T, i are complementary. Effects of R, r', r appear onlv in C-T- genotype. Order of dommance is

* Studies of crosses between L. sativa and L. serriola indicate that the inheritance of lobing may be more compiei.
LETTUCE AND ITS PRODUCTION

LETTUCE BREEDING

Lettuce flowers are normally selfpollinated. The structure of the

flower facilitates self-fertilization

but makes difficult the manipulation

of the flowers for crossing. Normally, the flowers open in early

morning, remain open for a short

time, and then wither. Pollination

occui's during this short period.

The style is enclosed in a tube formed by fusion of the anthers (fig. 2).

It grows up through this tube, and

the anthers dehisce as it pushes up

through the tube. When the stigmas emerge from the tube, they are

covered with pollen grains. It is

always necessary to wash the grains

off before applying pollen from another source to make a hybrid (fig.

3).

Some of the pollen grains, however, will have already germinated,

and complete elimination of selfing

in a flower head that is cross-pollinated is therefore virtually impossible. It is usually necessary to

carry a dominant marker gene in

the pollen parent in order to distinguish the crosses from the selfs.

The highest proportion of crosses is

obtained if the self pollen is removed and foreign pollen applied

when the stigma lobes have just

started to separate. Cross-pollinating before or after this stage will

result in a very low proportion of

crosses.

At least one form of male sterility

has been found in lettuce. This

may be useful to prevent selfing,

since a male sterile plant forms no

viable pollen. When a plant of this

type is used as a female parent, pollen from the male parent alone will

function and only crosses will be

obtained.

Little is known about the development of the old standard varieties

'Coro/h

anther ont/i,

Me,'eot/i

M-yOo¿}/:)¿js

_ Sfi^rr.

-û/ffhcr

- s//ç^na

-sry/ûr

'âr/sr/es

\M'.\— anî/ier

s/teath

—-/PO//en

-sMor

àr/sffes

~s7y/c

-ovu/e

FIGURE 2.—An mclividual lettuce floret : A, At an early stage of anthesi.?, just before

the stigma emerges from the anther sheath ; B, with the stigma emerging from

the apex of the sheath ; O, at the stage satisfactory tor pollen removal ; B, at a

late stage of deyeiopment. A flower at this last stage is too far advanced for

successful removal of pollen and cross-fertilization. (From (28).)
AGRICULTURE HANDBOOK 221, U.S. DEPT. OF AGRICULTURE

FIGURE 3.—The operator is washing the pollen from the styles and stigmas of the

composite lettuce flower : it will then be ready for application of foreign

pollen.
LETTUCE AND ITS PRODUCTION

of lettuce before the introduction oí

the Imperial varieties. Most of

them were listed as introductions

by various seedsmen and farmers in

the United States, as foreign introductions, or as selections from otlier

varieties.

The first major completely documented breeding of lettuce was initiated by I. C. Jagger of the U.S.

Department of Agriculture in 1922.

Brown blight, a disease whose cause

is still unknown, threatened the existence of the western lettuce industry in the 1920's. The main variety

at that time, New York, was severely damaged by the disorder.

Resistant plants were selected

within the variety New York, and

crosses between New York and

White Chavigne, a resistant French

variety, were made in an effort to

obtain additional adapted, resistant

strains. Crosses were made between New York and Blonde Lente

a Monter to obtain resistance to

downy mildew, also a serious disease

in the West. Starting with Imperial F in 1930, a large group of

varieties with the name Imperial

and a letter or number were released

and soon became the major group of

crisphead varieties grown in the

United States. They were resistant

to brown blight and to the known

strains of downy mildew.

A variety released by the U.S.

Department of Agriculture and the

Michigan Agricultural Experiment

Station in 1941 was a different horticultural type named Great Lakes.

Further selection within the type

have resulted in numerous varieties

and strains, which make the Great

Lakes type the most widely

adapted crisphead lettuce in the

United States. Because of this wide

adaptation, its extreme solidity and

weight, and its favorable shipping

qualities, it became the most extensively grown lettuce type in the

United States and largely replaced

varieties of tiie Imperial type. Its

eating quality, however, is only fair

to poor.

Horticultural improvement continues as an important lettucebreeding goal. Introductions of

new varieties by the U.S. Department of -agriculture and the California Agricultural Experiment

Station liave been made with this

in mind. Climax, Vanguard, and

tlie four Golden State varieties (see

p. 13) are examples.

Problems of disease resistance,

liowever, are of major importance.

Since the introduction of tlie Imperial varieties, another race of the.

fungus causing downy mildew appeared. None of the commercial

varieties except Imperial 410 was

resistant to this race. Much of the.

lettuce research of the U.S. Department of Agriculture, the California

.agricultural Experiment Station,

and the Texas Agricultural Experiment Station has been to obtain resistance to this new race and to

breed resistant varieties. Two varieties have been introduced so far:

Valverde, by the Texas Agricultural Experiment Station and the

U.S. Department of Agriculture in

1959, and Calmar, by the California

Agricultural Experiment Station

and the U.S. Department of Agriculture in 1960.

Another lettuce disease of tremendous economic importance in

the West is mosaic. An extensive

search for varieties resistant to this

disease has been made, but up to

1962 no satisfactory source of resistance has been found.
AGRICULTURE HANDBOOK 221, U.S. DEPT. OF AGRICULTURE

CLASSIFICATION AND ADAPTATION

OF LETTUCE VARIETIES

Thompson (SO) suggested a division of lettuce varieties into tíve

classes, as follows: (u) Crispliead

varieties; (b) butterhead varieties;

(e) cos, or romaine, varieties; (d)

looseleaf. or bundling, varieties:

and I e} stem lettuce.

Thompson's classification is a

simple and practical means of segregating tile vast number of lettuce

varieties ( an estimated 150 or more )

into easily identifiable groups. It

has been accepted by most horticulturists in this country and will be

followed in this handbook.

Crisphead Varieties

Tlie crispliead varieties are distinguished by their firm heads and

brittle texture. The leaves form a

head by overlapping one another in

a smooth, regular manner. The

veins are coarse and the ribs are

prominent. The head is usually 6 or

more inches in diameter.

Crisphead is the most important

commercial type of lettuce in this

country. There are numerous crisphead varieties, each with some specific character or set of characters

that make it desirable for culture

in a particular region. Their firm,

compact, solid heads make them

popular. Such heads withstand

long-distance transportation and

the stresses and shocks associated

with harvesting and marketing

practices. Since the crisphead varieties can be delivered to the consumer in good condition even after

long-distance transportation, they

can be produced in areas best

adapted to lettuce culture. Nearly

all the lettuce harvested for market

in the western region is of the crisphead type, and probably one-half or

more of that produced in the other

regions is of this type.

Some common varieties of crisphead lettuce are Great Lakes Eegular, Premier Great Lakes, Imperial 101, and Imperial 61.5. Most

of the modern crisphead varieties

have been derived By selection in

progenies evolved from the old variety Xew York (synonyms—Los

Angeles Market and "Wonderful)

crossed with various other varieties,

botli foreign and domestic.

Butterhead Varieties

The butterhead varieties are distinguished by their soft pliable

leaves and delicate buttery ñavor.

The leaves form a head by overlapping one another in a smooth regular manner. The veins are finer

and the ribs are less prominent than

those of the crisphead varieties.

The head is less firm and somewhat

smaller than that of the crisphead

type.

Because of the soft, pliable texture of the leaves of the butterhead

varieties, they bruise and tear

easily. The damaged tissues become discolored, often blackened

and unsightly, before the lettuce

arrives in the retail markets. Consecpiently. butterhead varieties are

not adapted to long-distance transportation. Even for local markets,

the produce must be handled with

great care to place it before the consumer in good condition.

The butterhead varieties are

traditionally grown near the centers of popuktion in the East,

along the east coast of New York,

New Jersey, Virginia, and North

Carolina, and in the Middle West.

Prior to the development and ex-
LETTUCE AND ITS PRODUCTION

paiisioii of tlie lettuce industry in

the western region duiing the first

and second decades of this century,

the butterhead varieties were predominant in eastern markets. They

rapidly lost favor, however, to the

crisphead varieties, until it became

imperative to develop crisphead

varieties adapted to eastern conditions.

During the 1950's, because of

consumer dissatisfaction with the

quality of some of the crisphead

varieties, production of the butterhead varieties revived, especially

for local and nearby markets.

Principal varieties of butterhead

lettuce are Big Boston, White Boston, Bibb, and May King.

Cos, or Romaine, Varieties

The cos, or romaine, varieties are

easily recognized by the upright

character of the plant, the long,

loaf-shaped head, and the long, narrow leaves. The varieties of cos are

divided into self-closing and looseclosing types. The leaves of the

self-closing varieties curve inward

at the tips and form a well-blanched,

closed head. The loose-closing varieties will not form a closed head.

The leaves appear coarse, but they

are tender, sweet, and tasty and

have less bitterness than other varieties. The table quality of the cos

varieties is excellent.

The growing popularity of the

tossed salad has increased the demand for cos varieties. The darkgreen outer leaves and the goldenyellow inner ones are attractive and

flavorsome ingredients of such

salads.

However, the cos varieties, like

the butterhead varieties, are ill

adapted to long-distance transportation. The elongated heads with

spatulate leaves are difficult to fit

into conventional packages. The

stiff, but tender, leaves are easily

torn, punctured, and crushed.

Therefore, the cos varieties are

grown mostly for local and nearby

markets, but there has also been a

small but steady demand for these

varieties in the winter-production

areas of the western region for more

distant shipping. From these areas

CDS lettuce is sliipi)ed mainly in cars

with other vegetables. Common varieties of cos lettuce are Dark Green

and White Paris.

Looseleaf, or Bunching,

Varieties

The looseleaf, or bunching, varieties do not form heads. The leaves

are clustered or pressed together,

but only the young ones at the center of the plant overlap to any extent. The older leaves are arranged

loosely around the stem.

Looseleaf varieties are popular

for the home garden and are the

principal varieties grown in greenhouses. They are not adapted to

long-distance transportation and

have a short mai-ket life. Their soil,

water, and temperature requirements are not so exacting as those

of the three preceding classes;

therefore, they are better adapted to

the needs of the home gardener.

Common varieties of looseleaf lettuce are Black-Seeded Simpson,

Prize Head, Grand Eapids, and

Salad Bowl.

Stem Varieties

The edible part of stem lettuce is

the enlarged stem, or seedstalk. It

may be peeled and eaten raw or it

may be boiled, stewed, or creamed.

Stem lettuce is an ingredient of

many Chinese dishes. It is little

used in this country except by people of Chinese extraction. Celtuce

is the only variety offered for sale

in the United States.
10 AGRICULTURE HANDBOOK 221, U.S. DEPT. OF AGRICULTURE

DESCRIPTION OF THE IMPORTANT

VARIETIES OF LETTUCE

Crisphead Varieties

The crisphead varieties of lettuce

can be separated into two distinct

types: (a) Imperial and (b) Great

Lakes.

Varieties of the Imperial type are

mostly large and normally medium

green. The leaves are thin, entire

margined, savoyed, and crumpled.

They form symmetrical, partly exposed heads, with many wrapper

leaves. The quality is good to

excellent.

Great Lakes types are generally

chai'acterized by dark-gi'een, thick,

crisp, tough, serrate leaves. They

form extremely firm, heavy, mostly

exposed heads. They are moderately slow bolting and resistant to

tipburn. The quality is only fair

to poor; the texture is generally

coarse and tough.

Varieties of the Great Lakes type

were derived from crosses between

varieties of the Imperial type and

the little-known variety Brittle Ice.

Unfortunately, during World War

II stock seed of Brittle Ice was

allowed to deteriorate : as a result

this variety ceased to exist. Lack

of seed of the Brittle lee parent has

frustrated efforts of plant breeders

to produce new varieties of Great

Lakes type with superior quality.

Except for poor eating quality and

inability to make adequate size

under moderately low temperatures,

the Great Lakes varieties are ideal

for the grower who must depend

upon surface transportation to send

his produce long distances to

market.

Varieties of Imperial T^i^je

New York.—Large : late maturing ; mature plant spreading ; leaves broad,

slightly saToye<i, crumpled, stiff, coarse.

with heavy veins and protruding midribs; a slow bolter; quality good; seeds

whitish. Thi.s variety was named and

introduced by Peter ¿lenderson and Co.

in 1.S96. It was grown almost exclusively

during the development of the lettuce industry in the western region. Siace

about 1926, New Yorl¿ has been replaced

with disease-resistant and better adapted

varieties. Xearly all the conunercialiy

important varieties of the Imperial type

have descended from New Yorls through

a combination of hybridization with other

varieties, bacliorossing, and selection.

New Yorii was first shipped to eastern

marlsets as "Iceberg" or "Western Iceberg." This was a misuse of the name,

as the Iceberg variety is altogether different from Xew Yorl:. There are several seedsman's strains of New Yorli.

One of them, New Yorli 515, is supposedly

much more resistant to tipburn than

other Xew Yorli strains.

Imperial 44.—Medium size ; for summer or fall maturity ; medium green ;

leaves broad, long, much savoyed. crumpled, with smooth margins ; heads

symmetrical, spherical, nearly always

covered with wrapper leaves ; a slow

bolter ; not so susceptible to tipburn as

other varieties of this type : quality excellent—crisp and tasty ; seeds whitish.

This variety was developed by the U.S.

Department of Agriculture and the Xew

York (Cornell) Agricultural Experiment

Station and released in 1938. Imperial

4-i was originally selected for cxilture on

the muck lands of northern N'ew York,

but it has been replaced by varieties oí

the Great Lakes type, which are less

su.sceptible to tipbtim. It is grown

mostly in the western region, but there

also it is gradually losing favor to yarieties of the Great Lakes type.

Imperial 615.—Large, vigorous; for

late-tall or winter harvest ; plant spreading at maturity ; dark green ; leaves

broad, slightly savoyed. crumpled; heads

well formed, solid, almost spherical,

covered with wrapper leaves except for

the apex: bolts quickly and tipboms

badly if exposed to high temperatures;

quality excellent ; seeds whitish. This

variety was developed by the U.S. Department of Agriculture and the California Agricultural Experiment Station,

and released in 1934. Imperial 615 will

produce good-sized heads under fairly

low temperatures. Until the late 1950's

it was essentially the only variety grown

for midwinter harvest in the western re-
LETTUCE AND ITS PRODUCTION 11

gion. It has been almost entirely replaced with Imperial 101.

Imperial 101,—This variety closely resembles Imperial 61Ö, except that the

plants are slightly larger and more

vigorous (flg. 4). It requires slightly

longer to mature than Imperial 615.

Most strains of this variety are less

smooth and less uniform than those of

Imperial 615. Like Imperial 615, it is

adapted for late-fall and midwinter harvest in the western region. The history

of this variety is obscure. It is obviously a selection from Imperial 615, but

the person or firm responsible for the

selection is not known with certainty.

There are reports that a Japanese grower

near HoUister, Calif., made the original

selection and increased the seed. In

some seed catalogs the HoUister Seed

Co. of HoUister, Calif., is listed as the

originator.

Varieties of Great La}{es

Varieties and strains of tlie Great

Lakes type are similar in appearance and so numerous that some

system of classiñcation is needed

for clarifying the differences among

them. They differ in size, color,

season of maturity, tipburn resistance, and other characters. Zink

and Welch {4^) studied 25 varieties

and strains of Great Lakes type

during one harvest season. As a

result of their studies they proposed

a division into four subtypes. The

Golden State varieties (introduced

in the late 1950's) appear to be different enough, especially in quality,

FIGURE 4.—Mature head of Imperial 101.
12 AGRICULTURE HANDBOOK 221, U.S. DEFT. OF AGRICULTURE

to warrant the addition of a fiftli

subtype. Tlie subtype groupings

used by Zink and Welch may not be

the best nor final answer to classification of the Great Lalves varieties,

but tlieir system brings some order

out of what otherwise would be

chaos. The similarities and differences are much easier to compreliend when they are arrayed in

groups similar in appearance and

environmental response.

Subtype 1 (subtype variety

Premier Great Lakes).—Characterized by few wrapper leaves,

exposed heads, color similar to subtype 5 but lighter green than other

subtypes, smooth leaf surface and

margin, early maturity, wliitisli

seeds. Principal varieties are Imperial 456, Pennlake, and Premier

Great Lakes.

Subtype 2 (subtype variety Great

Lakes -1:28).—Characterized by few

wi-apper leaves, exposed heads,

darker green color than subtype 1,

smooth leaf surface and margin,

larger heads and later maturity

than subtype 1, whitish seeds.

Principal varieties are Great Lakes

i-2x. Great Lakes .59, and Emerald.

Subtypes (subtype variety Great

Lakes Regular).—Characterized by

adequate wrapper leaves, good

cover, slightly savoyed leaf surface,

dentate ruffled leaf margins, darker

green color than subtypes 1 and 2,

late maturity, whitish seeds (fig. 5).

Varieties in this subtype are the

most widely grown of the Great

Lakes type. Principal varieties are

Great Lakes Regular, Great Lakes

118, Great Lakes 66, Great Lakes

366, Great Lakes A-36, Great Lakes

FIGURE 5.—Mature head of Great Lakes Regular.
LETTUCE AND ITS PRODUCTION 13

407, Great Lakes R-200, Oroverde,

and Grandeverde.

Subtype 4 (subtype variety Great

Lakes 659).—Characterized by

abundant wrapper leaves, excellent

cover, markedly dentate frilled or

ruffled leaf margins, darker green

color and later maturity than other

subtypes, slow bolting, heat resistance, whitish seeds. Principal varieties are Great Lakes 659, Wespak, Primaverde, and Gem.

Subtype 5 (subtype variety

Golden State A).—Characterized

by adequate wrapper leaves, good

cover, about the same color as subtype 1, late maturity, good quality,

black seeds except Golden State

D—which has white. Principal varieties are Golden State A, Golden

State B, Golden State C, Golden

State D, and Climax.

Other Crisphead Varieties

Two new distinctive crisphead

varieties, Vanguard and Valverde,

do not fit into either the Great

Lakes or Imperial category.

Vanguard.—Large, vlgorou.s ; adequate

wrapper leaves ; fair cover ; leaves grayish green, smooth, with regular margins,

thick, good texture; Interior leaves yellowish ; quality good ; seeds black (fig.

6). Vanguard is different in origin from

other lettuce varieties. E. C. Thompson,

who developed Vanguard, combined a

wild species of lettuce. Lactuca virosa,

with cultivated lettuce. Like the L. virosa

parent. Vanguard has smooth, grayishgreen leaves. The basal ones are retained

throughout the life of the plants.

Valverde.—Intermediate in appearance between Great Lakes and Imperial

types. Few wrapi>er leaves ; partially

exposed heads ; leaves dark green, somewhat blistered and savoyed ; heads firm

but not hard ; quality good ; seeds white

FIGURE 6.—Mature head of Vanguard.
14 AGRICULTURE HANDBOOK 221, U.S. DEPT. OF AGRICULTURE

(fig. 7). It is resistant to the physiological races of downy mildew occurring

in the lettuce-growing areas of Texas,

Arizona, and California.

Butterhead Varieties

Bibb.—Small; early to luidseason;

plant loose and spreading when young,

compact when mature ; leaves short,

spatulate, closely clustered to form a compact globular head, not overlapping, dark

green with some dark-red pigment, thick,

smooth, brittle ; a rapid bolter ; quality

excellent—delicate buttery flavor, sweet.

tender ; seeds black. It is a good variety

for the home garden, greenhouse, and

local market. It was introduced into

this country about 1890 as Half Century.

It disappeared for a time from trade

channels : later it returned under the

names Bibb and. erroneously. Limestone.

Big Boston.—Medium large ; intermediate in season ; young plant with upright

twisted leaves : mature plant compact,

with firm, well-denned, broad, slightly

pointed head ; leaves broad, smooth,

slightly savoyed, somewhat twisted, thick,

stiff, hard, dull light green except for the

borders of the outer leaves—which are

tinged reddish brown ; quality fair—not

so sweet and tender as other butterhead

varieties ; bolts easily and is very susceptible to tipburn damage ; seeds whitish

(fig. 8). It is widely adapted, but grown

mostly for local markets and short-distance shipping in the East and Southeast

White Boston.—Except that it is light

green and lacks any trace of red pigment

in the leaves, this variety is almost identical with Big Boston. It has the same

range of adaptation, and is preferred to

Big Boston when an all-green head is desired. It is known in Europe as Unrivalled and appears to be the same as

the French variety Sans Rival.

Continuity (Crisp as Ice).—Large;

late ; spreading ; forms loose, weUblanched beads ; leaves loosely overlapping, broad, cup shaped, savoyed, twisted,

thin, soft, limp, with entire margins, deep

green overlaid with brown ; a slow bolter; quality good—^soft texture, sweet;

seeds black. A good summer variety in

some localities, but used more as a novelty. It was introduced into this country about 1878,

May King.—Small; very early; light

green with slight red tinge when grown

outdoors ; leaves spatulate, thin, brittle,

clustered into small, round heads ; inside

leaves creamy yellow ; quality goodbuttery flavor, sweet, tender ; seeds

whitish. It can be used for forcing and

is a good, early variety for home gardens.

It bolts easily and scorches readily at

high temperatures.

FiGUEE 7.—Mature head of Valverde.
LETTUCE AND ITS PRODUCTION 15

Midas.—Large ; intermediate in season : leaves light green, slightly f^avoyed ;

a slow bolter; cinality good; seeds whitish. Similar to Big Boston except that

it has no red pigment in the leaves, bolts

slowly, and matures later, and the heads

are not so firm. A good variety for the

home and market garden. It was developed by the U.S. Department of Agriculture and released in li)54.

Salamander.—Medium large; early to

intermediate in season ; compact or

slightly spreading; leaves broad, savoyed,

crumpled, thin, limp, soft, with entire

margins, light green ; heads globular,

fairly firm ; quality excellent—delicate

buttery flavor, sweet ; a medium bolter ;

seeds brownish black. A popular variety

for the home garden and local market.

It withstands high temperatures better

than most butterhead varieties. Salamander was introduced into this country

from Europe about 1854. It is sold under

more names than any other variety ;

Tracy (32) listed 38 synonyms.

Cos, or Romaine, Varieties

White Paris.^Large ; self-closing ; intermediate or late in season ; leaves oval

to spatulate or slightly spatulate, curving inward at the ends and overlapping

to form a loaf-shaped, rather loose head ;

a slow bolter ; quality excellent—firm,

crisp, sweet ; seeds whitish. It Is an old

variety of European origin ; it has been

listed by American seedsmen since 1860.

This is one of the best varieties for the

home garden.

Dark Green.—Similar to White Paris,

except perhaps a little darker green,

slightly shorter, and more open at the

top; seeds whitish (fig. 9). The origin

of this variety is unknown.

FIGURE 8.—Mature head of Big Boston.

617152 O—62 3
16 AGRICULTURE HANDBOOK 22 1, U.S. DEPT. OF AGRICULTURE

Parris Island.—Medium size ; not completely self-cLosing, often tied close with

a string in retail markets; 1 week to 10

days later to mature tlian White Paris

or Dark Green ; leaves more coarse and

tough than either of those varieties, dark

green ; quality medium ; seeds whitish

i fig. 10 ). This variety was introduced

by the South Carolina Agricultural Experiment Station in 19Ó1. In many districts it is replacing the older varieties

because it manifests some tolerance to

the virus causing lettuce mosaic. Plants

usually will produce marketable heads

even though infe<:ted with mosaic ; whereas other varieties fail t(j mature a

crop.

Looseleaf, or Bunching,

Varieties

Black-Seeded Simpson.—Large; midseason ; compact plant : leaves broad,

savoyed. crumpled, twisted, thick, stiff.

coarse, with serrate margins and prominent midribs, light green : easily grown,

tolerant of high temperatures ; a slowbolter ; quality only fair ; seeds black. It

is adapted to home gardens. This variety was first listed in this country by

Peter Henderson & Co. about 1880.

Early Curled Simpson-^iledium size;

early to midseason ; leaves short-spatulate to broad, savoyed, crumpled, thick,

stiff, with serrate or frilly margins, light

green; quality only fair; seeds whitish.

It is well adapted to the home garden

but bolts readily. According to Tracy

Í32), it has been known in this country

since about l.SßO.

Grand Rapids.—^Large; early to midseason ; plant spreading when yonng,

fairly compact when mature ; leaves long,

somewhat spatulate, crumpled, blistered,

thic'k. with heavy midribs and frilled

margins, light or yellowish green; bolts

easily : quality poor outdoors, but very

good when grown under glass ; seeds

black. It is adapted to greenhouse culture (31) and is used more extensively

for this purpose than any other variety.

It is a very satisfactory variety for spring

sowing in the Northwestern States. It

bolts easily and the quality is poor farther south. This variety was first listed

FIGURE 9.—Mature head of Dark Green cos lettuce.
LETTUCE AND ITS PRODUCTION 17

by seedsmen in 1890, but it is said to

have been cleveloi^ed by Eugene Davis of

Grand Rapids, Mich., at a much earlier

date. It is reported to have been obtained after 15 years of selection within

the variety Black-Seeded Simpson.

Prize Head.^—Large ; early ; plant

spreading ; leaves thin, short, spatulate,

savoyed, crumpled, with serrate margins,

frilled at borders, tinged red but bright

green where not exposed ; wilts easily ;

a slow bolter; quality good—sweet, tender, crisp; seeds whitish. It is a good

variety for home gardens (fig. 11). The

ori^iin of this variety is unknown; it was

named and introduced by D. M. Ferry &

Co. of Detroit, Mich., about 1868.

Salad Bowl.—Large ; medium early ;

leaves much divided and lobed, medium

green; very attractive; tolerant of high

temperature ; a .slow bolter ; quality fair ;

seeds black (fig. 12). It was developed

by the U.K. Department of Agriculture

and released in 1950.

SEEDBED PREPARATION, PLANTING, AND

THINNING

Where the crop is grown under

furrow irrigation, the land should

be properly leveled to permit accurate control of water (1-5}. If the

land is not leveled tlie beds will be

variable in height ; uneven areas are

difficult or impossible to irrigate

properly. Tlie soil will be too dry

in some areas and too wet in others

and tlie crop will germinate and develop irregularly. Tliese conditions often lead to disease infection,

a prolonged harvest period, and a

product of variable quality.

TIQUEE 10.—Mature liead of Parris Island cos lettuce.
18 AGRICULTURE HANDBOOK 22 1, TI.S. DEPT. OF AGRICULTURE

The usual routine in preparing

the seedbed is to plow, disk, and

float. This sequence of tillage operations should leave the soil in a

soft, mellow condition and free

from large clods and trashy organic

material. If the sequence is ineffective, it may be necessary to repeat one or all of these operations

or to vary the sequence. The character of the soil and the preceding

crop determine the tillage sequence

required.

In the northern region and coldest parts of the Southeast, the land

is often plowed in the fall and left

rough imtil spring when the seedbed is prepared. Thus, the soil is

exposed to alternate freezing and

thawing during the' winter, which

makes it in a fine, mellow condition

in the spring, excellent for further

tillage operations.

Most of the lettuce crop in the

West is planted on raised two-row

beds. The beds are furrowed with

a lister, usually with two or three

14-inch bottoms, to depths of 4 to

8 inches. The beds commonly are

40 or 42 inches from center to center. The seedbed is 18 to 22 inches

wide; the furrow 20 to 22 inches

wide. The seed is ordinarily sown

in rows .3 to 4 inches from the edge

of the bed, which leaves 14 to 16

inches between the rows (fig. 13).

The beds are ordinarily shaped,

smoothed, and planted in one operation.

FIOUEE 11.—Mature plant of Prize Head.
LETTUCE AND ITS PRODUCTION 19

^ll#wf

FlGUEE 12.—Mature plant of Salad Bowl.

* r'*

H1

. Bed..

40

--20'-y-;Rpw

Furrow 4° /'":3"*l-:-

1

1

1

--I4"

•.Bed

Row

' -.' ' . '." ' . '

FIGURE 13.—Diagrammatic sketcli of the double-row planting system on slightly raised

beds. The dimensions may yary slightly with local conditions. If soluble salts

are troublesome, peaked beds can be used.
2Û AGRICULTURE HANDBOOK 221, U.S. DEPT. OF AGRICULTURE

If good, viable seed of over 90

percent germination is used, the rate

of seeding depends primarily upon

climatic factors. Lettuce seed is

very sensitive to high temperatures

during germination and emergence.

Soil temperatures higher than 75"

to 80° F. for prolonged periods

greatly reduce emergence.

For early-fall planting in the

desert areas of the western region

when temperatures are high, 2i/¿ to

31/2 pounds of seed per acre is necessary. Heavy seeding at this time is

insurance against poor stands.

Later in the season as the weather

becomes cooler, li/G to 2 pounds of

seed per acre is sufficient. In cool

weather cover the seed to a depth of

one-fourth to one-half inch. In hot

weather, barely cover the seed. The

action of irrigation water will help

to settle the soil enough to cover the

seed.

When the young seedlings have

reached the two- to three-leaf stage,

thin and space them 12 to 16 inches

apart in the row. The spacing between plants in the row will vary

with the variety, soil fertility, and

climatic factors. Complete thinning the plants before they are affected by competition for space or

nutrients. The time from germination to thinning varies from 3 to 8

weeks; the time depends mostly

upon temperature.

Thinning is an expensive handlabor task, usually done with shorthandled hoes. The plants may be

blocked out mechanically in clusters

to appropriate distances and then

further thinned by hand to a single

plant at each point. Care must be

taken not to leave two plants side by

side, as such plants develop asymmetrically and almost never make

marketable heads. Some growers

draw a heavy (about 1,000-pound)

roller, approximately 10 feet long

and spanning three beds, parallel

with the beds over the young plants

immediately before thinning. This

operation firms and smooths the

space between the seed rows, which

makes thinnirig easier. Whether

rolling is harmful to the plants has

not been determined.

SOIL MANAGEMENT AND FERTILIZATION

Lettuce is very sensitive to the

acid-alkaline balance in the soil.

The soil reaction should be nearly

neutral (about pH 6). Lettuce is

intolerant of acid soils, but it has

a fairly high salt tolerance ; for this

reason it does well on the alkaline

soils of the West. In many parts of

the East the soils are too acid for

maximum growth. Apply lime to

correct the acid reaction before

planting such soils to lettuce.

Lettuce is surprisingly modest in

its nutrient uptake. Lorenz and

IVIinges (19) estimated that a sum--

mer crop of lettuce in the Salinas

Valley of California will contain 47

pounds of nitrogen, 7 pounds of

phosphorus, 117 pounds of potash,

and 30 pounds of lime per acre.

The relatively low mineral nutrient

content of lettuce does not mean

that it can be gi'own successfully on

soils of poor fertility. In fact the

opposite is true ; the best crops of

lettuce are produced on soils of high

fert ility.

Among the methods of increasing

and maintaining the fertility and

good physical structure of the soil

are the use of proper tillage, greenmanure crops, animal manures, and

commercial fertilizers including

synthetic ones. Usually an appropriate combination of two or more

of these methods produce the desired result most efficiently and economically. Specific rates and kinds

of fertilizer to be applied cannot be

given here because the needs can
LETTUCE AND ITS PRODUCTION 21

vary from farm to fann and even

from field to field on the same farm.

However, general principles of fertilizer practice for lettuce on individual farms and fields follow.

Green Manures

Green-manure crops, when properly grown, can furnish an abundance of organic matter. In regions of heavy rainfall, gi'eenmanure crops aid in preventing

erosion and in conserving the soluble nutrients in the soil. At the

time tliese cx-ops are turned under,

additional nitrogen may be needed

to aid in decomposing the organic

material. Furthermore, in the

western region, green-manure crops

are usually watered by the flood

method of irrigation, which tends

to leach out the soluble salts from

tlie zone that Avill later be occupied

by the roots of the lettuce plants.

Grasses and leguminous crops sucli

as alfalfa, clover, cowpeas, soybeans, and vetch are good sources

of organic matter. In the western

region, sesbania and papago peas

are often used. Legiunes add nitrogen to the soil, but some grasses

produce more organic matter than

legumes in a short time and may be

preferable. Grasses require the

addition of more commercial nitrogen than do legumes; but, because

of the higher tonnage produced,

grass plus nitrogen is more efficient

than legumes alone in many situations.

Animal Manures

Lettuce shows a marked response

to animal manure. Pew and coworkers (S4) found that lettuce

grown on manured soil matures

earlier, is of better quality, and

yields more than lettuce grown on

comparable nonmanured soils. Use

manure with caution on lettuce

grown for summer or fall harvest in

the hot areas of the western region.

Excessive manure and high temperatures may result in plants with

loose heads or premature seedstalk

formation.

Well-decomposed barnyard manure, free from weed seeds, is a

most desirable source of organic

material. In applying manure,

sjjread it evenly over the field and

plow it under or disk it in promptly.

Even witli generous applications of

animal manures, some nitrogen is

usually necessary to produce the

most profitable crop. The nitrogen

is conunonly added as a side dressing shortly after thinning.

Commercial Fertilizers

Of the three elements—nitrogen,

phospliorus, and potassium—applied as commercial fertilizers,

nitrogen is the one likely to be deficient in most soils. This is especially true in soils of the western

region where there is usually sufficient native phosphorus and potassium in the soil for the crop.

Xitrogen can be applied in several

ways and in many forms. Method

and rate of application are generally more important than the source

of the commercial forms of nitrogen

commonly used.

For Arizona, Pew and coworkers

(2,5) suggest applying about onehalf to two-thirds o"f the total nitrogen before planting. Tliis may be

broadcast in granular or fine crystalline foi-m, injected into the soil as

a gas or solution, or applied in the

irrigation water. The rest of the

nitrogen is applied in solid form as

a side dressing or in irrigation

water immediately after thinning

and later as the needs of the plants

dictate. This method is probably

applicable for most soils in the

western region.

During cool weather, organic nitrogen, except synthetic urea, is

available to the plants very slowly.
22 AGRICULTURE HANDBOOK 221, U.S. DEPT. OF AGRICULTURE

if at all. Therefore, some source of

synthetic or natural inorpxnic nitrogen is essential to maintain good

growth during cool weather. The

common forms of inorganic nitrogen are nitrate of soda, sulfate of

anunonia, ammonium nitrate, and

agricultural ammonia. Urea, a

synthetic organic form, is sometimes used. In the western region,

do not use nitrate of soda because

its sodium ion tends to make the

soils too alkaline. Wiere the pH

of the soil is near, or above, neutral,

sulfate of ammonia is beneficial ; on

acid soils do not use it or use it

sparingly. In the western region,

anhydrous ammonia is frequently

applied in the irrigation water.

The ammonia gas is metered into the

water. Careñd control of the dispensing apparatus is necessary ; the

companies that furnish the material

usually supervise its application.

On some soils in the western region lettuce responds to added

phosphorus. Responses to phosphorus are especially marked when

the plants are in the seedling stage.

"WTien adequate phosphorus is available, the seedlings become established more quickly and grow faster

than they do on soils low in this element or on soils in which much

phosphorus is present but fixed in

unavailable form. For soils that

are deficient in available phosphorus, supply this element before

planting the seed or in the first side

dressing. Phosphorus may be applied in several forms, such as phospliate, superphosphate, treble superphosphate, and liquid phosphoric

acid.

Most soils adapted to lettuce culture have enough potassium for lettuce production. Evidence is lacking that applications of fertilizer

potassium improves yield or quality

of lettuce. But there is evidence

tliat excess potassium may be toxic

within the plant and thus be harmful to its growth.

Correct fertilizer placement is

important. The movement of

phosphorus in the soil is extremely

limited; therefore, place phosphorus fertilizers where the roots will

grow quickly into the fertilized

zone. In contrast, inorganic nitrogen fertilizers, especially the nitrate form, move freely in the soil

solution. Consequently, plac« such

nitrogen material at a level in the

soil at which leachiiig below the root

zone will be minimized. For the

preplanting application, place the

fertilizer no deeper than 6 to 8

inches. In sidedressing young

plants, place the fertilizer 11/2 to 2

inches below the seed level and cultivate about 2 inches into the furrow. In sidedressing large plants,

place the fertilizer I14 inches deep

and 4 to 6 inches to the furrow side

of plants.

ROTATIONS

In selecting crops for a lettuce

rotation, keep in mind the purposes

of a rotation. In general they are :

( 1 ) To reduce or eliminate soilborne

diseases; (2) to maintain or increase soil fertility; (3) to improve

the physical structure of the soil;

and (4) to control weeds and insect

pests.

Deep-rooted leguminous crops,

such as alfalfa and clover, are valuable as alternate crops with shallowrooted crops, such as lettuce, for

they tend to open up the soil to

greater depths. Furthermore, the

zone of nutrient absorption for

these roots is deeper than for shallow-rooted crops. They also add

nitrogen to the soil. Small grains

and many leguminous forage crops

are well adapted and valuable for

use in a lettuce rotation as they are
LETTUCE AND ITS PRODUCTION 23

not subject to the same root diseases

that attack lettuce. Of the vegetable crops, cucurbits, beets, spinach, tomatoes, onions, and sweet

corn are usually not subject to diseases that affect lettuce.

Soil on which lettuce is produced

inevitably receives much compaction from heavy equipment in tillage and harvesting operations.

Many times compaction is aggravated by the necessity to have heavy

equipment on the soil when it is so

moist that it compacts badly. To

counteract these harmful effects,

select a crop for the rotation that

requires less heavy equipment for

culture and that requires the equipment to be used only when the soil

is in a more nearly ideal condition.

IRRIGATION

Regulating and maintaining favorable soil moisture is necessary to

produce a successful crop of lettuce.

For optimum growth, lettuce requires a constant and relatively

abundant supply of soil moisture

throughout the growing period.

Sharp fluctuations in soil moisture,

especially in the later stages of development, are severely detrimental

to normal growth.

In the western region where the

crop is raised almost entirely without rainfall, the grower has close

control over soil moisture. In the

eastern and southern regions this is

not true. Some control can be exercised over drought, however, by

supplemental irrigation: Whenever rainfall is inadequate in these

areas, irrigation water will be

needed to produce a satisfactory

crop. Thompson (30) estimates

that in the East and South 5 to 8

inches of rainfall or the equivalent

in irrigation water is necessary during the growing, period of the

spring crop. The amount required

depends upon such factors as character of the soil, the amount of

cloudy weather, and the prevalence

of winds. Supplemental irrigation

water, usually applied through a

sprinkler system, can insure adequate moisture for the crop. Considerable skill and experience are

necessary to integrate rainfall and

irrigation so that soil moisture is

maintained at favorable levels

617152 0—«2 1

tliroughout the growing season.

Unpredictable rains following soon

after irrigation may I'esult in a serious excess of water.

In the western region, lettuce is

irrigated by open furrows. The

water is taken from the head ditch

into the individual furrows by

means of metal tubes used as siphons, short pieces of rubber tubing, or wooden conduits made from

lath. With each of these methods

the flow of water from the head

ditch into the furrow can be regulated easily.

Under some circumstances an irrigation before furrowing is justified; it germinates weed seeds that

are later destroyed by light disking

and it leaches the soluble salts out

of the surface layers of the soil

where they might interfere with

seedling emergence.

Usually start irrigating immediately after planting; however, if

a delay in germination is desired

start irrigating later. For the initial irrigation, apply enough water

to moisten the entire soil mass of

the beds. Do not permit the surface soil to dry until the young seedlings have emerged. Some growers

believe tíiat running water has a

cooling effect upon the soil, so m

warm weather they allow the water

to flow long after the soil is saturated. This is a questionable practice because it tends to waterlog the
24 AGRICULTURE HANDBOOK 221, U.S. DEPT. OF AGRICULTURE

soil, which reduces the air content

and prevents good germination.

The result may be a spotty, uneven

stand. Often the crop must be

disked up and the field reseeded.

The fall crop in the western reiriou may need an irrigation prior

to thinning to soften the soil as an

aid in thinning. Immediately after

thinning an irrigation is beneficial;

it firms and moistens the soil around

the plants, which aids them in making a quick recovery from the thinning operation.

The freciuency of irrigation after

thiruiing depends on the character

of the soil and climatic factors. In

parts of the Monterey Bay area of

California where the climate is cool

and rate of evaporation moderate to

low, Veihmeyer and Holland (Jß)

found that irrigation need not be

more frequent than once every 30

days after thinning to produce a

summer or fall crop of lettuce. In

Arizona where the climate is hot and

evaporation is high, Pew and coworkei-s (25) state that intervals of

11} to 14 days in the early stages of

development and 7 to 10 days as the

lieads mature are about the proper

frequencies for the fall crop. They

state, however, that the spring crop

may go as long as -30 days without

irrigation in the early stages of

development but that it requires irrigation every 20 to 23 days in the

later stages.

After the first han-est, it is customary to give the remaining plants

a light irrigation to insure enough

moisture for continued normal

growth. Excessive irrigations or

rain just before or during the har-

\-est season, especially if temperatures are high, may result in loose,

putîy heads. Excessive moisture

when heads are approaching market

maturity or are overmature also

may cause bursting of the heads.

CULTIVATION AND WEED CONTROL

The primary purpose of cultivation after the lettuce crop has been

planted is weed control. In fact,

Veihmeyer and Holland (36), on

the basis of several experiments in

the Salinas Valley of California,

suggest that this is the only reason

for cultivation. But most horticulturists insist that there are other

valid reasons for cultivating the

crop. These are: (a) To replace

the soil around the young seecUings

after thinning; (b) to open the furrow and loosen the soil surface for

improved penetration of irrigation

water; (c) to dust mulch the bottom

and sides of the furrow to facilitate

the application of fertilizers; and

(d), in nonirrigated areas, to break

up the surface crust after rains.

Horticulturists are generally agreed

however, that these are far less important than weed control. Many

growers undoubtedly have cultivated lettuce excessively.

"Weed control is necessary

throughout the growing season.

Weeds compete with lettuce plants

for nutrients and water and are

hosts for disease-producing viruses,

which can be transmitted to lettuce

plants by insects. Weeds are best

controlled when young. If the field

has been properly prepared and

cultivated, normally only one hoeing is necessary to control weeds

after thinning.

Lettuce is shallow rooted; therefore, cultivate, as nearly as possible,

only the surface layere of the soil (2

to 3 inches deep when grown on

irrigated beds ; 1% to 2 inches when

grow-n "flat.") Deep cultivation

tends to root-prune the plants; this

mechanically restricts the area from

which the roots may obtain nutrients and water.
LETTUCE AND ITS PRODUCTION

HARVESTING AND MARKETING

25

In 1948 a method of dry packing

and vacuum cooling was developed

in the Salinas-Watsonville district.

It is now the standard method in

the West and is often used in most

major production areas of the Midwest, East, and Southeast.

This new method liad extensive

effects on nearly all phases of harvesting and marketing. Until that

time head lettuce was cut in the field

and hauled in bulk to packing sheds

where it was trimmed and paclied

with crushed ice in wooden crates,

usually 4 or 5 dozen heads to a crate.

Crates were then loaded into railway refrigerator cars, and more

cruslied ice was blown on top of the

load before shipment.

With the new method, the lettuce

is cut and packed in the field (fig.

14). The first cutting, or topping

out, is made when the earliest maturing heads are hard. Usually

three or four cuttings are made, but

sometimes only one or two are made

if the market is slow or if quality is

poor. Cutting is ordinarily done

late in the morning and during the

afternoon when the plants are less

turgid than in early morning and

tlie leaves are less likely to break.

Labor crews in the West are

usually supplied by a labor contractor. The crews are divided into

groups, each with a specific job.

The harvesting-packing method

varies with the packing company,

area, and potential yield. Many

companies use the following

metliod.

The first group consists of the

cutters, who select mature heads, cut

them off close to the ground, trim

to six wrapper leaves, and set them

upside down on a bed. Each cutter

usually covers two beds.

Cartons are prepared by men on

a specially designed, slow-moving

FiGUBE 14.-A lettuce-harvesting crew at work in the field.
26 AGRICULTURE HANDBOOK 221, U.S. DEPT. OF AGRICULTURE

truck with wide-track wheels,

spaced SO inches to span two doublerow beds. These men fold and

staple cartons by machine and toss

them to tlie ground where other men

distribute tliem over the area to be

harvested.

Usually packers, pushing light

wheelbarrows carrying cardboard

cartons, follow the cutters. A

packer places a layer, usually of 12

heads, top up in the carton and

another layer of lí¿ top down (fig.

l."i). He places the full, open carton in the furi'ow and sprays water

on the exposed butts. A closer with

a poi-table tool closes and staples

the carton. Sometimes, a sheet of

paper is placed over the exposed

butts before the carton is closed.

FiGUBE 1.5.—A lettuce carton packed with 2 dozen heads, just before the carton is

closed and stapled
LETTUCE AND ITS PRODUCTION 27

When packed the cartons are

placed in two long rows, separated

so that another wide-track truck can

be driven between them; this permits simultaneous loading from

both sides of the truck. Two men

load the cartons onto pallets that

cover the truck bed. Two other

men on the truck arrange the cartons on the pallets. The load is

then tied down with rope and taken

to the cooler.

Some companies use a packing

machine carried on a truck. With

this, the cartons never touch the

ground. As the truck is driven

along the rows, the workers place

the lettuce heads on conveyor belts,

extending laterally from the moving truck, that carry the heads onto

the truck. There they are packed

into cartons. The packed cartons

are conveyed to the closing machine

and then to a second truck that

takes them to the cooling plant. ^ In

some areas, tractor-pulled, widetrack trailers rather than tracks are

used for hauling the packed cartons

to the cooler.

Crew sizes vary from 50 men to

as many as 300 men. The standard

50- to 60-man crew in the SalinasWatsonville district covers 14 beds

in a sweep. Larger crews fill up

to four or five truckloads at a time.

For light cuttings, fewer than 50

men are used.

In the West, the standard-size,

corrugated paperboard carton is

934 X 14 X 21 inches. It is designed

to hold 2 dozen heads, but it may be

packed with II/2 dozen larger heads

or 2i/¿ dozen smaller heads. Cartons are printed, usually in two or

three colors, with a shipper's label.

This gives a brand name, the name

and location of the shipper, the

number of heads, and the product.

Most shippers have two to four different labels, with a favored label

used for the highest quality lettuce

packed as 2 dozens, or "2's." Other

labels are used for lower quality lettuce, or lettuce packed as "21/2's" or

"fi/a's." Some head lettuce is still

packed in wooden crates, either in

sheds with icing in some parts of the

East or dry-packed in the field in

many parts of the country for local

shipment. To accommodate larger

heads, cartons used in the East and

Midwest are slightly larger than

those used in the West.

At the cooling plant the pallets

carrying the lettuce are removed

from the truck by enormous trucklifts and loaded side by side on

long, flat, low-wheeled dollies.

These are pushed into the vacuum

cooler, which is a large steel tube

(fig. 16). It may have doors at

both ends or a single door with the

other end permanently closed.

Wien loaded, the tube is tightly

sealed and air is removed rapidly

from the chamber by a powerful

vacuum pump or steam jet. Evaporation of the surface and tissue water of the leaves occurs in the lowered pressure of the chamber. This

causes a cooling of the entire head.

The vacuum is maintained until the

temperature is lowered to about 33°

F. The time required depends upon

the initial temperature of the load;

it usually takes 15 to 30 minutes.

After the cartons are removed from

tlie tube, they are placed on a conveyor belt that carries them to precooled railroad cars, which contain

only bunker ice and fans or a dieselpowered cooler, or to refrigerated

trucks for shipment.

There are several sizes of cooling

tubes. The standard size holds 320

cartons, or one-half of the normal

capacity of a railroad car. Some

tubes are half this size, and portable tubes are one-fourth this size.

Others will hold as much as one or

two railroad cars or truck trailers.

Vacuum cooling of lettuce is more

economical than the old shed-packing method; i.e., cardboard cartons

are cheaper than wooden crates, the

cost of crate ice and load ice is eliminated, and labor costs are lower.
2S AGRICULTURE HANDBOOK 221, U.S. DEPT. OF AGRICULTURE

Also, ice in the crate kept the lettuce wet and clrippinii, which made

the ciLite lieavier ancl more difficult

to liandle. The jagged edees of the

ice caused some damage to the

leaves. The water also facilitated

the growth aufl movement of various organisms catising breal-cdown.

In tlie dry pack, these factors are

eliminatecl, although the load occasionally warms up enough to

cause some wilting and breakdown.

In 1961, a new packing metltocl

was developed in the Salinas-AVatsonville district of California. Its

impact on the industry has yet to be

fully determined. The ijrocess consists of slirink-wrapping lettuce

heads in plastic film before cooling.

This is done eitlier in tiie field or in

a shed.

Ill the field, lettuce heads are cut,

stripped to a single wrapper leaf

and conveyed to a group of workers

in a truck-type vehicle driven

through the field. There each head

is wrapped in a thin plastic film

either by liand or by an automatic

or a semiautomatic wrapping device. The wrapped heads are conveyed through a lieat tunnel, where,

at a temperature of about 300° F.,

the film shrinks and hardens around

each head, which gives a tight clear

wrapping. The heads are then

packed in cartons, 20 or 24 per carton, and vacuum cooled.

The essential dilïerence between

the field and shed methods is that

in tlie latter the lettuce is cut in the

field and hauled into a shed wliere

the heads are trimmed to one wrapper leaf and then fed into a stationary machine that wraps and lieat

slirinks tlie wrapper automatically.

Tliey are then packed in cartons and

vacuum cooled as in the field

method.

The future importance of shrinkwrapping in lettuce marketing

ITiGTJEE 16.—A vacuum cooler for lettuce being filled with lettuce cartons. After the

produce is brdu^ht to the desired temperatures, the cartons are loaded directly

onto refrigerated railway cars or trucks from the tubes.
LETTUCE AND ITS PRODUCTION 29

probably "will depend upon two

factors :

The economic factor.—A greater

proportion of workers at higher

wage levels is needed either in the

shed or on the truck than for the

standard field packing operation.

It will, however, be possible to reduce the size of the crew. In addition, it is expected that shipping

costs will be lower with the reduced

weight of the lettuce. It has been

estimated tluit the removal of all

but one wrapper leaf will reduce the

weight per carton by 10 to lit

pounds. Other factors include the

cost of the equipment and materials

and the possible savings in reduced

deterioration of the lettuce.

Consumer afpeal.—A serious

problem is the red discoloration of

the butt. This can be trimmed otï

with unwrapped lettuce but not

with wrapped lettuce. Another

concern is whether there will be an

increase or decrease in the amount

of russet spotting, pink rib, and

other types of breakdown in transit.

These and other problems must be

solved satisfactorily before shrinkwrapping becomes a successful

marketing practice.

Butterliead, cos, and looseleaf lettuce are grown almost entirely for

local or relatively nearby markets.

Far less attention is given to harvesting and packing of these types

than to crisphead lettuce because of

their relatively low volume and

short distance to market. Nevertheless, they are even more exacting in

the care needed to keep them in

first-class market condition. In the

West, these varieties are dry packed

in western crates in the field and

shipped by refrigerated truck to

local outlets. In tlie East, they are

packed in wooden crates or bushel

baskets and shipped in refrigerated

trucks.

SEED PRODUCTION

Most lettuce seed produced in the

United States is grown in California. Arizona, Idaho, Utah, and

Oregon are also proclucing areas.

According to the Agricultural Marketing Service {33), 660,000 pounds

of seed of heading varieties, 158,000

pounds of seed of looseleaf varieties,

and 25,000 pounds of cos seed were

produced in 1958.

Two unusual features make the

production and harvesting of lettuce seed different from the procedures with most other annual

plants. First, most of the seed must

be indexed for mosaic virus. Second, the head lettuce plant matures

in two stages : From seed to market

maturity, and from market maturity to flowering. The transition

from market maturity to flowering

necessitates cutting open the heads

or otherwise removing the tight leaf

covering to allow the flower stem to

break through readily.

For head lettuce, seed-to-seed

production usually requires 6 to 8

months but sometimes as long as 10

months. Good weather, high soil

fertility, freedom from clisease and

insect damage, and sufficient water,

as required for a head lettuce crop,

are also necessary for growing seed.

In addition, temperatures too warm

for normal head formation are

needed to encourage bolting. During the maturing of the seed, calm,

dry weather is important. High

winds or heavy rains will result in

the loss of much seed. Seed plants

of lettuce are shown in figure 17.

At market maturity, bolting is

facilitated by one of several methods of removing the tight covering

of leaves. The usual method is to

make two slashes at right angles
30 AGRICULTURE HANDBOOK 221, U.S. DEPT. OF AGKICULTUEE

acrcjss tlie top of the head, not deep

enough to cut the ^-owing point,

.another method is to slice off the

top of tlie liead but not deep enougii

to harm the growing point. A

tliird method is to press down

tírmly on tlie top of the head, breaking the leaf ribs near the stem, and

pull off the entire mass of head

leaves without damaging the stem.

If the crop is grown for market

lettuce before harvesting seed, cut

the heads fairly high on the stalk ;

this will leave enough productive

lateral buds to form seedstalks and

a crop of seed.

With the recognition of the economic seriousness of lettuce mosaic

since 19.50, with its seedborne characteristic, modification of seed-production procedures has become

necessary (.9).

Zink. G-rogan, and Bardin (Ja)

showed that planting of seed containing a maximum of 0.1 pendent

seedborne infection of the virus offers the only feasible control of mosaic. The following procedure has

been developed for producing seed

of low mosaic content.

For a seed crop, plant the seed at

about the same rate as for a market

crop or a little heavier (see Seedbed

Preparation. Planting, and Thinning ). Rogue offtypes, diseased

plants, weaklings, and other undesirable plants before blooming.

Two roguings are the minimtmi requirement : one about 2 weeks before

market maturity and one at seed

maturity. More extensive roguing

is required for growing mosaicindexed seed (see p. -31).

A most important first step in

producing mosaic-indexed seed is

the use of stock seed free of infection. This seed is ustially obtained

from plants grown in a greenhouse

where infected plants can be removed and spread of vims can

l^irT^^77

riGUEE IT.—Lettuce plants maturing seed in the greenhouse. To produce stock

seed free from lettuce mosaic it is usually necessary to grow the plants in a

greenhouse in order to assure close control of the insect vector of the disease.
LETTUCE AND ITS PRODUCTION 31

be almost completely prevented.

Clean stock seed insures against infection starting in the field; thus

control measures are reduced to

those directed against outside

sources.

More roguings are necessary for

growing low-mosaic seed than for

regular seed. Since every infected

plant is a source of virus for infecting virus-free plants, remove each

as soon as possible after it is recognized. Plants may show symptoms

at any time ; therefore, examine all

plants in the field frequently and

remove infected plants as they appear. After thinning, rogue twice

a week for 3 or 4 weeks. After

that, rogue once a week until bolting. Kogue again before harvest,

since a new set of symptoms appears in the seed-head stage.

Control of weeds (p. 24) and

insects (p. 43) is highly important

in seed production. Many common

weeds ( p. 34) are hosts of the lettuce

mosaic virus and can act as sources

of infection. Eliminating these

weeds, both within the field and

around the edges, will reduce the

possible infection centers. Insect

carriers, especially the green peach

aphid, are responsible for spread of

the virus.

In order to be labeled as mosaicindexed seed, the percentage of

seedborne infection should not exceed 0.1 percent. This is determined by a seedling indexing procedure after the harvested seed has

been cleaned and packaged. Lots

of seedlings raised in flats are inspected for mosaic symptoms, and

the number of infected plants is

determined. No fewer than 3,000

plants per lot of seed should be indexed in this way, and more than

10,000 need to be indexed in doubtful cases. California allows the

same tolerance as for weed seeds,

that is, as high as 0.3 percent infection may appear and the lot may

still be legally labeled as mosaicindexed. Seed that does not fall

within the tolerance level of 0.3

percent must be sold as regular seed.

Through the 19.50's progi-essively

more mosaic-indexed seed was produced each year. In the SalinasWatsonville district of California,

a county ordinance was passed in

19.58 making it illegal to plant nonindexed seed of any variety of head

lettuce. The use of mosaic-indexed

seed will necessarily continue to expand imless resistant varieties can

be developed.

Since greater care is taken with

all production practices and disease

control throughout the growing of

mosaic-indexed plants, seed yields

have increased markedly. Yields

of nonindexed seed usually range

from 200 to 400 pounds per acre;

whereas, indexed seed may yield as

high as 600 to 800 pounds per acre.

Isolation is important in successful seed production. Lettuce is

naturally self-pollinated, but some

crossing can occur. Contamination, after several generations, can

lead to loss of varietal integrity.

As high as 17 percent crossing can

occur between plants standing next

to each other. The average is about

3 to 3.5 percent. This is sufficient

to cause considerable loss of uniformity in a variety. To prevent

crossing, plant different varieties at

least 10 feet apart, with several rows

of a taller crop, such as corn or sunflowers, between them. If possible,

grow a white-seeded variety next to

a black-seeded one, to enable detection of seed mixtures.

The growth regulator gibberellic

acid may shorten the seed-production period and perhaps increase

seed yields. Several experiments

in the greenhouse and a few in the

field have indicated that seed may

be harvested up to a month earlier

and that yields may be somewhat

higher than normal {12).
32 AGRICULTURE HANDBOOK 2 2.1, U.S. DEPT. OF AGRICULTURE

Until the 1950's, lettuce seed was

harvested entirely by hand. The

tops of the standing plants were

bent over the mouth of a canvas sack

and the seed shaken into it. This

was usually done two or thiee times

at intervals of about 7 to 10 days

since the seed heads matnre progressi\'ely. Some lettuce-seed fields are

now being harvested by machine;

a faster and more effiiient process

because less hand labor is needed.

The plants are cut, elevated to a side

belt, and delivered to a wagon.

They are spread out on a canvas on

tlie ground to dry. Later, the seed

is removed with a stationary

thresher. Some seedsmen still prefer to harvest by the "shake''

method. Small lots are always

harvested by this method. After

harvest, the seed is cleaned with

standard equipment to remove

chaff and unfilled seed.

DISEASES OF LETTUCE AND THEIR

CONTROL

Mosaic is tlie major lettuce disease

in the United States. The nature

and seriousne.ss of diseases differ

widely among the growing areas.

In the "West, mosaic causes by far

the most damage. Downy mildew

is also important in some areas, and

big vein may have serious economic

effects during a poor market when

only high-quality lettuce is salable.

In the Midwest, Xortlieast, and

Southeast, aster yellows, dampingoff, and mosaic are of major importance. Tipburn and rib blight

sometimes may be very severe in any

area. Other diseases" of occasional

importance in specific areas are

damping-off. spotted wilt, stemphyllium, anthracnose, powdery

mildew, bottom rot, sclerotinia, and

botrytis. Brown blight was once

serious in the West (1).

Damping-Off

Diseases that affect young seedlings before and sliortly after emergence are designated 6v the collective term "damping-off." The

causal agents of darnping-off are

species of PeUicuIana and Pythiiim, usually Pythium filamentosa

(Pat.) Rogers and P. idtimum.

Trow. These fungi destroy the

stem tissue at the soil line, girdling

the stem and causing the collapse

and death of the seedling. Damping-off may cause serious reduction

in plant stands during cool, wet

weather, especially in localized

areas where the soil is poorly

drained and poorly aerated. Where

young seedlings are grown in plantbeds for transplanting to the field,

damping-off can be a serious problem. Crowding and lack of proper

ventilation in the beds are icfeal

conditions for the growth of damping-off organisms.

In the field, good sou and water

management practices and the use

of seed disinfectants will control

damping-off. In plantbeds, water

control, proper ventilation, evenly

spaced plants, and the use of soil

fumigants will all be beneficial in

reducing losses fi'om damping-off

organisms.

Mosaic

Common lettuce mosaic is caused

b_v the virus Marmor lactucae H.

It was first recognized and described as a virus disease in Florida

in 1921. Since then, it has often

caused serious loss in many lettuce

areas in the world.

Tlie symptomatology of mosaic

is known in some detail, although

some aspects of it are not fully understood. Differences in symptom

expression are caused in part by differences in stage of plant growth at
LETTUCE AND ITS PRODUCTION 33

which infection occurs ; it is this

relationship that still needs exploration.

On a plant carrying seedborne

virus, the first true leaf is usually

mottled (fig. 18) with two shades

of green, normal and yellowish, and

is irregularly shaped. Sometimes

this symptom may not appear until

the second or third leaf or possibly

even later. As the plant develops,

the leaf edges show excessive frilling and the leaf tips roll back. In

the early-rosette stage, vein clearing occurs. Later the entire plant

will be stunted and yellowish and

may fail to head (fig. 19).

Plants infected after emergence

may or may not show the same sequence of symptoms ; this probably

depends on the stage of growth at

which they are infected. If infection occurs late, the head may form

and mature without any symptoms

showing. If the plant bolts, however, symptoms appear on the seedstalk. The stem leaves become

mottled and frilled. The seedstalk

is stunted, with its branches shortened and yellowed. The involucres

are distorted and show necrotic

spots, particularly on the tips of the

bracts. Necrotic girdling of the

pedicels is common.

FIGURE 18,—Leaf from mosaic-infected

lettuce plant, showing the typical mottling caused by the disease.

FIGXJEE 19.-Healthy lettuce plant on the left; mosaic-infected Plant on 'he right.

The light color, misshapen leaves, and small size of plant are typical symptoms

of this disease.
34 AGRICULTURE HANDBOOK 221, U.S. DEFT. OF AGRICULTURE

Some varieties of lettuce (Jade, seed planted at the rate of 1 pound

Imperial D, and Chesthunt Early per acre would mean on the average

Giant) are hypersensitive to mosaic 4,000 infected seeds per acre, or 1

and show necrotic flecks on the head infected seedling per G feet of bed.

leaves when infected. They also Mosaic is carried from plant to

have necrotic lesions on the'flower plant by several species of aphids.

stalks in addition to the involucre Dickson and Laird (5) have shown

spotting. that the most important of these is

In seneral, symptoms are most the green peach aphid (Mysws pei-

¡jronounced during periods of rapid x'leae Sulz.). The percentage of ingrowth when the plants have ample fection that appears in a field is renitrogen and the leaf tissue is dark lated to the seed infection and the

green. It is difficult to detect syrap- size and movement of the aphid

foms on slow-growing, light-green population. One-hundred percent

plants. infection may be reached when the

Plants infected relatively early in aphid population is large and acthe life cycle are sparingly or com- tive. Aphids may pick up the virus

pletely sterile. Seed yields are of- from seedborne infected lettuce

ten extremely low; severely infected plants in the field or from infected

plants may yield nothing. weeds. The number of weed hosts

Mosaic vii'us may be spread as a is not known nor is their individual

seedborne infection or from plant importance as sources of inoculum

to plant by insect vectors. completely understood. Several of

In nonindexed commercial lots of the known weed hosts are common,

seed, seedborne infection commonly however, in lettuce-growing areas

ranges from 1 to Z percent. In some and may be major soiu'ces of inocuit may be as high as 14 percent ; in lum. Thirty-five species of plants

a few strains, however, it may be at besides common lettuce are reported

or near zero. Indexed seed must as hosts of the ^nrus. Some of these

carry no higher than 0.1 percent in- may be at most only token hosts,

fection ( 0..3 percent in California). Others, such as groundsel and prickAn example of the importance of ly lettuce, maybe significant sources

the percentage of seedborne infec- of the virus (see following tabution: A 1-percent infection in the lation).

Reported weed hosts of lettuce mosaic virus : ^

Scientific name Common name

A-rmgallis arvensis L Scarlet pimpernel.

Aster spp Common aster.

Capsella bursa-pastoris (L.) Medic Shepherds-purse.

Chenopoflitim aibum L Lamt)squarters.

Chenopodijim niurah' L Sowbane.

Cichorium spp Chickory. endive.

Eroifiuyn ciciitqriiim fL.) L'Her Filaree.

Lactuca .serviola L Prickley lettuce.

Lamiiim amplexici/ile L Henbit. deadnettle.

Lathprns otioratiis L Sweetpea.

Maha parrifiora L Cheeseweed.

Picris echioides L Oxtongue.

Pisum sativum L Garden pea.

¡Senecio vulgaris L Groundsel.

Silyhiim niarianwm (L.) Gaertn Milkthistle.

Bonchus asper L Sowthistle.

Spinacia olerácea L Spinach.

BteUaria media ÍL.) Cyrill Cbickweed.

Tagetes erecta L African marigold.

Tetragonia expansa Murr New Zealand spinach.

Zinnia elegans Jacq Zinnia.

- From Keener and Foster (14)
LETTUCE AND ITS PRODUCTION 35

There are several possibilities for

control of lettuce mosaic. However, only the planting of mosaicfree or mosaic-controlled seed is

feasible at the present. Wiile this

does not eliminate the disease, it retards extensive infection long

enough that much lettuce can be

harvested from fields that would

otherwise yield no marketable

heads. By using indexed seed, fields

that are ordinarily severely damaged sliould liave only liglit infection. Since mosaic lias a cumulative

effect in areas where lettuce follows

lettuce and adjacent fields are

planted almost in sequence, the effectiveness of the low-mosaic seed is

eventually lost, particularly when

many apliids are present. As tire

virus is carried from field to field,

the inoculum level rises and infections get heavier and heavier and

may reach 100 percent.

Spray programs to control the

apliid population are more effective

in restricting the number of aphids

in the lettuce head than in preventing the spread of the virus from

plant to plant. Ultimate, reliable

control of the disease must come

from genetic resistance. This is

not known to exist at present, but

research is being conducted to find

or create it.

Spotted Wilt

Spotted wilt sometimes causes

heavy damage in lettuce. This is a

virus disease that is widely distributed. It is especially important on

tomatoes and peppers, but it also

infects celery, nasturtium, dahlia,

and several other cultivated species ;

it also attacks some weeds.

Symptoms on lettuce are characteristic and usually fairly easy to

distinguish from those of other diseases. Young plants that are infected turn yellow, wilt, and die,

often within a week. Symptoms

on older plants are marginal wilting, marked twisting, yellowing,

and necrotic spotting of the leaves.

Tlie infection is often most serious

on one side of the plant; this produces a typical lopsided appearance

(fig. 20). The younger, heart

leaves may have necrotic spots or

turn completely black and die. Infected plants eventually die.

Several species of thrips, including Frankliniella spp. and Thrips

tahaci Lind., are the vectors of the

virus. The virus, once acquired by

the thrips, persists during the lifetime of the insect.

No effective control of the disease

is known. All commercial varieties

of head lettuce are susceptible, although the variety Mignonette may

have some resistance.

Aster Yello-ws

Aster yellows is also a virus disease. At least two strains of the

virus are known; an eastern strain

that causes severe losses in lettuce

and a western strain that has not

been serious on lettuce in the important market-lettuce districts but

causes extensive damage to celery.

The first symptom of aster yellows on lettuce is a chlorosis, or loss

of green color, in the young leaves.

Center leaves develop abnormally as

short, thickened stubs. The outer

leaves are twisted and yellowed.

Light-brown latex deposits form on

the affected leaves. Late infections

on lettuce may show only latex spots

in the interior of the head. Infected

plants that bolt have distorted, discolored flower buds and bushy outgrowths of axillary buds (fig. 21).

Aster yellows is not seedborne ; it

is transmitted in the field by leafhoppers: The six-spotted leafhopper, Macrosteles fascifrons (Stâl.),

in the East and several species in the

West. No source of genetic resistance is known. Control of the leafhoppers will help prevent the

disease.
36 AGRICULTURE HANDBOOK 221, U.S. DEPT. OF AGHICULTUHE

June Yellows

June yellows is the name that has

been given to a complex of lettuce

diseases. Grogan, Snyder, and

Bardin (S) stated that it was caused

chiefly by mosaic but also by other

infectious diseases, faulty nutrition,

excessive salinity, poor root development, the use of unsuitable varieties, and unfavorable climatic and

soil conditions. Costa, Duffus, and

Bardin (4) and Duffus (G) stated

that at least two viruses are responsible for part of the complex. One

is the radish yellows virus, which

affects radish, spinach, sugar beet,

and other crop and weed species as

w-ell as lettuce. The other is the

malva yellows virus, which also affects cheeseweed, sugar beet, and

other species. Both are transmitted

by the green peach aphid as is the

lettuce mosaic virus, although they

are not related to the latter.

Radish yellows virus and malva

yellows virus cause similar symptoms in lettuce and may occur

separately or together. Irregular

chlorotic blotching appears on older

leaves. These areas tend to coalesce

and later produce severe interveinal

yellowing, especially near the leat

margins and base. Eventually the

whole leaf, except for narrow bands

along the main veins, turns yellow.

Some varieties, notably Prize Head,

show a reddening of tissue rather

than yellowing.

Severe yellowing ruins the appearance and quality of a head of

lettuce. Beyond this it is not

known what economic damage may

be done by the viruses. There is no

known source of resistance to either

virus, although the severity of

symptoms among varieties differs

markedly.

FiGUEE 20.—A lettuce plant infected witli the spotted wilt virus, showing the typical

necrotic pitting of the midrib of the larger veins of the exterior leaves of the

head and the lopsided appearance of the plant.
LETTUCE AND ITS PRODUCTION

FIOUEE 21.—Flowering shoot irom a plant infected with aster yellows (upper)

shoot from a comparable uninfected plant (lower).
3S AGRICULTURE HANDBOOK 221, U.S. DEPT. OF AGRICULTURE

Big Vein

The big vein disease of lettuce

was first reported in 1934:. Prior to

1958, the causal agent was assumed

to be a soilborne virus ; however,

tliere was never convincing proof.

In 1958, Grogan and coworkers

[10) demonstrated a significant

association between big vein disease

and Olpidiuni hra-ssicae (Wor.)

Dang., a primitive fungus. Some

recent work by Campbell and coworkers (S) suggests that (37/i;V/iMm

may be a vector of the virus that

causes big vein.

The leaves of plants infected w ith

the organism when small show a

pale-yellow clearing along the

veins. Such plants are apt to be

stmited and never develop into marketable heads. Leaves of plants infected in later stages show severe

vein clearing—actually a disappearance of chlorophyll adjacent to

the major veins—arid ruffling of the

outer, more mature leaves (fig. 22).

The infected plants have a peculiar

upright position. The leaves fold

in to form a head, but infected

lieads are never so large nor so solid

as comparable uninfected heads.

Such heads can be marketed, but

they are light and loose and the

quality is poor. Big vein is most

likely to occur on heavy, wet soils,

and symptoms develop most conspicuously when the air temperatures are between 42° and 60° F,

The symptoms gradually fade out

at higher temperatures.

Xo effective means of control are

available. Avoid big vein infested

soil if possible, but this is not very

practical because the organism remains infective in the soil for at

least 8 years. For small areas,

steam sterilize the soil or treat it

with certain chemicals. None of

the common commercial varieties

FiGTTBE 22,—Lettuce leaves showing mild to severe vein clearing, a characteristic

symptom of big vein.
LETTUCE AND ITS PRODUCTION 39

are resistant to the disease. Now

that the causal agent is known, a

large-scale screening program to

search for resistance is feasible.

Downy Milde-w

After lettuce mosaic, down}' mildew is probably the most destructive disease of lettuce in this

country. It is reported to liave

damaged the crop at one time or another in all the chief producing

areas in the western region, and it

can be serious in greenhouses and

plantbeds in the East and South.

The disease is incited by the fungus

Bremia lactiicae Regel. This organism is favored by cool, moist

weather, with humidity at or near

the saturation point. It is spread

by windblown spores. These spores

require free water to germinate and

become infective. Kain, fog, or

dew will usually provide enough

water for the spores to germinate.

At first pale-colored spots appear

on the undersurface of the older

leaves. These spots are irreg-ular,

sharply angled, and confined within

the main veins. AVhen conditions

are favorable, the pale areas become

covered with white, fluffy spore

masses (ñg. 23). These spores are

spread from plant to plant by wind

or by splashing rain. The infected

areas become brown and necrotic,

and eventually the entire leaf is destroyed. At liarvest, damaged

outer leaves have to be trimmed off

before the head is placed in the carton. When most of the wrapper

leaves are heavily damaged, the

head is unmarketable. If weather

conditions are conducive for rapid

growth of the fungus and infection

is widespread, entire fields can be

destroyed within a week or less.

FIGURE 23.-Part of a lettuce leaf infected with downy mildew, f'i°"'i,°f ,*«.";,'»''*

powdery masses of spores. The mycelium of the fungus tends to be limitecl Dy

the veins, hence the lesions are usually angular in outline.
40 AGRICULTURE HANDBOOK 22 1, U.S. DEPT. OF AGRICULTURE

The fungiis frequently develops

during transit ; this provides an

entry point for slime- or rot-produeino; organisms. As a result the

produce may be decayed and unfit

for market before it arrives at its

destination, or shortly thereafter.

Chemical control of downy mildew through the use of sprays or

dusts is inetiective and impractical

because of poor coverage and excessive costs. The best means of control is through the use of diseaseresistant varieties. At present

(1961), disease-resistant varieties

are available for only two producing areas and seasons: Valverde,

adapted to winter culture in thb

Lower Eio Grande Valley of Texas,

and Calmar, adapted to summer culture in the Salinas Valley of California. Plant breeders are developing resistant varieties for other

areas and seasons.

Po^vdery Milde^v

Powdery mildew (Erysiphe cichoraceanim DC.) is not a serious

disease of lettuce. It is reported to

do sporadic damage to the crop in

the Salinas-Watsonville district of

California. The disease first appears as white, powdery masses on

both surfaces of the older leaves.

These white, fluffy masses consist of

both spores and mycelium. The infected leaves later become brown

and dry and have a scorched appearance. Infected plants are usually

small, poor in ciuality, and unmarketable. Grogan and coworkers

(8) have recommended dusting

with sulfur to control the disease,

if enough damage is occurring to

warrant control.

Tipburn

Tipburn is one of the most common diseases of lettuce (¿7). It occurs in nearly all the lettuce-producing sections of the country in

both field and greenhouse (31).

Little has been learned about the

environmental and genetic factors

important in the etiology of the

disease. Most of the information

about tlie disease comes from scattered observations and impressions

rather than from carefully controlled experimental work. However, investigators generally agree

that: (a) The disease is nonparasitic, i.e. the causal agent is not an

insect, fungus, bacterium, or virus;

and (b) there are distinct genetic

differences among varieties with respect to susceptibility to the disease.

For example, varieties of the Great

Lakes type are less susceptible to

the disease than other varieties;

some of the Imperial types are

moderately resistant and a few are

very susceptible ; the butterhead and

cos varieties are extremely susceptible. But, mider favorable conditions for the development of the

disease, even the most resistant

varieties show tpical symptoms.

Initially dark-bro\vn spots appear between the larger veins and

along the margin of the leaves; at

first these commonly occur on the

outer leaves, but later they also develop on internal leaves (fig. 24).

These spots usually coalesce, and

the entire margin of the leaf becomes brown and necrotic. Ordinarily only a few leaves of the head

are infected. The necrotic areas are

ideal places for the invasion of rotand slime-producing micro-organisms that cause secondary decay.

Tipburn seldom occurs in young

plants, and it rarely occurs in the

young leaves of older plants. The

disease tends to appear just before

the plants reach market maturity.

The following environmental conditions are reported to favor the development of tipburn :

1. Conditions that support rapid,

succulent growth, such as an abundant supply of nutrients and excessive water.

2. High relative humidity and
LETTUCE AND ITS PRODUCTION 41

liigii temperatures, particularly

night temperatures that exceed

65° F.

3. Fluctuating soil-moisture content.

i. Low temperatures during

early development followed by relatively high temperatures at maturity. (These conditions frequently prevail at the time

late-spring lettuce is grovt'n in the

western region.

If these environmental conditions

are important in the etiology of the

disease, it is clear that most of them

cannot be controlled by the grower.

Therefore, about the only etïective

means of control is to plant varieties known to have some resistance

to the disease.

Sclerotinia

Sclerotinia, or drop, is a fungus

disease caused principally by Sclerotinia solerotiorum (Lib.) DBy.

It occurs during cool, moist parts

of the growing season in most lettuce-growing districts.

In late stages of the disease, a

soft, watery, brown rot that severs

the stem is produced. In damp conditions, a white mycelium appears

on the underside of the leaves. Finally, the hard black resting bodies,

or sclerotia, are formed. These

average about one-fourth inch in

diameter. They persist in the soil

for several years. Cup-shaped

fruiting bodies arise from the sclerotia. Although these produce

spores, it is the growth of mycelium

from sclerotia lying near lettuce

plants that is responsible for most

of the spread of the disease.

Eliminating sclerotia from the

soil is extremely difficult. Nonsusceptible crops may prevent increase

of the sclerotia in the soil, but use

FiGTJBE 24.—Part of a lettuce plant showing marginal tipburn. The dark necrotic,

fragmented area extends from the margin into the leaf tissue as much as onequarter inch or more.
42 AGRICULTURE HANDBOOK 22 1, U.S. DEPT. OF AGRICULTURE

of these in a short rotation is ineffective as a control. Application of

calcium cyanaraid to the soil or

flooding for long periods will eliminate some of tliem. No resistant varieties are known. Lettuce grown

on raised beds and not overwatered

is less likely to be infected tlian that

grown on muck, subjected to heavy

watering, or both.

Anthracnose

Anthracnose, sometimes called

shot hole or ring spot, is caused by

the fimgus Marssonina panattoniana (Berl.) Magn. It is not a

major lettuce disease, but it occasionally causes serious damage in

cool, damp seasons.

At first, small, brownish, watersoaked spots appear on the lower

leaves. Tliese enlarge and become

straw colored. The centers later

fall out, forming the characteristic

shot holes. Similar small, sunken,

water-soaked spots appear on the

underside of the midrib. Affected

leaves eventually shrivel and die.

Moisture is required for the

spread of the fungus spores.

Splashing rain, sprinkler water, or

water from dew-covered plants carried by cultivation practices will

spread the spores. Their germination is favored by cool temperatures, around 60° to 65° F., as well

as moist conditions.

In California, wild lettuce, Lactuca serriola, is the main source of

inoculum. It usually becomes infected with anthracnose in the early

spring. For preventing anthracnose, keep the areas around lettuce

fields free of wild lettuce and rotate

lettuce with nonsusceptible crops,

since the fungus can live only about

a year in dead material. In addition, do not cultivate when fields

are wet, since the spores can be

spread by the tools. No source of

resistance to anthracnose is known.

Bottom Rot

Bottom rot is a disease of maturing lettuce plants that is caused by

PeUicuJuria filamentosa (formerly

Rhhoctoni-fi solani), one of the

organisms responsible for dampingoff of seedlings. It may be destructive on muck, especially on varieties

that grow close to the ground.

Necrotic lesions first appear on

the petioles and midrib of the lower

leaves. These may spread over the

entire petiole and cause the death of

the leaf. Successive leaves may be

infected until the head is a slimy

mass ; eventually this drys down to

a blackened munamy.

The fungus lives in the soil and in

plant remains. It develops rapidly

during warm weather when there

is much moisture under the plants.

To control bottom rot, rotate lettuce with nonsusceptible crops or

dust the soil under the plants with

a fungicide. In greenhouses, where

the disease may be a major problem,

sterilize the soil.

Botrytis

Botrytis, or gray mold, is a fimgus disease especially important in

the greenhouse, but it also appears

in the field and in transit. It is

caused by Botrytis cinérea Fr.

Botrj'tis sometimes appears as a

secondary infection in fields of lettuce infected with sclerotinia.

On older leaves, lesions appear as

water-soaked areas, which turn yellow. Soon a gray mold appears.

On the stem at the base of the leaves,

a brown to black, dry, firm decay

appears that may rot through the

stem and cause the head eventually

to break off.

In the greenhouse, soil sterilization, adequate ventilation, and cool

temperatures will control the

disease.
LETTUCE AND ITS PRODUCTION 43

Rib Blight

Like tipburn, rib blight is a

somewhat mysterious disease of lettuce. No organism has been associated with it, and the conditions

that cause it are not understood.

Ribblight is also known as brown

rib or rib discoloration. A darkbrown or black elongated spot appears in the midrib of the outer

leaves as the head nears maturity.

More severe symptoms include several spots along the midrib and a

brownish discoloration extending

through the smaller veins of the leaf

blade. In transit, secondary infections may set in and cause the entire

head to rot.

Rib blight seems to occur most

frequently during periods of warm

weather. It is not known whether

its appearance coincides with that

of tipbum and it is caused by the

same environmental conditions.

Some varieties are less severely

damaged by rib blight than others,

but no resistant varieties are known.

Methods of control are unknown.

INSECT ENEMIES OF LETTUCE AND THEIR

CONTROL

Lettuce may be attacked by several kinds of insects, but the principal ones are caterpillars of various kinds and leafhoppers. Other

insects, such as crickets, grasshoppers, and wireworms, are sometimes

destructive. Aphids are injurious

to the crop not only directly because of their feeding but also

because they spread virus diseases,

especially lettuce mosaic. In the

eastern part of the country leafhoppers are a problem because they

transmit the virus disease aster

yellows.

Suitable insecticides for the control of insects on lettuce, effective

dosages, and appropriate precautions are published annually by

the Entomology Research Division

of the U.S. I)epartment of Agriculture (5.Í). In 1958 Harries and

coworkers (11) published control

methods for lettuce insects in

Arizona.

Insects Affecting the

Market-Lettuce Crop

Cutworyns and Armyworms

The beet armyworm, Spodoptera

exigua (Hbn.), the yellow-striped

armyworm, Prodenia ornithogalli

(Guen.), and various cutworms are

often a serious menace to young

lettuce seedlings. In the western

region this is especially true of the

crop planted in late summer or fall.

If insecticides are not used to protect the young plants, the stands

may be so reduced that it becomes

necessary to disk up the field and

reseed. Attack by these insects

usually is from worms that hatch

from eggs deposited by moths on

young plants, but sometimes the

worms move in from adjoining

fields.

Moths of the beet armyworm are

grayish brown with lighter gray

markings on the wings. They have

a wingspread of about 1 inch and

when at rest are % to % inch long.

Moths of the yellow-striped armyworm are somewhat darker and

have distinctive markings on the

forewings. They have a wingspread of about 11^ to 1% mches

and when at rest are % to % inch

long. These moths deposit large

masses of eggs on the small plants

almost as soon as they emerge.

The egg masses are covered with

scales from the body of the moth;

this distinguishes them from other
44 AGRICULTURE HANDBOOK 221, U.S. DEPT. OF AGRICULTURE

moth egps. Tlie eggs hatch in a

few days, and the young larvae

devour the phmts they are on and

spread to otiier phxnts. Armyworm

moths are nocturnal, but they may

be active in daylight.

Cutworm moths are all nocturnal,

dusty gray, and heavy bodied.

They have a wingspread of I14 to

2 inches and are 3/4 to 1 inch long

when at rest. Tliey deposit smaller

masses of eggs on the plants than do

the armyworm moths, and these

egg masses are not covered with

scales from the moth. The damage

to the young plants by the small cutworms is similar to that of the small

armyworms. Larger cutworms

tend to cut off the plants at the soil

surface, as the name implies.

In areas where these insects are a

menace, carefully watch the fields;

damage to stands can occur very

rapidly. If small worms and eggs

are present, dust or spray at once ;

if only eggs are present, prepare to

dust or spray within 3 or 4 days.

Cabbage hooper

The cabbage looper, THchoplusia

ni (Hbn.), is a pest in all the principal lettuce-growing regions in the

United States. In the western

region it attacks primarily the fall

crop. McKinney (2i) has shown

the importance of this insect to fallgrown lettuce in the Southwest.

Infestations of this insect usually

become critical near thinning time,

and may be serious immediately

after thinning.

The moths are grayish brown and

approximately 34 of an inch long

and have a wingspread of 114, to 1%

inches. They have a characteristic

silvery spot near the middle of each

of the front wings that looks like a

figure 8 (fig. 25). The moths ordinarily fly near the ground and are

most active at night. The pearly

white eggs are laid singly on the

leaves and hatch within a few days.

Wien first hatched, the caterpillars

have dark heads and almost colorless bodies. They begin feeding almost at once and soon become pale

green. ^Tien mature the caterpillars are about li/i inches long.

They crawl by doubling up, or

forming a loop, then projecting the

front part of the body forward.

They get the name "looper" from

this method of locomotion (fig. 26).

riGDBE 25.—Adult of the cabbage looper FIGUEE 26.—Cabbage looper feeding on

moth. lettuce leaf.
LETTUCE AND ITS PRODUCTION 45

If insecticides have been applied

to control cutworms and armyworms, they will probably control

loopers ancl there will be no need

to make further applications for

looper control until after thinning;.

However, closely examine the field

for loopei-s before starting the thinning operation. If loopers are present, apply insecticides before thinning so that the worms will not

concentrate on the remaining plants

after thinning. Watch the plants

closely after thinning and apply insecticides wlien necessary. Three or

four well-timed applications of insecticides are usually sufftcient. If

a thorough application is made just

before the plants commence forming heads, about 30 days before harvest, usually no further applications

will be needed.

Corn Earworm

Harries and coworkers (_7i) reported in 1958 that the corn earworm, Heliothis zea (Boddie),

causes considerable damage to lettuce some years in the Southwest.

Damage consists of the feeding and

presence of worms inside the lettuce

heads (fig. 27). Severe infestations

may make the crop unmarketable,

smce the infested heads cannot be

determined by inspection and discarded.

The corn earworm moth (fig. 28)

is about the same size as the cabbage

looper moth, but it is light olive

green to grayish or yellowish

brown. Eggs are laid singly on the

leaves; they are ribbed and shaped

like a flattened ball. After the eggs

liatch, the small larvae feed for a

short time on the outer leaves before

entering the head.

Control consists of applying insecticides to prevent the worms from

entering tlie lettuce heads. Once

thej' are inside the heads, the damage is done because they cannot be

reached with insecticides.

FIGURE 27.—Corn earworm feeding in lettuce head.
46 AGRICULTURE HANDBOOK 221, U.S. DEPT. OF AGRICULTURE

FiQXJEE 28.—Adult and larva of the com earworm.

Aphids

Aphids, primarily the green

peach aphid, Myzus persicae

(Sulz.), (fig. 29) and the lettuce

seed stem aphid, Ma-chrosiphum

harri Essig, may injure lettuce

plants in several ways. Eeid and

Cuthbert (S6) have pointed out that

aphids cause heavj' losses to growers of leafy vegetables by: (a) Eeducing vigor and yield of plants,

(b) contaminating edible plants,

(c) transmitting destructive virus

diseases of plants, and (d) killing

plants, if infestation is heavy. Although mosaic is seedborne (p. 32),

usually only a small percentage of

the plants are infected. However,

these plants serve as a source of infection from which the disease is

spread to other plants by aphids.

Control of aphids is therefore necessary to prevent the spread of disease

as well as to prevent direct damage

caused by the feeding and presence

of the insects in the lettuce heads.

Leafhoppers

The six-spotted leafhopper, Macrosteles fascifrons (Stal.), is a

major pest of lettuce in the eastern

and north-central parts of the

United States. This small, green,

wedge-shaped insect is likely to be

undetected until after the lettuce is

injured. This leafhopper damages

lettuce indirectly by spreading aster

yellows. Because of the activity of

the insect, small numbers are important in the spread of the disease.

This makes control difficult. Wallis

(37) in 1960 listed the host plants

of aster yellows, as well as other

leafhopper vectors of the virus.

Other Insects

Grasshoppers and field crickets

sometimes infest young lettuce fields

in sufficient numbers to threaten

stands. Parker (£3) has pointed

out that grasshoppers are one of

man's most ancient insect enemies.

Control of these insects was once dependent upon poison baits, but these

have now been replaced by more

eflîective sprays or dusts.

In many areas of the Southwest,

especially where lettuce is grown

near cotton, the salt marsh caterpillar, Estigmeme aerea (Drury), is

a problem. These insects breed in

cotton and move to lettuce. Aluminum-foil barriers around the fields

will prevent injury from these insects. Set a foil strip about 7

FiGDEE 29.. -Green peach aphid, wingless

form.
LETTUCE AND ITS PHODUCTION 47

inches wide upright in the ground ;

let about 5 inches protrude above

the soil surface. Then dig post

holes against the barrier about

every 20 feet, to trap the caterpillars as they travel along the foil

barrier.

Wireworms are immature forms

of click beetles, family Eliteridae.

They feed on the underground parts

of plantSj and can be recognized by

their shiny, wirelike, yellow to

orange bodies. Lane and Stone

(16) list lettuce as one of the crops

especially susceptible to attack.

Damage to the roots may kill young

plants or inhibit the development of

older plants.

Insects and Mites ' Affecting

the Lettuce'Seed Crop

Aphids

Aphids are common pests of the

lettuce-seed crop. Carlson {3) has

shown that heavy losses in yield of

lettuce seed may be attributed to the

lettuce seed stem aphid, Macrosifhwm harri. The green peach

aphid, Myzus persicae, is also capable of damaging lettuce seed, but

in the Southwest it does not usually

occur in large numbers at the season

of the year that the crop is maturing. In this region the primary

damage to the seed crop by the

green peach aphid is usually the

spread of mosaic before seedstalk

development.

Spider Mites

Spider mites, primarily the twospotted spider mite, Tetranychus

telarius (L.), (fig. 30) are sometimes serious pests of the lettuceseed crop. They are very small:

adult females average 0.42 mm. in

length; males and immature forms

are considerably smaller. They

vary in color from pale yellow to

'Mites, altbougb not true Insects, are

close relatives of them and, therefore,

are discussed here.

orange red or greenish. Infested

plants are heavily webbed, and

leaves are yellowish in color. Feeding of these mites devitalizes the

plants and reduces the yield of

seed.

Hyaline Grass Bug

The hyaline grass bug, Liorhyssus hyalinus (Fab.), often

causes considerable damage to the

lettuce-seed crop. Carlson {3a) reported experiments by McKinney

which showed that both adults and

nymphs of this insect were capable

of reducing yield and viability of

lettuce seed. Adults of this insect

resemble lygus bugs, and these two

insects are often confused. They

can, however, be easily identified by

a close examination of the antennae.

Those of lygus bugs are hairlike

and tapered toward the end;

whereas, those of the hyaline grass

bug are slightly clubbed at the end.

Lygus bugs may occur on the

lettuce-seed heads in small numbers,

but they are usually of little importance on this crop. Eggs of the

hyaline grass bug are red and are

laid in clusters usually on the

flowerbuds or flower stems. The

eggs hatch into reddish nymphs

that feed directly on the flowerbuds

and developing seed.

FIGURE 30.—Adult female of the twospotted spider mite.
48 AGRICULTURE HANDBOOK 221, U.S. DEPT. OF AGRICULTUHE

LITERATURE CITED

I 1 \ B<JH.\. (}. W.

10.j3. THE IMP»")RTANT DISEASES OF

LETTUCE. U.S. Dept. XgV.

Yearb.x>k 19.>i : 417-425.

{2) CAMPBELL, R. N., GROöAN, R. G., and

I'URCIFULL, D. E.

1961. GRAFT TRANSMISSION OF BIG

VEIN OF LETTUCE. VirolOgy

15: 82-85.

(3) CABLSO^f, E. C.

1959. CONTROL OF MACROSIPHUM

BARRI ESSIG AND ITS DAMAGE

TO LETTUCE SEED PLANTS.

Jour. Eron. Ent. 52: 411-

414.

(Sa)

1959. THE EFFECT OF LYGUS AND

HTAXINE GRASS BUGS ON LETTUCE SEED PRODUCTION.

Jour. Econ. Ent. 52: 242-

244.

(-Í) COSTA. A. S.. DUFFUS, J. E., and

BAEDIN. R.

1959. MALVA YELLOWS, AN APHID

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Amer. Soc. Sugar Beet

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(5) DICKSON, R. C, and LAIRD, E. F., JR.

1959. CALIFORNIA DESERT AND

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1960. RADISH YELLOWS, A DISEASE

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