• TABLE OF CONTENTS
HIDE
 Copyright
 Title Page
 Table of Contents
 Introduction
 Procedure for diagnosis
 Use of this manual
 Mineral deficiencies and toxic...
 Soil
 Pesticides
 Climate
 Genetic
 Senescence
 Miscellaneous
 Reference
 Key to nonpathogenic diseases of...






Group Title: University of Florida Agricultural Experiment Station bulletin no. 721
Title: Nonpathogenic diseases of lettuce
CITATION PAGE IMAGE ZOOMABLE PAGE TEXT
Full Citation
STANDARD VIEW MARC VIEW
Permanent Link: http://ufdc.ufl.edu/UF00028036/00001
 Material Information
Title: Nonpathogenic diseases of lettuce their identification and control
Series Title: Bulletin University of Florida. Agricultural Experiment Station
Physical Description: 39 p. : ; 23 cm.
Language: English
Creator: Marlatt, Robert B
Publisher: University of Florida Agricultural Experiment Station
Place of Publication: Gainesville Fla
Publication Date: 1967
 Subjects
Subject: Lettuce -- Diseases and pests   ( lcsh )
Lettuce -- Diseases and pests -- Control   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Bibliography: Includes bibliographical references (p. 27-34).
Statement of Responsibility: Robert B. Marlatt.
General Note: Cover title.
Funding: Florida Historical Agriculture and Rural Life
 Record Information
Bibliographic ID: UF00028036
Volume ID: VID00001
Source Institution: Marston Science Library, George A. Smathers Libraries, University of Florida
Holding Location: Florida Agricultural Experiment Station, Florida Cooperative Extension Service, Florida Department of Agriculture and Consumer Services, and the Engineering and Industrial Experiment Station; Institute for Food and Agricultural Services (IFAS), University of Florida
Rights Management: All rights reserved, Board of Trustees of the University of Florida
Resource Identifier: oclc - 18367536

Table of Contents
    Copyright
        Copyright
    Title Page
        Page 1
    Table of Contents
        Page 2
    Introduction
        Page 3
    Procedure for diagnosis
        Page 3
    Use of this manual
        Page 4
    Mineral deficiencies and toxicities
        Page 4
        Aluminum
            Page 4
        Boron
            Page 5
        Calcium
            Page 5
        Copper
            Page 6
        Iodine
            Page 6
        Iron
            Page 7
        Magnesium
            Page 7
        Manganese
            Page 8
        Molybdenum
            Page 8
        Nitrogen
            Page 9
        Phosphorus
            Page 10
        Potassium
            Page 10
        Zinc
            Page 11
    Soil
        Page 11
        Acidity and alkalinity
            Page 11
        Carbon dioxide in soil
            Page 12
        Salt injury
            Page 12
        Roof girdling
            Page 13
        Drought spot
            Page 13
        Puffy heads, burst heads (water)
            Page 14
        Germination
            Page 14
            Page 15
    Pesticides
        Page 16
        Antibiotic injury
            Page 16
        Fungicide injury
            Page 16
        Growth regulators
            Page 17
        Pentachlorophenol injury
            Page 18
        Sodium chlorate injury
            Page 18
        Insecticide injury
            Page 19
    Climate
        Page 19
        Smog
            Page 19
        Oxygen
            Page 20
        Sulfur dioxide
            Page 21
        Cold injury
            Page 21
        Light deficiency
            Page 21
        Ionizing radiation
            Page 22
        Pink rib
            Page 22
    Genetic
        Page 23
        Variegation, albinism
            Page 23
    Senescence
        Page 23
        Marginal browning
            Page 23
        Red cotyledon
            Page 23
    Miscellaneous
        Page 24
        Rib discoloration
            Page 24
        Spiraled heads
            Page 25
        Tip burn
            Page 25
            Page 26
    Reference
        Page 27
        Page 28
        Page 29
        Page 30
        Page 31
        Page 32
        Page 33
        Page 34
    Key to nonpathogenic diseases of lettuce
        Page 35
        Page 36
        Page 37
        Page 38
        Page 39
Full Text





HISTORIC NOTE


The publications in this collection do
not reflect current scientific knowledge
or recommendations. These texts
represent the historic publishing
record of the Institute for Food and
Agricultural Sciences and should be
used only to trace the historic work of
the Institute and its staff. Current IFAS
research may be found on the
Electronic Data Information Source
(EDIS)

site maintained by the Florida
Cooperative Extension Service.






Copyright 2005, Board of Trustees, University
of Florida





N IC











NONPATHOGENI DISEASES LETTUCE _
ITIO -AN CONTRqL

RO ER r BMA 'r ;T










ULT IME
ITUTE TIFOOD AND N ClENCES
J.. BA RECT










CONTENTS


Page


Introduction ....

Procedure for Diagnosis .....

Use of This Manual .....

Mineral Deficiencies and
Toxicities
Aluminum ...... .....
Boron ...........
Calcium ......... .
Copper .......
Iodine .. ....
Iron ...........
Magnesium -
Manganese
Molybdenum ........
Nitrogen ...........
Phosphorus .... .......
Potassium ..........
Zinc ........ ...--

Soil
Acidity and Alkalinity .
Carbon Dioxide in Soil ....
Salt Injury ............
Root Girdling .....-- ...
Drought Spot ......-
Puffy Heads, Burst Heads
(w ater) ...................
Seed Dormancy, Poor
Germination .............
Plant Residue Injury .....


3 Pesticides
Antibiotic Injury ....
3 Fungicide Injury ..
Growth Regulators .
4 Pentachlorophenol Injury
Sodium Chlorate Injury
Insecticide Injury .....

4 Climate
5 Smog ..... .. --
5 Ethylene-"Brown Spot"
6 Oxygen ...
6 Ozone ...........
7 Sulfur Dioxide .........
7 Cold Injury
8 Light Deficiency ...........
8 Ionizing Radiation ..
9 Pink Rib .. .......
10
10 Genetic
11 Variegation, Albinism .

Senescence
11 Marginal Browning ...
12 Red Cotyledon ..............
12 Miscellaneous
1 Rib Discoloration .........
13 Spiraled Heads ..
Tip Burn ......-
.14
References
14
16 Key ... ......-


Page








Nonpathogenic Diseases of Lettuce

Their Identification and Control

Robert B. Marlatt
Associate Plant Pathologist, Sub-Tropical Station, Homestead

INTRODUCTION
Although no figures are available, pathologists generally agree
that a large proportion of the problems encountered by lettuce
growers are nonparasitic. In attempting to diagnose them one
can easily find information pertaining to parasitic diseases, but
little is available concerning nonpathogenic maladies. It is hoped
this manual may help fill the gap.
The information contained herein is the result of a review of
the literature and the author's research and observations which
for several years were devoted primarily to lettuce diseases.
Since no literature review is ever completed and knowledge
gained by experience frequently is not published, the author
would be grateful to receive suggestions from other workers
which might be added to a later edition of this manual.

PROCEDURE FOR DIAGNOSIS
In distinguishing nonpathogenic from pathogenic diseases,
a few general observations may be useful. There are exceptions
to each of them but they will be helpful in making a preliminary
diagnosis.
1. With no sign of a fungus or bacterial ooze, one can suspect
a nonpathogenic or virogenic disease. Bear in mind, however,
that invasion by saprophytes is inevitable.
2. The recent history of a crop or a field often offers a clue,
i.e., application of fertilizer or pesticides, irrigation, or cultiva-
tion.
3. Inquiry about the possibility of injury by frost, strong
rind, hail, flooding, and lightning may be helpful.
4. Noting the distribution of diseased plants as to high or
)w spots in the field, soil changes, former roads, creeks, and
arrals may afford a hint.
5. Nonpathogenic diseases may appear suddenly and be
venly distributed, whereas pathogenic diseases often spread
rom the edges of a field or from an infection center within.








Nonpathogenic Diseases of Lettuce

Their Identification and Control

Robert B. Marlatt
Associate Plant Pathologist, Sub-Tropical Station, Homestead

INTRODUCTION
Although no figures are available, pathologists generally agree
that a large proportion of the problems encountered by lettuce
growers are nonparasitic. In attempting to diagnose them one
can easily find information pertaining to parasitic diseases, but
little is available concerning nonpathogenic maladies. It is hoped
this manual may help fill the gap.
The information contained herein is the result of a review of
the literature and the author's research and observations which
for several years were devoted primarily to lettuce diseases.
Since no literature review is ever completed and knowledge
gained by experience frequently is not published, the author
would be grateful to receive suggestions from other workers
which might be added to a later edition of this manual.

PROCEDURE FOR DIAGNOSIS
In distinguishing nonpathogenic from pathogenic diseases,
a few general observations may be useful. There are exceptions
to each of them but they will be helpful in making a preliminary
diagnosis.
1. With no sign of a fungus or bacterial ooze, one can suspect
a nonpathogenic or virogenic disease. Bear in mind, however,
that invasion by saprophytes is inevitable.
2. The recent history of a crop or a field often offers a clue,
i.e., application of fertilizer or pesticides, irrigation, or cultiva-
tion.
3. Inquiry about the possibility of injury by frost, strong
rind, hail, flooding, and lightning may be helpful.
4. Noting the distribution of diseased plants as to high or
)w spots in the field, soil changes, former roads, creeks, and
arrals may afford a hint.
5. Nonpathogenic diseases may appear suddenly and be
venly distributed, whereas pathogenic diseases often spread
rom the edges of a field or from an infection center within.








6. Several unrelated genera or species of hosts are often
affected by a nonpathogenic disease in the area inspected. This
is less often the case with parasitic diseases.
7. Nonpathogenic diseases often cause no lesions. If lesions
occur, they frequently have no water-soaked or chlorotic margins.
References.-14, 41, 108, 112.

USE OF THIS MANUAL
If the above procedures and information provided by the
grower suggest a specific disease, the Table of Contents will
direct one to the section containing a description of its cause,
symptoms and control or preventive measures.
Whenever such clues to the disease are insufficient for im-
mediate diagnosis, the key in the back of this bulletin will pro-
vide additional help. Diagnosis is an art; therefore, the key
will be helpful only when one is familiar with the crop and
with local conditions where it is grown.
All statements pertaining to the disease in this manual stem
from experimental results unless otherwise qualified as ob-
servations or opinions. Some of the problems discussed have
only been studied in solution cultures. They are included in hopes
that they will be helpful to persons experimenting with lettuce
or growing the crop hydroponically.

MINERAL DEFICIENCIES AND TOXICITIES
Aluminum
Toxicity
Cause.-Low lettuce yields in excessively acid soil may pos-
sibly be due to aluminum toxicity rather than to low pH. In
one solution-culture experiment the presence of aluminum was
definitely shown to be detrimental at low pH. Two and five-tenths
parts per million (ppm) of aluminum reduced yields 71% in
the presence of 1 ppm of phosphate. When phosphate was raised
to 50 ppm, yields were reduced only 23'%.
Symptoms.-Roots are stubby and there are few lateral roots.
The plants are smaller than normal, with chlorotic leaves.
Control.-Addition of phosphate will lessen the toxicity symp-
toms. If the soil is extremely acid, raising the pH by use of
soil amendments such as lime diminishes the plant's uptake of
aluminum.
References.-30, 80.








6. Several unrelated genera or species of hosts are often
affected by a nonpathogenic disease in the area inspected. This
is less often the case with parasitic diseases.
7. Nonpathogenic diseases often cause no lesions. If lesions
occur, they frequently have no water-soaked or chlorotic margins.
References.-14, 41, 108, 112.

USE OF THIS MANUAL
If the above procedures and information provided by the
grower suggest a specific disease, the Table of Contents will
direct one to the section containing a description of its cause,
symptoms and control or preventive measures.
Whenever such clues to the disease are insufficient for im-
mediate diagnosis, the key in the back of this bulletin will pro-
vide additional help. Diagnosis is an art; therefore, the key
will be helpful only when one is familiar with the crop and
with local conditions where it is grown.
All statements pertaining to the disease in this manual stem
from experimental results unless otherwise qualified as ob-
servations or opinions. Some of the problems discussed have
only been studied in solution cultures. They are included in hopes
that they will be helpful to persons experimenting with lettuce
or growing the crop hydroponically.

MINERAL DEFICIENCIES AND TOXICITIES
Aluminum
Toxicity
Cause.-Low lettuce yields in excessively acid soil may pos-
sibly be due to aluminum toxicity rather than to low pH. In
one solution-culture experiment the presence of aluminum was
definitely shown to be detrimental at low pH. Two and five-tenths
parts per million (ppm) of aluminum reduced yields 71% in
the presence of 1 ppm of phosphate. When phosphate was raised
to 50 ppm, yields were reduced only 23'%.
Symptoms.-Roots are stubby and there are few lateral roots.
The plants are smaller than normal, with chlorotic leaves.
Control.-Addition of phosphate will lessen the toxicity symp-
toms. If the soil is extremely acid, raising the pH by use of
soil amendments such as lime diminishes the plant's uptake of
aluminum.
References.-30, 80.








6. Several unrelated genera or species of hosts are often
affected by a nonpathogenic disease in the area inspected. This
is less often the case with parasitic diseases.
7. Nonpathogenic diseases often cause no lesions. If lesions
occur, they frequently have no water-soaked or chlorotic margins.
References.-14, 41, 108, 112.

USE OF THIS MANUAL
If the above procedures and information provided by the
grower suggest a specific disease, the Table of Contents will
direct one to the section containing a description of its cause,
symptoms and control or preventive measures.
Whenever such clues to the disease are insufficient for im-
mediate diagnosis, the key in the back of this bulletin will pro-
vide additional help. Diagnosis is an art; therefore, the key
will be helpful only when one is familiar with the crop and
with local conditions where it is grown.
All statements pertaining to the disease in this manual stem
from experimental results unless otherwise qualified as ob-
servations or opinions. Some of the problems discussed have
only been studied in solution cultures. They are included in hopes
that they will be helpful to persons experimenting with lettuce
or growing the crop hydroponically.

MINERAL DEFICIENCIES AND TOXICITIES
Aluminum
Toxicity
Cause.-Low lettuce yields in excessively acid soil may pos-
sibly be due to aluminum toxicity rather than to low pH. In
one solution-culture experiment the presence of aluminum was
definitely shown to be detrimental at low pH. Two and five-tenths
parts per million (ppm) of aluminum reduced yields 71% in
the presence of 1 ppm of phosphate. When phosphate was raised
to 50 ppm, yields were reduced only 23'%.
Symptoms.-Roots are stubby and there are few lateral roots.
The plants are smaller than normal, with chlorotic leaves.
Control.-Addition of phosphate will lessen the toxicity symp-
toms. If the soil is extremely acid, raising the pH by use of
soil amendments such as lime diminishes the plant's uptake of
aluminum.
References.-30, 80.








Boron
Deficiency
Cause.-Lettuce is considered to have a moderate boron re-
quirement, 0.1 to 0.5 ppm in the soil. Healthy lettuce has been
found to contain about 35 ppm in its leaves and boron-deficient
leaves contained only 12 to 28 ppm.
Symptoms.-In hydroponic culture, deficiency symptoms have
appeared within 2 to 4 weeks after planting. Root growth is
very slow. Leaves become chlorotic, except for the veins, which
retain their green color until just before death. Areas between
the veins are almost white and plants are stunted. The heart
leaves are thickened, small, cupped, and brittle and die back
from their tips. The growing point dies eventually, and margins
of older leaves become scorched later than those of the inner
leaves.
Control.-The optimum boron content in sand culture is 0.6
to 0.7 ppm. Broadcasting of 30 pounds of borax per acre has
been helpful. In solution culture one may add small amounts
of borates of calcium, copper, manganese, potassium, sodium or
zinc. Addition of 40-mesh Pyrex glass will also prevent the
deficiency. Other frits or soluble sources of boron could be tried.

Toxicity
Cause.-Lettuce is considered to be tolerant of boron; i.e.,
from 0.9 to 4 ppm in the soil have been harmless under some
circumstances.
Symptoms.-Plants are stunted or may be killed. Lower
leaves become chlorotic and have large, white or tan dead spots,
especially along their margins.
Control.-The availability of boron can be reduced by appli-
cations of lime. If excess boron is coming from irrigation
water, sometimes another source of water will solve the prob-
lem; thus, surface water may be used in place of well water
containing toxic quantities of boron.
References.-48, 65, 66, 68, 92, 116, 118.

Calcium
Deficiency
Cause.-One of the commonest causes of calcium deficiency
symptoms is excessive applications of potash, especially when
the soil is naturally low in calcium. Applications of potash have
been even more effective than magnesium in lowering calcium








Boron
Deficiency
Cause.-Lettuce is considered to have a moderate boron re-
quirement, 0.1 to 0.5 ppm in the soil. Healthy lettuce has been
found to contain about 35 ppm in its leaves and boron-deficient
leaves contained only 12 to 28 ppm.
Symptoms.-In hydroponic culture, deficiency symptoms have
appeared within 2 to 4 weeks after planting. Root growth is
very slow. Leaves become chlorotic, except for the veins, which
retain their green color until just before death. Areas between
the veins are almost white and plants are stunted. The heart
leaves are thickened, small, cupped, and brittle and die back
from their tips. The growing point dies eventually, and margins
of older leaves become scorched later than those of the inner
leaves.
Control.-The optimum boron content in sand culture is 0.6
to 0.7 ppm. Broadcasting of 30 pounds of borax per acre has
been helpful. In solution culture one may add small amounts
of borates of calcium, copper, manganese, potassium, sodium or
zinc. Addition of 40-mesh Pyrex glass will also prevent the
deficiency. Other frits or soluble sources of boron could be tried.

Toxicity
Cause.-Lettuce is considered to be tolerant of boron; i.e.,
from 0.9 to 4 ppm in the soil have been harmless under some
circumstances.
Symptoms.-Plants are stunted or may be killed. Lower
leaves become chlorotic and have large, white or tan dead spots,
especially along their margins.
Control.-The availability of boron can be reduced by appli-
cations of lime. If excess boron is coming from irrigation
water, sometimes another source of water will solve the prob-
lem; thus, surface water may be used in place of well water
containing toxic quantities of boron.
References.-48, 65, 66, 68, 92, 116, 118.

Calcium
Deficiency
Cause.-One of the commonest causes of calcium deficiency
symptoms is excessive applications of potash, especially when
the soil is naturally low in calcium. Applications of potash have
been even more effective than magnesium in lowering calcium








uptake. However, soils high in magnesium and low in calcium
very often cause deficiency. For instance, a clay soil with ex-
changeable bases consisting of 85.4% magnesium and 12.6%
calcium produced severe calcium deficiency, as did a peat soil
with a pH of 4.8 and 2.1% calcium.
Symptoms.-Symptoms may appear 2 to 3 weeks after
planting. Seedlings are chlorotic, and the young leaf tips bear
brown spots, are cupped upward, and may eventually die. Older
leaves have a marginal burn. The disease has been called
"rosette" because the plants were short and leaves formed a
rosette pattern. Seed from calcium deficient plants deteriorates
abnormally fast.
Control.-Addition of a calcium compound, such as lime-
stone or gypsum, is recommended. If potassium is not excessive,
a calcium saturation of 25% can prevent symptoms. Raising
soil pH to 5.7 has controlled the disease. In small-scale experi-
ments, application of calcium amberlite overcame rosette, where-
as adding magnesium amberlite increased the disease.
References.-34, 60, 107, 110.

Copper
Deficiency
Cause.-Peat and muck soils are most likely to be deficient
in copper. The deficiency is common on the sawgrass peat soils
of the Florida Everglades.
Symptoms.-Growth is reduced and leaves first become
chlorotic at the margins and on the petioles; later they may be
entirely yellowish. Leaves are stunted, narrow, malformed, and
limp.
Control.-The deficiency is overcome in Florida Everglades
peat by applying 20 to 30 pounds of copper sulfate (25.5%
copper) per acre. Muck with a very low copper content has re-
quired 100 to 300 pounds. Deficient mineral soils may be reme-
died by application of 8 to 16 pounds per acre of copper oxide
(79.6% copper).
References.-24, 48.

Iodine
Deficiency.-Under commercial conditions there is no report
of iodine deficiency. Normal lettuce contains 1.6-6.7 ppm of
iodine. In solution culture a growth response has been obtained
by providing 1 or 2 ppm of iodine but there were no obvious
deficiency symptoms.








uptake. However, soils high in magnesium and low in calcium
very often cause deficiency. For instance, a clay soil with ex-
changeable bases consisting of 85.4% magnesium and 12.6%
calcium produced severe calcium deficiency, as did a peat soil
with a pH of 4.8 and 2.1% calcium.
Symptoms.-Symptoms may appear 2 to 3 weeks after
planting. Seedlings are chlorotic, and the young leaf tips bear
brown spots, are cupped upward, and may eventually die. Older
leaves have a marginal burn. The disease has been called
"rosette" because the plants were short and leaves formed a
rosette pattern. Seed from calcium deficient plants deteriorates
abnormally fast.
Control.-Addition of a calcium compound, such as lime-
stone or gypsum, is recommended. If potassium is not excessive,
a calcium saturation of 25% can prevent symptoms. Raising
soil pH to 5.7 has controlled the disease. In small-scale experi-
ments, application of calcium amberlite overcame rosette, where-
as adding magnesium amberlite increased the disease.
References.-34, 60, 107, 110.

Copper
Deficiency
Cause.-Peat and muck soils are most likely to be deficient
in copper. The deficiency is common on the sawgrass peat soils
of the Florida Everglades.
Symptoms.-Growth is reduced and leaves first become
chlorotic at the margins and on the petioles; later they may be
entirely yellowish. Leaves are stunted, narrow, malformed, and
limp.
Control.-The deficiency is overcome in Florida Everglades
peat by applying 20 to 30 pounds of copper sulfate (25.5%
copper) per acre. Muck with a very low copper content has re-
quired 100 to 300 pounds. Deficient mineral soils may be reme-
died by application of 8 to 16 pounds per acre of copper oxide
(79.6% copper).
References.-24, 48.

Iodine
Deficiency.-Under commercial conditions there is no report
of iodine deficiency. Normal lettuce contains 1.6-6.7 ppm of
iodine. In solution culture a growth response has been obtained
by providing 1 or 2 ppm of iodine but there were no obvious
deficiency symptoms.








Toxicity.-Early growth was retarded by iodine, applied
as potassium iodide, if concentration reached 4 ppm. The ele-
ment may be absorbed by roots until foliage contains over 123
ppm.
References.-19, 76.

Iron
Deficiency
Cause.-Soils with a high pH, those which are highly cal-
careous, are commonly responsible for iron deficiency in lettuce.
When such soils are irrigated excessively, the condition is wors-
ened.
Symptoms.-Plants are stunted and chlorotic. Foliage is
crinkled and has a noticeable grayish bloom. Necrotic spots
near and on the midrib, especially on the upper side of its base,
have been attributed to iron deficiency.
Control.-The disease is rarely serious enough to warrant
control measures. In sand or solution culture, the deficiency
symptoms are less severe if manganese is also in short supply.
Toxicity
Spray applications of an iron chelate (EDTA) at the rate of
2 pounds per 100 gallons water has caused an unsightly brown
to black spotting on tops of head lettuce.
References.-58, 106, 118.

Magnesium
Deficiency
Cause.-If soil has a high calcium magnesium ratio, the
magnesium may be made less available. Lettuce is more tolerant
of magnesium deficiency than are some crucifers, cucurbits, and
solanaceous plants.
Symptoms.-Plants are stunted and heading is delayed.
Foliage is slightly chlorotic, and the older leaves have whitish
blotching not limited to interveinal areas. Affected leaves may
be asymmetrical and eventually develop necrotic areas. Roots are
stunted, and the entire plant decreases in dry weight and in-
creases in moisture content.
Control.-Application of magnesium in fertilizer or with
soil amendments is recommended. In solution culture, changing
the calcium: magnesium ratio from 11.8 to 4 increased dry
weight from 25 to 37 grams. (See also "Rib Discoloration".)
References.-7, 21, 48, 55, 110, 118, 124.








Toxicity.-Early growth was retarded by iodine, applied
as potassium iodide, if concentration reached 4 ppm. The ele-
ment may be absorbed by roots until foliage contains over 123
ppm.
References.-19, 76.

Iron
Deficiency
Cause.-Soils with a high pH, those which are highly cal-
careous, are commonly responsible for iron deficiency in lettuce.
When such soils are irrigated excessively, the condition is wors-
ened.
Symptoms.-Plants are stunted and chlorotic. Foliage is
crinkled and has a noticeable grayish bloom. Necrotic spots
near and on the midrib, especially on the upper side of its base,
have been attributed to iron deficiency.
Control.-The disease is rarely serious enough to warrant
control measures. In sand or solution culture, the deficiency
symptoms are less severe if manganese is also in short supply.
Toxicity
Spray applications of an iron chelate (EDTA) at the rate of
2 pounds per 100 gallons water has caused an unsightly brown
to black spotting on tops of head lettuce.
References.-58, 106, 118.

Magnesium
Deficiency
Cause.-If soil has a high calcium magnesium ratio, the
magnesium may be made less available. Lettuce is more tolerant
of magnesium deficiency than are some crucifers, cucurbits, and
solanaceous plants.
Symptoms.-Plants are stunted and heading is delayed.
Foliage is slightly chlorotic, and the older leaves have whitish
blotching not limited to interveinal areas. Affected leaves may
be asymmetrical and eventually develop necrotic areas. Roots are
stunted, and the entire plant decreases in dry weight and in-
creases in moisture content.
Control.-Application of magnesium in fertilizer or with
soil amendments is recommended. In solution culture, changing
the calcium: magnesium ratio from 11.8 to 4 increased dry
weight from 25 to 37 grams. (See also "Rib Discoloration".)
References.-7, 21, 48, 55, 110, 118, 124.








Manganese
Deficiency
Cause.-Manganese is less available to lettuce grown in
calcareous soils with a high pH. Its deficiency depends on the
availability and ratio of iron and manganese in the soil. Symp-
toms are apparent when the leaf iron: manganese ration is 13,
severe symptoms with a ratio of 32.
Symptoms.-Plants are stunted, are chlorotic, and exhibit
a grayish bloom on the undersides of leaves. Leaves may be
asymmetrical or distorted and sometimes have hollow midribs.
Irregular, necrotic spots occur on the midribs and small, well-
defined necrotic spots may appear on leaf margins.
Control.-Broadcasting 30 pounds per acre of manganese
sulfate has remedied a deficiency in soil with a pH of 7.8. Per-
haps lowering the soil pH. by addition of sulfur would remedy
the deficiency, provided that the soil is not too highly buffered.

Toxicity
Cause.-In solution culture, more than 0.025 ppm can be toxic.
Foliar application of manganese chelate (EDTA) at a strength
of 2 pounds per 100 gallons of water caused stunting of head
lettuce.
Symptoms.-Plants are stunted, are chlorotic, and may have
small necrotic spots near lateral veins of the leaf. The spots
may occur on chlorotic blotches.
References.-29, 60, 71, 106, 110, 117, 118.

Molybdenum
Deficiency
Cause.-Molybdenum may be insufficiently available from
soil with a low pH. Deficient leaves may contain 0.06 ppm
molybdenum, whereas normal leaves contain about 0.1 ppm.
Symptoms.-Lettuce is especially susceptible to a lack of this
element. Plants are stunted and spindly, and heading is de-
layed or prevented. Leaves are pale green with outer leaves
becoming yellow and developing a ragged marginal necrosis.
Occasionally interveinal areas become chlorotic, then necrotic.
The accompanying high nitrogen content of interveinal tissues
has been blamed for the necrosis.
Control.-The application of 2 to 4 pounds of sodium molyb-
date per acre will often eliminate the deficiency. With acid
soils it is necessary also to raise the pH. Sometimes the appli-








Manganese
Deficiency
Cause.-Manganese is less available to lettuce grown in
calcareous soils with a high pH. Its deficiency depends on the
availability and ratio of iron and manganese in the soil. Symp-
toms are apparent when the leaf iron: manganese ration is 13,
severe symptoms with a ratio of 32.
Symptoms.-Plants are stunted, are chlorotic, and exhibit
a grayish bloom on the undersides of leaves. Leaves may be
asymmetrical or distorted and sometimes have hollow midribs.
Irregular, necrotic spots occur on the midribs and small, well-
defined necrotic spots may appear on leaf margins.
Control.-Broadcasting 30 pounds per acre of manganese
sulfate has remedied a deficiency in soil with a pH of 7.8. Per-
haps lowering the soil pH. by addition of sulfur would remedy
the deficiency, provided that the soil is not too highly buffered.

Toxicity
Cause.-In solution culture, more than 0.025 ppm can be toxic.
Foliar application of manganese chelate (EDTA) at a strength
of 2 pounds per 100 gallons of water caused stunting of head
lettuce.
Symptoms.-Plants are stunted, are chlorotic, and may have
small necrotic spots near lateral veins of the leaf. The spots
may occur on chlorotic blotches.
References.-29, 60, 71, 106, 110, 117, 118.

Molybdenum
Deficiency
Cause.-Molybdenum may be insufficiently available from
soil with a low pH. Deficient leaves may contain 0.06 ppm
molybdenum, whereas normal leaves contain about 0.1 ppm.
Symptoms.-Lettuce is especially susceptible to a lack of this
element. Plants are stunted and spindly, and heading is de-
layed or prevented. Leaves are pale green with outer leaves
becoming yellow and developing a ragged marginal necrosis.
Occasionally interveinal areas become chlorotic, then necrotic.
The accompanying high nitrogen content of interveinal tissues
has been blamed for the necrosis.
Control.-The application of 2 to 4 pounds of sodium molyb-
date per acre will often eliminate the deficiency. With acid
soils it is necessary also to raise the pH. Sometimes the appli-








cation of three tons or more of limestone will be effective without
the addition of molybdenum. Molybdenum may be applied to the
foliage as a water solution of sodium molybdate providing ap-
proximately 0.07 grams of the molybdate per plant in 50 to 100
ml of water. Color and growth has been improved within 26
to 43 days. Although soaking seeds of some other plants in solu-
tions of molybdenum salts may be helpful, it has not worked
with lettuce.
References-37, 72, 74, 81, 82, 83, 94, 109, 110, 120, 121.

Nitrogen
Deficiency
Cause.-Most nitrified or applied nitrogen in the soil is
easily leached; therefore a deficiency may quickly occur after
heavy rains or excessive overhead irrigation.
Symptoms.-If nitrogen is completely lacking, as in soilless
or sand culture, symptoms may occur one week after emergence.
Usually considerably more growth occurs before a deficiency is
noticed. Heading is delayed; or if the deficiency is extreme, no
heads will form. Foliage becomes light green, and leaves are
abnormally smooth; they do not crinkle as much as normal
leaves. If deficiency is severe, leaves are small and yellowish,
especially at their tips and old leaves turn yellow prematurely.
Seeds are small, are fewer in number, and are more likely to
produce seedlings with "red cotyledon" disease.
Control.-Response is quickest when plants are fed by foliar
sprays of nitrate nitrogen.

Toxicity
Cause.-Toxicity of nitrogen compounds such as free am-
monia and ammonium hydroxide is often accentuated by wet,
cold soil in which nitrification is slow. Growing lettuce on former
corral sites may be hazardous because of the residual manure
plus poor drainage due to compaction. Inorganic fertilizers con-
taining high proportions of ammoniacal nitrogen are most likely
to be toxic. Other nitrogenous materials that may be toxic in
high concentrations include nitrites, nitric acid, and ammoniated
sodium nitrate. Urea-formaldehyde fertilizers have also caused
problems by killing seedlings or retarding growth.
Symptoms:--Plants are stunted and wilt during the hottest
part of the day, usually recovering at night. Imperial 456 is
very susceptible, but other varieties are also injured; Big Boston
and Cos are resistant. Frame (outer) leaves of head lettuce be-








come lusterless, yellowish or gray-green, and wilt, then die.
Occasionally necrosis first occurs as sectors in a leaf, as a margi-
nal burn or as brown specks. Small roots are killed; the tap
root is yellowish, corky, and sometimes split. A split tap root
shows yellowish, red, or brown xylem and may contain a hol-
low core which gradually extends to the crown.
Control.-Avoiding ammoniacal materials and using fertiliz-
ers such as calcium nitrate. In heavy soil it is wise to irrigate
cautiously, especially in cool weather. Raising the pH of muck
(above pH 5.5) has reduced symptom severity. Any cultural
practice that aids nitrification, such as aeration, is helpful.
References.-31, 32, 34, 40, 56, 61, 104, 110, 124.

Phosphorus
Deficiency
Cause.-Phosphorus does not move far from the site of ap-
plication. It is less available in soils high in calcium carbonate,
iron, or aluminum.
Symptoms.-Signs of severe deficiency may appear four
weeks after emergence. Lettuce generally has yellowish outer
leaves which may wither; heads may not form. Plants may be
stunted and die if deficiency is severe. Mild deficiency can delay
germination and rapid early development. Some green varieties
temporarily become darker green when young and later show
marginal chlorosis or russeting. Leaf edges may curl under
and leaves are toughened. Old leaves yellow prematurely. A
red variety may show leaf bronzing, with purple blotches and
crimson stalks. Fewer seeds are produced.
Control.-Apply a phosphate fertilizer where roots can easily
contact it; banding is more efficient than broadcasting.
References.-31, 34, 110, 115, 124, 126.

Potassium
Deficiency
Cause.-Deficiency is more apt to occur in dryer soil, in
contrast to some other elements which are less available in wet
soil. Potassium is less available in calcareous than in acid soils.
Symptoms.-Maturity is often delayed. Leaves, beginning
with the older ones, are dull, dark green with pale midribs and
veins. They may also be thickened and fleshy. Leaves become
more rounded in shape, some heart-shaped; and some crinkled
varieties may be nearly smooth. In severe cases, leaf margins
may be scorched or entirely killed. Root systems are often stunt-








come lusterless, yellowish or gray-green, and wilt, then die.
Occasionally necrosis first occurs as sectors in a leaf, as a margi-
nal burn or as brown specks. Small roots are killed; the tap
root is yellowish, corky, and sometimes split. A split tap root
shows yellowish, red, or brown xylem and may contain a hol-
low core which gradually extends to the crown.
Control.-Avoiding ammoniacal materials and using fertiliz-
ers such as calcium nitrate. In heavy soil it is wise to irrigate
cautiously, especially in cool weather. Raising the pH of muck
(above pH 5.5) has reduced symptom severity. Any cultural
practice that aids nitrification, such as aeration, is helpful.
References.-31, 32, 34, 40, 56, 61, 104, 110, 124.

Phosphorus
Deficiency
Cause.-Phosphorus does not move far from the site of ap-
plication. It is less available in soils high in calcium carbonate,
iron, or aluminum.
Symptoms.-Signs of severe deficiency may appear four
weeks after emergence. Lettuce generally has yellowish outer
leaves which may wither; heads may not form. Plants may be
stunted and die if deficiency is severe. Mild deficiency can delay
germination and rapid early development. Some green varieties
temporarily become darker green when young and later show
marginal chlorosis or russeting. Leaf edges may curl under
and leaves are toughened. Old leaves yellow prematurely. A
red variety may show leaf bronzing, with purple blotches and
crimson stalks. Fewer seeds are produced.
Control.-Apply a phosphate fertilizer where roots can easily
contact it; banding is more efficient than broadcasting.
References.-31, 34, 110, 115, 124, 126.

Potassium
Deficiency
Cause.-Deficiency is more apt to occur in dryer soil, in
contrast to some other elements which are less available in wet
soil. Potassium is less available in calcareous than in acid soils.
Symptoms.-Maturity is often delayed. Leaves, beginning
with the older ones, are dull, dark green with pale midribs and
veins. They may also be thickened and fleshy. Leaves become
more rounded in shape, some heart-shaped; and some crinkled
varieties may be nearly smooth. In severe cases, leaf margins
may be scorched or entirely killed. Root systems are often stunt-








ed. Fewer seeds are produced, and they deteriorate faster.
Control.-Apply a potassium fertilizer. Foliar sprays of
potassium sulfate (less than 2%) have controlled the deficiency
in Romaine lettuce.
References.-31, 34, 110, 122, 123, 124.

Zinc
Deficiency
Cause.-In alkaline soils, zinc may be in an unavailable form;
in acid soils it may be insufficiently available to lettuce.
Symptoms.-Symptoms are rarely noticeable in lettuce be-
cause a low supply merely delays maturity and size.
Control.-Foliar sprays of approximately 900 ppm of zinc
sulfate or side dressing young lettuce with 100 pounds per acre
have hastened maturity in Arizona. In California, growth rate
and head size were increased by soil applications of 10 to 34
pounds of zinc per acre (as zinc sulfate). The soil originally
contained 0.55 to 0.65 ppm dithizone-extractable zinc. A resid-
ual effect for a minimum of 4 years was obtained.
Toxicity
Cause.-Side dressing with several hundred pounds of zinc
sulfate per acre or with 200 pounds of zinc chelate (EDTA)
or spraying with 4 pounds of the chelate per 100 gallons have
resulted in similar toxicity symptoms.
Symptoms.-Lower leaves first become chlorotic, then wilt
and die. Plants are severely stunted.
References.-59, 128.

SOIL
Soil Acidity or Alkalinity
Acidity
Cause.-Extremes of soil pH may promote toxicities or defi-
ciencies of several elements, thus the actual effect of the pH is
difficult to evaluate. In solution cultures, with all nutrients pro-
vided, a pH below 6 is unfavorable for lettuce.
Symptoms.-Roots are dull gray in color and plants stop
growing. When placed into nutrient solution of pH 3 these root
symptoms appeared within one hour. Even at such a low pH,
the acidity of the plant sap did not change.
Control.-Addition of lime or other soil amendments con-
taining calcium is recommended. A pH of 6.5 to 7 has produced
the earliest and best heads in some experiments.








ed. Fewer seeds are produced, and they deteriorate faster.
Control.-Apply a potassium fertilizer. Foliar sprays of
potassium sulfate (less than 2%) have controlled the deficiency
in Romaine lettuce.
References.-31, 34, 110, 122, 123, 124.

Zinc
Deficiency
Cause.-In alkaline soils, zinc may be in an unavailable form;
in acid soils it may be insufficiently available to lettuce.
Symptoms.-Symptoms are rarely noticeable in lettuce be-
cause a low supply merely delays maturity and size.
Control.-Foliar sprays of approximately 900 ppm of zinc
sulfate or side dressing young lettuce with 100 pounds per acre
have hastened maturity in Arizona. In California, growth rate
and head size were increased by soil applications of 10 to 34
pounds of zinc per acre (as zinc sulfate). The soil originally
contained 0.55 to 0.65 ppm dithizone-extractable zinc. A resid-
ual effect for a minimum of 4 years was obtained.
Toxicity
Cause.-Side dressing with several hundred pounds of zinc
sulfate per acre or with 200 pounds of zinc chelate (EDTA)
or spraying with 4 pounds of the chelate per 100 gallons have
resulted in similar toxicity symptoms.
Symptoms.-Lower leaves first become chlorotic, then wilt
and die. Plants are severely stunted.
References.-59, 128.

SOIL
Soil Acidity or Alkalinity
Acidity
Cause.-Extremes of soil pH may promote toxicities or defi-
ciencies of several elements, thus the actual effect of the pH is
difficult to evaluate. In solution cultures, with all nutrients pro-
vided, a pH below 6 is unfavorable for lettuce.
Symptoms.-Roots are dull gray in color and plants stop
growing. When placed into nutrient solution of pH 3 these root
symptoms appeared within one hour. Even at such a low pH,
the acidity of the plant sap did not change.
Control.-Addition of lime or other soil amendments con-
taining calcium is recommended. A pH of 6.5 to 7 has produced
the earliest and best heads in some experiments.








ed. Fewer seeds are produced, and they deteriorate faster.
Control.-Apply a potassium fertilizer. Foliar sprays of
potassium sulfate (less than 2%) have controlled the deficiency
in Romaine lettuce.
References.-31, 34, 110, 122, 123, 124.

Zinc
Deficiency
Cause.-In alkaline soils, zinc may be in an unavailable form;
in acid soils it may be insufficiently available to lettuce.
Symptoms.-Symptoms are rarely noticeable in lettuce be-
cause a low supply merely delays maturity and size.
Control.-Foliar sprays of approximately 900 ppm of zinc
sulfate or side dressing young lettuce with 100 pounds per acre
have hastened maturity in Arizona. In California, growth rate
and head size were increased by soil applications of 10 to 34
pounds of zinc per acre (as zinc sulfate). The soil originally
contained 0.55 to 0.65 ppm dithizone-extractable zinc. A resid-
ual effect for a minimum of 4 years was obtained.
Toxicity
Cause.-Side dressing with several hundred pounds of zinc
sulfate per acre or with 200 pounds of zinc chelate (EDTA)
or spraying with 4 pounds of the chelate per 100 gallons have
resulted in similar toxicity symptoms.
Symptoms.-Lower leaves first become chlorotic, then wilt
and die. Plants are severely stunted.
References.-59, 128.

SOIL
Soil Acidity or Alkalinity
Acidity
Cause.-Extremes of soil pH may promote toxicities or defi-
ciencies of several elements, thus the actual effect of the pH is
difficult to evaluate. In solution cultures, with all nutrients pro-
vided, a pH below 6 is unfavorable for lettuce.
Symptoms.-Roots are dull gray in color and plants stop
growing. When placed into nutrient solution of pH 3 these root
symptoms appeared within one hour. Even at such a low pH,
the acidity of the plant sap did not change.
Control.-Addition of lime or other soil amendments con-
taining calcium is recommended. A pH of 6.5 to 7 has produced
the earliest and best heads in some experiments.








Alkalinity
Cause.-With all nutrients provided, lettuce may grow well
up to pH 8, but a higher reaction is unfavorable.
Symptoms.-Growth is slow and plants have gradually de-
clined at pH 9.
Control.-The most common cause of a high pH is a high
proportion of exchangeable sodium. Sulfur, gypsum, and sul-
furic acid, plus leaching, have been used to remove the sodium.
Fortunately, few soils are too alkaline for lettuce.
References.-3, 4, 7, 48, 77

Carbon Dioxide in Soil
Cause.-In an experiment to determine the effects of carbon
dioxide-treated soil, lettuce was grown in soils to which carbon
dioxide was applied for 8 and 24 hours. The soil was aerated
before planting.
Symptoms-Tap roots of lettuce grown in soil treated for
24 hours measured an average of 2.8 inches in length; in soil
treated eight hours they averaged 3.7 inches; untreated controls
averaged 5.2 inches. Reduction of growth was the only symptom
described.
References.-73.

Salt Injury
Cause.-Saline soil is often found in arid regions or where
salty irrigation water is applied. These soils contain excessive
amounts of sodium, calcium, and magnesium chlorides and sul-
fates. Soluble salts are responsible for the injury. Excessive
application of chemical fertilizers may have a similar effect by
causing high osmotic pressure, especially if the soil becomes
sufficiently dry to create injurious salt concentration.
Symptoms.-Lettuce has medium salt tolerance; a soil solu-
tion, extracted from a saturated paste with conductance of 4.4
millimhos reduced plant weight one-third; a higher salt con-
centration, 6 millimhos, reduced it one-half. Heads are less
compact and are slow in forming. Outer leaves are thick and
leathery and exhibit a bitter flavor. If soil becomes suddenly
saline, the leaf margins are often killed. Symptoms may be
confused with anthracnose.
Control.-If soil is saline but not excessively high in sodium,
leaching will usually correct the problem. In many soils, struc-








Alkalinity
Cause.-With all nutrients provided, lettuce may grow well
up to pH 8, but a higher reaction is unfavorable.
Symptoms.-Growth is slow and plants have gradually de-
clined at pH 9.
Control.-The most common cause of a high pH is a high
proportion of exchangeable sodium. Sulfur, gypsum, and sul-
furic acid, plus leaching, have been used to remove the sodium.
Fortunately, few soils are too alkaline for lettuce.
References.-3, 4, 7, 48, 77

Carbon Dioxide in Soil
Cause.-In an experiment to determine the effects of carbon
dioxide-treated soil, lettuce was grown in soils to which carbon
dioxide was applied for 8 and 24 hours. The soil was aerated
before planting.
Symptoms-Tap roots of lettuce grown in soil treated for
24 hours measured an average of 2.8 inches in length; in soil
treated eight hours they averaged 3.7 inches; untreated controls
averaged 5.2 inches. Reduction of growth was the only symptom
described.
References.-73.

Salt Injury
Cause.-Saline soil is often found in arid regions or where
salty irrigation water is applied. These soils contain excessive
amounts of sodium, calcium, and magnesium chlorides and sul-
fates. Soluble salts are responsible for the injury. Excessive
application of chemical fertilizers may have a similar effect by
causing high osmotic pressure, especially if the soil becomes
sufficiently dry to create injurious salt concentration.
Symptoms.-Lettuce has medium salt tolerance; a soil solu-
tion, extracted from a saturated paste with conductance of 4.4
millimhos reduced plant weight one-third; a higher salt con-
centration, 6 millimhos, reduced it one-half. Heads are less
compact and are slow in forming. Outer leaves are thick and
leathery and exhibit a bitter flavor. If soil becomes suddenly
saline, the leaf margins are often killed. Symptoms may be
confused with anthracnose.
Control.-If soil is saline but not excessively high in sodium,
leaching will usually correct the problem. In many soils, struc-







ture should first be corrected by manures or amendments such
as gypsum, followed by leaching. This is helpful if there are
considerable amounts of sodium. If salts have accumulated
above the roots, avoid insufficient surface irrigation which may
move salt only to the root zone, causing injury. If furrow ir-
rigation is practiced, planting on one side of a sloping bed
permits salts to rise above the root system. Resistant varieties
include: Imperial 615, Imperial 101, and Great Lakes. New York
515, K-l, and Dark Green Cos are very susceptible.
References.-5, 8, 9, 36, 125.

Root Girdling
Cause.-In western New York mucklands, strong winds may
have caused root girdling by bending and injuring the upper
portions of tap roots. In California, the drying surface of soil
with a poor, tight structure has been suspected. There was a
direct correlation between a high salt content of surface soil
and the disease in Arizona. The malady was restricted to those
sides of raised beds which had more soluble salts in the top
1 inch. Twisting and bending young plants by hand failed to
cause girdling in Arizona. After the constriction occurs, para-
sitic fungi may kill the plant.
Symptoms.-In New York the leaves often are inclined in
one direction and the roots the opposite. The damage most often
appears in very young lettuce as it develops its second true
leaves. The tap root is constricted just below the stem at the
soil surface or slightly below. Plants may survive only to be
stunted or slowly die just before heading begins. Outer leaves
become chlorotic and their margins turn brown. Later the
plant may wilt and die. Affected plants are easily snapped off
at the girdle, and nearby vascular elements may be discolored.
Control.-Windbreaks could be helpful if the damage is due
to mechanical injury caused by high winds. In cases where soil
structure is thought to be the cause, furrow-irrigated lettuce
should receive an extra irrigation before thinning. This keeps
the soil soft until plants are older, and tougher.
If girdling is due to excessive salinity near the soil surface,
decreasing the salinity will help. (See "Salt Injury".)

Drought Spot
Cause.-Maximum daily temperatures over 100 F have
caused the disease, along with a soil moisture only slightly above







ture should first be corrected by manures or amendments such
as gypsum, followed by leaching. This is helpful if there are
considerable amounts of sodium. If salts have accumulated
above the roots, avoid insufficient surface irrigation which may
move salt only to the root zone, causing injury. If furrow ir-
rigation is practiced, planting on one side of a sloping bed
permits salts to rise above the root system. Resistant varieties
include: Imperial 615, Imperial 101, and Great Lakes. New York
515, K-l, and Dark Green Cos are very susceptible.
References.-5, 8, 9, 36, 125.

Root Girdling
Cause.-In western New York mucklands, strong winds may
have caused root girdling by bending and injuring the upper
portions of tap roots. In California, the drying surface of soil
with a poor, tight structure has been suspected. There was a
direct correlation between a high salt content of surface soil
and the disease in Arizona. The malady was restricted to those
sides of raised beds which had more soluble salts in the top
1 inch. Twisting and bending young plants by hand failed to
cause girdling in Arizona. After the constriction occurs, para-
sitic fungi may kill the plant.
Symptoms.-In New York the leaves often are inclined in
one direction and the roots the opposite. The damage most often
appears in very young lettuce as it develops its second true
leaves. The tap root is constricted just below the stem at the
soil surface or slightly below. Plants may survive only to be
stunted or slowly die just before heading begins. Outer leaves
become chlorotic and their margins turn brown. Later the
plant may wilt and die. Affected plants are easily snapped off
at the girdle, and nearby vascular elements may be discolored.
Control.-Windbreaks could be helpful if the damage is due
to mechanical injury caused by high winds. In cases where soil
structure is thought to be the cause, furrow-irrigated lettuce
should receive an extra irrigation before thinning. This keeps
the soil soft until plants are older, and tougher.
If girdling is due to excessive salinity near the soil surface,
decreasing the salinity will help. (See "Salt Injury".)

Drought Spot
Cause.-Maximum daily temperatures over 100 F have
caused the disease, along with a soil moisture only slightly above








the wilting percentage during embryo development. The condi-
tion is aggravated by improper seed storage.
Symptoms.-Cotyledons have brown or black necrotic spots
on them, beginning adjacent to the vascular elements. In severe
cases the mesophyll and palisade tissues are affected. There are
often large dead areas near the cotyledon apex and at the point
of attachment.
Control.-Keep soil moisture no lower than one-half the avail-
able water supply during seed formation and store the seeds
properly (See Seed Dormancy, Poor Germination).
References.-26.

Puffy Heads, Burst Heads
Cause.-Allowing soil to become too dry before irrigating
mature heads or applying excessive manure before planting may
cause loose or puffy heads to form. This is especially likely
when temperatures are high.
Symptoms.-Heads may be unusually soft at the stage when
they should be firm. When torn apart, the puffiness is seen to
be due to imperfect overlapping of head leaves.
Heads may burst by splitting somewhere near their tops.
Many head leaves are also found with petioles severed near
the stem. This injury is typically caused by irrigating mature
lettuce in dry soil.
Control.-The problems may be prevented by allowing soil
moisture to drop no lower than one-half the available water
supply (difference between field capacity and the permanent
wilting percentage).
References.-17, 49, 115.

Seed Dormancy, Poor Germination
Cause.-After planting (seed barely covered with soil), the
condition of the soil is extremely important. Seed will germinate
poorly if the soil is kept continuously too wet, too dry, poorly
aerated, too salty or too hot or cold. Another very obvious cause
of poor germination is non-viable seed due to improper storage
or old age. At normal seeding rates, well-stored seed will pro-
duce a satisfactory stand after five years' storage. Seed must
have a maximum moisture content of 10% if stored at 40-50 F,
7% at 70F, and 5% if stored at 80 F. Seed from potassium-
or calcium-deficient plants may be especially susceptible to
deterioration.








the wilting percentage during embryo development. The condi-
tion is aggravated by improper seed storage.
Symptoms.-Cotyledons have brown or black necrotic spots
on them, beginning adjacent to the vascular elements. In severe
cases the mesophyll and palisade tissues are affected. There are
often large dead areas near the cotyledon apex and at the point
of attachment.
Control.-Keep soil moisture no lower than one-half the avail-
able water supply during seed formation and store the seeds
properly (See Seed Dormancy, Poor Germination).
References.-26.

Puffy Heads, Burst Heads
Cause.-Allowing soil to become too dry before irrigating
mature heads or applying excessive manure before planting may
cause loose or puffy heads to form. This is especially likely
when temperatures are high.
Symptoms.-Heads may be unusually soft at the stage when
they should be firm. When torn apart, the puffiness is seen to
be due to imperfect overlapping of head leaves.
Heads may burst by splitting somewhere near their tops.
Many head leaves are also found with petioles severed near
the stem. This injury is typically caused by irrigating mature
lettuce in dry soil.
Control.-The problems may be prevented by allowing soil
moisture to drop no lower than one-half the available water
supply (difference between field capacity and the permanent
wilting percentage).
References.-17, 49, 115.

Seed Dormancy, Poor Germination
Cause.-After planting (seed barely covered with soil), the
condition of the soil is extremely important. Seed will germinate
poorly if the soil is kept continuously too wet, too dry, poorly
aerated, too salty or too hot or cold. Another very obvious cause
of poor germination is non-viable seed due to improper storage
or old age. At normal seeding rates, well-stored seed will pro-
duce a satisfactory stand after five years' storage. Seed must
have a maximum moisture content of 10% if stored at 40-50 F,
7% at 70F, and 5% if stored at 80 F. Seed from potassium-
or calcium-deficient plants may be especially susceptible to
deterioration.








Even with ideal storage and soil conditions, dormant seeds
may not germinate. Seed dormancy is encouraged by: (1) low
temperatures a month before seed harvest; (2) exposure for
at least 30 seconds to far-red light (742 mu) ; (3) seed storage
at 86 F while moist and imbibing water, and (4) drying at
temperatures above 77 F. Seed produced in a cool climate may
germinate poorly at temperatures above 78 F, and all seed is
seriously affected by 85 F or above. Minimum germination
temperature is about 35 F. Low concentrations of indole acetic
acid have also inhibited germination.
Symptoms.-Poor stands will of course result when germina-
tion is low or exceptionally slow. Lettuce is, unfortunately, one
of those crops which vary considerably as to the period required
for seed germination. When a crop is being harvested, it is
not unusual to find young seedlings in the row. In severe cases
of dormancy, only 50% may'germinate after 18 months in the
dark.
Control.-After planting, try not to allow soil moisture to
drop lower than one-half the available water supply. Care must
be taken not to let the soil become too dry. However, continuous
irrigation may cause the soil to remain too wet, and poor aera-
tion will result. High salinity may be avoided by planting on
the sides of raised beds. Thus, salts rise to the bed peak with
furrow irrigation or are leached below the seed with sprinklers.
Seed destined for planting in a hot climate should be ob-
tained from a similar climate. Seed which matures under high
temperatures will germinate better in soil over 78 F. than seed
which matured in a cool climate. Exposure only to visible red
light (650 mu) for over 30 seconds helps break dormancy.
Also, one may soak the seed for a few hours in water at 40 to
60 F, then expose them to diffuse light for a few hours while
being aerated at 68 F. It should be dried slowly at 40 to 50 F.
If seeds are soaked in 60 ppm gibberellic acid (GAs) during
the red light treatment, germination may be improved. Up to
1500 ppm has substituted for the light treatment, germination
being proportional to the concentration of gibberellic acid. Some
varieties are more likely to have a dormancy problem than
others. Those which are more troublesome include: Grand Rap-
ids, Hubbard Market, Imperial D, Imperial 44, Imperial 615.
Less troublesome varieties include: Iceberg, Imperial 17, 101,
456, and Great Lakes.
References.-9, 20, 23, 35, 42, 43, 46, 102, 105.








Plant Residue Injury
Cause.-Root injury is especially severe when crop residues
(refuse) have been decomposing in soil. After 10 to 25 days in
saline soil, one such disease was especially severe. Soils were
found to contain water-soluble or ether-soluble toxins, one being
benzoic acid with an amine group. These crops formed injurious
residues: barley, broad-bean, broccoli, lettuce, rye, Sudan grass,
vetch, and wheat.
Symptoms.-Seedlings are especially sensitive to the malady.
Above-ground symptoms first appear as death of the apical
meristem and cessation of growth. Root examination reveals
dead areas where residues contacted the roots. A corky root
rot has also been caused by decomposition products of lettuce
residues. Seed germination is inhibited and seedlings are killed
by the toxins.
Control.-At temperatures conducive to lettuce growth, crop
residues are no longer toxic 25 days after they have been in-
corporated.
References.-79.

PESTICIDES
Antibiotic Injury
Streptomycin
Cause.-Experimental applications of streptomycin sulfate
to greenhouse-grown head lettuce showed that one application
of 1 part in 25 parts of water (by weight) was injurious. Four
applications of 1 part in 1000 of water at weekly intervals were
similarly toxic.
Symptoms.-Leaves are slightly chlorotic and treated plants
are more susceptible to freeze injury.
References.-57.

Fungicide Injury
Material Formulation Symptoms
Sprays:
Ammoniacal 21/ oz. copper slight burn
copper carbonate
carbonate 11/2 pt. ammonia
50 gal. water
Bordeaux 5-4-50 severe burn
mixture








Plant Residue Injury
Cause.-Root injury is especially severe when crop residues
(refuse) have been decomposing in soil. After 10 to 25 days in
saline soil, one such disease was especially severe. Soils were
found to contain water-soluble or ether-soluble toxins, one being
benzoic acid with an amine group. These crops formed injurious
residues: barley, broad-bean, broccoli, lettuce, rye, Sudan grass,
vetch, and wheat.
Symptoms.-Seedlings are especially sensitive to the malady.
Above-ground symptoms first appear as death of the apical
meristem and cessation of growth. Root examination reveals
dead areas where residues contacted the roots. A corky root
rot has also been caused by decomposition products of lettuce
residues. Seed germination is inhibited and seedlings are killed
by the toxins.
Control.-At temperatures conducive to lettuce growth, crop
residues are no longer toxic 25 days after they have been in-
corporated.
References.-79.

PESTICIDES
Antibiotic Injury
Streptomycin
Cause.-Experimental applications of streptomycin sulfate
to greenhouse-grown head lettuce showed that one application
of 1 part in 25 parts of water (by weight) was injurious. Four
applications of 1 part in 1000 of water at weekly intervals were
similarly toxic.
Symptoms.-Leaves are slightly chlorotic and treated plants
are more susceptible to freeze injury.
References.-57.

Fungicide Injury
Material Formulation Symptoms
Sprays:
Ammoniacal 21/ oz. copper slight burn
copper carbonate
carbonate 11/2 pt. ammonia
50 gal. water
Bordeaux 5-4-50 severe burn
mixture








Plant Residue Injury
Cause.-Root injury is especially severe when crop residues
(refuse) have been decomposing in soil. After 10 to 25 days in
saline soil, one such disease was especially severe. Soils were
found to contain water-soluble or ether-soluble toxins, one being
benzoic acid with an amine group. These crops formed injurious
residues: barley, broad-bean, broccoli, lettuce, rye, Sudan grass,
vetch, and wheat.
Symptoms.-Seedlings are especially sensitive to the malady.
Above-ground symptoms first appear as death of the apical
meristem and cessation of growth. Root examination reveals
dead areas where residues contacted the roots. A corky root
rot has also been caused by decomposition products of lettuce
residues. Seed germination is inhibited and seedlings are killed
by the toxins.
Control.-At temperatures conducive to lettuce growth, crop
residues are no longer toxic 25 days after they have been in-
corporated.
References.-79.

PESTICIDES
Antibiotic Injury
Streptomycin
Cause.-Experimental applications of streptomycin sulfate
to greenhouse-grown head lettuce showed that one application
of 1 part in 25 parts of water (by weight) was injurious. Four
applications of 1 part in 1000 of water at weekly intervals were
similarly toxic.
Symptoms.-Leaves are slightly chlorotic and treated plants
are more susceptible to freeze injury.
References.-57.

Fungicide Injury
Material Formulation Symptoms
Sprays:
Ammoniacal 21/ oz. copper slight burn
copper carbonate
carbonate 11/2 pt. ammonia
50 gal. water
Bordeaux 5-4-50 severe burn
mixture









Material
Sprays, Cont'd.
Copper sulfate
Lime-sulfur


Potassium sulfide
Soil Fungicides:
Calcium
cyanamide


Methyl
isothiocyanate
pentachloro-
nitrobenzene


Formulation

1, oz per gal.
2 parts of 32.09
Baume/100 gal.
112 oz. per 10 gal.


1000 lb. per acre


traces are toxic


15 lb. 75%
WP/100 gal as
a spray
200 lb. 75%
WP/acre soil
treatment


Symptoms

severe burn
slight burn


slight burn


enlarged midribs,
enlarged veins,
smooth leaf
margins, marginal
burn, stunting,
and poor heading


slight burn, more
frost injury

delayed heading,
stunting


References.-15, 93.


Growth Regulators

Cause.-Traces of low volatile esters of 2,4-D in spray equip-
ment or an accidental drift of this herbicide have caused injury.
Symptoms.-If plants are sprayed with such contaminated
equipment or receive such a drift when they are 3 to 5 weeks
old, leaves are twisted and distorted. If plants form heads,
these are loose and peaked, and do not store well. Drifted
indoleacetic acid (IAA) caused inner leaf margins to roll back
and laminae were thickened. Low concentrations can inhibit
seed germination. Seedlings grown in a solution containing
0.2 to 1 ppm 2,4-D developed very short, swollen roots with
excess hairs. A ring of swollen hypocotyl tissue also arose.
Control.-Avoid using the volatile ester of 2,4-D. Clean
metal equipment thoroughly with an alkali and change non-
metalic parts. It is best to use herbicide spray equipment only









for such a purpose. One must also bear in mind that irrigation
water may carry herbicides from treated ditch banks.
References.-23, 50, 75, 84.

Pentachlorophenol Injury
Cause.-Pentachlorophenol (a wood preservative), either as
crystals or dissolved in the customary solvents, injures lettuce
which is exposed to it, especially in a confined space. The ex-
tent of injury is proportional to the duration of confinement.
Solvents-pine oil, astral oil, deodorized kerosene, methanol
and mineral oil-also may cause similar injury. Wooden flats,
treated with pentachlorophenol, were injurious to plants grown
in them, even after drying for 8 months.
Symptoms.-The volatized material caused leaf edges to turn
brown and curl; later the entire leaf shriveled. Plants grown in
treated flats were stunted and yellowish.
Control.-Substitute copper naphthenate for pentachloro-
phenol.
References.-38.

Sodium Chlorate Injury
Cause.-The effects of various concentrations of sodium
chlorate were determined in a Dutch greenhouse experiment as
follows:

Symptoms

mg per kg soil
2 to 4 Veins are yellow in leaves, especially the
smaller ones.

8 Leaves are completely yellow and some veins
are brown.

125 to 250 Margins and inner lamina are necrotic,
brown.

500 Leaves completely killed.

In some cases this injury may resemble lettuce mosaic caused
by a virus.
References.-10.









for such a purpose. One must also bear in mind that irrigation
water may carry herbicides from treated ditch banks.
References.-23, 50, 75, 84.

Pentachlorophenol Injury
Cause.-Pentachlorophenol (a wood preservative), either as
crystals or dissolved in the customary solvents, injures lettuce
which is exposed to it, especially in a confined space. The ex-
tent of injury is proportional to the duration of confinement.
Solvents-pine oil, astral oil, deodorized kerosene, methanol
and mineral oil-also may cause similar injury. Wooden flats,
treated with pentachlorophenol, were injurious to plants grown
in them, even after drying for 8 months.
Symptoms.-The volatized material caused leaf edges to turn
brown and curl; later the entire leaf shriveled. Plants grown in
treated flats were stunted and yellowish.
Control.-Substitute copper naphthenate for pentachloro-
phenol.
References.-38.

Sodium Chlorate Injury
Cause.-The effects of various concentrations of sodium
chlorate were determined in a Dutch greenhouse experiment as
follows:

Symptoms

mg per kg soil
2 to 4 Veins are yellow in leaves, especially the
smaller ones.

8 Leaves are completely yellow and some veins
are brown.

125 to 250 Margins and inner lamina are necrotic,
brown.

500 Leaves completely killed.

In some cases this injury may resemble lettuce mosaic caused
by a virus.
References.-10.








Insecticide Injury
Cause.-Generally, lettuce is resistant to insecticide damage.
Leaf lettuce is very susceptible at all ages to parathion aerosols
and sprays in the greenhouse. Lettuce tolerates high rates of
DDT, aldrin, chlordane, and toxaphene in the soil.
Symptoms.-Young leaves show severe necrosis and older
ones have translucent spots when parathion is toxic.
Controls.-The enclosed conditions of a greenhouse are con-
ducive to pesticide injury that might not occur in an open field.
Adequate ventilation might sometimes avoid injury.
References.-16, 98, 99.

CLIMATE -GASES
Smog
Gaseous Component
Lettuce, including the cos type, is extremely susceptible to
smog.
Symptoms.-The underside of the leaf has a silvery glaze
as the protoplasts of the lower mesophyll collapse and are re-
placed by air pockets. Eventually the entire mesophyll is killed,
and leaves have necrotic areas that are not confined by the veins.

Aerosol Component
This is severe on some very foggy days.
Symptoms.-Usually the upper surface is the first to be
affected. There is no glazing or bleaching, merely necrotic spot-
ting scattered over the leaves.
References.-67

Ethylene-"Brown Spot"
Cause.-Improper storage or storage of lettuce with fruit
can cause severe symptoms as ethylene accumulates to a toxic
concentration. Twenty and 35 ppm of ethylene were found to
cause the disease at temperatures ranging from 35 to 60 F,
with most serious injury at 44 to 45 F. When head lettuce was
removed from the ethylene environment and stored at 70 F for
four days, the injury did not increase beyond the original dam-
age. In the Southwest, lettuce produced during the cooler "slow
growing" season is most likely to be affected.
Symptoms.-Most of the disease is found in hard, mature








Insecticide Injury
Cause.-Generally, lettuce is resistant to insecticide damage.
Leaf lettuce is very susceptible at all ages to parathion aerosols
and sprays in the greenhouse. Lettuce tolerates high rates of
DDT, aldrin, chlordane, and toxaphene in the soil.
Symptoms.-Young leaves show severe necrosis and older
ones have translucent spots when parathion is toxic.
Controls.-The enclosed conditions of a greenhouse are con-
ducive to pesticide injury that might not occur in an open field.
Adequate ventilation might sometimes avoid injury.
References.-16, 98, 99.

CLIMATE -GASES
Smog
Gaseous Component
Lettuce, including the cos type, is extremely susceptible to
smog.
Symptoms.-The underside of the leaf has a silvery glaze
as the protoplasts of the lower mesophyll collapse and are re-
placed by air pockets. Eventually the entire mesophyll is killed,
and leaves have necrotic areas that are not confined by the veins.

Aerosol Component
This is severe on some very foggy days.
Symptoms.-Usually the upper surface is the first to be
affected. There is no glazing or bleaching, merely necrotic spot-
ting scattered over the leaves.
References.-67

Ethylene-"Brown Spot"
Cause.-Improper storage or storage of lettuce with fruit
can cause severe symptoms as ethylene accumulates to a toxic
concentration. Twenty and 35 ppm of ethylene were found to
cause the disease at temperatures ranging from 35 to 60 F,
with most serious injury at 44 to 45 F. When head lettuce was
removed from the ethylene environment and stored at 70 F for
four days, the injury did not increase beyond the original dam-
age. In the Southwest, lettuce produced during the cooler "slow
growing" season is most likely to be affected.
Symptoms.-Most of the disease is found in hard, mature








Insecticide Injury
Cause.-Generally, lettuce is resistant to insecticide damage.
Leaf lettuce is very susceptible at all ages to parathion aerosols
and sprays in the greenhouse. Lettuce tolerates high rates of
DDT, aldrin, chlordane, and toxaphene in the soil.
Symptoms.-Young leaves show severe necrosis and older
ones have translucent spots when parathion is toxic.
Controls.-The enclosed conditions of a greenhouse are con-
ducive to pesticide injury that might not occur in an open field.
Adequate ventilation might sometimes avoid injury.
References.-16, 98, 99.

CLIMATE -GASES
Smog
Gaseous Component
Lettuce, including the cos type, is extremely susceptible to
smog.
Symptoms.-The underside of the leaf has a silvery glaze
as the protoplasts of the lower mesophyll collapse and are re-
placed by air pockets. Eventually the entire mesophyll is killed,
and leaves have necrotic areas that are not confined by the veins.

Aerosol Component
This is severe on some very foggy days.
Symptoms.-Usually the upper surface is the first to be
affected. There is no glazing or bleaching, merely necrotic spot-
ting scattered over the leaves.
References.-67

Ethylene-"Brown Spot"
Cause.-Improper storage or storage of lettuce with fruit
can cause severe symptoms as ethylene accumulates to a toxic
concentration. Twenty and 35 ppm of ethylene were found to
cause the disease at temperatures ranging from 35 to 60 F,
with most serious injury at 44 to 45 F. When head lettuce was
removed from the ethylene environment and stored at 70 F for
four days, the injury did not increase beyond the original dam-
age. In the Southwest, lettuce produced during the cooler "slow
growing" season is most likely to be affected.
Symptoms.-Most of the disease is found in hard, mature








heads, especially on the midribs of older, outer leaves. Severe
cases, however, are affected throughout the head, and inter-
veinal areas are seriously damaged. The effects of ethylene ap-
pear four days after exposure to it. Lesions are tan, brown, or
olive colored, beginning as numerous small (2 square mm) spots.
Both sides of a leaf are affected but the adaxial (inner) side is
more seriously damaged. Only the epidermis and two or three
underlying cells are killed but this may include the entire thick-
ness of interveinal tissue. At first, middle lamellae are reddened,
then cell walls become brown or olive green.
Control.-The disease is not as severe if lettuce is stored at
slightly above freezing temperature. Adequate ventilation and
air renewal are important. Lettuce should never be stored with
ethylene-producing fruits such as apples.
References.-27, 53, 78, 88, 95.

Oxygen
Cause.-This disease of head lettuce was first described as
occurring in cold storage that was poorly ventilated. It was
found to be caused by insufficient oxygen. More disease occurred
at 11 to 15 F than at 32 to 34 F. Lettuce stored in nitrogen for
four days was affected by the injury three minutes after heads
were removed to air. Bacterial soft rot was often associated
with this disease.
Symptoms.-Heart leaf lesions were walnut brown, neutral
red, Indian purple, to chestnut brown (Ridgeway colors).
Lesions are elongated parallel to the midrib axis. Occasionally
outer leaves develop small chestnut brown pits in midribs and
veins or between veins.
Control.-Air in storage should be circulated and contin-
uously renewed. Temperatures should be maintained slightly
above freezing.
References.-69.

Ozone
Cause.-Lettuce stored eight days at 36 F and 90% relative
humidity was injured by 0.05 ppm ozone.
Symptoms.-Leaves became transparent. Midribs showed
brown flecking, and veins and intravascular areas turned yellow
or brown.
Control.-Only the Romaine (cos) varieties are considered
to be resistant; they will tolerate 0.41 ppm of ozone.
References.-39.








Sulfur Dioxide
Lettuce is classed as very susceptible to injury by this gas
but no details regarding symptoms or gas concentrations were
reported.

Cold Injury

Cause.-In the field, early fall freezes are the most severe
because lettuce has not yet hardened. Long freezes are much
more injurious than short periods. Entire heads may freeze
briefly, if plants are hardened, with no serious injury. Heads
will thaw and appear normal, depending on variety. Tissues
freeze, on the average, at 31.2 F. However, when packed tightly
in a 2-dozen crate with ice on top, only the outer heads freeze
when stored at 20 F for 54 hours. These outer heads apparently
provide good insulation. Pentachloronitrobenzene or streptomy-
cin sulfate applications made the plants unusually susceptible
to freeze injury.
Symptoms.-A quick freeze of field lettuce causes the epi-
dermis to separate from the mesophyll and gives the leaves a
silvery appearance due to the underlying air space. Freezing
will cause subsequent slow growth and irregular head develop-
ment. Parts of leaves may be killed, especially on tops of mature
heads.
Control.-The average monthly cardinal temperatures for
lettuce growth are: minimum 45 F, optimum 60 to 65 F, maxi-
mum 70 to 75 F. The recommended storage temperature is 32 F
in a relative humidity of 90 to 95%.
Varietal susceptibility to freeze injury is as follows:

Excellent resistance Imperial 17
Good Imperial 615
Moderate Imperial D, 44, 152, 410, 847,
850, and Great Lakes
Poor Imperial F. and 456
Very poor Most butter head varieties
References.-11, 48, 89.

Light Deficiency

Cause.-Less than 300 foot candles of daylight or its equiva-
lent is insufficient for optimum growth. Even in a high-light








Sulfur Dioxide
Lettuce is classed as very susceptible to injury by this gas
but no details regarding symptoms or gas concentrations were
reported.

Cold Injury

Cause.-In the field, early fall freezes are the most severe
because lettuce has not yet hardened. Long freezes are much
more injurious than short periods. Entire heads may freeze
briefly, if plants are hardened, with no serious injury. Heads
will thaw and appear normal, depending on variety. Tissues
freeze, on the average, at 31.2 F. However, when packed tightly
in a 2-dozen crate with ice on top, only the outer heads freeze
when stored at 20 F for 54 hours. These outer heads apparently
provide good insulation. Pentachloronitrobenzene or streptomy-
cin sulfate applications made the plants unusually susceptible
to freeze injury.
Symptoms.-A quick freeze of field lettuce causes the epi-
dermis to separate from the mesophyll and gives the leaves a
silvery appearance due to the underlying air space. Freezing
will cause subsequent slow growth and irregular head develop-
ment. Parts of leaves may be killed, especially on tops of mature
heads.
Control.-The average monthly cardinal temperatures for
lettuce growth are: minimum 45 F, optimum 60 to 65 F, maxi-
mum 70 to 75 F. The recommended storage temperature is 32 F
in a relative humidity of 90 to 95%.
Varietal susceptibility to freeze injury is as follows:

Excellent resistance Imperial 17
Good Imperial 615
Moderate Imperial D, 44, 152, 410, 847,
850, and Great Lakes
Poor Imperial F. and 456
Very poor Most butter head varieties
References.-11, 48, 89.

Light Deficiency

Cause.-Less than 300 foot candles of daylight or its equiva-
lent is insufficient for optimum growth. Even in a high-light








Sulfur Dioxide
Lettuce is classed as very susceptible to injury by this gas
but no details regarding symptoms or gas concentrations were
reported.

Cold Injury

Cause.-In the field, early fall freezes are the most severe
because lettuce has not yet hardened. Long freezes are much
more injurious than short periods. Entire heads may freeze
briefly, if plants are hardened, with no serious injury. Heads
will thaw and appear normal, depending on variety. Tissues
freeze, on the average, at 31.2 F. However, when packed tightly
in a 2-dozen crate with ice on top, only the outer heads freeze
when stored at 20 F for 54 hours. These outer heads apparently
provide good insulation. Pentachloronitrobenzene or streptomy-
cin sulfate applications made the plants unusually susceptible
to freeze injury.
Symptoms.-A quick freeze of field lettuce causes the epi-
dermis to separate from the mesophyll and gives the leaves a
silvery appearance due to the underlying air space. Freezing
will cause subsequent slow growth and irregular head develop-
ment. Parts of leaves may be killed, especially on tops of mature
heads.
Control.-The average monthly cardinal temperatures for
lettuce growth are: minimum 45 F, optimum 60 to 65 F, maxi-
mum 70 to 75 F. The recommended storage temperature is 32 F
in a relative humidity of 90 to 95%.
Varietal susceptibility to freeze injury is as follows:

Excellent resistance Imperial 17
Good Imperial 615
Moderate Imperial D, 44, 152, 410, 847,
850, and Great Lakes
Poor Imperial F. and 456
Very poor Most butter head varieties
References.-11, 48, 89.

Light Deficiency

Cause.-Less than 300 foot candles of daylight or its equiva-
lent is insufficient for optimum growth. Even in a high-light








area such as Arizona, a slatted cover providing one-half shade
reduced the quality of Great Lakes head lettuce as compared to
unshaded beds in the same field.
Symptoms.-Plants are light weight and head leaves do not
overlap tightly. Leaves are narrower than normal, especially at
their bases, are irregularly cupped, and often bent to one side.
The result is a small, loose (soft)head.
Control.-Provide at least 300 to 500 foot candles of day-
light or neon and mercury-lamp illumination. These lamps were
found to be superior to sodium or fluorescent lamps. Plant
weight was directly proportional to such light up to 300 to 500
foot candles.
Lettuce leaves will absorb 10% sucrose solution which will
substitute for part of the light needed for photosynthesis. This
might be useful for laboratory studies.
References.-52, 111.
Ionizing Radiation
Cause.-Chronic gamma irradiation.
Symptoms.-Lettuce leaf blades gradually become thicker
than normal.
References.-33.
Pink Rib
Cause.-The cause of the disease as it occurs in unbruised
head lettuce in the field is unknown. In the Southwest the
disease is most likely to be found in crops harvested from Jan-
uary to May. The malady is common in lettuce which has been
harvested and packed tightly for shipment and storage. Such
heads often are pink wherever they are bruised. This type of
pink rib was less severe in storage for 6 days at 37 F than at
47 F.
Symptoms.-A diffuse pink coloration is seen in the base of
the other leaf midveins (ribs) of mature heads. It is especially
noticeable on the inside adaxiall) surface of the rib. The color
intensifies with age and, in stored lettuce, may extend through
the entire veination of a leaf far beyond its bruises. Microscopic
observation revealed three discolored areas: (1) parenchyma
surrounding the lacunae in the rib base, (2) parenchyma ad-
jacent to lactiferous ducts near the phloem, and (3) vessel
walls scattered or grouped in the xylem. The protoplasm as well
as cell walls becomes pink or red. At times, when the cortex
is stripped from a seed stalk, the underlying stele appears bright








area such as Arizona, a slatted cover providing one-half shade
reduced the quality of Great Lakes head lettuce as compared to
unshaded beds in the same field.
Symptoms.-Plants are light weight and head leaves do not
overlap tightly. Leaves are narrower than normal, especially at
their bases, are irregularly cupped, and often bent to one side.
The result is a small, loose (soft)head.
Control.-Provide at least 300 to 500 foot candles of day-
light or neon and mercury-lamp illumination. These lamps were
found to be superior to sodium or fluorescent lamps. Plant
weight was directly proportional to such light up to 300 to 500
foot candles.
Lettuce leaves will absorb 10% sucrose solution which will
substitute for part of the light needed for photosynthesis. This
might be useful for laboratory studies.
References.-52, 111.
Ionizing Radiation
Cause.-Chronic gamma irradiation.
Symptoms.-Lettuce leaf blades gradually become thicker
than normal.
References.-33.
Pink Rib
Cause.-The cause of the disease as it occurs in unbruised
head lettuce in the field is unknown. In the Southwest the
disease is most likely to be found in crops harvested from Jan-
uary to May. The malady is common in lettuce which has been
harvested and packed tightly for shipment and storage. Such
heads often are pink wherever they are bruised. This type of
pink rib was less severe in storage for 6 days at 37 F than at
47 F.
Symptoms.-A diffuse pink coloration is seen in the base of
the other leaf midveins (ribs) of mature heads. It is especially
noticeable on the inside adaxiall) surface of the rib. The color
intensifies with age and, in stored lettuce, may extend through
the entire veination of a leaf far beyond its bruises. Microscopic
observation revealed three discolored areas: (1) parenchyma
surrounding the lacunae in the rib base, (2) parenchyma ad-
jacent to lactiferous ducts near the phloem, and (3) vessel
walls scattered or grouped in the xylem. The protoplasm as well
as cell walls becomes pink or red. At times, when the cortex
is stripped from a seed stalk, the underlying stele appears bright








red. Such reddening has been seen in mature seedstalks in the
field.
Control.-Avoid unfavorable storage temperatures and bruis-
ing. No control is known for the field disease.
References.-28, 53, 62.

GENETIC
Variegation, Albinism
Cause-Chlorophyll deficiency can be an inherited character-
istic which is transmitted as a single, recessive gene or by
cytoplasmic inheritance. The latter is more common.
Symptoms.-Leaves may be variegated with white, complete-
ly colorless, or may show a blotchy absence of green pigment.
The plant's vigor is reduced in proportion to the amount of white
area. When most of the leaves are white the plant usually dies
before producing seeds. Seedlings which are completely white
(albinos) die as the seed's nutrients are depleted. Cotyledons
are affected as well as true leaves.
Control.-Rogueing the variegated plants helps eliminate the
disease from a seed stock.
References.-101, 114.

SENESCENCE
Marginal Browning
Cause.-"Senescence" of the plant in its vegetative stage is
not actually old age, but a relative term concerning the market-
ability of lettuce after heads have been harvested.
Symptoms.-The margins of older, outer leaves first become
yellow, then necrotic. Inner head leaves remain normal for a
much longer period. The necrotic areas widen with age.
Control.-Avoid harvesting overmature heads. Refrigerate
the heads immediately and properly. The storage life of head
lettuce at 32 F and 90-95% relative humidity averages three
weeks. Heads shipped at 35.5 F (average) show less marginal
browning after being moved to 70 F for three days than heads
shipped at 40.9 F (average).
References.-6, 48, 85.

Red Cotyledon
Cause.-Senescence of seed, which is usually due to improper
storage, is usually responsible for this problem. Seed from nitro-








red. Such reddening has been seen in mature seedstalks in the
field.
Control.-Avoid unfavorable storage temperatures and bruis-
ing. No control is known for the field disease.
References.-28, 53, 62.

GENETIC
Variegation, Albinism
Cause-Chlorophyll deficiency can be an inherited character-
istic which is transmitted as a single, recessive gene or by
cytoplasmic inheritance. The latter is more common.
Symptoms.-Leaves may be variegated with white, complete-
ly colorless, or may show a blotchy absence of green pigment.
The plant's vigor is reduced in proportion to the amount of white
area. When most of the leaves are white the plant usually dies
before producing seeds. Seedlings which are completely white
(albinos) die as the seed's nutrients are depleted. Cotyledons
are affected as well as true leaves.
Control.-Rogueing the variegated plants helps eliminate the
disease from a seed stock.
References.-101, 114.

SENESCENCE
Marginal Browning
Cause.-"Senescence" of the plant in its vegetative stage is
not actually old age, but a relative term concerning the market-
ability of lettuce after heads have been harvested.
Symptoms.-The margins of older, outer leaves first become
yellow, then necrotic. Inner head leaves remain normal for a
much longer period. The necrotic areas widen with age.
Control.-Avoid harvesting overmature heads. Refrigerate
the heads immediately and properly. The storage life of head
lettuce at 32 F and 90-95% relative humidity averages three
weeks. Heads shipped at 35.5 F (average) show less marginal
browning after being moved to 70 F for three days than heads
shipped at 40.9 F (average).
References.-6, 48, 85.

Red Cotyledon
Cause.-Senescence of seed, which is usually due to improper
storage, is usually responsible for this problem. Seed from nitro-








red. Such reddening has been seen in mature seedstalks in the
field.
Control.-Avoid unfavorable storage temperatures and bruis-
ing. No control is known for the field disease.
References.-28, 53, 62.

GENETIC
Variegation, Albinism
Cause-Chlorophyll deficiency can be an inherited character-
istic which is transmitted as a single, recessive gene or by
cytoplasmic inheritance. The latter is more common.
Symptoms.-Leaves may be variegated with white, complete-
ly colorless, or may show a blotchy absence of green pigment.
The plant's vigor is reduced in proportion to the amount of white
area. When most of the leaves are white the plant usually dies
before producing seeds. Seedlings which are completely white
(albinos) die as the seed's nutrients are depleted. Cotyledons
are affected as well as true leaves.
Control.-Rogueing the variegated plants helps eliminate the
disease from a seed stock.
References.-101, 114.

SENESCENCE
Marginal Browning
Cause.-"Senescence" of the plant in its vegetative stage is
not actually old age, but a relative term concerning the market-
ability of lettuce after heads have been harvested.
Symptoms.-The margins of older, outer leaves first become
yellow, then necrotic. Inner head leaves remain normal for a
much longer period. The necrotic areas widen with age.
Control.-Avoid harvesting overmature heads. Refrigerate
the heads immediately and properly. The storage life of head
lettuce at 32 F and 90-95% relative humidity averages three
weeks. Heads shipped at 35.5 F (average) show less marginal
browning after being moved to 70 F for three days than heads
shipped at 40.9 F (average).
References.-6, 48, 85.

Red Cotyledon
Cause.-Senescence of seed, which is usually due to improper
storage, is usually responsible for this problem. Seed from nitro-








red. Such reddening has been seen in mature seedstalks in the
field.
Control.-Avoid unfavorable storage temperatures and bruis-
ing. No control is known for the field disease.
References.-28, 53, 62.

GENETIC
Variegation, Albinism
Cause-Chlorophyll deficiency can be an inherited character-
istic which is transmitted as a single, recessive gene or by
cytoplasmic inheritance. The latter is more common.
Symptoms.-Leaves may be variegated with white, complete-
ly colorless, or may show a blotchy absence of green pigment.
The plant's vigor is reduced in proportion to the amount of white
area. When most of the leaves are white the plant usually dies
before producing seeds. Seedlings which are completely white
(albinos) die as the seed's nutrients are depleted. Cotyledons
are affected as well as true leaves.
Control.-Rogueing the variegated plants helps eliminate the
disease from a seed stock.
References.-101, 114.

SENESCENCE
Marginal Browning
Cause.-"Senescence" of the plant in its vegetative stage is
not actually old age, but a relative term concerning the market-
ability of lettuce after heads have been harvested.
Symptoms.-The margins of older, outer leaves first become
yellow, then necrotic. Inner head leaves remain normal for a
much longer period. The necrotic areas widen with age.
Control.-Avoid harvesting overmature heads. Refrigerate
the heads immediately and properly. The storage life of head
lettuce at 32 F and 90-95% relative humidity averages three
weeks. Heads shipped at 35.5 F (average) show less marginal
browning after being moved to 70 F for three days than heads
shipped at 40.9 F (average).
References.-6, 48, 85.

Red Cotyledon
Cause.-Senescence of seed, which is usually due to improper
storage, is usually responsible for this problem. Seed from nitro-








red. Such reddening has been seen in mature seedstalks in the
field.
Control.-Avoid unfavorable storage temperatures and bruis-
ing. No control is known for the field disease.
References.-28, 53, 62.

GENETIC
Variegation, Albinism
Cause-Chlorophyll deficiency can be an inherited character-
istic which is transmitted as a single, recessive gene or by
cytoplasmic inheritance. The latter is more common.
Symptoms.-Leaves may be variegated with white, complete-
ly colorless, or may show a blotchy absence of green pigment.
The plant's vigor is reduced in proportion to the amount of white
area. When most of the leaves are white the plant usually dies
before producing seeds. Seedlings which are completely white
(albinos) die as the seed's nutrients are depleted. Cotyledons
are affected as well as true leaves.
Control.-Rogueing the variegated plants helps eliminate the
disease from a seed stock.
References.-101, 114.

SENESCENCE
Marginal Browning
Cause.-"Senescence" of the plant in its vegetative stage is
not actually old age, but a relative term concerning the market-
ability of lettuce after heads have been harvested.
Symptoms.-The margins of older, outer leaves first become
yellow, then necrotic. Inner head leaves remain normal for a
much longer period. The necrotic areas widen with age.
Control.-Avoid harvesting overmature heads. Refrigerate
the heads immediately and properly. The storage life of head
lettuce at 32 F and 90-95% relative humidity averages three
weeks. Heads shipped at 35.5 F (average) show less marginal
browning after being moved to 70 F for three days than heads
shipped at 40.9 F (average).
References.-6, 48, 85.

Red Cotyledon
Cause.-Senescence of seed, which is usually due to improper
storage, is usually responsible for this problem. Seed from nitro-








gen deficient plants may sometimes be especially susceptible.
Symptoms.-At first a small red spot appears on the upper
surface of the cotyledon base. The spot may gradually enlarge
until the lower half or more of the cotyledon is reddened. The
growth of the plant is seriously retarded if the reddish area
exceeds one-half of the cotyledon.
Control.-Growing conditions rarely affect the disease and
no varietal differences are found. Proper seed storage, as de-
scribed under "Seed Dormancy", will prevent the disease.
References.-18, 34.

MISCELLANEOUS
Rib Discoloration

Cause.-The disease was studied in Arizona for several years,
and some of the possible causes that were eliminated included:
bacteria, fungi, fungicides, irrigation practices, root pruning,
soil-borne organisms, and temperature extremes or drastic
changes. The pattern of occurrence on raised beds or in the
field did not implicate sunburn, nor did infection centers occur
from which the disease spread in or around a field, as it might
if it were caused by a pathogen or insect. More recently the
disease incidence has been positively correlated with the mean
minimum temperature of the 7-day period preceding harvest.
Symptoms.-The second and third leaves from the top of
the head often show the injury three weeks before harvest. The
midrib and main veins are affected, particularly where the rib
branches. Rarely are outer frame leaves, or those near the
center, diseased. Although affected leaves are usually adjacent to
one another, the lesions are not. Lesion dimensions vary from 2 to
90 mm long by 1 to 20 mm in width, averaging 19 x 4.3 mm.
Lesions darken in storage at 37, 47 and 50 F, but neither the
size nor their number increases. Lettuce with rib discoloration
was more susceptible to decay than normal lettuce and had a
greater tendency to develop pink rib in storage.
Control.-Foliar applications of magnesium sulfate, with or
without acid as a chelating agent, at concentrations of 2 to 8
grams per gallon diminished the disease slightly but not suffi-
ciently to be of commercial significance. Daily overhead sprink-
ling late in the growing season reduces rib discoloration. No
resistant varieties are known.
References.-27, 28, 44, 45, 63, 97.








gen deficient plants may sometimes be especially susceptible.
Symptoms.-At first a small red spot appears on the upper
surface of the cotyledon base. The spot may gradually enlarge
until the lower half or more of the cotyledon is reddened. The
growth of the plant is seriously retarded if the reddish area
exceeds one-half of the cotyledon.
Control.-Growing conditions rarely affect the disease and
no varietal differences are found. Proper seed storage, as de-
scribed under "Seed Dormancy", will prevent the disease.
References.-18, 34.

MISCELLANEOUS
Rib Discoloration

Cause.-The disease was studied in Arizona for several years,
and some of the possible causes that were eliminated included:
bacteria, fungi, fungicides, irrigation practices, root pruning,
soil-borne organisms, and temperature extremes or drastic
changes. The pattern of occurrence on raised beds or in the
field did not implicate sunburn, nor did infection centers occur
from which the disease spread in or around a field, as it might
if it were caused by a pathogen or insect. More recently the
disease incidence has been positively correlated with the mean
minimum temperature of the 7-day period preceding harvest.
Symptoms.-The second and third leaves from the top of
the head often show the injury three weeks before harvest. The
midrib and main veins are affected, particularly where the rib
branches. Rarely are outer frame leaves, or those near the
center, diseased. Although affected leaves are usually adjacent to
one another, the lesions are not. Lesion dimensions vary from 2 to
90 mm long by 1 to 20 mm in width, averaging 19 x 4.3 mm.
Lesions darken in storage at 37, 47 and 50 F, but neither the
size nor their number increases. Lettuce with rib discoloration
was more susceptible to decay than normal lettuce and had a
greater tendency to develop pink rib in storage.
Control.-Foliar applications of magnesium sulfate, with or
without acid as a chelating agent, at concentrations of 2 to 8
grams per gallon diminished the disease slightly but not suffi-
ciently to be of commercial significance. Daily overhead sprink-
ling late in the growing season reduces rib discoloration. No
resistant varieties are known.
References.-27, 28, 44, 45, 63, 97.








Spiraled Heads
Cause.-Leaving two or more plants closely adjacent to one
another in the row often results in spiraled (peaked) heads.
Being closely pressed together, the margins of adjacent leaves
may graft together and thus prevent the heads from expanding
normally. Also, the margins of one leaf often becomes grafted,
and the enclosed head is even more likely to be spiraled. Pres-
sure exerted by such a graft for seven days will result in
spiraling. The longer the pressure, the more likely the malady
will occur. Certain varieties are especially susceptible: Great
Lakes (G. L.) S, G. L. 659, and G. L. 6238. More spiral heads
form in spring and fall than in other seasons in the Salinas,
California area.
Another practice which leads to conical heads is planting
transplants too deeply.
Symptoms.-Heads are of a conical shape with wrapper
and/or head leaves in a spiral-like fold. The 6th to 16th true
leaves can be affected, but the 8th to 12th are most commonly
involved.
Conical plants caused by deep transplanting are more densely
packed, elongate, and slightly flattened.
Control.-Planting less susceptible varieties, such as Premier
Great Lakes and G. L. 428, and being careful not to leave
"doubles" in the row at thinning time are good preventatives.
Transplants should have their leaves entirely above the soil.
References.-64, 127.

Tip Burn
Cause.-Many conditions have been correlated with tip
burn, but their great variety and the existence of numerous
conflicting reports lead one to conclude that some of the causes
are still unknown. Some of the reported causes are: warm weath-
er, (others report that it is not related to temperature), excess
water, deficient water, low light intensity plus high humidity
(the disease stopped when humidity was lowered), nitrogen
nutrition, vigor, age, head closure, soil vs. air temperature, and
accumulation of respiratory products. Head lettuce is more
susceptible than leaf lettuce, and there are varietal differences.
Symptoms.-The disease begins as a plant closely approaches
market maturity. Unlike some marginal burns, tip burn usually
begins on inner, mature leaves as small, translucent spots slight-
ly back from the leaf margin; these darken and then the margins








Spiraled Heads
Cause.-Leaving two or more plants closely adjacent to one
another in the row often results in spiraled (peaked) heads.
Being closely pressed together, the margins of adjacent leaves
may graft together and thus prevent the heads from expanding
normally. Also, the margins of one leaf often becomes grafted,
and the enclosed head is even more likely to be spiraled. Pres-
sure exerted by such a graft for seven days will result in
spiraling. The longer the pressure, the more likely the malady
will occur. Certain varieties are especially susceptible: Great
Lakes (G. L.) S, G. L. 659, and G. L. 6238. More spiral heads
form in spring and fall than in other seasons in the Salinas,
California area.
Another practice which leads to conical heads is planting
transplants too deeply.
Symptoms.-Heads are of a conical shape with wrapper
and/or head leaves in a spiral-like fold. The 6th to 16th true
leaves can be affected, but the 8th to 12th are most commonly
involved.
Conical plants caused by deep transplanting are more densely
packed, elongate, and slightly flattened.
Control.-Planting less susceptible varieties, such as Premier
Great Lakes and G. L. 428, and being careful not to leave
"doubles" in the row at thinning time are good preventatives.
Transplants should have their leaves entirely above the soil.
References.-64, 127.

Tip Burn
Cause.-Many conditions have been correlated with tip
burn, but their great variety and the existence of numerous
conflicting reports lead one to conclude that some of the causes
are still unknown. Some of the reported causes are: warm weath-
er, (others report that it is not related to temperature), excess
water, deficient water, low light intensity plus high humidity
(the disease stopped when humidity was lowered), nitrogen
nutrition, vigor, age, head closure, soil vs. air temperature, and
accumulation of respiratory products. Head lettuce is more
susceptible than leaf lettuce, and there are varietal differences.
Symptoms.-The disease begins as a plant closely approaches
market maturity. Unlike some marginal burns, tip burn usually
begins on inner, mature leaves as small, translucent spots slight-
ly back from the leaf margin; these darken and then the margins








die. Tipburn seldom occurs on young leaves. Butter head and cos
types are especially susceptible. Tip-burn free lettuce does not
develop the disease in storage, but if lettuce is affected before
storage, the disease becomes worse.
Control.-Plant resistant varieties, such as: Alaska, Avenue,
Black pool, Great Lakes types, Imperial 44, New York 515,
Premier Great Lakes, Resistant Early French Frame, Rudiger,
and Trocadero. Most seed is now selected for tip-burn resistance,
and there are continually newer varieties with superior charac-
teristics for each lettuce-producing area. As yet, no variety is
immune.
References.-2, 12, 13, 17, 47, 54, 58, 70, 85, 86, 97, 100, 115.









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31









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KEY TO NONPATHOGENIC DISEASES OF LETTUCE


SYMPTOMS
Al Plants missing (poor emergence)
B1 Brown root lesions adjacent to PLANT RESIDUES
B2 Seeds fail to germinate or radicle barely emerges;
POOR. GERMINATION ----...................- -

A2 Heads conical or flattened
B1 Wrapper leaves grafted, forming a spiral;
SPIRALED HEADS --.-----
B2 Heads flattened, deeply planted;
SPIRALED' HEADS ----- --...-.---.

A3 Roots obviously abnormal
B1 Taproot stele discolored
C1 Upper taproot girdled; ROOT GIRDLE --
C2 Taproot pith hollow; stele yellowish, red,
greenish or brown; surface corky or split,
yellowish; NITROGEN TOXICITY --
B2 Roots with dull, gray surfaces, soil or nutrient
solution of extreme ALKALINITY ..--


Page


B3 Roots stubby
C1 Excessive root hairs, leaves assymetrical,
thickened, cupped; GROWTH REGULATORS -
C2 Few lateral roots; leaves generally chlorotic;
ALUMINUM TOXICITY ...............--...... --

A4 Roots relatively normal
B1 Leaves with necrotic areas
C' Small brown, necrotic spots on leaves
D1 Spots on chlorotic blotches near lateral veins;
MANGANESE TOXICITY ..........-....-..
D2 Spots on underside of leaf only, where leaf
surface has a silvery glaze; SMOG ------
D3 Spots on upper surface; no glaze; SMOG ....










C2 Large necrotic areas
D1 Necrosis starts in veins, later is also inter-
veinal; history of herbicide application;
SODIUM CHLORATE INJURY ......- 18
D2 Leaf tips, or margins are first to be necrotic
El Heads large, practically mature
Fl Tips and/or margins of outer head
leaves are first affected
G1 Heads overmature; SENESCENCE _23
G2 Inner head leaves are first affected;
TIP BURN -..-..- .........- 25
E2 Plants somewhat stunted, symptoms may
appear long before heads are mature
Fl Heart leaves stunted, thick, cupped
G1 Found on alkaline soils; heart
leaves with tip burn; interveinal
chlorosis; BORON DEFICIENCY 5
G2 Found on acid soils; plants rosetted;
a general chlorosis; CALCIUM
DEFICIENCY .............. ...----... 5
F2 Heart leaves not stunted, thick or cupped
G1 Leaf undersides with silvery glaze;
freeze injury; COLD .---- 21
G2 Leaves unusually smooth; a history
of application of high rates of cal-
cium cyanamide; FUNGICIDES .... 16
G3 Leaves temporarily darker green
than normal and with a heavy
grayish bloom; POTASSIUM
DEFICIENCY ...............- ........ 10
G4 Leaves evenly chlorotic; some inter-
veinal necrosis; acid soil only;
MOLYBDENUM DEFICIENCY .. 8
H1 May occur in any soil;
PENTACHLOROPHENOL 18


SYMPTOMS


Page










D3 Necrotic areas not primarily marginal
El Necrosis develops in stored heads
F1 Disease more serious at temperatures
below freezing; Accompanies insuffi-
cient oxygen; Lesions throughout
head; OXYGEN DEFICIENCY ..... .20
F2 Disease most serious at 44-45 F; ac-
companies exposure to ethylene; ne-
crotic lesions throughout head;
ETHYLENE ........ ..... 19

E2 Necrosis develops before harvest
F1 Cotyledons affected only
G1 Red spots on cotyledons;
RED COTYLEDON ...- .23
G2 Black or brown spots on cotyledons;
DROUGHT SPOT .- :....... ....13
F2 Necrotic spots only on exposed leaves,
not inside head
G1 Necrotic spots only on leaves
receiving a spray or dust
H1 Typical pesticide injury, i.e.,
necrotic spots sunken, papery,
abruptly adjoining healthy
tissue. Necrotic areas where
spray could accumulate.
IRON TOXICITY or
FUNGICIDES ... ... 7 or 16
G2 Plants may not have been sprayed;
outer leaves become translucent;
OZONE ............ ............20
F3 Necrotic spots not restricted to outer,
exposed leaves
G1 Necrotic spots restricted to midrib
and major lateral veins of a few
outer head leaves; RIB
DISCOLORATION ..... ...... -. 24


SYMPTOMS


Page








SYMPTOMS Page
G2 Midribs hollow; leaves with heavy,
grayish bloom; necrotic spots near
leaf margins; MANGANESE
DEFICIENCY .
G3 Leaves often assymetrical, with
white, chlorotic blotches;
MAGNESIUM DEFICIENCY 7
G4 Necrotic spots near margins; severe
stunting; BORON TOXICITY 5
G5 Translucent spots on older leaves;
INSECTICIDES 19
B2 Leaves rarely, if ever, necrotic
C1 Red areas at base of cotyledons;
RED COTYLEDON 23
C2 Leaves chlorotic or white
D1 Leaves with white sectors or white marbling;
VARIEGATION ...23
D2 Chlorosis as yellowing
El Veins remain distinctly green;
IRON DEFICIENCY ................ 7
E2 Chlorosis as a general yellowing,
beginning at leaf margins or tips
Fl Leaves narrowed; COPPER
DEFICIENCY 6
F2 Leaves tough with edges curled; "red"
varieties with bronze and purple
blotches; PHOSPHORUS
DEFICIENCY ............ 10
F3 Leaf edges curled; leaves narrow and
assymetrical; head soft; LIGHT
DEFICIENCY ........ 21
F4 Leaves unusually smooth and somewhat
stunted; maturity delayed or no heads
formed; NITROGEN DEFICIENCY 9
F5 Similar to above but following strepto-
mycin application; adequate nitrogen
applied; ANTIBIOTICS ....... 16








SYMPTOMS Page
C3 No chlorosis
D1 Plant not stunted
El Heads puffy and soft or hard and split;
PUFFY AND BURST HEADS ..... 14
E2 Base of petiole of mature head with a
diffuse pink color, especially in adaxial
view; in storage, color deepens and all main
veins can be affected; PINK RIB ...22
E3 Leaf blade thickened; IONIZING
RADIATION .......- ....... 22
D2 Plant somewhat stunted
El Leaf edges curled; leaves narrow and
assymetrical; head soft; LIGHT
DEFICIENCY ......... ... 21
E2 Very susceptible to freeze injury; history
of PCNB application; FUNGICIDES
(PCNB) --... --................- 16
*E3 Roots stunted
*F1 ACIDITY ......... 11
*F2 CARBON DIOXIDE .....- 12
*E4 Roots not stunted
*F1 Maturity delayed; ZINC
DEFICIENCY .. .... 11
*F2 History of excessive zinc applications
ZINC TOXICITY .. .. 11
*F3 History of excessive iodine applications
IODINE TOXICITY ...... 6






* Symptoms for these disorders are not sufficient for diagnosis; knowledge
of the crop history, soil or nutrient solution, and tissue content is neces-
sary.




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